Hi Chris,
Perhaps you could explain this to me further. From my understanding flat grain and epitaxial grain are not mutually exclusive. Because of the technology in achieving the crystallization of the silver halide, an epitaxial layer may be thicker than a "flat" crystal layer, this isn't a necessity, right? I have worked in the CVD industry and an epitaxial layer was one in which the crystals in the layer all had the same orientation.
In photographic film, this would likely have the effect of maximizing the crystal's cross section as photons passed through the emulsion. While a flat grain that is not epitaxial might have silver halide crystals at differing orientations, some of which would be less than optimum. Because of this, the epitaxial arrangement of the Delta films would be more efficient photon grabbers. Am I thinking about this correctly?
You're right up to a point. As occurs often the use of the terms isn't precise and as usual manufacturers tell us what they think will impress us.
A t-grain emulsion has flat grains:
and in Kodak's t-grain films are monosize emulsions (the spread of grain sizes is narrow) with the grains in the same (or a narrow range of) planes. But Kodak don't tell us how they lay the emulsion down.
Ilford's "core shell technology" emulsion has rhomboid grains that are grown around a core of a slightly dissimilar material, with more depth than t-grains - I can't find a picture of it right now, but I know there used to be some on the web. Epitaxy refers to the method of depositing a monocrystalline film on a monocrystalline substrate, and epitxial refers to of, pertaining to, or employing epitaxy. This describes well how the Ilford film emulsions are made and is a way to orient the grains in the emulsion. Ilford's Delta emulsions most correctly are monosize core shell epitaxial emulsions, but are called epitaxial by many, even by some engineers involved with making it who I've spoken to. The main reason to make an epitaxial film emulsion is to ensure that the grains all have a similar orientation, but there are other ways to achieve the same thing. So we know a bit about Ilford's grains and how they are laid down.
In that sense you could make an epitaxial t-grain film, but Kodak don't tell us if this is how they apply the tgrain emulsions. The rhomboid grains that Ilford uses makes their films ever so slightly grainier than flat grain films, but with better latitude. Kodak's flat grains have less time, temperature and developer latitude than Ilford's core shell ones.
You can also make a monosize cubic / traditional grain film. Acros is one of these.
The grain orientation does not necessarily make them automatically more sensitive, because sensitivity is largely determined by sensitization dyes and incorporated development accelerants in modern films.
Additionally, strictly speaking, when your chemical vapour deposition industry people talked about an epitaxial layer, they were referring to the means to get to the end, not the end (the crystals in the layer all having the same orientation) itself.
I hope I explained this clearly.
Marty
