Atmospheric Ozone

i::r'~~O~~PIJERIC O~ONE

~ thesis nres8nt~a for the deerGs of

by

'fhe development of a I)]:.!.;y:.::ic£uly compact, light, robust ':met rel l,ti vel~y cheap f11 ter ozonometer for me;J8uring the ;;llilOllnt of ozone in t (Le atmosphere is discuf; sed.

Jzone uu.t::t fr'.);-n the filter instrument and a Dobson np8ctro,;>hotometer were found to 8}~~re~~ both in term:_ of relative change and 8oso1ute l1IDOunt. i t CLm '~le concluded from this close correlation bet~een ta for the complete ranGe 01' airmasl;j avai lable, th".t the filter instr'Jment con-~?tructed in this proj ect gives ozone values wi tb a.n

c:iccurhc,y cOillYdrableto that of the Dobson spc:ctrophotometer.

1), good correlation,\lCi3 found, in the limited ;3a~ple of dll t,3 available, between the daily mea:l 0 one values from the fil tE:r instrument and meteorolotr,ical upper air par::lmeters.

11..Il. .?utoir,,<1tic mo(~;e of oj)erution for the filter instrument

vzonc C:1U 1:;8 c.')IlSi ered a COl) serv~ti ve c~JDsti tu(~nt in the lo':!nr stritospherE; becau~')e of the consid.er ble r;hotochem-ieal shielding bJ the air ab0~e i t . ~ ll'-reforf", vD,rL::tions in UL;Olle act as a usefuli,r!;1.c8r of a tITlO E;pheric motion in this rb',;i)H ( ) f tlw .tmo<?here.

~his thesis describes the develoument of D filt~r ozone epectro hotoIDeter that ~as successfully use~ to measure at-mospheric ozone concentr tions and includes on account of the 1ni ti9.l d:-;!,ta ootclined fro::;] this instl'uwent.

~hc f i l t0 r ozone specTrophotometer Rnd itA associated

electronics were C0nstructed by the author with the exception

) f the equ'Jtorial mount and :::;Ull follower teleseo-~)e, '"hieh

'i. ere constructed in the ,Iilechanical orkf"\ho;J of the ,h;ysics

L;epClrt.~Jentt and the !)hasine; pulse ::;ener,;'.tor whieh Vv8,!:, c::evelop-eel by

i.:,.

~~orrish in the i~lee tronici, ;,orkshop.

i':"nis Drorjt~ct \lve',i:,: the su;,';e tion of my ~::-,n')crvisor, :Jr

ih,rin this l:;roj0ct, '!.lsefL'l

c.is-CussiOIla ~'Je, e held '.vi th mClny peo,)le C:Jnci I would;~articularly.

L::.ckn:):,;l(;d;1;(~ tYe time ;'0 '~;e()8rou:.:ly f~lven by ;.:"U tic -_;'~:9.rltEls,LJr

I am Bob',;,en:1ett, "i.eic Ba.~,ller , Olive .o,-;e anG l>,n,~orrish.

J. hysics Uepartment of the ~ni versi ty of Canterbury for

finan-d_31 suplJort in tl~e fDrrn Df .:3 !lleaching i;'e~ lowship. lv;y thanks also to i,irs Marf;aret 3el:ell for typing ttis nanuscript ane. .. lr L3urie f:unter for photo raphic services.

I am indebted to the [';ew ,~ealand ;!eteorolopical iService for providing tbe facilities at invercargill that m:~deposs­ ible comparison measurements wit~ the Dobson spectrophoto-meter, f ')r the rGQucti on of additional ozone datil and for

;::-rovidinF; U;iper air and urfg.,ce pressure d;:l.ta. l'inancial a:: foistance for er-lui pment 1;'m,;3 received from the [; ew ,jei.lland

CliAl<l:hH. 1: Introduction

1.1 history

1.2 ·.l!heory

1 • .3 Jiim of lroject

2.1 introduction

~.2 er

2.~ ~hotomultiplier

3.1

Introtuction

3.2

Jilters

3.3

Losarit~nic HDplifier

3.4

~pectral ~ensitivity

Ulul1.l'I:ER 4; Instrument 0011stants and lJata h(;duction

), ? '"1".1.:..

4.4

IntroQuction

.~bsorption '-'oeff'icients

x:';xtra ierrestl'ial Constant

13. t~ l':eauction

1 .1 1.3 1 .11

2.1

2.11

4.1 4.1

C .. l;.PT.EII.

5:

Comparison vd th Doi)son ~ipectrophotoli.leter

5.1

Introduction

5.2

Dobson ~~ectropbot0meter

5.3

tesign Differen~es

5.

4Sxperimental r~esul ts

5.5

Effect of Cloud

C:.lAF": t::H 6: llelation bet1.ciieen '..Lotal Ozone and ,iieteoro-logical ~arameters

6.1 Introduction

6.2 l'ressure Corlel';-tion

6. L~ Upper Air li

e:ilperntures

6.5

Longer rCerm V:}rLltions 6.6 Short Term Variations

ClllUl_{TER 7: Conclusion}

J~PPt:NDIX.it: Cnlculation of Airmass

F'il ter ,)pectroohotometer

B.2 iilters

~ic~el ~ulphete Uexahydr~te 1ilter 13.4

" 5

.u. ,I ew Interference fil tars

5.7

6.1

6

7-• .,1

G.8

6.10 6.13 6.16

B.1

b.9

.~lect.r()riics

r lhotomultiplier C.1

C.3

Ihasin~ fulse G or C.6

C.4- ,;un Follower .cr:vo ve

e.G

uJ L'.t::rlDIX D: Automatic Lata rroce!"sing

"'ieure

=ro~.011er ~1965)

2.3

~8rcent~~e tr~nsmi8sion o~ iaterference filter

2.4 Cross-section of simrle interfer3ncc filter, ~rom

Croes-section of ;Jltilayer interfere~ce £ilter,

2.12 2.6 ,,:Jl,£,r 2pec tru'l e t ths o;roullCi

2.7 2.14

violet into ozo~e ata 2.16

j\iount 2.20

Gairs of interference ~ilter2

:.;. b

C 1.1i br tiou curve .t"() r lOt,::r:ri tlli:lic am.;;lif ier 3.8

..Figure

;~hot')lneteI' ant: f ~l teI' ozonorne1JeI' at .i.nvercare;ill,

f'ronI :·""t'·'n=':IS (l'li nr.=.c.s")

.L. ' d l~ t) _ L'C; .11 . _ !~. ... '-'

.5

.10

.1 T)aily me '.1.0. p::L'esE.ures at tne 3urface, 1() ~CL:l and

15

kn and I,'aily ~llea:n OZOlle amount in l.lecember 1970 6.4 (-).2 ])::iily ll18au p:::.'e"sures at the nuri':Jce, 10 Yilll and

15

~;:n; 811(} daily mean ozone amount in 2ebruary 1971 5.5 6.:3 Compurison D:' ·jaily iJe&.n t rOj)opauB8 height and

drlily i:16Em oJ,one GJJlount in lJecembeI' 1970

6.9

(;. il Compari son of ('Jl.ily llle~in tropopau~:e height and

daily me"lll OZ,Jne 3J.i10unt in ii'ebru3.r;y 1971

6.5 })3ily mean 100 iiloteorer tu.x-e,:; ".G:1d daily uean

6.12

ozone amount i~l ~:ebru,J.I'y 1971

6.7 .. _IllOottJeC_ v~o.11.1e2 of prer: :.;ure, te:uper" -cure, tro

po-G.C) "hart ter;, V:lrl2.tlOn[~ i:J oz;one over ChriEctchurch

in December 1970 6.12

.1.1

3-D

geometry for airmass calculations .H.,.2

~;ip;ure

D.1 Insi(e vie: 81' ~~-,;)Cct:::'Orj:lOtollieter unit n _,

.

