In my book, Camera Lenses: From Box Camera to Digital, Chapter Two is entitled Films and Emulsions. What follows here is an expanded and augmented version of Section 2.5, How a Photographic Emulsion Works.
Many photographers do not clearly understand what happens inside a photographic emulsion when it is exposed to light. They do not know about latent images and related photographic effects. So here I give first a discussion of the formation of latent and developed images. This is followed by an explanation of reciprocity failure and some of its consequences.
This discussion is recommended to anyone who is curious about emulsions and wants to know how they operate. Although what I say is generally applicable to ordinary photography, much can be understood best by examining the effects that astronomers encounter when recording faint images of stars and nebulae at the telescope.
The discussion applies only to classical photochemical photography. It does not include any effects encountered with the new photoelectric image detectors such as CCD and CMOS sensors.
Latent Image Formation
The light-sensitive part of a photographic emulsion consists of a myriad of tiny (about a micron) crystals of silver halide (mostly bromide). These crystals are suspended in a medium consisting mainly of a very pure form of gelatin (the same stuff that is in Jell-O). The resulting emulsion is thinly spread on a supporting substrate such as a glass plate or plastic film. Actual photographic products have additional constituents and features, but this is the essential form.
The atoms in a silver halide crystal exist as ions. Each bromine is a negative ion with an extra electron. Each silver is a positive ion with one electron missing. The combination, of course, has no net electrical charge.
During an exposure to light, a photon event occurs when an incoming photon knocks off the extra electron from one of the bromine ions. This photoelectron is now free and mobile, and it wanders about in its crystal until typically it finds a dislocation or flaw in the crystal lattice, where it becomes trapped.
A few of the silver ions are not locked in the crystal lattice, and these interstitial ions are also mobile. The negative electric field set up by the trapped photoelectron draws one of these positive silver ions over to it. When the two meet, the ion and the electron combine to form a neutral silver atom. This silver atom is the beginning of a latent image.
As the exposure continues, the same process is repeated several times until, in at least one place in the crystal, a compact group or aggregate of about four to six neutral silver atoms is eventually created. When this happens, a threshold is crossed. This clump of silver atoms is now large enough that it has become a fully formed latent image.
A silver halide crystal containing a fully formed latent image has become developable. Developable means that when an exposed plate or film is placed in a developing solution, the latent image aggregate acts as a development center. Starting around the clump of silver atoms, the chemical action of the developer progressively converts the whole crystal into a grain of metallic silver. The developer can be thought of as a chemical amplifier that multiplies the size of the original clump of silver atoms formed by light.
If the recommended development procedure is followed, and if the crystal contains no latent image aggregate of sufficiently large size, then the developer has no effect on that crystal. For each individual crystal, it is either a go or a no-go situation. Those silver halide crystals that are not converted into silver metal during development are removed from the emulsion during the fixing process that follows.
The final overall developed photographic image is thus a vast array of tiny silver grains (which are largely opaque) in the gelatin layer (which is otherwise transparent). Areas that received heavier exposure had a greater fraction of its crystals made developable and later converted into metallic silver.
The invisible latent image has thus been converted into a visible developed image. When the developed plate or film is viewed by transmitted light, more silver grains absorb more light. Thus heavily exposed areas appear darker. The result is the familiar photographic negative.
The Law of Reciprocity
Ideally, a given amount of light should produce the same photographic effect regardless of whether it is bright and arrives over a short period of time, or is faint and arrives over a long period of time. This yields the "law" of reciprocity, which states that the photographic effect of an exposure should be proportional to the product of the light intensity (image irradiance in photons/mm²/second) multiplied times the exposure duration (in seconds). In particular, the photographic effect should be independent of the individual values of intensity and exposure duration.
In normal photography, this law holds reasonably well. Photographers know that 1/250 second at f/4 yields the same exposure as 1/60 second at f/8. But very high and very low light intensities requiring very short or very long exposure times are a different matter. At these extreme intensities, there are significant departures from this law. These departures are referred to as failure of the law of reciprocity, or reciprocity failure.
It is interesting that the most widely accepted explanation of photographic reciprocity failure is completely wrong. This misconception asserts that during a long exposure (such as in astronomy), the sensitivity of an emulsion decreases, that is, the emulsion speed slows down over time. Somehow the ISO/ASA rating supposedly goes from, say, 1000 at the beginning of an exposure to something like, say, 200 or 100 or 50 after an hour or so.
