Jim Thompson wrote:
What current would one expect to see from a photodiode exposed to
typical bright room light (used to control a backlight proportional to
room light)?

As usual, my customer has no clue. My only concern, design wise, is
to build a micropower transconductance amplifier with sufficient
output drive to handle the maximum available photo current.

...Jim Thompson

As Tim said, "It depends". Tim mentioned the "obvious suspects" - how
big is the photodiode? (1 mm x 1 mm or 1 cm x 1 cm - it will make a
difference...), how bright is "bright"? (IIRC, the eye's photoresponse
is logarithmic, but a photodiode is linear over several orders of
magnitude in illumination), etc.

One thing Tim didn't mention is the frequency response of the
photodiode. Do you and your client only care about DC illumination, or
does the photodiode have to be able to deal with the illumination
switching on and off at several tens or a few hundred kHz. In general,
if only DC is of concern, the photodiode can be designed to have better
responsivity - the current out per unit optical power in.

Also, with the photodiode be reverse-biased or zero-biased? (Reverse
bias will improve the responsivity) but not applying an external bias
voltage across the PD will make things simpler.

Depending on what the client's needs are, you might want to consider
using a photoconductor instead of a photodiode.

Bob Pownall