Heat reflection filters (hot mirrors)
This special filter type ranks among the category of reflection filters. By evaporating several dielectric layers onto neutral filter glass, the reflection is achieved. The wavelengths of visible light are almost completely passed. As many infrared portions of the light are reflected, it is also referred to as hot mirror (mirror for the heat proportion of light). Typical reflection ranges are from 750 to 1100 nanometres. As they have a uniform transmission over the entire visible range, they are suitable for monochrome and colour cameras. Their heat resistance of up to 400 degrees Celsius allow for the use even in hot environments.
Heat absorption filters
A similar effect can be achieved by using heat-absorbing glasses. In this case, the heat proportion is not reflected, but absorbed. The absorbed energy must, however, be dissipated. Known for this are the Schott heat absorption glasses KG 1-5.
Transmission of Schott KG 2 and KG 3 filter glass
Application in machine vision
- as infrared elimination filters to avoid chromatic aberration by the optics which would lead to blurred images in case of a high infrared proportion
- as heat shield to protect cameras, e.g. as large protective pane in front of the camera housing.