Wavelength and optical performance

Many machine vision applications require a very specific light wavelength that can be generated with quasi-monochromatic light sources or with the aid of optical filters. In the field of image processing, the choice of the proper light wavelength is key to emphasize only certain colored features of the object being imaged.

The relationship between wavelength (i.e. the light color) and the object color is shown in the picture. Using a wavelength that matches the color of the feature of interest will highlight this specific feature and viceversa, i.e. using opposite colors to darken non relevant features .

For example green light makes green features appear brighter on the image sensor while red light makes green features appear darker on the sensor. On the other hand, white light will contrast all colors, however this solution might be a compromise.

Additionally it must be considered that there is a big difference in terms of sensitivity between the human eye and a CMOS or CCD sensor. Therefore it is important to do an initial assessment of the vision system to determine how it perceives the object, in fact what human eyes see might be misleading.

Monochromatic light can be obtained in two ways: we can prevent extraneous wavelengths from reaching the sensor by means of optical filters, or we can use monochromatic sources.

Relationship between object color and light color.
Relationship between object color and light color.

Optical filters allow only certain wavelengths of light to be transmitted. They can be used either to allow light of a specified wavelength to pass through (band-pass filters) or to block desired wavelengths (e.g. low-pass filters for UV light only).

Color filters can block other non-monochromatic light sources often present in industrial environments (e.g. sunlight, ceiling lights etc.), however they also limit the amount of light that actually reaches the sensor.

On the other hand, quasi-monochromatic sources only produce light of a certain wavelength within a usually small bandwidth. Either way, if we select monochromatic (e.g. green) light, every non-green feature will appear dark grey or black on the sensor, depending on the filter bandwidth and the color of the feature. This gives us a simple way to enhance contrast by using monochromatic light with respect to the use of white light.

Color temperature
One way to maximize contrast is to select the light color that is on the opposite side of the wheel of the feature color. In such case, features will appear dark on the image sensor.

Additionally, in some cases a specific wavelength might be preferred for other reasons: for example, Opto Engineering® telecentric lenses are usually optimized to work in the visible range and they offer the best performance in terms of telecentricity and distortion when used with green light. Furthermore, green light is a good tradeoff between the resolution limit (which improves with shorter wavelengths) and the transmission characteristics of common glasses (which in fact have low transmission at short wavelengths). In cases where any wavelength will fit the application, one might choose a specific LED color just based on cost considerations.

Polarizing filters consist of special materials characterized by a distinctive optical direction: all light oscillating in this direction passes through, while the other components of the wave are suppressed. Since light reflected by a surface is polarized in the direction parallel to the surface itself, such reflection can be significantly reduced or blocked by means of two polarization filters - one on the light and one on the lens. Polarizing filters are used to eliminate glare effects occurring when imaging reflective materials, such as glass, plastic etc.

Filtering and coloured samples.
Filtering and coloured samples: concept scheme and monochromatic result. On the left, Red light is reflected off the red background, but is absorbed by the blue circle. On the right, Blue light is reflected off the blue circle, but is absorbed by the red background.
Color camera
Color camera
Mono camera.
Mono camera.
Red filter.
Red filter.
Green filter.
Green filter.
Blue filter.
Blue filter.
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