_.:.... ?

0.1 . Lotomul tipli er ;:. 'J.:. supply C ~

.c::

.wOi:;nri -cI,(!Jic aJ',plifi er

\...j ,\ .a:,ang ;::mlE8 ~::;ener:.:1tor

1.1

C HAP T E R 1

INTRODUCTION

1.1 History

~he presence of ozone in the earth's atmosphere was

postulated about the turn of this century: firstly to explain

the very abrupt cut-off in the solar spectrum, observed at

the ground, at approximately

3000i,

and secondly to account

for the~rming of the atmosphere at an altitude of approxim-ately 50 kms, Lindemann and Dobson (1923). It had been shown in the laboratory thRt ozone has a very strong aL)sorption

band in the ultraviolet region, Hartley (1881), in the region

2000 to 3000~, now called the Hartley bands, and by huggins

(1890), in the interval between 3200 and

,600~,

the Huggins

bands. ~'he ozone absorption coefficients in the Hartley bands were first measured by }I'abry and BuisBon (1913), who

o .

extended. their measurements to 3340A in the Huggins band.

Fabry and .Buisson (1921) showed, by measurinr~ the solar

spectrum in the range 3000 to 3340i, for various solar zenith

3llgles, and. therefore various transmission path lengths

through the earth's atmosphere. that the major absorbing

sub-stance was in the earth's atmosphere, and that its absorption

spectrum coincided with that measured for ozone.

Later observern confirmed F":;. ry and Buisson' B conclusion that ozone existed in the earth's atmosphere and was the

1.2

depletion of ultra-violet radiation from the sun, at the

sur-face of the earth. fllhe method used by Fabry and Buis50n, and

developed further by Dobson and Harrison (1926), to estimate

the amount of ozone in the atmosphere, relied on measuring

the reduction in intensity of a band of radiation, absorbed

by ozone, for which absorption coefficients had been obtained,

wi th changing path length through the atliiosphere.

The region of the solar spectrum within the absorption

bands of ozone, chosen for such analysis; needs to be free

from the absorption of other atmospheric gases so that

changes in their concentration will not affect the estimate

of ozone concentrations. The chosen bands also need to be

free from strong absorption bands of constituents in the

sunts chromosphere, so that the total irradiated energy in

these bands can be considered essentially constant. ouch

bands must therefore be free from Fraunhofer lines in the

solar spectrum and consequently need to be situated in what

are called Fraunhofer transmission spaces.

Many bands in the ultra-violet have been used

success-fully for estimating ozone contents. Ten bands in particular

have been used in pairs, various pairs being better suited to

different atmospheric conditions. Of these ten bands, the

International Ozone Commission (IOC)

(1956),

a commission

formed under the auspices of the International Association of

these bands, namely those witb centre wave-length at 3254 and

3055~,

and

3398

and

3176~,

commonly designated the "A" and

"D" wave-length pairs, as the standard bands for Uf3e in ozone computations. 'rheae bands had been found to give the most

consistent results. the

IOC

therefore proposed th~t any

other wave-length bands used, should be compared with the

standard bands, and the results accordingly modified, so as

to agree with ozone values obtained from computations using

the "A" and "D" wave-length pairs. l!'or this reason, the "A" and liD" wave-length bands themselves were selected for this

project.

1.2 1heory

The "Fabry Buisson. method of ozone measurement, using

the relative reduction in intensity of two wave length bands

with path length, where the absorption coefficients are

mown, yields in f;eneral, the integrated amount of ozone in

this path. In particular, this path is usually taken as the

vertical distance between the surface of the earth and the

top of the atmosphere. Therefore, this method gives the

tot-al content of ozone in a unit vertictot-al colum .. L above the

observer. This quality is customarily called the "total

amount" of ozone, and is by far the most common ozone

para-meter .easured. 'llhe total amou.nt of ozone is usually

1.4

at ;3.T.P. ':rhis equivalent thickness has the units (10-3 cm

at ~~.T.P.) or (n atm.-em) and is given the symbol Q, after

Godson (1963).

Let

lA

represent the integrated solar flux in a narrow

spectral band with centre wavelength A. If the effective

ozone absorption coefficient for this band is (lA then the

fractional reduction in solar flux in this spectral interval,

due to ozone absorption in the distance between x and x+dx,

is given by the relation

...

(1.1)

where

Px

is the d.ensity of ozone at the height x and Xx is

the solar zenith angle :'Jt the height x which is, in general,

not independent of height. The reduction in intensity

however, does not capend solely on the amount of ozone in the

atmosphere. ;-cattering of radi[~tion by molecules and larger particles, reduces the intensity still further.

Juchmatter-ing can be characterised by the qU<ffitities ~A and Ox where ~A molecular scattering coefficient for wavelength

A

and

Ox

=

fraction of radiation scattered, per unit path length,

by particulate matter at a height x. To a first approximation

Ox is assumed independent of wavelength. Thus, including the scattering coefficient as well as ozone absorption coefficient, equation (1.1) becomes

dl A

p a~ sec X dx + ~~ P sec X dx +

°

where p

=

air density at height x.

Integrating equation (1.2) over the total path between the earth's surface and the outer edGe o~ the atmospbere gives:

=

• _log

• •

aArS:

px sec Xxdx +

SAS:

P sec Xxdx

+

s:

Ox sec Xxdx

IA

ro

Xx

dx - I3_A

S:P

sec lOA

;:;a

-aAJOpx sec Xxdx

-[0

_o

_x sec XxdX ( 1.4)

\Jhere IA now represents the intensity of radiation in the narrow spectral band of centre wavelength A at the,~round,

and lOA represents the integr.'3.ted in tensi ty in this band

outside the atmosphere. ~o simplify the integrals on the R.H.S. of equation (1.4), the following assumptions are made:

Because there is a ID::lximum concentration of ozone at

some height, thnt is. ozone is not evenly distributed with

heitSht in the :ltmosphere, it is more appropriate to replace

sec Xx in the first integral with a con~3t:·.lnt _{sec Xh , where} h is the zenith angle at the height of the maximum concentration,

assumed for standardisation purposes to be constant and equal

to 22 kIn for all sites. l'he second inte:'~;ral is replaced by

~Am where ~A' the molecular scattering coefficient, is

cal-culated from Iiayleigh's theory of 1::101ecular scattering and m

1.6

angle at the point of observltion, taken in practice to be

given by Bemporad's Function

(1907).

