Infrared filter

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The term infrared filter denotes completely different filter properties :

Infrared transmitting filters

In infrared photography , with infrared lamps and infrared receivers for remote controllers , and - photoelectric sensors is an infrared filter to filter out visible light and largely or wholly to absorb. These filters are made of glass or plastic.

Infrared blocking filters

The red curve shows the extinction coefficient of water

In front of the image sensor of digital cameras and video cameras, there is usually an infrared blocking filter (also infrared filter , "IR-Cut" , "Hotmirror" ), which is supposed to prevent the incidence of infrared light . This avoids disruptive influences of the IR radiation on the image quality of the sensor. This can be blurring or color distortions. These filters are interference filters or dichroic filters.

In headlights and projectors , filter glasses are often used in order to reduce the radiation of the light sources (especially incandescent lamps ) in the infrared and thus to reduce the heating of the illuminated objects (slide, film, speaker). These are also infrared blocking filters, they are called heat protection filters here .

A "water filter", a cuboid, water-filled cuvette in which the light runs through a layer of water at least 1 cm thick, is particularly easy to implement. Such filters were previously used to reduce the strong IR component of the light from a carbon arc lamp . The thermal load capacity is considerably greater than with thin film filters. In the picture you can also see that the extinction coefficient of water in the IR range is more than a thousand times higher than in the visible range.

See also: absorption behavior and color of water

Dichroic infrared filters

With dichroic filter layers (interference filters) on the glass bulb of incandescent lamps (IRC halogen incandescent lamps), it is possible to increase their light yield by selectively reflecting the infrared radiation component back onto the filament . Since the filters also somewhat limit the long-wave (red) part of the visible spectrum, this increases the color temperature . These coatings act as an infrared blocking filter.

The screen of cold light mirror lamps has a dichroic infrared filter that only reflects the visible part of the spectrum. The result is also an increased color temperature of the emitted light and a lower heat effect in the light cone, since the infrared portion of the incandescent lamp escapes through the dichroic reflector to the rear.