What material blocks IR, but passes visible light?

[zahc]

I built a visible light meter using a light-to-frequency IC and it totally works, but it's too IR sensitive. I knew the absorption spectrum of the silicon sensor was biased toward IR but I figured it might work anyway since lots cameras use Si sensors, plus I hoped that my glass lens in my meter would absorb some IR and of course it will be out of focus compared to the visible light. However grass makes the sensor respond much more than a sheet of white paper even though the paper is much whiter. So clearly I have terminal IR problems at this point.

The only solution I can think of is to get some kind of filter that will filter out IR and pass visible light, but I don't know what material to use. Most common "IR filters" are designed to pass IR and block visible, such as for remote controls and stuff.

I know that some projectors take the light from the lamp and bounce it off a special mirror that lets the IR pass and reflects the visible light, but I don't know what's special about the mirror. How do cameras that use silicon photosensors get around this problem?

Maybe I could find a broken digicam and steal the IR blocking filter off the sensor.

[Nick1911]

Maybe I could find a broken digicam and steal the IR blocking filter off the sensor.

This is a good approach, I think.  It's likely to be the ideal material.

[Physics]

Could you use a diffraction grating?  http://en.wikipedia.org/wiki/Monochromator

[Harold Tuttle]

Modding the Coolpix 900 series to make them IR cameras used to be the thing:
http://www.echeng.com/photo/infrared/wooten/

there must be a number of pulled out IR block screens out there

http://www.schneideroptics.com/industrial/filters/UV_&_IR_Cut.htm?gclid=CP2W3ue4xJwCFQxM5QodGwT3ow

[Headless Thompson Gunner]

There are plenty of camera lens filters that block IR and UV while passing visible light.  Hit up your favorite photographic mail order outfit and do a search.

[Brad Johnson]

You can get SLR-type lens filters at Best Buy.

Brad

[HankB]

. . . I know that some projectors take the light from the lamp and bounce it off a special mirror that lets the IR pass and reflects the visible light, but I don't know what's special about the mirror.

That's what's called a "cold mirror" . . . you can also get a "hot mirror" that reflects IR and passes visible. You can find them at places like Edmund Optics.  (  http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=3150  )

You might also get by using heat absorbing glass, but this stuff has a distinctly green color. (  http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2403  ) It's also usually tempered, so if the piece you get is too large, cutting it down will be a problem.

[roo_ster]

Some materials block all IR, some only block some IR.

[zahc]

I dissected my wife's old cell phone to try to harvest the IR filter, but it was glued in and I broke it getting it out. The largest piece covers the active region of my photodiode but I'm not happy because it's almost as thick as it is big and there could be nontrivial reflections. I put it on there anyway though and I'll test it tomorrow.

[Headless Thompson Gunner]

I don't know how much you're willing to spend, but the optical engineers at work buy their gear from www.thorlabs.com.  They'll have high pass filters with just about any cutoff frequency you could want. 

I bet they also sell all sorts of photodiodes with built-in IR rejection.

[zahc]

I'm not willing to spend anything! If I was, I would buy a lightmeter instead of resoldering 22ga solid core wires 32 times over, and writing hundreds of lines of C.

[Antibubba]

Try your local Freecycle.

[Nick1911]

I'm not willing to spend anything! If I was, I would buy a lightmeter instead of resoldering 22ga solid core wires 32 times over, and writing hundreds of lines of C.

http://www.dealextreme.com/details.dx/sku.20170

And that includes shipping!

I'd cry False Economy, but I can identify all too well with someone who has more time then money. 

[zahc]

That meter's no good anyway. I need a spot meter that can measure the light reflecting off of a particular part of a subject, over a 1-5 degree angle. They tend to start around $200.