This function takes

into account both the sphericity of the earth and refraction

by the atmosphere. 'I':.lbulated values of this function are

gLven by List (1958). ~ince most particulate matter exists in the troposphere, the third integral may be approximated by

assuming that sec ~ has a constant value sec

z.,

where ~ is the solar zenith angle at the point of observation.

'l'here-fore equ3tion

(1.4)

becomes:

I log --L

lOA

where & m ,

coefficient.

&

dx, the eff9ctive particulate scattering

x

Because it is easier to measure the relative reduction

in intensity due to absorption and scatterIng of two

wave-lengths, rather than the absolute reduction of one, in

practice, two wavelength bands are used. This pair of bands

is chosen so that the effective ozone absorption coefficients

of the bands are as different as possible, while the

wave-length interval between the bands is as small as possible.

This makes the intensity of the irradiated flux in each band,

outside the atmosphere, comparable. At the edge of the

liartley band system, absorption coefficients are changing

rapidly with wavelength, and narrow bands are therefore chosen

J:,lor a second narrow band with central wavelength X',

equation

(1.5)

becomes

IX'

log •

...;,.;..-lOA'

=

(1.6)

assuming the spectral intervals are sufficiently close to

allow 5 to be independent of wavelengt:l.. '.rherefore subtract-ing equation

(1.6)

froID equdtion

(1.5)

and rewriting in terms of 0, get

I 10

log ~

_r;:;

- log ~ - (~ ,-~ )m

~ X X

Q ...

l'he total ozone amount lllay be calculated from equat.Lon

(1.7)

IX loX

if the quantities aX' aXI, t.1X' ~AI' log ~ , log ~ , ill and X_h are known. Of these qUl:-ulti ties, aX and

ax

I way be

obtained from laboratory experiments, j3 and 13'\1 from IA f\.

L{ayleigh I s scattering theory, and log ~ is measured

.1. A. 1

directly with an ozone spectrophotometer, such a device

providing the basis of discussion in subsequent chapters.

The quantities sec X

h .3.::J.d ill, which are functions of

solar zenith angle, depend on the time of observation and

known way ..

the location of the site in a

loX

The quantity log ~o may be evaluated from a series of I

- ..L XI

direct sun measurements of log

IX

versus airmass, on a day

X'

when condi tiona remain uniform throughout the period of

1.8

The evaluation involves extrapolating a graph of log

IA

S -

(~A'-~A)m _{versus sec Xh back to where sec Xh :; O. buch}

a procedure necessarily requires that Q and h remain constant 10

over the observing period. necause log ----I A represents the

°A'

log ratio of the incident intensity outside the atmosphere,

which is 9.ssumed to rema.in constant, such 9.n evaluation needs

only be done once and for all. In practice, several dqys of

uniform conditions are chosen so that a better value of 10

log ~ may be obtained. ~,;i th all the quanti ties on the

A'

R.I-1.f;. of equation (1.7) determined, a value for the total amount of ozone may be calculated.

The assumption that

&

is independent of wavelength,

(i.e. the scattering particles are. large compared with the

wavelength), is not true in all conditions. This assumption

can thus introduce sizable errors. However, these can be

minimised using two pairs of wavelenp;tLl bands. This is

because the separation of the centre w~'j.velength in each pair of bands is approximately 200~, and over a small wavelength

interval the ::?cattering coefficients are changing almost

linearly, the absolute value of the difference in scattering

coefficients for each pair is almost equal. Consequently,

8ubsti tution of scattel'ing coefficients in the two pair

formula almost completely removes the dependence on

scatter-ing from the ozone calcul~tions.

in section

(1.1),

equation

(1.7)

becomes:

where

and

f., log I AD - II lo@; lOAD - 6t3ADm

lla_{AD sec} !h

-~ log rOAD :::

b. (I'D _A

(1.8)

'rhe laboratory calcul::1tion of ozone absorption

coefficients has received very active scientific interest,

since the tLne of Fabry and Buisson

(1913),

with notable

work by NJ' Tai Z.e and Choong ~hin Fiaw

(1932),

Inn and Tanaka

(1959) and Vigroux

(1953,1967).

The values accepted by the IOC at its meeting in

Lucerne in

196'1,

IOC

(1968),

_{retain the 6cxAD value calculated}

by ~;ir Charles l~ormand, from th~? results of Vi~roux (1953).

and published in the "Observer's Handbook" (Dobson

1957)

for

use in the International Geophysica.l Yec:tr. The value

1.388

had been maintained for uniformity by the

roc

since the most

recent wor.k by Vigroux (-1')67), g~ve B. value of

1.396

for

1.10

differences for each wavelen:'th pair, now accepted by the lOC, eTe given as 1.74-8 for (0:

3055 - 0:3254) and 0.360 for (0:3176 -0:

3398), thus maintaining the 1.388 value for LlClA.D.

'l'he values of Hayleigh scattering coefficient, tabulated

by 'Dobson (,'1957), are ~Jhown in 'l'able 1 below:

J-'air Centre ~avelen~th :'~olecular lesignation in band in .1 Scattering Coeff.13

~' ~. _S'-S

3055 0.491

A 0.116

3254 0.375

3176 0.416

D

0.104-3398 0.312

from Te,ble 1, 6~f~D ::; 0.012.

Using these experimental values and making the aSS1 rnp-tion that m ... sec 'X_h, which is accurate to within :3 percent, provided ~ is less than 74°, that is sec 'Xh is less than

3.5,

equation (1.8) becomes:

Q ==

Ll log lAD - 6 log _{lO kD}

1.388

_{sec 'Xh}

-

0.009 (1.9)

where ~2 is in the standard units (m atm-cm).

fl1he amount of ozone in a unit column between the

by m.ea;;:3urint~ -the value of b.

lAD'

with an ozone spectro-,:-l::..0tometer, at Borne value of sec 'X

h, when b. log lOAD has been determined.

1.3

Aim of :Pro;ject

In the Heport by the Panel on Ozone to the Committee on

Atmosph€'ric ;3ciences, l'lational Academy of ~.~cience - National

Hesearch Council

(1965),

with regard to the International

observation programme, it is suggested "that considerable

effort be Diade to increase its (the Dobson ozone

spectro-photometer) reliability and sensitivity, make it an automatic

recording instrument and reduce its size and pricell •

'1'he suggestions of this report provicied the aim for this

project. It war:; immediately obvious that the physical

cimensions of a double ':lu',rtz monochDomator could not be

reduced significantly without IOf;s of resolution and because

this resolution also depends on precision engineering, the

possibili ty of lowerinl'S the cost on. this conventional

instru-ment seemed unlikely_

i~ new type of spectrophotometer was required, that could in fact be smaller, as relL~ble,and cheaper than the conven-tional system.

In Chapter 2, the design of such a spectrophotometer is

1.12

Chapter ) compares its .performance with that of the standard

Dobson ozone spectropho1;ometer, a:ld in Chs..pter 6 the correl-ation between the limited filter O~OD.e data available and

2.1

IN S'l'RlTMEN'r AT I ON

2.1 Introduction

'l'he method for measuring ozone has been described in

Chapter 1. l.t has been shmvn tlwt the values for the logar-ithm of the intensity ratio of two wavelength Lands in the

ultraviolet for different values of airmass, must be obtained

in ord.er to estimate the amount of ozone in a column

extend-ing vertically from the observer, to the outer edge of the

atmosphere. fl'llis is called the total anl01lnt of ozone.

