I built a spotmeter 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 would absorb some IR and of course it will be OOF on the measurement plane compared to 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.

How do cameras that use silicon photosensors get around this problem?