Quantification of the unsharp masking technique of image enhancement

Theses Thesis/Dissertation Collections

6-19-1981

Quantification of the unsharp masking technique

of image enhancement

Larry A. Scarff

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Recommended Citation

TECHNIQUE OF IMAGE ENHANCEMENT

by

by

Larry A. Scarff

B.S. Rochester Institute of Technology

(1981)

A thesis submitted in partial fulfillment of the requirements for the degree of

Master of Science in the School of Photographic Arts and Sciences in the College of Graphic Arts and Photography of the Rochester Institute of Technology

Signature of the

June, 1981

Larry A. Scarff

Author ••••.•••..••••••••••••.•••••••.•.••••

Photographic Science and Instrumentation

Ronald Francis

Accepted by . . . • . . .

Rochester, New York

CERTIFICATE OF APPROVAL

MASTER'S THESIS

The Master's Thesis of Larry A. Scarff has been examined and approved by the thesis committee as satisfactory

for the thesis requirement for the Master of Science degree

Edward Granger

·

... .

Dr. Edward M. Granger, Thesis Advisor

John F. Carson

• • • • • • • • • • • • • • • • • • • • • • j • • • • • • • • • • • • • • • • •

Professor John F. Carson

John Schott

· ... .

Dr. John Schott

by

Larry A. Scarff

Submitted to the Photographic Science and

Instrumentation Division in partial fulfillment of the requirements for the Master of Science degree at the Rochester Institute of _Technology

ABSTRACT

The technique of _{unsharp masking} is described and its

use as an image enhancement technique discussed. A mathe

matical model for the _masking process is _developed; exper imental _testing and MTF measurements of the masked and

sharpened images are made to test the _validity of the

mathematical model as a predictor of the mask and final

image characteristics. The effect of _contrast, mask

unsharp-ness, and source spread function size on the _resulting MTF

are presented. Subjective evaluations are used to determine the _visually optimum image. It is shown that the _visually "best" _{image is}

not _necessarily the one with the largest MTF value or _area; suggestions are made for _adjusting

existing image quality specifications to incorporate the

The author wishes to express his appreciation to

his thesis _advisor, Dr. Edward M. Granger of the Eastman

Kodak _Company for his _{patience, understanding,} and guidence

throughout the period of this research. Mr- Richard Norman

of the Rochester Institute of _Technology Was instrumental

in the construction of the pin-registration apparatus used

in this _thesis; the author is _very greatful for all of his

help and knowledgeable suggestions _regarding the design of

the apparatus used for this project.

This research project was _partially supported _by a

grant from the Central Intelligence _Agency to the Photo

graphic Science and Instrumentation Division of the

Rochester Institute of Technology.

List of Tables v

List of Figures vi

Introduction 1

Background 7

Mathematical Development 14

Experimental 36

Objective Results 58

Subjective Analysis 65

Conclusions and Recommendations 73

List of References 76

Appendix A: Image Analysis Principles 78

Appendix B: Computer Program for

MTF Determination 82

Appendix C: Scene Content and Calculations

of Defocus 86

Appendix D: Methods of _Producing

an _Unsharp Mask 95

Appendix F: Subjective _Quality Factor 105

Appendix G: _{Microdensitometry} 108

Vita 115

Table 1 Exposures produced and analyzed for the scenes and sinusoidal target images. Sinusoidal

target images were produced at two contrast

levels for each indicated blur value; mask contrasts of 0.4 and 0.6 and the corresponding

print contrasts of 1.67 and 2.5. (PM = Pan Masking Film; FP-4 = _Ilford FP-4 Film. All enhanced images were produced on Contrast

Process Ortho Film.)

Table 2 Response of eighteen judges when asked about

the sharpness of an _originally "sharp" image with various amounts of image enhancement from

unsharp masking. (See Table 1 for an explana

Figure 1 A graphical analysis of the edge enhancement

produced _by the _{unsharp masking} technique. Figure 2 Graphical representation of _{unsharp masking,}

showing the effects of defocus on two _frequency regions. Note the absolute enhancement in (b)

in the _frequency region where spurious resolu

tion occurs.

Figure 3 Diagram _showing the type of edge enhancement

produced _by the subtraction of first and second

derivatives from a one-dimensional represen tation of a degraded edge.

Figure 4 The use of a mask to lower the contrast and

decrease the output _density range of a negative.

Figure 5 Graph _showing the _{theoretically} determined

enhanced modulation transfer functions produced using various levels of mask blur.

Figure 6 Effect of changes in mask contrast on the

resulting theoretical MTF for a "perfect"

original MTF of 1.0. Mask contrast: (a) = _0.2;

(b) = _0.4; (c) = 0.6.

Figure 7 Various mask MTF shapes and the associated enhancement produced _by each mask MTF.

Figure 8 Three original MTF shapes _representing various degrees of original scene blur.

Figure 9a Possible enhancement scheme for a "sharp" original MTF.

9b Possible enhancement scheme for a "just blurry" original MTF.

9c Possible enhancement scheme for a _"very blurry"

original MTF.

Figure 10 Effect of the original input modulation (MQ) on the predicted enhanced MTF.

are (a), _{0.2; (b), 0.4; (c), 0.6; (d),} 0.8.

(From _{Kriss, Nelson,} and Eisen - _reference 20.)

Figure 12 The image formed on the _masking film from a

point on the negative. The exposure distri

bution on the mask film is the mask image

point spread function.

Figure 13 Apparatus used for _{producing unsharp} masks

and enhanced final images. _Top photograph

shows the individual elements of the system;

bottom photograph _illustrates, the use of the three-point registration system.

