Colour filters

Colour filters are absorption filters made of coloured glass which absorb light in specific wavelength ranges in varying degrees and let other ranges pass to a major extent. Depending on the technical function and translucence in specific wavelength ranges, these absorption filters are called colour filters, daylight elimination filters, high pass filters, etc.

Classic colour filters

They are typically used for monochrome cameras in order to filter light from the entire spectrum in such a way to produce something like coloured illumination.

Principle of a colour filter

The filter glasses (very often coloured glasses by Schott or Hoya) are red, green, orange, blue, or yellow. They let the wavelengths of their own colours pass to the greatest extent (approx. 90 per cent) and block any other. A portion of the rays are reflected at the glass surface. If you use a colour filter with white illumination, you must expect a significant loss of light. They work according to the principle of subtractive colour mixing and due to their design they are not as narrow-banded as interference filters.

Transmission of some Schott colour glasses (e.g. blue, green, orange, red)

Example: the red filter - an absolute must

In this way equally bright green or red which could not be distinguished in the monochrome camera image can be differentiated by using a red filter: the red is lightened up, the green of the test object is darkened. However, the prerequisite for this is that white light is available as a basis, from the spectrum of which suitable ranges are filtered out.

Examples of colour filtering:

Colour image

Illuminated with fluorescent light and with performed white balance.

Monochrome image

Illuminated with fluorescent light. The differentiation of the Colours in monochrome image is very difficult.

Mono & red filter

Reds and oranges appear very bright, green and blue colors very dark.

In addition to the accentuation of test object colours, particularly a red filter (091 / RG 630) can render further important services:

Transmission of Schott red filter glasses OG 590 and RG 630

It lets pass nearly the entire light quantity of a red LED lighting or a red laser, but blocks many daylight influences and fluorescent lighting effects. In this way, interference due to effects of extraneous light can significantly be reduced. This standard filter should be used on any red LED lighting. Much more robust applications are available for very little money!

Laser line with daylight

Red laser line with ambient light - laser line is sometimes difficult to recognize on object surface.

Laser line with red filter

Disruptive ambient light (fluorescent tubes) is almost completely blocked. Only bright laser line remains visible.

Daylight elimination filters

This usually refers to high pass filters which let pass high wavelength ranges of the spectrum. They are normally used in combination with infrared lighting. The transmission in infrared radiation is at the maximum, daylight and UV are blocked and not registered by the camera sensor. Daylight influences are almost completely avoided in this way.

Transmission of Schott infrared filter glasses RG 695 and RG 830

Non-visible infrared lighting is used wherever there is much extraneous light (machines close to windows or skylight) or when industrial lighting could disturb the operators (at a manual work station). Depending on the wavelength ranges of the infrared lighting used, there are suitable filters which let pass the light only from 800 or 900 nm.

These filters have a deep red colour and are therefore not suitable for colour cameras.