All about Filters
The filter is as critical to the operation as the camera itself as it blocks visible light and allows only infrared light to enter the lens.
The basic principal and reasons are described in How Infrared "X-ray" Works.
The information presented here is based on information submitted by the makers and reports from users or actual testing of the filter in question by the Guide authors.
The important factor in a filter specification is the point at which transmission begins. This refers to the wavelength of light which is allowed to pass the filter at more than around 50%. Wavelengths (for our purposes) are measured in nanometers or "nm" for short. The higher the nm figure, the longer the wavelength and the better the filter will perform. Any filter which lets light through with a wavelength shorter than around 780nm will generally perform poorly when used for "x-ray".
Please note that no filter has an absolute "cut off" point. As the light wavelengths increase they gradually let more through. The result is a curve of light transmission increasing until it approaches 100%. Also note that even the best filters reduce the light level a little even when they are supposed to be letting 100% through.
Infrared Filter Index
The Kodak Wratten 87 Range
The Hoya R72
The Hoya RM90
The Hoya RM100
The M&K 1000
The M&K 093 and 095
The Home Made Filter
Where to buy
These filters can often be difficult to locate. Some more than others.
I have gathered information about filter vendors to try and make this easier. Your locality will, of course, have a large bearing on ease of purchase. However, I have myself ordered filters from abroad and had no problem in delivery and pricing.
Check out the Infrared Filter Buyer's Guide.
You have probably drawn your own conclusions at this point, but in summary I will say that if you wish to spend as little as possible then the Kodak/Wratten 87C is the one to choose. However, this depends largely on your camera's state of modification.
Take into account your intended filming locations. Remember that the 1000nm rated filters generally give a poor picture under anything but bright sunshine.
If you're likely to be filming in artificial light or overcast weather look to a filter of 800-900nm rating which will be a good all-rounder.
If you already have a filter and are not sure of its performance, take a look at Checking your filter.
One last word of advice - be careful with gelatin filters. They are very sensitive to moisture, scratching and finger-prints. Clean them carefully with a soft, dry cloth or purchase the proper cleaning material.