Figure 14 Equipment required for enhancement system:

Diffuse light source (enlarger,) non-contact

registration _apparatus, and vacuum _pump (to

insure firm contact and proper registration.)

Figure 15 Detail of final enhanced image production.

Original negative and print film are placed in _contact; mask image is again separated a distance "d"

from the _negative, with lens

aperture and s'

distance adjusted to produce

a similar cone angle 6 as was used when _making

the mask.

Figure 16

Figure 17a

Figure 18

Figure 19

Relationship between spread function width and zero modulation values for various spread

function shapes: _{Square, Circular,} and Annular

apertures.

Gray scale reproduction for enhanced "Sharp" images.

Gray scale reproduction for enhanced "Just Blurry"

images.

Gray scale reproduction for enhanced _"Very Blurry"

images.

Experimental and computer predicted values of

the mask MTF for two different _{masking films;} Kodak Pan _Masking Film and Ilford FP-4 film.

Experimental and computer simulated enhanced

MTF values for two levels of mask contrast.

Figure 21a

each ranked value is ione standard _deviation,

based on the ranks assigned _by 11 consistent

observers from a total sample of 16 judges.

"Sharp"

scene used for subjective analysis of

image sharpness improvement through _unsharp

masking.

b "Just Blurry"

scene used for subjective analysis

of image sharpness improvement.

c _"Very Blurry" scene used for subjective analysis

of image sharpness improvemeot.

Figure 22 Diagram _showing the _relationship between object

plane, plane of _{sharp focus,} and a defocused

image plane with a blur circle diameter of a1.

Figure 23 Method proposed for _controlling the

negative-to-film separation _using collimated light to

maintain unit magnification between the

negative and mask.

Figure 24 Characteristic curve for Kodak Contrast Process

Ortho _Film, Kodak HC-110 _developer, dilution _A, 68 _F.

, nitrogen-burst agitation; 1 second bursts _every 10 seconds.

Figure 25 Characteristic curves for Kodak Pan _Masking Film

DK-50 _developer, 68 _F., nitrogen-burst

agitation; 1 second bursts _every 10 seconds.

Figure 26 MTF for the visual system _showing Granger's

SQF passband. The three scales shown give

the _frequency scale in terms of (a) cycles/mm.

on the retina; (b) cycles/degree; Cc) cycles/mm.

on the print viewed at 30 cm.

Figure 27 Scheme of test object used for the objective

MTF analysis. The figures for the sinusoidal

targets indicate the specified and (experimen

tally determined) frequency in cycles/mm.

Values for the _gray scale patches indicate

nominal net _density above base + fog.

Figure 28 Experimental and calculated values for sinu

soidal transmittance vs. distance distribution.

Modulation = _0.7, average transmittance = 18.5?o.

density

Contrast Process Ortho _film; F-wedge used

in the microdensitometer.

The desired result of the photographic process is a

"perfect"

recreation or reconstruction of an original _scene,

complete in _every detail. Because of the limitations inher

ent in _{any imaging system,} this ideal result cannot be

achieved, and some compromise must be made. in the _quality

of the final image. All photo-optical systems suffer some

degree of image _degradation; the _severity of this degredation

being dependent on the type of equipment used and the care

taken to insure optimum results.

Once the image has been recorded on the photographic

material, however, it would seem that little (if anything)

could be done to restore or reconstruct image information

that was lost due to the inefficiencies in the photographic

process. This statement is _{strictly true;} information about

a scene cannot be retrieved from an image if the detail was

not recorded when the photograph was made. However, _by

using proper _techniques, faint or _barely recognizable detail

can be enhanced or _improved; in essence it becomes _visually

easier to extract information from the image, thus _improving

the overall _quality of the photographic record.

Common methods for image enhancement _today center

around digital image processing by computer. These tech

the signal is converted back into tones of gray. Computer

picture _processing is _{currently being} studied _by numerous

researchers and will not be addressed _directly in this paper, Other methods for image enhancement - _{what might} be termed

the optical _analog of digital image enhancement - _are the

photographic methods of image _sharpening or edge enhancement,

It is possible to improve the visual appearance of an

image _{by sharpening} the _edges, or _boundry, between distinct

objects in the image _{by exaggerating} the _density difference

across the edge. With increased edge definition (up to a

point) the visual appearance of the image is improved and

it is judged to be of a higher _quality than an identical

image without the edge sharpening. One form of _accentuating

the edge _gradient, known as _{adjacency effects,} ~

is pro

duced _{by processing} the film _using a high solvent/low

activity developer that produces a greater development

effect (larger _density differences) at the _boundry of different exposure regions than the _density difference

obtained in the region _surrounding the boundry. This

procedure requires that the enhancement be done in the

development stage and allows for no additional _sharpening

after the image has been chemically developed. Thus, any

and degree of _sharpening is difficult to control with this

procedure.

Another photographic method for image enhancement is

known as _unsharp masking. It is analogous to the _adjacency

effect in the results _obtained; the method of _producing

these results is _{completely dissimilar,} however. Once a

photographic image of a scene has been obtained, another

image can be produced on film from the original image.

