Help required for surface reflection circuit (IR/ PHOTODIODE)

[milovisk]

Hi

I am new to this forum but have been interested in reading many threads and would like to know if any of the experts could help.

I am looking to make a circuit that I can measure different output levels of a photodiode from the ammount of "signal" reflected onto it from a particular glass surface

My idea is to set the angle of the transmitter / receiver to get the optimum reading on a known surface coating on the glass, I would then like to measure a range of coatings and see if I can establish a significant measurable difference between them.

Ideally the transmitter and receiver would be about 5cm apart and I coulkd make some sort of light tube to optimize the reflected signal if possible.

I am looking for advice on a circuit so that I can measure the output of the photodiode as I do not want to use it as a switch like high or low , I want to be able to measure the output in mV so that I can then process the different levels into some meaningful data. I would also like advice if it would be better to use a very bright LED to reflect off the glass or an infrared LED?

Sorry if I have not explained myself very well but I hope someone can understand what I want to do.

Thanks in advance

 

[MikeMl]

Configuring a photo diode as an analog light detector is well documented. Driving emitters (LEDS) is well documented. Emitters and detectors with built-in optics to get a narrow beam is possible and desirable to limit the effect of ambient or scattered light. You will likely have to build your experiment into a light-tight box to eliminate the effect of ambient light.
You will likely have to devise a method of substituting a high quality mirror for your sample to calibrate the high end of surface reflectivity. You will likely have to use micrometers (micro-positioners) to align the optical axes of the emitter, the detector, and the height above the reflective surface.

Get an Edmunds Scientific Catalog to see what is available for some of the hardware.

 

[milovisk]

Thanks for the reply, would you say that a high intensity narrow beam LED would be better that a IR LED for this application

 

[MikeMl]

I know little about how a surface reflects IR vs visible light. I suspect that there is a difference, because the glass in my fireplace front blocks IR but lets visible light through...

 

[milovisk]

Any further ideas from anyone would be most appreciated