2.2 Spectrometer

A spectrometer-is therefore required to select these

ultraviolet bands from the solar spectrum, and to enable the

measurement of their intensities. A serious problem is

immediately introduce~ as only E'Jbout

5

percent of the sun's

irradiated energy re~ching the earth is in the ultraviolet. Ii'his means thclt any device used for the selection of these

ultraviolet bands must be very selective indeed and. ideally

"blindrt

, that is, not :'.ensi ti ve, to radiation on the long wavelen~th side of the required band.

'rhe problem is further complicated by the fact that

') ' ) L_ 1'-_

belo',v

:;300~

a'.':ld c',)!1,9p.qll.8ntl;r <l'1Y transmission optics in the

s~)ectrophotometer Must be qU'3.rtz, or more r;enerally, made of

so.rne ultraviolet transmitting material. (ligure 2.1 shows the trailsmission spectrum of ordinary window glass from Koller

(1965)).

2.3

Filters

:in interference filter, first conceived by Geffcken (1942) is i;; efect, a specialised form of the Fnbry J'erot etalon. In this latter device, shown schematically in figure

(2.2),

after Andrews

(1960),

circular interference fringes

are observed on the scree~l

:P,

for monochro'J1lltic radiation from source ;~~, due to the superposition of rays produced by the rnultir;le reflectious froID the artially silvered glass plates that bound an air gap of width d. Since the series of rays )roduced through multi()le reflections are parallel, a lens .L is required to ma..1(e the seri as superi'.npose, so that interference may be oo;:;erveo.

~ssuming n:::1 for the refractive index of air in the gap, reinforcement occurs for wav~len~th ~ when

2 d cos

e

=

m" (2.1)

100r, ---c---~

c : )

80

~ 60

E

", c

m

~

3500

Wavelength In angstroms

4000

Transmission for two thicknesses of window glass. (Data supplied

courtesy of Pittsburgh Plate Glass Co.)

2.#-A _ 2d

m (2.2)

where .m is any inte£l;er. For a white light source with d ==

5

mm, 12,000 frinBes would be produced in the ranp:e cf the vi::;ible spectrum. However, if d is very small, only one or two narrow wavelength bands would be transmitted in the visible spectrum

ana.

the rest would be reflected.

The pair of semi-transparent metallic films therefore act as a filter, passing nearly monochromatic light. .i.'he transmitted energy, as a function of wavelength. is shown in

/

figure (2.3), after Va~iC:ek (1960), .for v;,riOllS reflectivit-ies R. If' the transmitted maxims. are to be widely separated, a small value of m is required.. cl'hi.s can be obtained only if the reflecting surfaces are very close together. ~'or the maximum m=2 at wavelenp:;th ~, by 8Qu0tion (2.2), the metal

films would need to be separated b,' a distance~. ·.the maximum ill==1 will then appear at 2~. I:,>mall separations of the order of a wavelength are now possible using sophisticateu. vacuum deposition techniques.

~o make a simple single layer filter, a semi-transparent metal film is evaporated on to a substrate. A tbin lo.yer of some dielectric rnateri al, such as cryolite (3NaF .1I.1F 3)' is evaporated over thi s £il,n, which in turn is coated. with :1I1O-ther layer of metal film. Usuall;t another glass plate is placed against this second film to provide mechanical

Figl:!.re 2.3: rc ssion of I~terfererice

F'il ter various Film Reflections, from

Vas{c ('1 )

GLASS

GLASS

EVAPORATED

METAL FILM

2.6

in figure (2.4), af'tel' Xndrevs (1<)60). :':"he thicknesE of the dielect;rlc fi 1m is greatly eXEgrjerated in this figure.

If the dielectric m9terial has refr2ctive index n, equation (2.2) becomes:

i\. :0:

-

2nd m

'i'i"here this is the condition for maximum transmi ssioc of wave-length A at norma.l incicetlce. If more than one band is tranf~-mitted in the region of interest, this can be removed by

using a sui table ~{bsorbing filter, in series wi tl1 the inter-ference filter.

Figure (2.3) shows that as the thickness ()j~ the metal film is increased, (tbat L:>, 9.h the reflectivity is increas-Ed) the ha.lf width of the filter oecreases, but simultaneous-ly the greatest transmission of the filter also d.ecreases. A compromise must therefore be ree-lched betweer half wiclth and transmission. Also, since the abeorbing filter used t:J block out some unwanted maximum. will not be 100 percent trans.itt-ing in the wavelength blmd of interest, this 1A"ill further reduce the nett transmission of the filter.

The great advantage of interference filters, over the older absorption type, is that they are not heated. by the passage of intensive radiation. I~eally, all radiation not tr&nsmitted, is reflected. Yhere is a small amcunt of

than in the older filters.

Any absorbin[~ fil ters used in conjunction ""'i th an inter-ference filter, to remove side lHUlds, are placed on the

transmitting side of the interference filter so that tbey are subject to a much lower tota.l flux and consequently experience less heating.

This considerably reduces the problem of shi£tin~ band-passes with changing light intensities. ~rhig problem could have produced sizable errors in this particular application when the sun i~ the Rource of the radiation being weasured.

Interference filters, 8.8 well as absorption filters, have

a finite tempAr?ture coefficient for shifting bendpB8s. A tempernture coefficient of between. 0.1 and

0.3

~/oO was found by Blifford when testing a series of comruercial filters - Blifford (196r»). The increase iT'. e.entre w3vEllength was reported to be linear with increasing temperature over the range -60°C to +600C. 'rhe maximum values of the temper-ature coefficients of bandwidth and transmission fou.nd. in the

( 0 )-1 sample tested, were less than 0.01 C •

The peak w:.ivelength of the transmission band t 9.6 can be

seen in equation

(2.3),

depend.s linearly on both the refract-ive index and the thickness of the layers of material used in the filter.

of refractive inil.ex fo:, .substances sL:nilar to those usually used in interference filter2, i t may be assumed that the tem-ner>-1tUI'e chanfr,es ar;;) due to v,Clriations in refractive index,

(~Jolfe, :"I~llard 3Ild 1,]cCarth:r, 1963). Blifford also reports

:'i ndinl' little chan[~e in the trLiIlsmission characteristics when tiltinf5 the filter relative to the incident rAdiation, ')rovidec1 the t i l t is less than 5 degrees.

Using the aver'c~e tomperature coefficient obtf:.ined from the series of ril ttirU tested by Blifford ~ 1966) and the

absorption coefficients of Vilr,roux (1953, 1967) and Inn and rranaka (1959), a chance of 10°C in the a.mbient temperature '\\'0'.110. ;,roduce a 2?~ cr ::=,.:2ge in th~· 2.bsor;,tion coe:'::ficient of t~e

3055Y.

1:'ancl filter Emi~ a COrlsecment 31' c.he.nge in tne estim-sted vslue of c~one cont9nt.

'itL tt:.C ~icld of view oI t:tG in.strurr.ent set c:.t 1;)

d€f;recc, t i l tine th,c.: 1'il te:!:'E eVer teis r;~.Lge produces no Et~~'preci!O,ble ('.:C;-m~e in transmissioL chare,cteristics.

