The brewster (symbol B) measures the photoelastic effect in optical materials like glass or plastics. One brewster equals 10⁻¹² square meters per newton. It quantifies how much a material's refractive index changes when subjected to mechanical stress, which is critical for analyzing lenses and optical components for residual manufacturing stresses.
Heard on the job
The polycarbonate blanks show a stress optical coefficient of about 80 brewsters, which will cause unwanted birefringence under mounting pressure.
Insider tip
While rarely used in general architectural lighting, the brewster is a critical unit when designing injection-molded optics where internal stresses can distort the final beam pattern.