I hesitate to do this, but this information is wrong and goes against every principle of sensitometry.
Originally Posted by Thilo Schmid
The densitometry has nothing to do with the type of light the enlarger uses. Densitometers do not use diffuse or colimated light. all they do is pass a beam of light through a small section of the film and the diference in opacity from the reference beam is detected by a photo sensor. If anything all densitometers are "diffused light" as the light gets scattered by the film.
The toe of the film curve is affected very little by developing. ALthough Robert is very close to the treshold, small amounts of developing changes would not affect the zone I value. And even if they do, is easy to compensate for this by reading the zone I value and adding the appropriate extra exposure. For example if he gives 3 minutes less developing and notices his Zone I value has gone from 0.11 to 0.09, all he has to do is add 1/3 stop to the film rating to bring him up to the adequate zone I values. He certainly does not have to make paired test every time he changes developing times.
I stress again the density range one aims for to use with diffused or colimated sources has nothing to do with the light source used in the densitometer.