This duplicate image of opposite sign _(e.g., a positive if

the original image was a negative) is made _slightly "blurry"

or _unsharp, either _{by defocusing} (if a projection _imaging

system is used) or _{by physically separating} the original

image and _duplicating film (if a contact _printing system

is used.) The contrast and amount of unsharpness of this

duplicate are two critical factors that determine the final

image _quality, and will be discussed in detail later in

this paper. The original and _unsharp duplicate are now

placed in contact and a final image produced from the

combination of the two images. _{Paradoxically perhaps,} the

image _resulting from the _unsharp mask and original image

combination will be _visually sharper than an image produced

from the original record alone. The image enhancement is

due to the edge _sharpening that occurs when an _unsharp mask

is used; this result is depicted _graphically in Figure 1.

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ing the selection of the amount of _sharpening to be produced

and the spatial _frequency (object size) region in which the

sharpening will have the greatest effect. _Also, since it

is not _necessary to produce this form of _{sharpening during}

the image development _process, it can be altered or added

when _required, based on the individual requirements for

the final image.

We note that an optimum degree of unsharpness must

exist that will produce the "best"

visual image. It is

suggested that this optimum will be dependent on _many

factors; image _{magnification,} image and object _contrast,

and the sharpness of the original image. It is the purpose

of this research to determine a theoretical model for the

unsharp masking process, predict the _{theoretically} optimum

mask parameters for a specific _image, and to test the

theoretical predictions through experimentation. Subjective

evaluations of picture sharpness will then be correlated

with objective measurements of the experimental images to

determine the _validity of the theoretical predictions for

The technique of _{unsharp masking} was _originally des

cribed _by Spiegler and Juris and _by Yule . Yule proposed

a method for _increasing the visual sharpness of a photo

graphic image _{by printing} together a photographic image

and an _unsharp or blurred reversed duplicate of the _image,

called a mask. The results obtained from _printing this

combination gave an increase in the apparent sharpness of

edges in the _image, when the contrast of the print film

was adjusted so as to produce a normal _density range on

the final output image. The mask in combination with the

original image _{significantly} reduces the low _frequency

contrast, but does not affect the high _frequency contrast;

since it is the high _frequency contrast that affects the

apparent _sharpness, the masked image appears sharper than

an image produced without the _masking process.

Yule outlines a procedure that might be used to produce

unsharp masks: A transparent or diffusion sheet is used as

a spacer between the original image and the _{masking film,}

and the original-spacer-masking film combination is rotated

below an off-axis point light source to obtain annular

illumination. This system produces a blurred image on the

masking film that is then registered with the original

shapes of point spread functions are also obtainable _by

altering the method used to produce the _unsharp mask .

The technique of _{unsharp masking} proposed _by Yule has

been modified _by several other researchers to incorporate

other methods to produce edge enhanced images. Johnson

p

and _Kelly have proposed methods for _producing enhanced

images _using the Herschel effect. _Kelly suggests that

exposures be made on photographic film _exhibiting the

Herschel effect; a _sharp exposure is made with short wave

length _light, and an out of focus exposure is made with

long wavelength light. The exposure made at the longer

wavelengths will _effectively erase part of the original

exposure at short _wavelengths, (due to the Herschel effect )

and produce an enhanced _"unsharp masked"

image. This tech

nique can also be applied _{by using} a lens with significant

chromatic _aberration, focused at short _wavelengths, or _by

inserting into the light path _{appropriately} colored spatial

filters which will weight the spread function as desired.

Phosphor quenching is a similar application except that

it uses a phosphor screen that is excited _by ultraviolet

radiation and quenched _by infrared radiation. If both

sources are _used, the ultraviolet to _fog the screen and the infrared to quench the screen _selectively through a

Armitage and Lohmann have shown that an absolute increase

in high _frequency modulation can be achieved _{by making} a

mask that is defocused or blurred to such an extent that

spurious resolution for a band of frequencies is achieved.

In this _region, when the mask is combined with the original

image an absolute enhancement will be achieved. This

result is shown _graphically in Figure 2. Jhese and other

methods of photographic image enhancement techniques have

12

been reviewed _by Levi in a _very good _survey of current

enhancement techniques. Digital image enhancement _using a

method of derivative subtraction techniques have been

13 14

studied ' and the suggestion is made that these tech

niques correlate with _unsharp masking.

Rosenfeld suggests the _intuitively simple _analogy that

if an image can be blurred or smoothed _by an _averaging or

integration _process, then an image might be sharpened or

edge enhanced _by a dif ferentation process. A number of

differential operators have been used as edge _enhancing

functions, most _notably the _Laplacian, which is proportional

to the difference between the _gray level at a point and

the average _gray level in an annulus centered at the point.

In Figure _3, the second derivative of an edge function is

subtracted from the edge _itself; note the _similarity to the

XZVffi

Original Image

Mask

W\

Combination Image

Original Image

Mask

Combination Image

(a) "Usual"

unsharp masking technique _giving rela

tive enhancement in the

high frequencies.

(b) Phase reversal on mask in high frequencies gives "absolute"

enhancement.

Figure 2

Graphical representation of _{unsharp masking, showing} the

effects of defocus on two _frequency regions. Note the

absolute enhancement in (b) in the _frequency region where

spurious resolution occurs. (From Armitage and _Lohmann,

Figure 3

Edge Function

Position

First Derivative

Edge - First Derivative

Edge Function

Position

Second Derivative

Edge - Second Derivative

Additional investigation has been made to determine a

mathematical model for the prediction of the _resulting

density distribution when film is processed in a developer

exhibiting adjacency effects. The results obtained with

adjacency effects are similar to those produced _{by unsharp}

masking, though the mechanisms for the two processes differ

significantly. _Thus, the model _{describing adjacency}

characteristics is not _directly applicable- to describe

masking effects. The reader is referred to references 1

and 2 for additional information _{concerning adjacency} effects

and the _{experimentally} derived mathematical model.

