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Filters

Filters can dramatically alter the character of the light that enters a lens and they can have a wide variety of uses, from simple protection of the lens surface to restricting certain wavelengths of light or polarising the light etc.

Filters are defined by their spectral transmission, and most of the types used in the machine vision industry will have associated transmission graphs like the one shown below.

This specific graph shows the spectral transmission for a range of yellow and red filters.

Shortpass filters

Shortpass filters are designed with sharp cut-off wavelength from 615 nm to 850 nm. A broadband anti-reflection coating avoids troublesome reflections. Shortpass filters are ideal for fluorescence and wavelength sorting applications.

Longpass filters

Longpass filters are designed with sharp cut-off wavelength from 400 nm to 820 nm. A broadband anti-reflection coating avoids troublesome reflected light. Longpass filters are ideal for fluorescence and wavelength sorting applications.

Neutral density filters

Neutral density or grey filters are used to reduce the transmission of light equally over the entire visible spectrum. Thus the light intensity is reduced without affecting colours and contrast. Typically used in front illuminated applications where light bleed can not be avoided simply by closing the aperture of the lens or by adjusting the camera settings.

Front light without (left) and with (right) grey filter

Daylight cut filters

Daylight cut filters block visible light from contaminating the image information. The resultant image is only generated by the IR ranges of the spectrum to which the camera sensor is sensitive. The main use of these lenses is to make the application independent of ambient light such as the changing intensity of sunlight. By combining daylight cut filters with IR illumination, the resultant image is very consistent while visible light levels are changing.

UV cut filters and IR cut filters

UV cut filters are longpass filters that block the UV wavelength range and transit the visible light. They are used in front of a lens on a camera to protect the sensor from UV light, in the case when the camera offers no internal protection. In addition they are often used to protect the front lens.

IR cut filters are shortpass filters that block infrared light and transmit the visible light and are typically used with or built-in to colour cameras to mimic the photo responsivity of the human eye which is not sensitive to IR hence making colours appear similar to human perception.

UV/IR cut filters

UV/IR cut filters are a combination of a UV cut and an IR cut in one single filter, using UV cut absorptive glass with a thin film layer system that reflects infrared light. These filters are also called daylight pass filters and are used in strong sunlight or when certain artificial light sources are used to make the camera's image perception similar to that of the human eye.

Coloured filters

Coloured filters in a wide range of tones are used to block certain wavelength ranges of the visible light. In machine vision applications they are used in front of a lens on a camera to improve image contrast.

The appropriate wavelength band (colour) has to be selected with respect to the colour of the object and the type or colour of the illumination.

Since they affect the composition of the transmitted light, they are usually not suitable for colour cameras. In the case where a colour filter is used on a monochrome camera, objects in the filter colour appear bright, whereas the complementary colour appears dark.