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The Relevant Properties of Silver Halide Grains - Grain Size and Distribution, Grain Shape, Spectral Sensitization, The Stability of the Latent Image—Reciprocity Failure, Sensitometry

sensitivity exposure reciprocity figure

There are a number of key parameters that influence the photographic properties of silver halide grains. First and foremost among these are the size, size distribution, and shape of the individual crystals. However, also of importance is the chemical and spectral sensitization of the grains.

Grain size is of great importance because in general terms the probability that a silver halide crystal will absorb a photon of light is a function of the volume of the crystal. As a general rule, large crystals make sensitive emulsions. However, although large crystals are good for sensitivity, they compromise the image quality parameters of the emulsion. This is because large crystals produce poor definition in the final image and also increase the perception of graininess. Grain size is therefore a compromise between sensitivity and image quality.

One very useful method of gaining extra sensitivity without increasing grain size is to employ chemical sensitization techniques. These use the addition of chemicals to increase sensitivity and because they do not imply an increase in grain size they carry none of the image quality penalties described above. However, there may be a penalty to pay in terms of background fog. Once more we are back to a compromise between sensitivity and image quality.

Grain Size and Distribution

The grain size and size distribution are key parameters determining the characteristics of an emulsion. In practice, all other things equal, the sensitivity to light increases with around the second or third power of grain diameter. The grain sizes used in commercial emulsions vary widely as the emulsions are designed for different applications. Figure 2 illustrates this with emulsion grains imaged with a scanning electron microscope.

The grain size distribution is therefore an additional variable to be considered. Rather than estimate this visually from pictures such as Figure 2, instruments are used to quantify particle size variations to produce data such as that in Figure 3.

Grain Shape

Figure 2 also illustrates the fact that silver halide crystals can be manufactured in different shapes. One of the reasons to use this variable in some products is that it can produce advantageous gains in sensitivity without undue increases in graininess. Whereas the intrinsic sensitivity of a grain is a function of the volume, the sensitivity of a grain with sensitizing dyes attached is more a function of the grain surface area. Thin, flat grains of the type illustrated in Figure 2b give a better speed: grain ratio when used with sensitizing dyes to increase spectral sensitivity.

Spectral Sensitization

The intrinsic sensitivity of silver halides is limited to the restricted wavelengths where they absorb light. This absorption spectrum is a function of halide type. Silver chloride appears almost colorless, absorbing (and therefore sensitive) predominantly in the ultraviolet. Silver iodide is yellow as the absorption (and therefore the sensitivity) extends into the blue end of the visible spectrum. Silver bromide is a pale yellow and has characteristics between the two.

This limited spectral sensitivity need not be a problem in some applications; however, products such as camera films require recording over the entire visible spectrum. In this case the wavelength sensitivity is extended by the use of sensitizing dyes. These are dyes adsorbed on the surface of the crystals that add additional sensitivity over their absorption spectrum. Judicious choice of these organic dyes can extend the sensitivity beyond the visible spectrum and into the infrared.

The Stability of the Latent Image—Reciprocity Failure

It was mentioned earlier that the latent image consists of small groups of neutral silver atoms in a silver halide crystal. These atoms accumulate at sites of imperfections within the grain, usually deliberately introduced during manufacture. The purpose of chemical sensitization is to facilitate the process of the accumulation of these latent images and to promote their stability. Types of chemical sensitization are covered in a later part of this essay.

One important aspect of latent image stability concerns the phenomenon of reciprocity failure. The Reciprocity Law states that the amount of photographic exposure is dependent on the amount of energy employed, irrespective of the time over which it is delivered. Put in equation form this can be written as

Photographic exposure = light intensity × exposure time

For exposures typical of consumer cameras this relationship works well. However, for very short (below around 1 ms) or long (above around 1 second) exposures this relationship breaks down and higher overall exposure than expected is required to achieve the same recorded result. This is a very important factor in the calculation of exposure in certain circumstances. This breakdown in the Reciprocity Law is known as Reciprocity Failure.

High exposure reciprocity failure is associated with short exposures of high intensity. Extreme examples include the exposure of holographic films and plates using pulsed lasers for times measured in nanoseconds (10 -9 ). Products designed to minimize the effects of high-intensity reciprocity failure tend to be chemically sensitized with gold.

Low-intensity reciprocity failure is associated with very long exposures under low-light intensities. A typical application would be astronomical photography. Silver halide emulsions in general exhibit low-intensity reciprocity failure, but chemical sensitization using sulfur is effective in minimizing this. However, low-intensity reciprocity failure does have virtues in normal circumstances as it tends to reduce the long-term accumulation of fog due to thermal effects, even in complete darkness. For astronomical photography, where low-intensity reciprocity failure must be eliminated, hypersensitization techniques are used.

Sensitometry

The usual way in which to define the sensitivity characteristics of a photographic process is a graph showing optical density as a function of the logarithm of exposure. This S-shaped plot is often referred to as the characteristic curve of the product. The use of logarithmic scales approximates the response of the eye, giving a presentation that is closely related to the visual impression of the image-recording characteristics. The shape of this curve is determined by many variables, not just the characteristics of the emulsion and the coated assembly. It is also a function of exposure and development conditions and the method used to measure the resultant optical density. Because of this it is important to note that the form of the characteristic curve is a measure of the complete photographic system, not just the recording emulsion.

There are a number of key parameters that can be extracted from the characteristic curve. The first of these is the fog level, the optical density of the processed material at zero exposure. This is a strong function of the emulsion type and the development conditions. Fine-grain emulsions such as illustrated in Figure 2a tend to have a low fog level. This is particularly important in a paper product where the process is often designed to give a clear white image in the unexposed areas.

The slope of the characteristic curve is a measure of the image contrast of the recording system. Emulsions with crystals all of very similar size (see the nuclear emulsion in Figure 3) are known as monosize emulsions and tend to have very high contrast, whereas those with different crystal morphologies and sizes (see Figure 2b) tend to have lower contrast. The slope of the characteristic curve can also vary across the exposure range giving the subtly different characteristics sought by creative photographers.

The exposure range giving rise to densities between fog level and the maximum density (known as D max ) is a measure of the exposure range over which the system can record detail. High-contrast products, with very steep characteristic curves, therefore, tend to have small exposure ranges.

The photographic speed of the system is usually expressed by some measure of the exposure required to achieve a specific developed density. There are various International Standards to define this for pictorial photography. Because of the application these are defined in photometric units. However, for scientific applications it is sometimes more applicable to define these in radiometric units.

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