🚀 go-pugleaf

"George R. Gonzalez" <grg1@flash.net> wrote in message news:<aa6ogm$ia3$1@laurel.tc.umn.edu>...
> > John Popelish <jpopelish@rica.net> wrote in message
>  news:<3CC62328.1B2300D2@rica.net>...
> > > Adam Seychell wrote:
> > > >
> > > > I am trying build a photometer that will measure absorption of light
> > > > from an high intensity red LED through a substance. Knowing that LEDs
> > > > and photodiodes are quite temperature sensitive devices (in terms of
> > > > luminescence vs temperature for fixed current) I designed a
> > > > compensating photodiode circuit so the LED puts out constant
> > > > brightness.
> 
> How did you determine you are getting constant brightness?
> If you used the same type of photodiode as your leveling detector,
> then you have a tautology.
> 
> How are you compensating for your detector's temperature sensitivity?


The LED passes light to both feedback and measruing photodiode. So
assuming both photodiodes are matched then any change in LED bightness
or photodiode sensitivity will cancel out to give a constant output
level from the measuring photodiode amplifier. For example if I put a
220 ohm resistor across the LED the feedback circuit compensates for
this extra current drive and keeps the output level constant.


> 
> How about keeping the LED at a constant higher temperature with a little
> heater resistor?
> 
> How about using a 4x brighter LED source-- use one of those new 30Kmcd
> LED's,
> or just cluster a few LED's.
> 
> 

30kmcd !  They just keep getting brighter. 
Who makes this one ?

Adam