If A is a substance of high refractive index and B of

low refractive index, figure

(2.5)

shows a typical

cross-.r

section of a multilayer interference filter, after Vasicek

(1960) •

'.l:he centre film is made of substance B and has thickness

d

=

~ _t:::n

,

which determines the wavelength for which the

B

transmission will be a maximum. On either side of the cen-tral filter are films ~ thick which have alternately high

and low refractive index in the order (A,B,A,B, ••• I~,A).

'l'he optical thickness of one quarter wave1eniJ:th is chosen

since the reflected waves tr'lve1 an optical path of ';2

wave-length and also undergo a phase reverse! on reflection. 'l.'he

physical

....L

dA "" 4n_A The

thickness of the A films is therefore given by .".

and of the B film by dB

=

4n

n

•

(A,B,A,B, "', B,A)

series provide reflections

com-narable with those obtained using metal films, but with much

less absorption. The multilayer interference filters have a

central layer and a reflective set 6n either side just as in

the classical B'abry-Perot type.

lnterference filters of this type have been available

in the visible part of the electromagnetic spectrum since

Geffcken (1942). Imwever, the availability of film forming dielectrics with the required high ultraviolet transmitting

properties has been the limitation in the development of

2.10

Osherovich

A.L.

et ale (1969) report using

27

layer

interference filters, with alternate layers of cryolite and

lead fluoride, to obtain narrow band filters for the

ultra-violet.

Narrow band pass interference filters with transnl.ission

peaks at preselected wavelengths in the ultraviolet became

available commercially only in the late 1960's.

In September 1968, the first pair of narrow band

ultra-violet interference filters with transmission peaks specified

at 3254- and 3055~, correspondin!'; to the ItAII wavelength pair,

was oraered.

Detail of these filters, others subsequently purchased,

calibration procedures, the need for further blocking to

eliminate transmission leaks, and the manufacture of such

blocking filters by the author, is given in Appendix B.

It is sufficient to say that two pairs of narrow band

filters (half power band width of approximately 30~) were

eventually obtained. iJ.lhese pairs had centre peak wavelengths

corresponding to the "All and liD" wavelength pairs. After

testing and combining these filters with selective absorption

filters to increase the signal to noise ratio, a system was

available to enable the wavelength bands used in the

calcul-ation of "total oz:·ne" toL,e separated from the solar

2.11

2.4 l~hotomul tiplier

Jrbe integrated intensity of solar radiation in the selected ultraviolet bands, passed by the interference filters, has to be measured. A photomultiplier with an

ultraviolet sensitive photocathode was chosen to convert this ul traviolet radiation into an electric current signal. ,I. gain , this particular region of the spectrum, between 3000 and

3400~, shows itself as an awkward region i'rom which to obtain data. f.ilost photomultipliers, sensi t i ve to radiation in this region, the near ultraviolet, are produced simply by encap-sulating the photocathodes and dynodes in a quartz envelope rather than the normal silica glass envelope. This means that rather than the photomultiplier being sensitive just in the near ultraviolet, the response function is an extension of some larGe visible response. Consequently, a

photo-multiplier of this type, shows a spectral preference for

radiation outside the near ultraviolet, and any noise in this region is consequently preferentially amplified.

lihotomul tipliers usually used in ul t;raviolet work are of the ",solar Blind" type, having a cutoff at approximately

o

3000A, to make them IIblindll _{to solar radiation and of course} useless for ozone applic3tions. ~ecause of the rapidly

decreasing intensity with decreasing wavelength. in the solar ultraviolet reacbing the ground, i t is only radiation wi th wavelengths greater than the signal band that cause

r-'-"

t. .. ) ""

I- f',

\ . J

" ,'-., .-'"

v:J

4:".~

0=

i"I 'I I· t'\ I:

, r

"~;;"~-"'J '

"'~-'-'

c ... r:::-'-'" , W ... ,

'-'

-<, ''''

'--:J

~ ~

l"J

.

_,J

!'i

; i :i

, ;

2.5: _- Cross-sec filter D A B A B A B A 8 A B A B A

e

A D

on of multilayer int

Vasicek (1960).

~; ~,-:-,I~.~r"

<F:: ,:,;..~ !:-.;:::. [j<;;\,

c, -. i.":!1 L d ?

~<;;J;U

ar spectrum at the

erence

-:::::::;;-\ )

2.13

Therefore the ideal photomultiplier for ozone measure-ment is one that has a high wBvelen;;-;th cutoff at about

'500~.

To the author's knov,ledge, no tubes of this type are

commercially available to the Western .(orld at this time. Osherovich A.L. et ale

(1969),

report using a tellurium-potassium-rubidium photocatilode th;o.t does not respond to radiation ',bove

5000~. ~'lon-production

line developmental tubes using rubidium-telluride photocathodes are now becoming available, and the advent ~f such tubes on the commercial market can do nothing but improve the signal to noise ratio

of the system.

rl'he photomultiplier chosen for the spectrophotometer, built by the author, was a nine s'tage, side-on type, 11-28

photomultiplier ~ manufactured by rWA. r:L'he tube uses a cesium bismuth opaque photocathode and cesium-antimony dynodes in an ultraviolet transmi ttint-~ envelope. 'l'he nominal peak spectral response is at 3400 ± 500R. This ture is cl&ssified as

baving an C5 type response and the resi,onse function is shown in figure

(2.7).

'llhe high wavelem::;th cutoff at 6500A o can be seen in this figure. ':;:the stabili sed hi~~h voltage supply for the photomultiplier is detailed in Appendix C. Modifications to the standard bleeder chain configuration for the photomultiplier are discussed in Ehapter 3.2.

01

2~

_•

S

t

2.15

The current produced by the photomultiplier, proportion-al to the incident light intensity, was passed through a

log-arithmic amplifier to convert this signal into a voltage

propol,tional to the logari thIn of this input current. 'II hi

system allows the meaGurement of intensity differences over a

d;y-namic x'ange of four decades, that is, an intensity

differ-ence of 10. 000 to 1, on a single scale. A d;Y"11.amic range of

this magnitude, although theoretically possible, is not in

practice Observable. ;.)uch an intensity difference would lJe

measured when ;:J was approximr:.<tely equal to 860 t by which time

the incid.ent light intensities would h·Jve dropped to such a

degree that the signal levels would be buried in the clark

current of the system.

lrhe output voltage, which was directly measurable on a

chart recorder, or could be digitised and recorded on paper

tape. was directly proportional to the logarithm of the

intensity input. which is the basic parameter used in ozone

calculations. 'llhe transformation from solar ultraviolet

radiation, reaching the earth, to recorded signal is shown

in block diagram form in figure

(2.8).

The details of

circuitry and design of the logaritbmic amplifier used in

this project are included in Appendix C.

The transfer of ozone intensity data on to punched tape

I .

I

SOLj~\R

I

UL TRAVIOLE

T

i

I

i 1

::l,7_'

_ d .