Most of the practical application of the _unsharp

masking technique is found in the graphic arts _industry

for _improving the sharpness of reproductions , and in

radiography for improving detail and image _quality in

radiographs . Currently, masking techniques used by

workers in these fields have been developed through trial

and error; no method for _predicting the optimum degree

of mask unsharpness or contrast to use for a specific set

MATHEMATICAL DEVELOPMENT

The explanation of the edge enhancement _resulting

from _{unsharp masking} is found _{by noting} some fundamental

concepts of Fourier analysis and their applications to

the photographic process. For a brief summary and review

of these _principles, refer to the discussion in Appendix A.

We can _apply the concepts of MTF analysis to form a _simple,

elementary model for the _masking process. _Masking has been

used _primarily as a method for dynamic range Ccontrast)

compression, as shown in Figure 4. A superposition of the

negative and positive images will produce an image _having

a transmittance equal^ to T x T at all points; thus, the

n _p r

densities of the two images are additive. In the process

known as _{sharp masking,} the transmittances of the positive

are _inversely proportional to those of the negative at all

points, altered _{only by} the contrast of the _masking film.

When an _{unsharp ,} or blurred positive image is made from a

sharp negative, the fine detail (high frequency information)

is degraded _by the larger point spread function caused _by

the _blurring process. This "smearing" of the exposure in

high _frequency regions decreases the contrast _{significantly}

in those areas as compared to the low frequencies (large

areas) in the scene. When the film is processed, the con

ong.

Log Exposure

Figure 4

The use of a mask to lower the contrast and decrease the

is less than that of the low frequencies. When the two

images are _{superimposed,} a subtraction of negative and mask

density range _effectively occurs C since the images are

of opposite _sign; one positive and one negative,) and con

trast _(modulation) is more _{significantly} reduced in the low

frequency region. When the combination is printed onto a

higher contrast film to restore the low _{frequency modulation,}

an effective increase in the high

frequency*

modulation _occurs,

producing "enhanced" high frequency detail.

We note that if the MTF characteristics of the mask

could be _{selectively adjusted,} various degrees of enhance

ment could be produced at each level of frequency. For

example, Schade has suggested that an optimum mask would

produce a flat output MTF curve over a wide range of frequen

cies. _Thus, we would _{theoretically} produce a mask with an

MTF _which, when used in combination with the negative MTF

will yield the greatest region of level response. Such an

MTF can be obtained _{by adjusting} the point spread function

used in the production of the _unsharp mask and the image

contrast of the mask. For increased enhancement (greater

than the original scene modulation) other point spread func

tions and/or mask contrast values _may be _{used, depending}

on the desired result.

The effects of _masking revolve around a frequency-wise

change in contrast, associated with the spread function size

to the final result are factors _involving the original

negative contrast and point spread _function, and the mask

and print material contrast. The effect of this _"frequency

selective"

contrast control can be observed _{mathematically}

by the _following arguments:

We note that the photographic process can be regarded

as log-linear in a finite region of the _D-Log H response

curve. The requirement of _linearity exists if the principle

of superposition _{holds; i.e.,}

fCx + y) = fCx) + f(y) . (1)

For a photographic system this is true provided the system

transformation is the linear approximation of the _D-Log H

curve, or more _commonly referred to as gamma (Y . ) Thus,

for this case superposition _holds, since the _density result

ing from the sum of two _log exposure values is equivalent to

the sum of the densities produced _by each exposure indivi

dually.

With this assumption of _{log-linearity,} we can begin an

analytical examination of the effects of _masking on the

final image characteristics. We start with an original

target of modulation M . By definition, modulation is

given _by the equation

CT /T . ) - 1

max min

CT /T .) + 1

max mm

where T and T . are the maximum and minimum

trans-max min

mittances of the target. We can solve equation (2) for

the ratio of maximum to minimum transmittance:

T /T .

max min (1

+ M ) / (1 - M

)

o o (3)

Exposure of this target on film will produce an incident

exposure ratio equal to the target transmission ratio.

The _log exposure difference (ALog-H) can thus be expressed

by :

ALog-H = _Log

max

min

LogCl + M ) - LogCl - M

) (4)

If the negative material has an MTF _{significantly} less

than _unity for some frequencies of _interest, then ALog-H

becomes _{frequency dependent,} and the effective _log exposure

range on the negative is given _by the expression

ALog Hn(l>) = Logll +

MoMn(!;)]

-Log

[l

-M0Mn(l0]

where the subscript "n" refers to the _negative, and M (v)

is the MTF value for the negative material at _frequency V.

When the film is developed to a contrast *Y _, the _log

exposure difference will produce a _density range on the

negative given _by

AD = 7,

n n[ALog

HR

This negative _may now be Ci) exposed onto a material to

mask produced and the combination printed in register to

obtain the final image. We now examine the results obtained

using both methods. (Note: In the _{following discussion,}

the subscript "n" will indicate a _quantity that refers to

the _negative; the subscript "m" indicates a value for the

mask; "p" refers to the unmasked print _material, and "p,M

refers to the masked print _material.)

Case i: Normal _Printing

To restore the modulation to the value of M , the print

o ' r

material must have a contrast ₇ such that 7x7= _1.0;

P n _p

thus, the required gamma for the print is the reciprocal

of the negative gamma. The _log exposure difference for

the print material will be equal to the _density range of

the _negative, given in equation (6). _Thus, we see:

ALog_a H ₌₇

ALog

p 'n a n (7)

AD = ₇

7n

We have assumed here that the MTF of the print material

will be _{essentially unity} over the _frequency region of

interest.

