I’ve been a bit of a shutterbug since I was a kid, and one of the things that always fascinated me was infrared photography. I shot a lot of film from 24 ASA slides to 1000 ASA night shots. Fascinated by the sample images I’d seen in books, and armed with a lack of knowledge and resources, I bought some IR film at a local camera shop, where the higher end photo enthusiasts liked to hang out. I did not know that I also needed an IR filter to place on the SLR camera I was using, so my images did not turn out as I had hoped.
Below is an example of a nice IR photograph I found with google. Click on the pic to go to the web page where I found this nice photo.
I’ve seen some hacking projects use sections of floppy disk material as a cheap infrared filter, so I thought I’d compare it to a “real” infrared filter used for photography.
The picture below shows an old 3.5 inch floppy disk, with the media from another 3.5 inch floppy disk laying on top for illustration.
The next photo is of a HOYA R72 infrared filter used for photography.
It’s pretty widely known that digital camera sensors are sensitive to IR light just outside the visible spectrum. Remote controls usually use IR LEDs, and an old trick to see if a remote is working is to point a digital camera at a remote control and see it light up on the camera even though the naked eye cannot see anything.
The 3 images below show an IR LED in a remote control viewed with no filter, with the R72 filter, and finally with the floppy disk filter. I simply held the filter materials in front of the lens of a Sony HX5V camera. Many cameras have a filter in front of the sensor to filter out IR, so some digital cameras don’t work so well for this. However it appears that this camera works fine to see the wavelength.
Floppy disk filter
As you can see, there is almost no attenuation of the IR when using the R72. It looks almost the same as with no filter. All of the images were taken with the same shutter/aperture settings. In the third image, you can see that the floppy disk filter blocked most of the light. I actually boosted it a little in photoshop just because it was so dim.
Ok, so the attenuation is pretty bad with the floppy disk, but how about the spectral response? For that, I took a couple of test shots of sunlight falling on some trees. I removed the color in the photo and optimized the images for comparison of spectral response.
First, the R72:
Now, the floppy disk:
As you can see, the floppy disk has a much different spectral response. I would not even call this an IR filter at all, but more of a neutral density (gray) filter.
I found a graph of the R72 spectral response, compared to the R90. The R72 is a good choice for people wanting to do IR photography. The RM90 probably would not work so well for a digital camera, but would provide more of a gap between visible light an IR.
Here’s a picture of the EM spectrum around visible light:
You can see that the R72 starts passing light just beyond the red cutoff around 700nm.
While infrared filters can produce amazing photographic results, the sensors are not adequate to detect the much longer wavelengths that are used for things like thermal imaging. For that, a special type of image sensor called a bolometer is used.
With a bolometer based imaging system, you can get some really neat effects. These imagers prices begin in the sub $2000 range and extend upward from there in price. Resolution of the image sensors is pretty poor, typically in the range of 120 x 120 or 160 x 160 pixels. Since not much detail is really needed to see the areas of interest, it ends up looking surprisingly good.