The English word photography is most commonly understood to mean, “ writing with light, ” as derived from the Greek, Φ ως , “ phos, ” the root of the word for light; and Γ ρα ϕ ω , “ grapho, ” the base of the verb, to write. So it is logical that to better understand the processes involved, the nature of light should be examined, as well as the device that allows it to enter the camera. After all, it is the light that is doing the writing.
A good start to learning about light is to consider how it is generated and what it is made of. The place to start is the simple Bohr model of the atom, which is made up of a nucleus surrounded by a group of moving electrons. In this simple model, the electrons are in circular orbits around the nucleus similar to the way in which the planets circle the sun (see Figure 3.1 ). Newer models of the atom are more complex, but the basics of light generation are similar enough so that we can use the simple model. In the atom, the orbits have distinct energy levels. If energy is applied to an atom at the right level, it can cause an electron to move from a near orbit to a more distant orbit. But this is not necessarily a stable condition, and the electron will eventu- ally fall from the high energy orbit to one at lower energy, often the one from which it came in the fi rst place. As the atom goes from a higher energy state to a lower one, it must give up some energy. This is done by emitting light. Each such transition produces a packet of energy in the form of an electro- magnetic wave packet called a photon. The photon is considered a particle of light. When a charged particle (in this case, an electron) moves rapidly, it results in the creation of an electromagnetic wave. This is what is hap- pening in the atom. Electromagnetic waves do not need a medium in which to propagate (as water waves and sound waves do). The photon will travel in a straight line until it is either sent off course by a physical obstacle or absorbed. Photons are absorbed by the reverse of the process by which they are created. They impart their energy to an electron and send that electron to a higher energy orbit. The speed of the propagation (in a vacuum) is a
universal constant, the speed of light, c. In materials other than a vacuum, the speed of light will be lower, and the ratio of c divided by the speed in the particular medium is called the refractive index . This is a property of the specifi c material.
A light source is a device that causes the release of photons. In an ordi- nary light bulb, electricity causes the metal fi lament to become very hot so that the atoms in the metal are violently banging into each other at a rapid rate. This causes the atoms to take on energy and move electrons to higher energy orbits. After a brief time those electrons will fall back to their ground state and emit a photon. The process repeats as long as power is applied. The emission of photons is a random process in which millions of atoms are active and the amount of energy is different from atom to atom. The result is a fl otilla of photons emitted per second, and although each photon has its own level of energy, in aggregate they cover a wide range of energy levels. The energy of a photon depends on its wavelength. The property that humans call color is attributable to wavelength. Put this all together and we can say that the light from the light bulb will cover a wide range of wave- lengths and colors. There are obviously other mechanisms that can result in the creation of light, such as fi res, the atomic processes of the sun, the acti- vation of phosphors in fl uorescent tubes or computer monitors, and so on. The basic story is the same, but the processes differ a bit as do the colors produced.
FIGURE 3.1 Depiction of the Generation and Nature of Light. On the left is a simple model of an atom showing that the act of an electron transitioning between orbitals/energy levels causes the creation of an electromagnetic wave. Shown at the bottom is an electric wave (vertical) and its corresponding magnetic wave (horizontal) pushing each other along. Above this is a shortened wave packet, showing that the photons do not have infi nite length but have a beginning and an end.
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If we were to increase the level of electrical power to the light bulb, it would glow more brightly and produce more photons per second per unit area. Also, the increase in power would cause the fi lament to get hotter. This means that the atoms are banging into each other with more energy on average. The higher energy in some of the electron transitions means that those cases will impart greater energy to the resulting photons. Higher energy in photons is accompanied by shorter wavelengths. Humans perceive the higher energy, short-wavelength photons as blue. A bit lower and they become green, then yellow, and fi nally red. So if we put more electrical power to a light bulb, the light becomes brighter and more bluish. Conversely if we decrease the power to the bulb, the light will become less intense and more reddish. This effect is conceptualized as color temperature . It turns out that the higher the temperature of a black body source, such as the fi la- ment, the more blue light there will be compared to red light (see Figure 3.2 for approximations of light outputs from typical sources). So, for example, we can say that the color temperature of normal, midday, northern daylight is about 6000 degrees Kelvin (°K) and a normal incandescent light bulb is about 3000°K. Just knowing the one number gives the spectral quality of the light from the source.
The concept of color temperature is derived from the work of Max Planck, who advanced the theory of light emission and derived the basic
Basics of Light
FIGURE 3.2 Color Temperatures of Common Sources. The curves show the approximate energy distribution by wavelength for each of several common light sources.
equation for determining the amount of energy available at each wave length from a black body radiator or a certain temperature.
S hc ehc KT λ Π λ λ
( )
⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ( ) 8 1 1 5 / where:■ h is Planck’s constant, 6.626 exp -34 joule seconds
■ c is the velocity of light, 2.99792 exp 14 microns per second
■ K is Boltzman’s constant, 1.380622 exp -23 joules/degree Kelvin
■ λ is the wavelength in microns
■ T is the temperature in degrees Kelvin
■ Planck’s equation gives energy values in S λ watts/square meter at each
wavelength, λ
The values for the numerical constants are dependent on the system of units being used. Planck’s equation is a cornerstone of physics, but use caution when employing it. Most light-measuring devices, such as cameras and light meters, actually are photon counters. They do not measure energy, that is watts per square meter, but rather they count photons per second per square meter. And since they have a fi xed area of exposure and read for a fi xed time, they simply report the photon count for that area and exposure time. To convert Planck’s equation to photons per second per square meter, simply divide the basic equation by the energy of each photon, which is given by:
Energy per photon h * c/ λ
This gives the energy of a photon in joules. And since a watt is equal to one joule per second, the modifi ed equation gives the number of photons per second per square meter. The result is:
M e T λ λ
( )
⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ 25 1327 1 1 4 . ( ) λ 14,383/In this statement of the modifi ed Planck equation, the constants already have been evaluated.