FJLTERS

PHOTOMULI PLIER

LOGARITHMiC AivtP

LJ NEAR AMP

I

CHART

IRECORDER

I

CONVERTER

l

\~

TAPE

PUNCH

, POPS

.4 .I

Block diagram of

ultraviolet into ozone

, from solar

2.17

signal from the logarithmic amplifier consists of a four

level step function which is repeated with every revr:,lution

of the filter chopper in [rout of the photomultiplier. The

interference filters are mounted, as is shown in figure (B.1)

in Aj::.pendix B, on an assembly that rotates in front of the

photomultiplier and thus the photomultiplier views

sequential-ly, radiation in each of the four w,I.velengtrJ bands.

'110 identify each particular band, a phasing pulse is

generated by a reed rela.y switch, activated by a magnet

attached to the rotating filter assembly. This phasing pulse

is shortened in duration and s.l'iaped by means of a Bchmi tt

trigger, monostable multivibrator and differentiator. The

details of the electronic circuitry to perform this function

are given in AI)pendix C. 11

118 phasing pulse is used to trigger "interrupts" on the l'D1.18 and cause the analog-to-digital

converter, into which the output of the log amplifier is

being fed, to be read and the log intensity value to be

recorded.

~he computer programme used to effect this data collect-ion operatcollect-ion and its subseauent output on to paper tape, is

given in Appendix D.

The .PDP8 system, in the IJhysics Department at

Christ-church is equipped with a real time clock. It was therefore

2.18

time intervals, and al{~o store the real time value at which this data was recorded. On command, this data was punched on

paper tape and listed on a teleprinter.

In the automatic mode of operation, it was of course

necessary for the ozone photometer to maintain aligrun.ent on

the sun automatically. ~his was acl:ieved by bolting the l.mit containing the filters and photoml.lltiplier on to a

mount, free to rotate in the equ!'!.torial plane and !}djustable

in declination. A three inch Newtonian telescope \"a9 Illounted

beside this unit with an ORP61 cadmium sulphide photocell,

used as a light dependent resistor (LDR) at the focus of the obj 8cti ve. 'rhe LDH was in the load arm of the power supply to the motor drive on the equ8torial mount. Uonsequently,

changes in resiEltance due to changing incident light levels

on the LDH, changed the vol tat~e, supplied. to the drive motor. This servo mechanism therefore adjusted the drive speed so as

to maintain the alignment of the telescope, and consequently

the filter spectrophotometer, on the sun.

fl'he spectrophotometer was designed to have a viewing

angle of 1~ degrees. A viewing cone with this angle was

recommended by Osherovich and hodionov (1961). '.rhe tracking telescope, tracking the leadinl; edge of ·the sun, could

main-tain alignment to within two seconds of arc with clear skies.

2.19

speed was provided, slightly less than the optitlU;Il drive, and

t;he LDR provided perturbations to this speed. rbis allowed

complete clouu cover to persist for twenty minutes before

trackillg was lost. If the cloud cleared within this tima,

the trac.r.:iog device would. drive the sys't:;em back into correct

alignment. If however the cloud did persist for longer than

this period, the sy,£!tem would have drifted out of range of the

sun and thus have lost the power to drive it back. Data

recorded when cloud obstructed a direct view of the SUll could

easily be detected by the drop in light levels.

'1he apparent diameter of the sun is 32 minutes of arc

at the earth. and with the cllanges of declination of 12

min-utes of arc in twelve {.lours in the equinoctial period,

radiation frow the whole solur disc can be viewed throughout

the day without chan!3'ing the day's initial declination

setting.

1'0 ensure an unimpeded view of the sun, from sunrise to sunset, throughout 'the yei..r, the equatorial mount was fixed

on the roof of the eight storey ,Physics Department bullding.

The equatorial mount, tr':!.cking telescope

and

the small

ozone spectrometer mounted beside it are shown in figure

2.2()

~ - ~ - -

--

-r;

...

,...4~ -

.... _

'

[image:42.554.40.485.51.570.2]

, "" ... "

Figure 2.9: lfilter Ozone p _ctropllOtometer on E~ll[\tori::tl

2.21

In the instrument testing stage, the teleprinter

list-ings were used to produce ozone values manually. .ti.outine

ozone measurements are now being made by :'!lr ii.. Basher, and values obtained usinp;j the paper tape and a data processing

system on the IB1\i] 360/44 computer.

II'he way in which the ozone values were obtained from

3.1

C HAP T E R 3

CALIBRATION

3.1

Introduction

To transform the ozone data obtained from the instrument

described in the previous chapter to real ozone

concentrat-ions in the atmosphere, it is necessary to perform tests to

calibrate the vRrious instrument parameters affecting the

data recorded.

In Section 2, the calibration of the interference

filters to obtain the centre wavelengths is described. The

calibration of the logaritttlIlic amplifier, to enable recorded

signals to be converted into standard logarithms to the base

10, is contained in Section

3,

while dection 4 deals with the

spectral sensitivity of the photomultiplier.

3.2

Filters

The composite filters are described in Appendix B. For

calibration, each interference filter was placed in a Cary 14

Hecording Spectrophotometer (qliartz optics). In this

instru-ment the spectrum is automatically scanned and the optical

transmissivity recorded simultaneously. :rhe \rmvelength

marker on this photometer was calibrated using blown

was found to be

5~

too low on the forward drive and

6~

too loV{ I)n the reverse drive. I-~he 1~ difi'erence was attributed to backlash in the drive system. Consequently, all wave-length markers on the recorded curves were adjusted by these amounts. The transmission slide wire was used in the photo-meter. This allowed a full scale deflection of either 0 to 100 percent transmission or 0 to 10 percent transmission.

'l',ith no filters in the sample cavitJT , the comparison

beam in the spectrophotometer was balanced ~tith the sample beam to p;ive a tr~,nsmission scale reading of 100 percent. This balance was found to be constant over the range 3000 to

3500~

and consequently no separate adjustment was required for each filter.

A spectral transmission curve foI' each filter was then obtained and. thl")se are shown in figure

(3.1).

In this proto-type ozone spectrophotometer, two additional blocking filters were used. These were a Corning CS7-54 filter and a nickel sulphate hexahydrate crystal filter.

%

(2

(j)

~fl

:?: (f)

z

«

n::

r

w

<:)

<.{

.-Z

w

r:

_{" "} 0

IX

LlJ

a..

0

10-r

2'c

22GO

"305S"

B.W.

,d.

"3254"

______________

B_~_~

__ . ________________ _

,1.

30[\1

'PrGf1:3:oi ~)ion npoctra of I I'> I

tnt erOTIce filters

"33sa-B.w.231

3.4

!J.lherefore, a bloci.(ing- fil ter wit h good transmission in

the ultraviolet between 3000 and 3400~ is required, but with a very sharp cut-off between 3400 and 3800~. In his paper on ultraviolet filtering tec;miques, Kasha (1948) showed the

transmission char~cteri8tics of nickel sulphate hexahydrate.

'r.his substance has the required spectral characteristic to block these unwanted sidebands.

