Measuring light reflection in very tight space.

[eTobey]

Hi,
i have to measure the reflection of a disc, but i dont know, how i can properly align the LED to the phototransistor. The LED does not distribute the light at 180° degrees with an even intensitiy (see the picture). Anyone has an idea how to get some good illumination?

Cheers
Toby

[Twoflower]

You can just by a ready made device like this: ELI TR9904 Reflexlichtschr 3,0mm PCB-Mt

Not sure how strong the sensitivity falls off if you reduce the distance below the focal point (the device above has a working distance of 3mm).

Edit: Just noticed Reichelt mentioned the package weight in scientific notation 

[eTobey]

This is waaay to huge, but i also found some smaller ones of this kind. But now i wonder, where the center point of detection is. Is it in the middle of the two elements, or just under the phototransistor?

With my seperate elements, i was thinking about how i could put the LED in a slight angle, to fully iluminated the surface under the phototransistor.

[eTobey]

I found another funny thing:
I wonder if this part is good, when the datasheet is already kinda funny.

[Twoflower]

Your picture showed the 1...3mm form the top of the photo-transistor no limitation of the sensor dimensions ;-) But yes there are also SMD types (e.g. GP2S700HCP). The GP2S700HCP datasheet has also a diagram showing the relative sensitivity over distance. The 1mm is probably to close.

I would assume that both emitter and receiver are looking inward that the sweet-spot is aligned to the center of the device.

[eTobey]

I have read a few times those elements "facing the same directions" or "both are in the same plane". So it would look like in my drawing. I just ordered one, and i will investigate.