Quote Originally Posted by Thilo Schmid
I beg to differ. The measurement charachteristics is important and is usually indicated with the technical data (and is the reason why you can change the aperture on some densitometer models). The Callier Effect will not change the basic shape of the caracteristics curve (well - there are people, who believe it does), but it will for sure change the grade. This is the reason why you usually have different development times for condensor and diffuse light enlargers. You may, of course, apply arbitrary correction factors in this case. But this would indeed be "against every principle of sensitometry". The Callier Effect depends on light aperture, grain size and grain shape and you will need different correction factors for different types of film/developer combinations. To be exact, you would have to calculate backwards from paper densities to receive the desired negative densities you should tune your development process to.

There is a good document about this availiable under: http://www.gigabitfilm.de/download/callier_effekt.pdf (esp. the second part about the different measurement geometries on transparent targets)
Unfortunately, it is written in German, but some translation program might help.
Yes, but the callier effect you are talking about applies to enlarger light source not to densitometer light source. The reason some densitometers have the ability to vary the sampling aperture is so that you can sample a smaller area of the film, not to try to simulate "difuse or colimated" light sources this is very well explained by the late Dr. Henry. Now if you are talking about spectrometry in thin film applications then that is another matter, but then we get into micro meter densitometers which is entirely a different matter.

One of the best books in this matter is photographic sensitometry by Todd and Zakia, and I will quote " The present United States Standard specifies diffuse measurements, and all commercially available instruments are said by their manufacturers to conform to this standard in terms of results. Consequently, if we wish to make specular density measurements, we must design our own equipment, or modify existing equipment." So even if you wanted to make specular measurments you would have to have a detector that was away from the film, not in contact as all densitometers work presently. trying to simulate density measurements by specular or difuse source is not practical, and since there are no correction factors to convert either way, for all practical purposes diffuse measurements are the rule.