Pixiq

When trying to describe what an IR filter is actually doing, we often talk about transmission rates. A filter is designed to absorb certain wavelengths. For IR filters, we are concerned with blocking near infrared wavelengths (from approximately 700 to 1100 nanometers (nm) and the shorter visible wavelengths (about 650 to 700 nm).

If a filter (such as an Wrattem 89B) has a 0% transmission rate at 680 (nm), it is cutting out all wavelengths below that number, thus most of the visible spectrum. In general, filters don’t suddenly turn on or off at a given wavelength, but block wavelengths at a progressive rate of change. This will vary from filter to filter. Therefore, the filter with 0% transmission of wavelengths of 680 nm might exhibit a 50% transmission of wavelengths of 720 nm, meaning that 50% of the radiation with a wavelength of 720 nm is transmitted (and 50% is not transmitted). Typically the rate of change rises quite sharply, so this particular filter may be transmitting 85-90% or more of radiation with wavelengths of 750–775 nm.

It would be nice if all filter manufacturers adopted a logical numbering system to identify their filters, but that is not the case. For example, a Hoya R72 starts to transmit wavelengths above 720 nm, but the company’s RM90 transmits wavelengths above 820 nm. By contrast, the Wratten 89B, which is similar to a Cokin 007, allows wavelengths beginning at 680 nm to pass.

A comparable filter is called 092 when manufactured by B+W. A Wratten 87 is the equivalent of a Heliopan 5780 (sometimes called am RG 780) and a Tiffen 87! See what I mean?

While this lack of a standardized numbering system can be a source of confusion, it is just the way that filters and lots of other stuff have been handled in photography, with each manufacturer creating a numbering system as they go (you’ll need a Rosetta Stone to help you decipher it all).