Since the _density range on the final print CAD )

is _{logarithmically} related to the output transmittance

(T /T . )

max mm 10(AV

=

0

Therefore, the output modulation on the print as a function

of _{frequency is} given _by

M _{v)

P

0- 1

0+1

(10)

and the effective system MTF is _simply M (v)/M

Case ii: _{Unsharp Masking}

When an _unsharp mask is made from the negative _using

a mask contrast 7 and an MTF for the mask of M {v) , we

note that the output modulation for the masked image will

be the product of the negative3 outputK modulation M .(>)

out

and M (J>). To determine M

+ (v) we begin from equation (6):

(T /T . )

max min 10

(ADn)

(11)

M .(P) out

1 - (T /T

. ) max mm

1 + (T /T . ) max mm

(12)

Now, using the same principles as were used for the calcu

lations of original negative exposure _range, we can deter

(H /H . )

max mm

1 + M .(v) M (y)

out m

1 - Mnilf(i>) Mm(-y)

out m

(13)

ALog

Hm

-Log [l - M

fc(P) M^)]

(14)

The _density range of the _mask, AD _, is then _simply

AD.

7m

m 3 m (15)

When the original negative and the _unsharp mask are

placed in register with each _other, the _resulting comb

ination _density range (AD . ) will be the difference

3 y

comb

of each _density range:

AD

comb AD

AD ₍₁₆₎

When the combined mask and negative are exposed onto a

print material with contrast _{7 i} (and again _assuming that

the print material MTF is _{essentially unity} over the range

of frequencies of interest) the _{resulting density} range on

the print (AD ,) will be given by:

AD ,

P 7 i

Ad ,

#p'

Again, the logarithmic _relationship between _density range and transmittance ratio gives

(T /T . )

max mm ,

P

CAD.)

10 P = _CO (18)

Thus, the output modulation as a function of _frequency

for the masked image is given _by

Mp,(>) CO

- 1

CO + 1

(19)

The system' MTF for the masked case is thus M , (&0/M

p o

If we now consider the proper print contrast for the

mask + negative combination _print, we note that the final

density range on both the masked and unmasked prints will

be equal for some chosen _{frequency; hence,} we can equate

equation (8) and equation (17) and obtain after substitution

V

7-7

m

/ALog Hm(f)T

\ALog H(-y)/

C20)

7 i will, in most cases, be chosen such that the print

density range Cand hence the print _MTF) will be equal for

This specification will produce equal _{macro-density} levels

for both images. If it is assumed that the values of the

negative and mask transfer functions are _unity at zero

frequency, then the calculation of the required print gamma

for the masked image can be simplified. Through substitution

it can be shown that equation (20) will reduce to:

7 .

( _7n

-7m

n m n

(21)

since for _unity MTF values, ALog H = AD = 7 ALog H

' 3

m n n n

A _relatively simple computer program was developed for

an Apple computer that will solve equation (10) and

equation (19) and determine the _resulting MTF values for

both the masked and unmasked _prints, for various values of

mask contrast and negative and mask MTF shapes. For a

listing of the computer _program, consult Appendix B.

Discussing now some specific results obtained from

the computer _analysis, we will begin _{by examining} the

affects due to the _{unsharp masking} process on a "perfect" .

negative _having MTF values of 1.0 for all frequencies of

interest. The affect on the final system MTF _{by masking}

with a mask MTF proportional to sinCx)/(x) and scaled to

provide various levels of "unsharpness" is shown in

different levels of mask contrast Cand _hence, final print

material contrast) are used. Note that the region of

enhancement becomes greater as the mask is made _increasingly

unsharp (poor MTF,) and that the degree of enhancement

(system MTF values above _1.0) is increased with increased

mask contrast. There _{is, however,} a practical limit to

the unsharpness in the mask that will produce acceptable

results; this is due to the flare and _scattering problems

when _exposing the mask image and will be discussed further

in the next section. Figure 7 shows comparisons of the

effect when various types of MTF shapes are used in the

mask _making process. It can be seen that _{by changing} the

shape of the mask _MTF, a slight degree of control over the

range of frequencies to be enhanced is possible.

When actual photographs are made of a target or _scene,

some amount of _{blur, defocus,} or other source of image

degredation _may cause the negative image to have modulations

significantly less than 1.0 in the frequency range of

interest. Three levels of degraded negative MTF values

have also been used to examine the effects of _masking on

more realistic, optically-degraded images. The _optically

degraded MTF values were obtained from the work of Levi

1 8

and _Austing who calculated MTF values at various levels

of defocus for a perfect lens. The levels of defocus used

in the _following analysis represent "sharp," "just blurry,"

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d-are shown in Figure 8. Figure 9 illustrates the amount

of enhancement available with the aid of _{unsharp masking}

techniques applied to these defocused originals.

In the _preceeding examples we have assumed that the

input modulation CM ₎ was constant at a value of 0.70.

Because of the non-linearities introduced when _determining

the final modulation from _log values (density) the _resulting

output modulation and hence the final MTF values will be

dependent on the original input modulation. Figure 10

shows the calculated values of MTF for different levels of

input _modulation, with all other factors held constant.

