Illumination

Illuminate features & make defects visible

Physical charateristics of light

‘Fiat lux’ - let there be light. But light is not just ‘light’ for machine vision experts. Even the famous physicist Isaac Newton investigated the properties of sunlight with prisms in the 16th century and discovered: ‘Coloured light is hidden in white light’.

Machine vision uses white light, monochromatic light of a specific colour, but also UV or infrared radiation. These are invisible to the human eye, but can be easily captured by industrial cameras.

Electromagnetic spectrum

VIS

Light visible to the human eye is electromagnetic radiation with wavelengths between about 380 and 760 nanometres.

UV

Below 400 nanometres we speak of ultraviolet (UV) radiation, which is extremely hard UV radiation down to 10 nm. The UV range, which includes sunlight, is called UV-B from 280nm to 320nm and UV-A from 320nm to 400nm.

IR

Above the visible range, we speak of infrared radiation, also incorrectly referred to as 'IR light'. It covers the wavelength ranges from 780nm to 2500mm in the near IR, and the ranges from 2.5µm to 50µm are referred to as the mid IR.

Violet 380-450nm

Blue 450-495nm
Green 495-570nm
Yellow 570-590nm
Orange 590-620nm

Red 620-700nm

The properties of light can be described in terms of both wave and particle characteristics, and a large number of phenomena and effects that can be directly derived from this also occur in machine vision.

The wave properties of light are the theoretical basis of geometric optics: the path of light rays through a lens, caused by refraction, is subject to these laws. Vision engineers use these principles to select optics and calculate working distances. Reflections from surfaces are also subject to the principles of geometrical optics. When working with structures whose size is in the range of the wavelength of light, further interactions occur. Possible effects of theoretical wave optics are interference, diffraction and polarisation.

Quantum optics analyses and describes the particle properties of light. These quantum mechanical aspects of light explain the emission and absorption effects of bodies, Compton/Raman scattering and the formation of spectral lines, e.g. from illumination sources such as LED lighting, fluorescent lighting or metal halide lamps in cold light sources.

Most of these theoretical principles can be observed by the user in machine vision practice, as the inspected objects...

absorb certain wavelengths of light (i.e. they are coloured)
emit certain ranges of wavelengths
reflect incident light
sometimes produce interference patterns (usually disturbing)
refract light at their surface
produce polarisation effects

Conclusion:

Some of these effects are undesirable and make it difficult to evaluate the inspection of parts.

On the other hand, these specific properties of objects can be used in combination with the right lighting, lens or filter to detect defects or flaws in the object being inspected. A suitable camera sensor can also help to solve the problem.

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