X Series IR720 67mm

In contrast to the naming convention of optical filters where the name of the filter denotes the wavelengths that are blocked, photographic filters are named for the color of light they pass. Thus a blue filter makes the picture look blue. A black filter marginally allows more light in the blue wavelength to pass resulting in a slight shift of the color temperature of the photo to a cooler color. Because of this, the term "IR filters" is commonly used to refer to filters that pass infrared light while completely blocking other wavelengths. 

Unlike the eye, sensors based on silicon (including CCDs and CMOS sensors) have sensitivities extending into the near-infrared. Such sensors may extend to 1000 nm. Digital cameras are usually equipped with IR-blocking filters to prevent unnatural-looking images. IR-transmitting (passing) filters, or removal of factory IR-blocking filters, are commonly used in infrared photography to pass infrared light and block visible and ultraviolet light. Such filters appear black to the eye, but are transparent when viewed with an IR sensitive device.

These filters block UV and Visable light in the spectrum up to 720nm. This while appearing black to the eye, they transmit infrared light. When taking pictures interesting effects can be acheived, since infrared is actually heat. Obvjects which are alive and transmit or reflect heat will appear to be bright. 

This made infrared photography invaluable in discerning living objects from painted camoflage, for example.

Today infrared photography opens up a whole new way of looking at the world from conventional visible light.