[SOLVED] Photoresitor - narrow reception angle - Thanks liudr

Hello there ladies and gentlemen, I need some help and hope this is the right category. I am attempting to build....wait let me rephrase, I will be building a photographic exposure meter. The basics of the meter is not an issue for me at this time, however the one function which I am concentrating on seems to be a bit harder than I thought, especially if I can't find a ready made component.

Photographic exposure meters, or rather many good one's allow you to take spot meter readings. All this means is that you can take a reading of reflected light on a subject/object a distance. For this to give an accurate reading, spot meters read in a 1 degree angle of reception, meaning you don't want all the reflected light around you to influence the reading, only the object in the distance you are interested in.

You can almost think about it in terms of LED lights. Some LED's will shine light in a wide angle, where others will shine in a narrow angle. I need the narrow angle, or as narrow as I can get.

My other thinking was, in case no manufacturer makes a photo(resistor or transistor) with a very narrow angle, is to use a combination of a normal photo-transistor and a monocular. Its just a thought for now since I rather hope I can find a suitable component.

Just note I am not interested in selenium cells. I hope someone here has knowledge of these components and can a least point me in the right direction.

Thank you!

The LEDs that are focused within a narrow angle have a circular head that act as a lens. If you put a point light source like the LED's junction, at the focal point of a lens, the output from the other side of lens is going to be parallel rays, that have very small diversion. Same goes backward. If you have a photo transistor made with a spherical head in front of it that is designed to focus parallel rays, you will only see rays coming from one angle, ie. rays coming off a small spot from a "far away" object. I'm not aware of who makes them but should not be too hard to find from a large store.

Thanks Liudr. From my research and noting some of the comments made on so many phototransistors which all go something like this “has a wide reception angle” as was starting to think some lens system will have to do.

Thanks for sharing your knowledge. I will have to scratch up again on my high school science about light and refraction.

I did find a phototransistor with a lens on in the RS catalogue locally, however the two they listed are both IR.

I’m getting more excited now since I can see a path emerging.


If you purchase a small cheap lens with a focal length say 10cm, then mount the wide-angle photo transistor at 10cm behind the lens and make a tube for the entire thing. This way only the direction you point the tube to will get measured, thus translating to a small area on a large object.


Interesting. Thanks liudr. I see some intersting tests in the near future, some might even include my Takumar 50mm lens, mind you to be silly I might test the 200mm lens. I just looked at the spot meters on ebay and they all have some sort of lens system.

I appreciate your advice a lot!