Optics

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Optical Coatings & Spectral Transmission

High light transmission is important for modern machine vision lenses. Small camera pixels require a lot of light. Many C-mount lenses meet this trend with very small apertures such as f/1.2 or f/1.4.

However, it is also important that the lens has a high level of spectral transmission.

Optical coatings - prevention of reflections

Losses occur as light passes through the glass lenses of the optics: It is reflected at the surface, absorbed within the material and reflected at the light exit surface. Transmittance defines the total amount of light exiting the material, which is generally made up of directed and scattered portions. For example, the transmittance of ordinary glass is only 95 percent.

The use of high quality optical glass and coatings can greatly reduce this loss. Ideally, several very thin layers of metal compounds are vapour deposited on the glass to form an anti-reflective coating. These are visible to the naked eye as greenish or reddish shimmering coatings. The thickness of the coating and the material are chosen so that the reflections that occur at the individual interfaces overlap and cancel each other out (destructive interference) and the continuous light beams reinforce each other.

Because individual interference layers only work optimally for certain wavelengths of light, a large number of such layers have to be vapour-deposited on high-quality glass in a high vacuum.

Antireflective coatings: Destructive interference principle

This explains the difference in price between basic and high quality lenses. A light loss of 4 to 5% with uncoated lenses seems harmless at first. However, as a lens can quickly consist of 4 to 7 elements, such values add up considerably. The resulting stray light is extremely disruptive to image processing and greatly reduces image contrast.

Spectral transmission

The light transmission of optical glass is also known as spectral transmission. As already described, it depends on the absorption and reflection properties of the lenses and coatings, but also on the wavelength of the light. Normal imaging lenses are mainly designed and corrected for the visually detectable range of light from 400 to 700 nm.

Inspecting in the UV:

Crown or flint glass lenses cannot be used for inspection in the UV range. The light transmission of these common types of glass drops extremely sharply from around 350nm, and at around 310nm the transmission is less than 5%.

The only solution is to use quartz glass lenses. Here, the spectral transmission does not drop sharply until a wavelength of 180nm. However, the production of quartz glass lenses is technically complex and very expensive, so they are only used in special applications.

Inspecting in the IR:

For inspections in the IR range, care should also be taken to ensure that the optics have a high spectral transmission here too.

Although the lenses also have good transmission in the NIR, the optical coatings in particular must also be adapted to these wavelengths in order to avoid stray light (anti-reflection coatings).
Without these broadband anti-reflective coatings, the amount of stray light in the IR would increase dramatically.

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