In spectroscopy, filters are usually made by placing a

solution of the required substance in a quartz vessel.

however, in this ar\,Jlication, the author decided to use a

crystal filter of nickel sulphate hexahyd.rate. rJ:he proposed

long period of operation, the inconvenience of using a liquid

filter ·jnd. the possibility of evaporation producing a change

in transmission, because of increased concentr&tion, were the

reasons for this choice.~jowever, manufacture of such a

filter proved rather more difficult than was expected because

of the preferential formation of nickel sulphate

hepta-bydrate, a crystal form that dehydrates readily in air. The

anhyclrouG salt does not transmit the ultraviolet. A descrip-tion of the manufacture of nickel sulphate hexahydrate

crystals is given in A;.pendix B. r:ehe transmission curve for

a polished nickel sul"phate hexahydrate crystal filter is

given in figure

(3.2).

The other blocking filter used was a Corning

CS7-54

spectro.scoplS t~:; for ul tra.v-.i.olet wor~{:. It has a tmiform

transmL:'Eion of SUiv 'oet'Neen 2900 and 3500~, and good blocking '»-.l.' d b-r ... · ,0' 0-'j(.

cliar8.cteristics bet~'leen (~'3UO a...i ___.n. It does bowever have

o

Cl sizable red leaK, starting'lt 6600.11 and exte,ldin.(;; into the near infrared. l'h~) transmission curve for trlis filter,

obtained from the Gary ·V~·, is given in figure l3.3).

Combining the transmission characteristics of the two

blocking filters witb each interference filter, a value for

centre wavelengtil of each band was obtained, to be used in the cai.culation8 for the effE)cti ve a-bsorption coefficient •

. 'j:he value8 are listed in table (.3.1) below:

TABLE 3.1

l:'air Designation centre.WBlelength Half .Fow~r ~andwidth

J.T) ~ In _

3052 46

A

3252 28

3178

32

D

3402

₂₃

The nett transmission through each of the combined

filters was between 1 and

3%.

However, the important quality

is not the absolute trAnsmission, since there is plenty of

energy available from the sun, but rather the signal to noise

[image:48.553.65.507.83.556.2]

i

WI.

t- '

Z

WI

,...,

, - ¥

._ I c,:· I

,,;:.

L1

3Joo

Ir

ran ss ctru;r.

0:

NiS crystal filter

o

A

7000

F" 3 "\

t

g. '') :.

ssion ectrum of

filter Corninp; C

3.7

outside the passband had a transmission greater than 10-

5

of the peak signal transmission. This was extremely important

as the photomultiplier views the integrated energy through

the filters. Because the intensity of solar radiation is

increasing rapidly outside the signal passband, a small

trans-mission leak may pass a large total energy flux: and

consic!er-ably reduce the signal to noise ratio. ~'his problem is dis-cussed in more detail in Appendix B.

3.3 Logarithmic Amplifier

rrhe almost infinite input impedance of tbe lor::;ari thmic

amplifier, designed to match the almost infinite output

imped-ance of the photomultiplier, neceGsitatecl t:he use of a

con-stant current source to drive the amplifier for calibration

purposes. 'rhe constant current generator used in these

cali-brations provided a supply of current, in preset decade

steps, between 1 nano amp and 100 micro amps, to an accuracy

of better than three percent. ;.rhe';enerator itself was

calibrated ngainst a l'hiJ,.lips 1:;M2435 micro-voltmeter.

'Iihe resulting output for this range of input current is

plotted in the form of a calibration curve in .figure

C:'.4).

It can be seen from this fi~;ure that the res;-:onse is logar-ithmic to within one percent over the range

25n

amps to

7.5

t -Z W Q:: 0::

=>

u

l}'A (O·1na) l

-=>

n...

z

3.3

i ~ I I

-l

i

J

....,

-1

-; !

!

I

I

I ~ I -; --{ I - ; ... J l -i

j

I

J

I

I

10na~--~2--~~4----~6---8~--~10~---Output Vol ts

becoming non-lo:~ari thmic ::-~.t both the top and bottom end.s of the range shown.

Tille line through this line:3r portion was taken as the characteristic respons~ function for the amplifier. This assumed. charhcteristic response m,eans that all losari thmic ratios experimentally measured, are t) the base

5.5

and con-sequently need to be multiplied by the factor 0.7143 to con-vert them to standard base 10.

All data recorded outside this linear re~ion was corrected for its deviation from the assumed lORarithmic

function. 1'he metrlOd, for applyin,,; this correction to the raw data is detailed i1'l ;3ection 4 of this chapter. In real

terms, this means a difference in intensity levels measured as 1.4 volts, when the individu.:.l.l intensity levels have been corrected" corres'conds to a f"ctor of 10 difference in light 18vels.

3.~ Spectral Sensitivity

It is particularly im:"lOrtfmt that 3. £pectrometer used

3.10

parameter. In bstimati.ng the dependence of measured intensity differences on instrument parameters, care !!lust be takan to i"wla te Flach par;-:.meter. i: or instance, increasi np; the photo-Tn'll tiplier supply vol bige rr,~.q cause some cb ansa in spectral sensi tivi ty. hut bGcausf~ of the associated increase in signal level t an observed chanfjO may he due to the latter effect. Therefore, care must be taken in such calihJ:'!:ltion procedures to change only one vilriablp. t a timA and observe the effect of such a change.

It was established not long after tl].e introduction of photOIllul tipliers tlFlt most ~Jhotomul tipliers res'pond differ-ently to different wavelenGths when the ap)lied voltage is changed; that iS_t the spectt'8J. 88nsitivity is voltage depend-ent. A theoretical explanation of this ?~enomenon is th~"lt

chfin?:;int~; the dynode vol t::l',;el'3 produces a Chan!?;6 in the contour

pattern of the 'potential field within the photomultiplier. }juch ::t ch3..n;:;e means th;~.t the optimum tr~1.j ectori ~s of the e13ct;r.::m8 between the )hotocathooe and fi.rst ely ~lode and bet-ween the fir8t and second 'l~ode8 are cl1anged. Electrons

thf1t reach thA first liynone t:J.erefore orir;inp.te from a

Because the photosensitive coating on the photocathode is not absolutely uniform, the spectral response of tfie hihotocathode is not uniform, and thus a chan?~e in spectral sensitivity is observed with changing dynode volt~tges. Iro eliminate this effect, the trajectories of the photoelectrons from the photocathode and first dynode need to be fixed with respect to changes in applied volta~e. Dynodes two to eight do not enter into the spectral response cbanges since most of their ser.si t i ve surface is being bombarded anyway, owing to the increased numoer of photoelectrons and the distribution of their trajectories. This means the spectral response for the rest of the tube is 'flat I to changes in voltage. Iro fix the trajectories_t the potential differences acro[s the first two stages need to be fixed and this can be accomplish-ed by using zener diodes between the photocathode and the first dynode and between the first and second dynodes, rather than the standard bleeder chain ~esistors.

Changing the applied voltage across such a bleeder chain configuration changes only the potentials between dynodes two and eight and the trajectories of the photoelectr:Jns OVer the first two staf2~es are fixed. Consequently, the spectral res-ponse of the photomultiplier is not dependent on applied voltage changes.

practice in prBcision spectrophotometers and in recommended by some photomultir,lier manufn.cturflrs (E.rr.I., 1970).