As can be _seen, the MTF values increase as smaller input

modulation values are used. This result is consistant

20

with that shown _{by Kriss, Nelson,} and Eisen who derived

a model to predict the results obtained with films processed

to produce a large amount of _adjacency effects. Since the

results produced _{by adjacency} effects and _{unsharp masking}

techniques are _similar, correlations with Kriss1 work would

be expected. Figure 11 shows the MTF values obtained _by

Kriss for various input modulation values. It should be

remembered that when an actual scene is used, an entire

spectrum of modulation levels will be _present, and _hence,

a _family of MTF values will _result, one for each input

modulation and _frequency combination present in the scene.

Since the _following experimental work was performed with

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lA o LA CD LA O LA

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10 20 30 40

Frequency (cycles/mm.)

50

Figure 11

The effective modulation transfer function as a function

of input modulation (M ) for films with _adjacency effects.

Input modulation values are (a), _0.2; (b), _0.4; (c), _0.6; (d), 0.8. (From _{Kriss, Nelson,} and Eisen

values will be _strictly valid _only for this scene. Further

study would include an analysis of various scenes with

different Weiner _Cpower) spectra to determine if the optimum

values presented here differ _{significantly} for various

scene power distributions.

Summarizing, by knowing the range of frequencies that

are to be enhanced and also the degree of enhancement

required, a selection of proper MTF shape acid _scaling,

mask _contrast, and final print contrast _may be made to

provide an increase in image sharpness not possible through

normal _printing processes. It has been suggested _by several

19-21

researchers that a reasonable objective measure of

image sharpness and _quality is a comparison of the areas

under portions of the system MTF curves for various images.

With the aid of _{unsharp masking} it now becomes _{theoretically}

possible to increase the modulation level of a range of

frequencies much higher than _1.0, thus _increasing the area

contained under the MTF curve and hence _increasing its

measured _"quality factor." _However, images produced with

an extraordinary amount of enhancement become _visually

displeasing due to an exaggerated edge sharpness beyond

the level that is visually acceptable. _Thus, a visual

optimum must exist and the definition of _quality through

area measurements alone might be modified somewhat to

incorporate these _masking effects. Through experimentation

and subjective analysis this question of a visual optimum

EXPERIMENTAL

In order to correlate the theoretical models for

unsharp masking effects with subjective judgements concern

ing image _{sharpness, unsharp} masks and enhanced images

were produced under a _variety of conditions and _subsequently

examined _by a panel of judges to determine the degree of

improvement in sharpness for each image. The two major

objectives of this experimentation were as follows:

i. Determine whether the proposed mathematical model

adaquately predicts the effects observed when image

enhancement is performed _using the _{unsharp masking}

technique.

ii. _{By varying} some of the parameters associated with

the original negative _image, mask _production, and

film sensitometry , determine the optimum conditions

to provide the best degree of improvement in image

sharpness for negatives of different quality.

To obtain a solution to the first _objective, a sinu

soidal test target was used as the original negative and

enhanced _using this technique. Objective measurements of

mask and system MTF could then be compared with the values

predicted _by the mathematical model. The second objective

conditions and that a matrix of _differently enhanced images

be produced and each image _subjectively judged for improve

ment. The techniques used to produce these "scene" images

(as well as the enhanced sinusoidal target images) will

now be discussed.

Two basic parameters were varied to produce original

negatives with _varying sharpness _levels; image blur due to

errors in focus _position, and the signal-ta-noise ratio

of the original negative image Cadjusted _by selecting films

that differ in granularity.)

It was decided that the scene to be photographed should

contain a _variety of objects _varying in texture and size

in order to provide the viewer with a broad range of

subjects with which to judge sharpness improvement. A

scene _containing text _material, as well as a _variety of

objects with different spatial _frequency information were

used in the scene. A person was not included in the _scene,

as it was decided that edge _sharpening would not be appro

priate on portrait-type _images, and the subjective impress

ion of "optimum" might be altered. _Thus, the subjects for

all of the tests were inanimate objects and the results

obtained are based on the subjective impressions of _only

these objects. However, since most scientific applications

for image _sharpening do not include photographs of _people,

A _gray scale was also included in the scene in order

to monitor the final image _contrast, but it was located

in an area of the object field that could _easily be excluded

from the final area to be viewed. The absence of a _gray

scale in the final scene required judges to base their

decision upon picture sharpness and scene _contrast, rather

than on _gray scale reproduction. For a complete description

of the _scene, refer to Appendix C.

In order to _vary the signal-to-noise ratio on the

negative _images, two film types _{having significantly} diff

erent _granularity values were _used; Kodak Professional _Copy

Film, type _4125, and Kodak _{Recording Film,} type 2475 provided

high and low signal-to-noise _ratios, respectively.

It can be seen from equation ₂₁ that a _relatively

high negative contrast is desirable if mask contrast values

around 0.5 are to be used and the final print contrast is

not to exceed 3.0. The value of 3.0 was considered to be

the maximum contrast that could be obtained from an emulsion

whose contrast could be _easily altered through processing.

Emulsions _having infectious development characteristics

such as "Lith-type" films were considered undesirable for

this experiment as contrast is not _easily adjustable for

these films through development.

A film format for the final images of 4" x 5" was

chosen for ease in _{handling, processing,} and mask making.

inter-positives were produced on Fine Grain Release Positive

Film, type _7302, and then 4" x 5" negatives made from the

inter-positives _by contact _printing onto Professional _Copy

Film. This procedure also served to enlarge the grain

structure on the final _negative, thus _providing a lower

signal-to-noise ratio for the tests.

Three focus positions were used for both film images.

The original negatives made with the 4125 film were photo

graphed _using a Plaubel view camera and a 235 mm. focal

length Schneider _{lens, visually} focused on the subject.