This description of what happens is so widespread and repeated so often that few question it. However, the theory of latent image formation refutes it. Something else is happening. The correct explanation is actually quite different.
Low-Intensity Reciprocity Failure
The formation of a developable latent image, as described above, actually occurs in two distinct stages. In the early part of the first stage, the clump of neutral silver atoms contains only one or two atoms. At this time, the process forming the latent image is reversible. The silver aggregate is unstable because thermal agitation (Brownian motion) can reionize one of these atoms. If this happens, then the reionized atom is lost from the latent image.
Not until the clump contains three or four atoms is it finally stable against thermal disintegration. When this number is reached, this stable, but not yet developable, latent image is called a latent sub-image.
During the second stage, the stable latent sub-image is converted into developable latent image. To do this, more exposure is necessary. The additional photoelectrons add one or two more silver atoms to the growing silver clump to finally give it the required threshold number of four to six atoms.
Thermal instability during the first stage of latent image formation has consequences for photography of faint objects. Because the photons arrive slowly during the long exposure, there is plenty of time for the reionization process to work. The potential latent sub-image can therefore fall apart almost as fast as it is being formed.
Not only are there fewer photons/mm²/second at low light levels, but the emulsion's net response to these photons is less. In other words, the efficiency of the process is reduced. This loss of efficiency (and thus sensitivity) at low intensities is the cause of low-intensity reciprocity failure.
Note carefully that this process is unchanged throughout the length of an exposure. In fact, the properties of an emulsion remain (nearly) the same from the time of its manufacture until it is placed in the developer. The emulsion has no built-in clock that decreases its responsiveness with time.
The controlling variable is the intensity (brightness, irradiance) of the incident light. Reciprocity failure is a light intensity effect, not an exposure duration effect.
It is of further interest that reciprocity failure can be occurring differently at different locations across the emulsion at the same time. Under the image of a bright star, emulsion efficiency is relatively high. But simultaneously in a nearby region under the image of a fainter star, emulsion efficiency is lower.
These differences in efficiency yield a rule of thumb: if it is desired to photograph stars one magnitude (2.512 times) fainter than those that can be barely recorded with a given exposure, then the exposure length must be increased about three times (neglecting the sky fog limit).
High-Intensity Reciprocity Failure
In addition to reciprocity failure at low intensities, photographic emulsions also exhibit reciprocity failure at high intensities. The mechanisms producing the two effects are entirely different.
If the light intensity is very high during an exposure, then the finite mobility of the silver ions becomes a source of inefficiency. When photoelectrons are liberated so rapidly that the sluggish silver ions cannot neutralize them fast enough, then more than one photoelectron can be active in a given crystal at the same time. Because of their mutual repulsion, no more than one photoelectron can reside at a given crystal lattice dislocation at once. Thus with high intensities, the forming latent image is dispersed to several dislocations at different places throughout the crystal.
But only one sufficiently large clump of silver atoms is needed to make the crystal developable. Any others that may be formed are redundant and useless. This lack of concentration of the latent image requires extra photon events to create at least one clump having the minimum-required four to six silver atoms. The resulting reduction of efficiency is the cause of high-intensity reciprocity failure.
This effect is well known to photographers who use very-high-intensity electronic (strobe) flash units having flash durations of about 1/10000 second. When using these super-bright flash units, extra exposure is required to compensate for the loss of emulsion efficiency.
Thus for a given emulsion, there is one intensity of the incident light that gives maximum efficiency and maximum sensitivity. Both higher and lower light intensities are less efficient in forming a developable latent image.
This optimum intensity is not the same for all emulsions. For astronomical work, it is possible to make special emulsions that have their maximum efficiency shifted during manufacture to lower light levels. Those from Eastman Kodak (many now discontinued) have an "a" in their names, such as IIIa-J, IIa-O, and 103a-E.
Prior to an exposure at the telescope, if an emulsion is briefly and uniformly exposed to just the right amount of light to create lots of latent sub-image all across the plate or film, but not enough exposure to create much developable latent image, then the exposure at the telescope need only be sufficient to convert the sub-image into developable image. The technique is called pre-flashing. Pre-flashing has the potential of greatly increasing the effective sensitivity of photographic emulsions.
