Yee olde exposed film IR PASS FILTER

Everyone knows that a couple of layers of developed exposed color print film makes a good Infrared Pass filter that blocks visible light.

The question is, how good does it block visible light and pass Infrared?

The following link:

> Film makes infrared filter - descripton <

clearly characterizes how well these IR PASS FILTERS work.

NOTE: This is not what you want to put on your laser pointer to block IR. These filters WILL NOT BLOCK IR. They block visible light and pass Infrared.
In other words, these will change your Green DPSS laser into an IR Laser if you do not have an IR blocking filter installed. This should not usually be done.

A good use for an Infrared Pass filter is for testing/estimating the effectiveness of your installed IR Blocking filter by using a DIY detector.

If you do a G--gle search of "How to make a Webcam into an Infrared Camera" you will find numerous methods to do such a "Hack" that can be used as an INFRARED detector that will give you an indication of how good your IR Blocking filter is working on your DPSS laser. For me this works better than destroying your cell phone camera or other digital camera when used as an IR detector. Also you can do this without looking toward the beam because you will be viewing beam images on a computer screen. Use a TV remote on your DIY detector to check that the detector is working.

An imaginative mind should lead to other applications of these IR Pass filters.

FINAL WARNING: Do not experiment with any type of laser without using proper SAFETY GOGGLES.

While exposed film negatives are cheap, I've never liked using them for IR filters. The negatives aren't all that clear, and its filtering characteristics are not well characterized. Real IR filters are only like $10 so it's not too expensive for something better.

Even so, while the visual inspection of the IR output may give an indication of the IR output, but it's not very accurate, after all most cameras have IR-cut filters as well. Even with a digital camera with its IR cut filter removed and using a professional IR filter, it only shows the relative amount of IR that comes out of a laser, and the shape of its spot. It doesn't give you a measurement, and the visual differences can be misleading.

I've also heard the media from a 3.5" floppy will pass IR while attenuating visible light.


Edit: I just tried it with a low powered green. I can still see green after the filter, however it's very dim. I'm going to open the floppy up so I can cut pieces from it and double the filter.

Edit: I was wrong. After trying 2 pieces together I see nothing behind the filter both visibly and with the modified webcam. I did noticed that the floppy media is actually very reflective to both visible and near IR. The detector shows a spot in the same location as the reflected green spot.

In addition to reflections, a lot of that film isn't very transparent for its non-filtered wavelengths compared to real filters as well. There'll sometimes be "grain" or opaque particles in the film (in the case of disks).

I too have tried using floppy disc material as an IR pass filter. It works for me when two layers are used.
I have never seen any characterization graphs for it as in the link I included in my original post for exposed Kodacolor film.

Unexposed slide film (Ektachrome) developed with the Kodak E-6 process works equally as well as unexposed Kodacolor film. Slide film has specific dyes that allow IR transmission so that they don't absorbe IR and melt in the projector. The lamps in such projectors are typically around 500 Watts and give off enough heat inspite of condenser lenses and heat absorbing glass to melt normal undyed film stock.

Badger, you mentioned that you can get real IR filters for $10. Share with me and others the source that you have. I assume that there are specifications included.

Those IR filters are on Amazon.com (2). They're not Hoyas or something, but for these applications they'll do.