One image was made with the subject in _focus, and then

two levels of defocus were used to produce images that

represented "just blurry" and _"very blurry" reproductions

of the scene. An identical procedure was followed for the

2475 _film, except that the camera used was a Nikon F-2

with a Nikkor 55 mm. macro-lens. For a more complete

discussion of the calculations involved in _selecting the

defocus positions, refer to Appendix C.

The design of the apparatus used to produce the _unsharp

masks from the original 4" x 5" negatives was based on the

following requirements:

It was desired to adjust the degree of blur introduced

into the mask image through an adjustable physical

separation between the negative and mask film. The

geometric effects of this alteration on the

can be seen in figure 12. The equation _relating

the image point spread _function, "a," to the source

point spread _function, "A," is given by:

A

L

a x n

(22)

where "L" is the separation between the _exposing

source and the _negative, "d" is the separation between

the negative and the _mask, and "n" is the index of

refraction of the medium between the negative and

the mask film.

The source point spread function was to be altered

by inserting a variable _density filter between the

diffuse light source and the film. This filter could

be adjusted to provide different source point spread

functions, thus _controlling the shape of the mask MTF.

Adjustments in the scale of the MTF could thus be made

not _{only by} the size of the filter but also _by adjust

ing the distance between the filter and the negative.

The registration between negative and mask must be

accurately controlled to allow for their re-registra

tion when _making the enhanced image. _Thus, a system

that allowed longitudinal movement between the mask

and negative but maintained the lateral registration

Light Si

Figure 12

The image formed on the _masking film from a point on the

Figure 13 shows a diagram of the completed apparatus.

Negative-mask separation was adjusted through the use of

5"

x 7" glass plates which varied in _thickness, while

lateral registration was preserved _by the three-point

registration system. A larger negative-mask separation

was possible with this system _{by using} a metal spacer

that provided an air _gap of _{approximately} 10 mm. between

the negative and mask film. A 4"

x 5" Omega condenser

enlarger was used as the _{exposing unit;} with the lens

removed from the _enlarger, a piece of opal glass located

at the negative gate provided a uniform diffuse source.

Variable size and _density filters could also be located

at the negative gate to adjust the spread function size

and shape. Figure 14 shows the entire _exposing system

ready for operation. Some important design considerations

for _producing an acceptable mask _exposing system that were

investigated _during this research are discussed further

in Appendix D.

Another consideration for the _exposing system is that

the geometries of the mask _making and print _making light

paths be complementary to each other. Because of the

separation of the mask film and _negative, and the diver

gence of the light from the exposing source, the mask image

will be slightly magnified. Upon printing, the position

of the negative and mask are _reversed; the negative is

Figure 13

Apparatus used for _{producing unsharp} masks and enhanced final images. _Top photograph shows the individual elements

of the _system; bottom photograph illustrates the use of

Figure 14

Equipment required for enhancement system: Diffuse light

source Cenlarger,) non-contact registration _apparatus,

and vacuum _pump Cto insure firm contact and proper

separated from this negative-print film sandwich. The

diffuse _exposing source is now focused on the film _by a

lens and the mask-to-negative separation adjusted so that

the cone angle of the _exposing source is identical with

the angle of light when the mask was exposed. This adjust

ment accounts for the slight difference in magnification

and provides exact mask-to-negative registration. These

principles are outlined in Figure 15.

In order to _strictly control _{processing variability}

and _accurately determine the _necessary development time

to obtain the desired _contrast, a deep-tank nitrogen-burst

development system was used. A complete description of

the sensitometric tests conducted with this system to

determine _processing characteristics can be found in

Appendix E. The _masking film used for the initial tests

was 5" x 7" Kodak Pan _{Masking Film,} type 4570 developed

in Kodak DK-50 _developer, diluted 1:1 with water- For all

enhanced _images, the print film used was Kodak Contrast

Process Ortho _Film, 4154 (4" x 5" sheets) developed in

Kodak developer _HC-110, dilution A.

As previously mentioned, the mask MTF shape and scale

were adjusted via the aperture filter. To obtain a

(sin x)/(x) or "sine" function shaped MTF for the _mask,

square aperture filters were _{used, varying} in width to

provide different _scaling values. Scaling was adjusted

Diameter D

Diffuse Source

mask

image

original negative

print film

Figure 15

Detail of final enhanced image production. Original

negative and print film are placed in contact; mask

image is again separated a distance "d" from the _negative,

with lens aperture and s'

distance adjusted to produce

be equal to the first zero value for the MTF (see

Figure 16. ₎ Circular and annular aperture masks were

also _{used, providing} Bessel- and Cosine- type MTF shapes.

Originally, the desired cutoff values for the mask

MTF were selected for their _relationship to the SQF band

19

proposed _by Granger when the final print was viewed at

a distance of 30 cm. (see Appendix F.) The values chosen

for the mask cutoff _frequency were _{2, 4, 8,} and 16 cycles/mm.

It was also decided that the separation between negative and

mask film would be held constant at a 3/8 inch air _gap,

and _scaling adjusted _by aperture size alone. _Using equa

tion _22, one can select a value for L that is convenient

and will allow convenient aperture widths to be used.