Furthermore, because the latent sub-image is stable and its conversion to developable latent image is not subject to thermal reionization, the subsequent exposure to faint light at the telescope is not affected by reciprocity failure.
For the greatest amount of sub-image and the least amount of developable image, flash with a light intensity that is the most efficient for the emulsion. At this intensity, there is a maximum difference between the time it takes to make sub-image (stage 1) and the time it takes to make developable image (stages 1 plus 2). For most emulsions, the flash exposure duration matching this intensity is about 1/100 second.
Unfortunately, statistical variations in the arrival rate of photons and statistical variations of the silver halide crystals in the emulsion mean that some developable image fog is always created when pre-flashing. Even when using the most efficient intensity, the margin for error is not enough to avoid fog and its associated granularity. Later when analyzing the developed image, the general fog level can be subtracted off, but not the random noise (variations) in the fog. This grain noise can obscure subtle image details (the image signal-to-noise ratio is reduced). Thus, pre-flashing is rarely done in practice.
If low-intensity reciprocity failure is caused by thermal disintegration of the early latent image as it is forming, then cooling the emulsion during an exposure to faint light might be expected to reduce this loss of efficiency. This indeed happens, and it is the principle behind the cold camera.
Cold cameras do work, although they are complicated by practical concerns such as preventing the condensation of moisture. But do not cool too much. Temperatures around –40°C or so are about right. Further cooling will cause a loss, not a gain, in sensitivity. If the emulsion is overcooled, then the mobility of the silver ions will be so severely reduced that the emulsion will suffer from high-intensity reciprocity failure at all intensities. Cooling with dry ice (–78°C) works, but not liquid nitrogen (–196°C).
A Double-Exposure Effect
Many astrophotographers may question the above explanation of reciprocity failure, citing the observed photographic images of long-exposure star trails (or widened objective-prism spectra) that are clearly more exposed at the beginning of the trail than at the end. But this is a double-exposure effect.
At the beginning of the trail, the emulsion at this location on the plate or film is given a quick exposure to the bright star followed by a long exposure to the faint sky background light. The bright star forms a developable latent image in some of the silver halide crystals, and a latent sub-image in others. During the subsequent long exposure to the low-intensity sky, much of this sub-image is converted into developable image without reciprocity failure.
At the end of the trail, the order is reversed. At this location, there is first a long exposure to the faint sky background followed by a quick exposure to the bright star. The exposure to faint light suffers severely from low-intensity reciprocity failure, and relatively little latent image of any kind is formed. The final short exposure to the bright star is nearly the only effective exposure. And most important, any latent sub-image created by the star at the end of the trail has no subsequent opportunity to be converted into developable image.
The result is that the trails seem to be more heavily exposed at the beginning than at the end. Nevertheless, the properties of the emulsion have remained the same throughout the exposure duration. There was no decrease in speed.
A Blessing in Disguise
Most astrophotographers regard low-intensity reciprocity failure as an unmitigated disaster with no redeeming virtues. But in fact, if it were not for this effect, photography as we know it would be impossible.
Light is not the only thing that can form a latent image in a photographic emulsion. A long exposure to ordinary heat can do it too. The result over time is fogged plates and films.
Fortunately, the instability of the early latent sub-image causes nearly all of the thermally generated silver atoms to be reionized and lost. Most of the fog is thus reversible and goes away almost as fast as it is formed.
This is a good thing because it allows manufacturers to make and sell fast emulsions that have reasonably long shelf lives. Without reciprocity failure, all emulsions would be like the special infrared plates that must be made slow, shipped refrigerated, and hypersensitized just prior to use. However, with reciprocity failure restricting fog, widespread amateur photography, motion pictures, photojournalism, and many other applications outside a laboratory setting all become possible.
© Copyright 2006, Gregory Hallock Smith
Gregory Hallock Smith
About the author: Gregory Hallock Smith is an optical engineer and lens designer. His serious interest in photography and cameras began when he received his first good camera, a Zeiss Contaflex, when he was 15. He has a Ph.D. from the Optical Sciences Center, University of Arizona, and has held several professional positions at major corporations and research institutions. As a consultant, he was invited by JPL to design all the camera lenses for the two Mars Exploration Rovers now on the surface of Mars. Previously he has written a textbook on lens design, Practical Computer-Aided Lens Design, published in 1998.
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