A stone blinks in a polariscope when it turns light and dark four times per turn between crossed polarizers. Blinking shows that the stone is doubly refractive.
In more detail
The dark positions come every 90°, where the stone's vibration directions line up with the filters. Looked at down its optic axis, a doubly refractive stone does not blink. This “blink” is not the blink ray class of our light census.
How we measure it
Our Polariscope calls a stone blinking when its darkest position over a full turn is under half its lightest. It says a stone “stays dark” when its lightest position stays under Y = 0.003. These thresholds are our own choice.
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.
- Doubly refractive: A doubly refractive stone splits light into two rays that vibrate at right angles and travel at different speeds. Each ray has its own refractive index.
- Strain: Strain is internal stress that makes a singly refractive stone weakly doubly refractive in patches. Between crossed polarizers it shows as patchy light that never goes dark all at once.
- Optic axis: An optic axis is a direction in a doubly refractive crystal along which light does not split into two rays. Seen along it, the stone behaves as if it were singly refractive.
- Census ray classes: In our light census, every ray that enters a stone ends in one of five classes. The classes are useful, blink, leak, grazing and lost.