[rec.photo] CdS or Si, which is better for a photometer? and why

johnw@watnext.waterloo.edu (John Wieczorek) (03/01/90)

	I am considering building as a pet project a single chip
computer controlled light/flash meter much like those spiffy 
Minolta III thingies. At first I thought of using a silicon photodiode,
but on later reflection, I think that CdS might be more suitable. The
resons are:

	1) CdS has a natural response curve very similar to the human
	   eye and therefore film. Silicon cells are maximally sensitive
	   in the IR region and begin to die around blue.

	2) The sluggish response of CdS seems to infer that the molecules
	   tend to remain in an conductive state for quit some
	   time. This allows the CdS cell to do short term integration
	   of fast incident radiation (flashes). With Silicon I'd have to
	   integrate either digitally after log conversion of straight off.
	   That would be a bitch because of the simultaneous requirements
	   for both wide dynamic range and speed.

	3) CdS seems to a have higher sensitivity to available radiation.

	4) A lot of hand held meters used to be CdS and with the exception
	   of their lack of sophisticated user interface seemed to do quite
	   well.

	Further, how do Si and CdS respond to temperature and age. The
linearity of repsonse in my application is of only trivial importance 
because the EEPROM of the MC68HC11 that I intend to use will contain
an emperically derived personality table.

	Input would be greatly appreciated. Thanks in advance.

						John Wieczorek
johnw@watnext.waterloo.ca