The beauty of this relationship between the number of photons per square meter per second at each wavelength and the temperature is that it is a single parameter model. That is, we can plug in the temperature of the source and solve for the output at any selected wavelength. This will play an important role in how digital cameras adjust the color response for each photo, which is discussed in Chapter 16.
Fluorescent lights are not so easily characterized since their light is pro- duced in a very different way. For lasers and LEDs, the whole concept does
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not apply. Color temperature is a shorthand way for stating the relative amount of light being emitted at each of the wavelengths from a source as long as the source behaves reasonably like a black body.
In the light bulb example introduced earlier, the given photon was created by a distinct energy transformation, so the photon has exactly that amount of energy captured in its electromagnetic wave. Photons have no mass, so the energy is not a kinetic energy like that of a bowling ball rolling down an alley. In order for the photon to be absorbed, it must fi nd an atom in which it can effect a transformation of the exact amount of the energy that it carries. If the photon carries X joules, it needs to fi nd a location that will accept X joules— not 0.9 * X joules, and not 1.1 * X joules, but exactly 1.0 * X joules. It is pos- sible to create molecules of materials that have the selected energy bands for electrons and therefore are able to selectively interact with certain wavelengths (colors of light). These molecules are used to make dyes.
In some cases there is a two-stage process. The molecules absorb a cer- tain amount of energy and elevate electrons to an allowable higher energy state, as before. But those electrons drop back to a different lower state. This lower state may not be as stable as the original lower (ground) state; it is called a meta-stable state. The lower state is not the ground state, but is higher than that. This causes the molecule to emit a photon, but since the transition is not as large as the energy absorbed, the photon will have a lower energy and a longer wavelength. In summary, the molecule absorbes a photon at a relatively high energy level—let’s say, blue. This imparted energy raises an electron accordingly. Then the electron drops to a new state that has higher energy than the original ground state, but less than the elevated one. The transition of the electron results in the emission of a photon. But since the downward transition involves less energy than the original photon, it has a less energy and a longer wavelength—let’s say, orange. The material is said to fl uoresce or phosphoresce. In fl uorescent materials the electrons fall to a lower state fairly quickly, whereas in phosphorescent materials the time interval is somewhat longer. In both cases, the electrons do not fall all the way back to their ground states, thus the energy released is lower than that absorbed. The electrons in the meta-stable states will ultimately drop back to the ground state, but the energy transitions are small enough that emissions are not visible.
In fl uorescent light bulbs, the exciting light is ultraviolet (shorter than blue, which is the shortest wavelength that people can see), and is cre- ated by the breakdown of mercury in an electrical arc. The ultraviolet light then strikes the inside of the tube. The light that we see from these tubes comes from the coating of fl uorescent dyes on the inside of the glass tube. Several dyes are used to get a color that generally is perceived as “ white. ” Many body fl uids also are fl uorescent. They tend to absorb ultraviolet (black) light and emit in the range of the green to orange portion of the spectrum.
Photography based upon this effect is referred to as alternative light source photography.
When a photon is created, it starts to move along a fi xed path at a very high velocity, the speed of light, c. This velocity—300,000 meters per second in a vacuum—is very fast! It is so fast, in fact, that nothing can move faster than this: not matter, not energy, not information, nothing. However, when light moves through a medium other than a vacuum, it moves more slowly. Air is a medium that is very close to a vacuum with respect to the velocity of light, so the reduction in velocity is generally negligible. However if the air is more dense (cold), the velocity will be slightly lower. When light shines through air that has signifi cant local variations in temperature, the objects that are seen through it appear to shimmer. In solid materials, gener- ally the effect is signifi cantly greater. Glass for example might slow the light by as much as 20% or so, depending on specifi c composition. The veloc- ity of light in a vacuum divided by its velocity within the medium is called the index of refraction . The larger the index, the lower the velocity in that material.
When light moves from one medium to another a number of changes to the light beam occur at the boundary. As indicated previously, the light slows down. If the incoming beam is at some angle other than perpendicular to the surface, it will change direction as it enters the new medium. Also, some of the light will be refl ected back off the front surface. These three changes will depend on the differences in the refractive indices of the two materials. The light beam will also be chromatically dispersed. Blue light changes more sharply than green, which changes more than red. The angle at which photons ’ paths are altered depends upon their wavelengths. This is to say that the refractive index of a material is different for the different col- ors. The degree to which the beam diverges is measured by the Abbe num- ber. The refractive index and the Abbe number are somewhat independent of each other. So a glass can be formulated with a preselected refractive index and a preselected Abbe number. Figure 3.3 shows some values for different glasses taken from specifi cations in advertising literature.
The bending of a light beam by means of refraction is one of only a few ways to effect change. Refl ection, as off a mirror, is another. Diffraction is the bending of a light beam as it passes close to the edge of a mechanical member, such as the metal ring that holds a lens together and provides for mounting. Light beams can also appear to be bent by the distortion of space due to the gravity of massive bodies such as stars, but this is an issue for astronomers and not forensic photographers and examiners.