Coloured filters with a wide range of tones are used to block certain wavelength ranges of the visible light. These filters absorb the blocked light and are therefore insensitive to the angle of incidence.
Blue and green filters are bandpass filters.
Red and yellow filters are long-pass filters that pass light at wavelengths longer than their characteristic cut-on wavelength – typically the wavelength at which the transmission is 50% of the maximum. These filters use absorptive glasses, that are robust, cost-effective and insensitive to angle of incidence.
In machine vision these filters are typically used in front of a lens on a monochrome CCD or CMOS camera to improve image contrast. The appropriate type of filter has to be selected with respect to the colour of the object and the type and / or colour of the illumination.
Since they affect the composition of the transmitted light, they are usually not suitable for use with colour cameras.
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