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Archive for March 27th, 2009

Quick-and-easy IR-passing / Visible-blocking Optical Filter

Gel filters - normal visible light

Gel filters - normal visible light

When you don’t need high optical quality for an IR filter, you can superimpose red and blue stage-lighting filters: pure black to the eye, transparent to IR.

You know they’re IR-transparent because they’re generally snuggled right up against huge incandescent bulbs: if the filter material absorbed any IR, it’d burn right up.

I’ve used Lee Filters Congo Blue (181) and Primary Red (106) to good effect. They may be available from a stagecraft outlet near you, but around here that stuff is a mailorder deal. You’ll get a lifetime supply, so maybe you can pass some out to your cronies; techies always enjoy odd presents like that.

A more optically flat (and durable and expensive) option would be a photographic-image-quality glass IR filter suitable for camera mounting. I got one after I dropped a homebrew plastic filter down a sewer grate.

For examples, go to Adorama, click on Filters in the left column, then select Infra-Red Filters, then maybe refine the search to the cheaper Hoya brand before your budget runs away in fear.

Gel filters - IR view with visible light

Gel filters - IR view with visible light

Make sure your camera doesn’t have an IR blocking filter behind the lens. I think most consumer-grade digital cameras do have an IR-blocking filter and most video cameras don’t, but I’m sure those general rules don’t hold in all cases. Indeed, I bought a Sony DSC-F717 specifically for its IR mode; fortunately, the CCD sensor failed shortly before the factory recall ended.

The pictures show the same scene under normal lighting, with the camera set to its IR mode, and IR mode with an IR filter in front of the lens.

The gel filters appear dark-gray in the middle image because the camera sets the exposure (1/60 f2.4 ISO100) based on the visible light entering the lens. They’re transparent in the bottom image because the exposure (1/30 f2.4 ISO1000) is based on only the IR illumination, which is pretty dim. The gratuitous greenish cast is how Sony reminds you that the image was in IR mode

Gel filters - Pure IR view

Gel filters - Pure IR view

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