Photo System Engineering III, B&W Film 2
In the last section, we described the formulation of a blend of 3 monodisperse emulsions that served as fast, medium and slow components of a single film. We also described a single emulsion to serve that purposed with a less than normal (old fashioned) curve shape. We could have coated the 3 emulsion system as a one, two or three layer film. As we saw though, there were some problems. Here are a few more.
If the emulsions differ substantially in size to achieve graded speeds, then the developability goes down as size increases. If we use iodide to assist in gaining higher speeds at more similar grain sizes, then the developability again can go down. So, during development the fine grained component goes up in contrast faster than the coarse grain component does and this leads to a low contrast in the toe, and a high contrast in the shoulder. In effect, you must increase silver of the slow component and decrease silver of the slow component in order to retain the same contrast in a given development time. In fact, you must retain this relationship over many developers and many development times to keep the curve shape from falling apart.
Now, the graded iodide emulsion may cause you problems due to the reequillibration tendencies of the iodide to do this during keeping, so it would probably be best to use a fixed iodide and use 3 grain sizes for the three emulsions needed in a modern film.
Letís try to fix this by designing a 3 layer film to prevent reequillibration. If we do that, we observe that even under the best circumstances, the interfaces between the layers themselves become something akin to partially silvered mirrors which reflect and scatter light. So, the three layer system might need more absorber dye to attain the same sharpness level as the single layer material.
Now lets try our 3 layer material. We find that the diffusion of developer through this somewhat thicker 3 layer structure causes the bottom layer (fast) to be underdeveloped relative to the other two and we have to readjust the level of silver yet again when compared to the previously readjusted single layer 3 component version.
There is no right solution to this. You can have either a one, two or three layer structure and it will work with the appropriate adjustments in grain type, size, amount and addenda.
Having said this, you have to include testing in this sequence to tell how well off you really are. The two main tests for film imaging quality are RMS (Root Mean Square) granularity, and sharpness. Of course, the proper ISO speed, contrast, reciprocity and latent image keeping have been taken care of already in the individual emulsion design, right? Right???? Of course you have done all of that testing before we get to this stage!
So, the first test is for grain (RMSG) which is done by giving a stepped scale exposure to the film with 21 steps each spaced 0.15 log E apart which is then processed. The resultant silver image is then scanned with a micro aperture densitometer to measure light intensity, and the deviation from the average at each macro density level. At zero density, there are no grains and you get some intermediate noise. As density increases, the fluctuations due to grain increase until they peak and then begin falling off. At Dmax, this noise falls again to nearly the same level as we see at Dmin. This is due to the fact that the grain is so tightly packed in the Dmax that it is one solid mass to the instrument you are using. If we have designed an ISO 100 film, then the mid scale of this film curve is ISO 100 by definition, and if it has better grain (Lower RMSG) than your previous product, you are doing very well. In fact, it might reach the aim that the manager told you you had better achieve! If it did, you can go on to sharpness.
Sharpness is measured by using a set of square wave charts with different frequencies. The exposure is a set of light and X-ray exposures that are about Ĺ stop apart, or 0.15 LogE again. So, you have a variation in frequency (measured in lines / mm for example) and in exposure. After processing and comparison, you can compare each X-ray and visible light exposure with the same density to measure line width and height. The width will give you a measure of scatter if you compare the exposures from the two energy sources and will tell you if you have enough acutance dye and AH layer. The height of the lines (or density) will be a measure of edge effects induced by developer and iodide content (or other addenda) in the emulsion. A plot of the wide lines, narrow lines and plain step scales allow you to compare macro and micro contrast of the negative.
Micro contrast is best to be higher than macro contrast so that as objects become smaller, they can appear at higher contrast and be seen as higher in sharpness (an illusion).
Now, the above sharpness experiments are done with both positive charts and negative charts. There is a reason for this. A large white area in film can expose into a narrow black line causing fill in, and a large black area around a clear line can allow the line to bloom outward into the dark surround. So this bloom and fill are critical issues in judging the film. Sometimes it is due to light scatter, and other times it is due to infectious development. This latter takes place when one crystal can cause adjacent crystals to develop even though they were not exposed and usually comes about if you have not used enough gelatin in the coating.