Additional optical filters: short-pass and long-pass filters, color filters and grey filters

 

In addition to the optical filters already described, there are many other ways to optimize the test image using filter techniques.

 

1. Shortpass filter

 
Shortpass filters transmit wavelengths that are below the cut-off wavelength of the filter. Wavelengths above the cut-off wavelength are blocked. Shortpass filters are often used to separate wavelengths and increase the contrast in the image.

They can also be used as illumination filters for fluorescence applications. If such a shortpass filter with a very steep edge is used as a replacement for the already described Bandpass filter,  then a similar effect can be achieved. When combined with an appropriate a corresponding longpass filter, a special band-pass filter can also be generated.

2. Longpass filter

 

In contrast to shortpass filters, long-pass filters transmit wavelengths above the cut-off wavelength. Like shortpass filters, longpass filters can be used to separate wavelengths from each other.

In fluorescence applications, for example, longpass filters can be used in front of the optics to block the excitation wavelengths and thus increase the contrast. Read more about this under the topic “Fluorescence applications in industrial image processing”.

3. Color filter

 

Color filters are often used in conjunction with monochrome cameras. They are suitable for increasing the contrast in the image, separating, highlighting or suppressing colors. Color filters are bandpass filters that allow a specific color band to pass through. When using a red color filter, red light is allowed through while other wavelengths are blocked.

We have already described the interaction of colored objects with visible light under  the topic “Influence of light colors in industrial image processing”: An object reflects certain wavelengths and thus appears colored to our eyes. An object that we perceive as red therefore reflects the red part of the light. Other wavelengths, on the other hand, are absorbed. This characteristics can also be exploited when working with color filters. If you use a color filter in the same color as the object, it will appear bright in the image. A color filter in a different color, on the other hand, makes the object appear dark.

 

You can also find more detailed information on how bandpass filters work and what they are used for in the section  bandpass filter.

4. Grey filter


Grey filters allow the incident light to be attenuated and are used to avoid overexposure in the image. The light reduction is uniform across the entire light spectrum, so that the use of the filter has no influence on the representation of light or object colors


Find out more about other optical filters here: