High-order white is the near-white light that a thick doubly refractive stone passes between crossed polarizers. Its two rays are so many wavelengths apart that the interference colors mix into white.
In more detail
A ruby 5.8 mm thick, across its axis, puts its rays about 46 µm apart. That is some 80 wavelengths of green light. The white takes on the stone's own body color. True interference colors show only near the optic axis or in the conoscope.
How we measure it
The Polariscope takes the exact average of each 5 nm band. So it does not invent colors that are not there.
Related terms
- Polariscope: A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.
- Birefringence: Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.
- Interference figure: An interference figure is the pattern of colors and dark bands that a doubly refractive stone shows through a conoscope. Each point of the figure is one direction through the stone.