Ghan~e3 in flpcct'Hl sRnflitivity could also be introduced through changing ah301ute .sii;nr;.1... lGvel~. ev'?rRJ. s<:lries of tp.sts werA pe:;:'formed in which the ef:""ect of chauc;ing the ap;;li8rl photor:,ul t i pli er 1J. T. for constn.nt si;';nal levels, and changing t;h"~ signp,l level '1t R ccnsi:;:71nt su;"ply voltage, for a range of photomnlti;llier nUr,lply voltages, was r(~corded. Of courRB, nIl these tests needed. to be performed with a source

source ill which the inte~r:::i tAd flux Q'J,tput in each waveleng·th in terval iA const nt '~li th tim;~. G1c 7--1rly the sun cannot

ea,<=;ily 'r:>e used as A. source of r:ldiation :ar theso tests since

the ctJangin~:; relative int'3nsi t~l levaL, in the 301ar radiation at the ground iG the very eff'3ct thn t the ~~t,')ctroD:1.otometer ."i 11 Tile:18Ure w"len cali b-;-:"lted. _i'or t hi oS re8.~')on t R ,1euterium

'j:his ;:,;ource -;:;as left running for twenty minutes to :~ar"r, :lP, (double the r:l'.1nUf'l~turers·

specif-the output :;:-eILainad con.st ~:nt t::-JIO';J.3hout tbe .3GI'ics of te::-ts,

dt d ;)hotoraul tl:ylL~-;:, suppl.; va 1 t:l ~e o~ (:~,y; volts, 'llld at a

3.13

However, to check that the measurements made in this

calibration procedure were Dot a function of the source used,

all the tests were repeated using a 100 watt quartz-iodine

lamp. The approximate spectral output from the deuterium source and the quartz-iodine lamp are shown in figure

(3.5).

i.s this figure shows, the spectral content of the 'luartz-iodine lamp is similar to that of the sun, shown in figure

(2.6), and very different from that of deuterium.

II'he difference in spectral composition of the deuterium

l3Jllp tha~'efore provides a check on the source dependence of the system. 'The greater luminosity of the quartz-icdine lamp in the ultraviolet enabled calibration at high signal levels, comparable with those measured in direct sunlight.

\:'/hen the apparent lu.minosi ty was reduced 1 a region of

overlap with the deuterium lamp was o[)tained allowi..ng the

source comparisons to be made. rerfect agreem.ent W':H~ found in the region of overlap showing the system was independent of

the source used. 'rhis test justifies the assumption that calibrations performed using tJle deuterium and quartz-iodine

lamps will be valid for data colJ.ected using the sun as a

source. Walshaw et

ale (1971),

discussing the results of a

previou3 absorption coefficient experiment (Dobson

1963),

also

report finding no difference in the relative reduction in

intensity of various wavelength bands when either skylight

or radiation from a tungsten ribbon lamp was passed through a

I

>-1

_{J - '}

- I

01

z

W

J-Z

o

w

...

<.

_ I

o

<t

a=

3000

DEUTERIUM

3500

4000

QUARTZ - 1001 NE

LAMP

5000

7000

3.15

output from each lamp was kept constant throughout the meas-urin~ period.

However. apparent chans-';es in luminosity. observed by the filter system. were obtained by reducing the source output with c;,librated neutral density filters. Neutral density filters for the ultraviolet are more difficult to obtain, especially in New Zealand, than those for the visible region, since the normal silica glasl'l substrates are ul traviolet

absorbing.

The partial silvering of ouartz plates to make effective optically "greyll filters was considered, but very fine stain-less steel e:auze was finally used. lJ..'he particular gauze chosen, 100/100 mesh, is a square mesh with 152 micron ~ole size and 102 micron wire (42 gauge) which has a theoretical transmission of approxj .. mately 36 peDcent. The wavelength transmission characteristic of filters made by usin~ this gauze sheet were measured in a Csry 14 Recording ~pectro­ photometer. A single thickness of gauze was found to have a transmission of 32.0 percent. fhe transmission, as expected, proved to be the same for all wavelengths (that is, the

spectral response was 'flat').

,.16

in this orientation produced neutral density filters with a

density of 1.0, where density is defined as the logarithm, to the base 10, of the f:) ctor by which the inteDsi ty is reduced. Therefore, a reduction b;,' a factor of 10 in the transmi tted radiation means the filter must have a c;ensity of 1.0. When two of these filters were superimposed, Moir: fringes were

observed if the distance between the filters was less than

0.1 m. ~he nett transmission with a spacing of less than 0.1

m was found to be depenjent on the relative orientation of

the filters. However, with 8 spacing of 0.15 m, or greater,

one filter could >e rotated relative to the other, without

fringes being observed. or the nett transmission changing.

'l'his procedure was checked during the calibration tests and

no chanf;e in the absolute level waf. observed.

By using a stack of three filters with inter-filter

spacingsof 0.15 m, it wa~, possible to reduce the intensity of the source, as seen by the p:Cotomultirlier in the filter instrument, by a factor of 1000 since each neutral density filter was 10 percent transmitting. In this way, the radiant flux seen by the filter f:;ystem could be reduced by knovm

steps without fear of changing the spectral con.tent of the

source.

11.educing the arc current would o.lfjo reduce the radiated

source energy but would produce a change in the colour

temper-ature indicative of a change in spectral output. It was

3.17

It cen

tbere-ltiSll enc.rEeJ·, liLl:t leve::"F', tte rmoeie current became comparable

effecti ve dJrLode vol tc-ge on th.e 1 af:~t stHC€ ancy consequently d.roppeG_ the aEIJIL~·icLtic·n ()f' the photorillJ.l tiplier. AS a

rc,ul"t the tunr] ificc~ticn f ct.:.)r of the ptotOPil •. lt;iplier becaJIie

cr€D.sint; the bleeder CUlrer:.t b~T 9. f:;;.c-vcr of

15.

IJ~his change

bl.eeder cur::.'(::nt ::l.ud '::;he [;;CJ,::-:irmL eX;'Gc-::;ed clUOe' e current,

U~cI:o:l1;:eren 19?O). 'Ii th ~;uch R r:-ltio, the 8B)lification

,.18

been collected. Thus all data collected before this bleeder

current fault was corrected, needed to be corrected for this

systematic error, introduced by the changing amplification

factor. 'ro calibrate the old system, with the corrected

system that gives a constant amplification even at high light

levels, the new set of dynode resistors was connected in the

chain so that it was possible to switch between the old and

the new bleeder currents.

~J.io obtain a calibration relationship, data was recorded at many different light levels. 3ince the spectral

sensitiv-ity did not change with changing H.T., or with changing

absolute light levels, data from each of the four

interfer-ence filters could be used in this calibration. At anyone

setting of

H.'r.,

and for BIly particular neutral density

con-figuration, the old and new system could be compared at four

different light levels because of the intrinsic difference in

transmission of the interferonce filters, due to their

diff-erent bandwidths and transmission characteristics, and

because of the non-uniform spectral content of the source.

The results obtained from this procedure at

555

volts H.T.

(after the logarithmic amplifier corrections to intensity had

been applied) are presented in figure

0.6}

The figure

demonstrates the ;,:,pectral insensi ti vi ty to change by the

way in which the data from each w':lvelength bana lie on the