After some initial tests it was discovered that the

scattering effects caused _by the silver grains in the

negative and mask caused a _noticeably degraded final image

to be produced. This result required that smaller values

for the width "d" be used and the values of L and A re

adjusted to provide the same geometric angles. New values

for "d"

of 2 mm. and 6 mm. were selected _using glass plates

to provide the separation, and it was found that the 2 mm.

separation provided the best results, allowing geometric

scaling through changes in L and A, but limiting the degree

of scattering caused _by the Callier-Q factor associated

with the silver images used in this system. _During these

MTF

MTF

1.22

a

freq,

MTF

2 _\JJ>4q

Figure 16

Relationship between spread function width and zero

modulation values for various spread function shapes:

(A) Square aperture, Sin(x)/(x) frequency spectrum;

to provide the system with an _inherently lower Q-factor

because the dye clouds that replaced the silver grains in

the color images would cause _{significantly} less _scattering

to occur. _However, this option was discarded since the goal

of this research was to optimize a practical method for

unsharp masking; since most of the current applications

of this technique _rely on silver _materials, the decision

was made to continue the tests with black-and-white silver

photographic materials.

Using the 2 mm. separation and _adjusting "L" to

638 mm. , aperture sizes of 2"

and 1" were used to expose

the sinusoidal targets to produce masks with scaled MTF

shapes. Also exposed was the _focused, high signal-to-noise

ratio _scene, and the masks processed to a gamma of 0.6.

From these _images, enhanced positives were produced and

examined. It was found that although there was a slight

difference in the mask MTF values for the two aperture

sizes, it was not significant enough to produce a visual

difference between the scene images made with _differently

shaped apertures. For this _reason, the use of variable

density aperture masks for MTF _shaping became _unnecessary

since both the mathematical model and experimental evidence

indicated that there was no observable difference between

images produced with different aperture shapes (see

Figure 7.) An annular aperture mask was used to expose

effects of this _radically different source spread

function.

A much greater effect is noticed when mask contrast

is altered. Contrast levels of 0.4 and 0.6 were used for

masks made from each high signal-to-noise ratio negative.

From _these, the proper print contrast was selected to

restore the low _frequency contrast to 1.0. A series of

images were made _varying the contrast of tire mask for the

cases shown in Table 1. These were then examined _by judges

to obtain a _ranking of their sharpness. The _gray scale

contrasts for all images were maintained as uniform as

possible; Figure 17 shows the small deviations in _gray

scale reproduction inherent between the prints produced.

In addition to the scenes indicated above, sinusoidal

target exposures were made for the cases shown in Table 1

in order to provide objective information about the enhance

ment produced at each level. In the next _section, the

objective results obtained through analysis of the _resulting

enhanced MTF values and the subjective results determined

through observations of the scene images will be correlated

and compared with the mathematical model results presented

earlier.

When the low signal-to-noise ratio negatives were

enhanced _using the system described, a significant decrease

in overall image quality was obtained. This degredation

Table 1

Exposures produced and analyzed for the scenes and

sinusoidal target images. Sinusoidal target images were

produced at two contrast levels for each indicated blur

value; mask contrasts of 0.4 and 0.6 and the _{corresponding}

print contrasts of 1.67 and 2.5. CPM = Pan _{Masking Film;} FP-4 = _{Ilford FP-4 Film. All} enhanced images were produced

Table 1

a) ORIGINAL SCENE EXPOSURES

Original

Contact

Print

-no mask

Contact

Mask

Blurred mask:

a = 0.10 Cd = 2 mm. )

7 = 0.4 7 = 0.6

m m

Sharp A B C D

Just _Blurry E F G H

Very Blurry J K L M

b) SINUSOIDAL EXPOSURES

0 mm.

Separatio

2 mm.

n "d"

6 mm. 9.5 mm.

0 PM

FP-4

- -

-0.05 - PM

FP-4

PM PM

CO

0.10

x: -p

, ,

- PM

FP-4

- PM

FP-4 X

s 0.20 - - PM PM

0.5 1.0 1.5

Original _Gray Scale _Density

Figure 17a

Gray scale reproduction for enhanced "sharp" _images;

sharp mask, 7= 0.4; unsharp mask, 7= 0.4;

-unsharp mask, 7= 0.6; positive,

0.5 1.0 1.5

Original _Gray Scale _Density

Figure 17b

Gray scale reproduction for enhanced "just blurry"

images:

sharp mask, 7= 0.4; unsharp mask, 7= 0.4;

0.5 1.0 1.5

Original _Gray Scale _Density

Figure 17c

Gray scale reproduction for enhanced _"very blurry" images:

sharp mask, 7= 0.4; unsharp mask, 7= 0.4;

structure) _by the masking process. So severe was the

noise amplification that further tests with the low

signal-to-noise ratio images were discontinued in order that a

more detailed analysis of the high signal-to-noise ratio

OBJECTIVE RESULTS

Measurements of the sinusoidal target images produced

on the _masking film and the enhanced image values were

obtained _using a Joyce-Loebl microdensitometer- _From

this data the MTF values for both the mask and enhanced

images were determined. The microdensitomefcric procedure

used to determine the MTF data is discussed in Appendix G.

It was noted after calculation of the mask MTF values that

the values never reached 1.0 at low spatial _frequencies;

in fact the maximum value obtained with the 4570 film was

0.65. This low value was attributed to three factors:

1. Light _scattering within the emulsion of the negative

and between the negative and mask _film;

2. Lack of an adaquate anti-haliation _backing on the

masking film to inhibit back-scatter of the light

in the _{masking film;}

3. Flare in the _exposing system.

Of the three factors, (1) could not _easily be reduced,

but (2) and (3) were adjusted _{by selecting} a camera-type

film with properties of fine grain and an anti-haliation

coating. Ilford FP-4 film was selected, and the _exposing

these _{modifications,} low _frequency MTF values o