Dichroic coatings

Most of todays optical design engineers are qualified in MIL spec coating applications while using the latest computer technology available. Layering and substrate masking can make a big difference with regard to the consistency of the end product.

A controlled and clean environment is essential. Tits & pits produced by dirt and scratches can weaken the coating.

There are several minerals or materials that can be used to produce the same color. Some materials cost less. This can effect the hardness of a coating. Post coating processes can also differ. Coating procedures are a must. Substrate cleaning and handling is always important.

Dichroics And How They Work

If you remember there are four types of color filters used to produce colored light. Dyed polyester gel, tinted or resin or Lexan coated glass and dichroic coatings on glass.

Polyester types hold up well for short term applications, but in time they fade. The same is true for tinted glass. Resin dipped or coated glass tend to flake and crack.

The problem with dyed polyester, glass and resin coatings is that the wavelengths of the light spectrum which do not pass through the filter are being absorbed by the coating, thus breaking down the organic dyes used for the color. The filter becomes dull and lifeless.

Vacuum coated materials on glass is the key. We fabricate a mirror like lens which reflects certain wavelengths of light while transmitting the selected wavelengths we require to achieve the desired color filter. This is known as the dichroic or band pass color filter.

For example: A blue transmitting dichroic transmits the blue while reflecting the red & green (= or yellow). A red transmitting dichroic transmits red while reflecting green and blue. The end result is extremely steep color separation that could never be reproduced with organic dyed substrates.

Off Axis Tweaking In the process of creating a color separation mirror we are also able to tweak or pivot the mirror off of a O° axis (or normal incidence) and create other colors. For example: When tweaking a dark blue dichroic at approximately 22° we get a lavender, tweak to 80° you get a flesh pink. With a light red at 22° you get a rich amber, go to 80° you get a bright yellow. Using Color Separation Filters Additive: Red, Green, Blue, (R,G,B,) Subtractive: Cyan, Yellow, Magenta (C,Y,M,) Reflective: Red, Blue
We can also produce color and white light by adding and subtracting light with dichroic filters. A total of five filters would be used. Additive colors (#1) green, (#2) red and (#3) blue. Each additive filter to have a source of its own at 0°. Reflective colors would be (#4) red reflector at 45° and (#5) blue reflector at 45° . NOTE: The blue reflector equals a yellow transmitting filter at 0° and the red (or magenta) reflector equals a blue green transmitting filter at 0°. Alternatively we could also produce white light by using a (#1) red, (#2) green and (#3) blue, each additive filter to have a source of its own at 0°. Reflective colors would be (#4) yellow green reflector at 45° and (#5) blue reflector at 45°.
NOTE: The yellow green reflector equals a dark purple transmitting filter at 0° and blue reflector equals an amber transmitting filter at 0°. (example not shown) Color Separation Application Display Green 0° Red 0° Red 45° Blue 45° Blue 0° LightWhite