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A Brief, Simple and Approximate Explanation
of Photographic Film and Reciprocity Failure

Photographic film is still probably a commonly used medium for astrophotography. There are many types of black and white film and color film as well. This discussion is devoted almost entirely to black and white (gray scale) film. Color film behavior is a sort of sub-set of B&W.

For a detailed discussion of various practical hypering techniques see:
A manual of Advanced Celestial Photography by Wallis and Provin. There is a 22 page chapter on the topic written in their usual clear and concise style. This is still the best book ever written on astrophotography in my opinion.
Other references of value are:
Astrophotography for the Amateur
by Michael Covington and
The Guide to Amateur Astronomy by Jack Newton and PhilipTeece
And a nice reference manual with much information is: class:
Astrophotography II by Patrick Martinez

Black and White (gray scale film)

The great variety of B&W films available narrows down considerably when doing photography of very dim objects. For ordinary photography, there is generally enough light to form an image with an intensity that enables gathering an image with relatively short exposures. Normal exposure length is usually in the range where the camera can be hand held. That is from 1/25 of a second to 1/1000 of a second. Thus films are designed to give predictable results for this range of exposure times. For exposures that are much shorter, such as with high speed electronic flash, or much longer there is what is called reciprocity failure. Reciprocity failure is often badly understood as will be explained in the following.

Let us first get an understanding of how a photographic film captures a latent image which can be later developed chemically. The sensitive elements in the film are crystals of, most often, silver halide which can change their structure when excited by light (photons). The relationships between these crystals, the details of the use of so called sensitizers and the sensitivity of the crystals to photons of different energies is very complex. We need only a basic understanding however to understand certain basic properties (behaviors) of films.

In general less sensitive films (slower films) have finer grains that are closely packed and more sensitive films (faster films) have courser grains. A film may have a distribution of grain sizes to obtain certain desirable properties. The reason for the sensitivity relationship to grain size is related directly to how the grains are converted from a stable non-developable state to another stable state (latent state) from which they can be developed chemically. This happens in something like the following way. When a photon of light strikes a grain it dissipates its energy in the crystal (grain). This energy may or may not be enough to flip the crystal into a latent state. Generally it takes a few photons to flip the grain (depending on its size and sensitivity). In the meantime, thermal energy is jiggling the grain and tending to drop it back into its normal state. If enough photons strike the grain in a given time, the grain flips to a latent state and sticks there. We then have a grain that can be turned opaque chemically. Thus the photons build up a latent image that is later developed. The darkness of the image is more or less proportional to the light striking the film.

It takes about the same number of photons to flip a large grain as a small one. Since the larger grain intercepts more light more of the larger grains will be flipped and thus less light is required to create a latent image. This later phenomenon makes course grained films faster (more sensitive). Now let us understand that there a great variety of other factors that control the sensitivity and graininess of emulsions the case is not as simplistic as stated here. Notice however that there are two controlling phenomena going on. One is the flux of photons in intensity and time that flips the grains and the other the thermal agitation that serves to reduce the grain tension that is built up by the photons. The latter effect is continually wiping out the effect of the photons to some degree.

For example, consider a single grain. Say it takes 3 photons to flip the grain to its latent state. Assume one and then another photon hits the grain but that the light flux is very small. Before the next photon can hit the grain and flip it, one of the previous photon actions is undone by thermal energy. The third photo then in not enough to do the job and the grain does not flip. The concept is that there is a "battle" between the photons accumulating fast enough to flip the grain into its latent state and thermal energy neutralizing the photon action. If the light intensity is small enough no image at all might form.

If the light intensity is strong enough so that the exposure to it requires a short time, say under a few seconds, the thermal effect is minimal and the exposure forms and sets the latent image. Once set, the latent image is very stable and will last for years even at room temperature.

It is now easy to understand "reciprocity failure." Reciprocity is the relationship between the "apparent" sensitivity of the film and the reciprocal relationship between light intensity and exposure time. What happens is this. When the light is strong enough for a normal exposure time as described above the, a few seconds or less, the grains are flipped and form an image without significant intervention of the thermal relaxation phenomenon. But when the light is very weak as is the case for many astronomical sources, the image is build up very slowly often in many minutes or even hours. Thus all the while the image is forming, the thermal action within the emulsion is destroying some of the results of the photon flux. The film appears to be less and less sensitive when weak photon fluxes are present. (i.e. long exposures are required to build up an image)

Note that the problem is not directly the long exposures required but it is the weak photon flux. We can now see how the cold camera works. The desensitizing effect of thermal agitation is reduced by cooling the film. Liquid nitrogen temperatures or at least dry ice temperatures are required to reduce the thermal agitation significantly. But film cameras with such cooling are made mainly for professional use. They are not easy to use because of the problems of insulating them from dew and frost.

We can, by considering one additional phenomenon, understand how "hypering" works. As it turns out in a typical emulsion, it is free molecular elements and especially water vapor which allows the thermal agitation in the emulsion to couple to the sensitive grains in it. Thus the coupling of the thermal energy "called phonons" to the grains can be reduced greatly by removing from the emulsion all possible water vapor and if possible molecules like oxygen and nitrogen. Nominally this is done by placing the film in a chamber, warming the film and evacuating the chamber. This dries out the film and allows oxygen and nitrogen to migrate out of the film. But the film will quickly take up the undesired contaminants when taken out of the chamber. A good solution would be to saturate the emulsion with hydrogen gas. Then it would take a long time for the unwanted agents to penetrate the film and it would stay "hypered" for a long time. (days to weeks) What is normally done is to use forming gas instead of pure hydrogen because pure hydrogen is so very dangerous to handle. Forming gas is mainly nitrogen with some hydrogen content, about 8% Hydrogen, so it can not explode. The forming gas is totally dry so that the worst offender, water vapor, is eliminated. Do not try pure hydrogen unless you have your insurance paid up.

Hypering film in this manner is commonly done by many amateurs and professionals. The "hypered" film then retains its sensitivity even though illuminated with very weak light since much of the thermal desensitizing effect is suppressed. The film will become 3 to 5 time more effective with very low light levels found in astrophotography. Instead of limiting exposures of one hour, three to five hours will remain effective.

There is still the question of why one does not start with a very fast film like TriX or equivalent instead of the Tech Pan that is so popular. The reasons are that the fast films because of their grain size are not as effectively "hypered" and have significantly poorer reciprocity failure in the first place. They are also of low contrast and grainy. A film like Tech Pan, which has a normal ASA speed of only 25, has inherently good reciprocity characteristics and takes very well to hypering. It has in addition very fine grain, high contrast and good sensitivity to longer wavelengths, which are important for astrophotography.

The above discussion is a very simplified explanation of the processes but it does describe the results you find with cooling and hypering film.

Color films show the very same phenomena as B&W films. Color films can also be "hypered" with good results. Because of the complex structure of color films, being three layers of emulsion with build in filters, they may show significant color shifts because the three emulsions hyper differently.

From the above discussion, it can be seen that film sensitivity can be improved by cooling and by drying (hypering) it in some way. The hypering process is not dangerous when done with forming gas but it is complex and tedious, take up to several days and must be done shortly before exposure to be most effective. Pre-hypered film is available but should I feel be used within days of hypering. I have not seen data on the long term (many days) stability of hypering. I would assume the film should be sealed in forming gas until time of use and when exposed in the camera, to air, be used completely and promptly. (in hours)

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