I would like to regulate the light flux output of an LED or laser diode over its entire operating temperature range by varying the current.
What are some different methods to achieve this?
Edit: This is for a sensor application. I'm attempting to measure position by blocking the light over a photodiode. If the LED flux changes however, it will seem like position is changing, when in fact it is not.
Best Answer
Your original question mentioned a sensor, which now seems to be gone. How precise do you want to keep that luminosity constant? If it's for visual it's pointless: your eye's sensitivity follows a logarithmic curve, so that it can cope with both 1 lux moonlight and 100 000 lux sunlight. Therefore it is not very sensitive to small changes in luminosity.
Most LED manufacturers will give a linear relationship between current and luminosity, so to get the latter constant you only have to keep current constant.
Note however that you have to derate current at higher temperatures:
So for this particular LED you can't go higher than 5mA if you want the same luminosity for 25°C and 85°C.
The Supertex CL25 is a temperature compensated constant current source for LEDs.
This has a 0.01%/°C typical temperature coefficient. It is 25mA however, so you'll have to use a LED which doesn't need to be derated below that.
Rocketmagnet makes an important remark: "It may be the phototransistor which is sensitive to temperature." (RM, I hope you don't mind that I copy it here.)
Indeed, it's no use to have a temperature controlled LED driver if the sensor's reading varies highly with temperature. You'll have to look into that too.