brewster
A unit of stress optical coefficient which is equal to 10⁻¹² m²/N or 10⁻¹² Pa⁻¹. Symbol: B.
brewster: unit measuring how stress warps light through materials
The brewster is a unit that quantifies the stress-optical effect, the property describing how mechanical stress changes the refractive index of a transparent material. It measures the stress-optical coefficient, written as C, in units of 10−12 m2/N or equivalently 10−12 Pa−1. When you compress or stretch glass, plastic, or other optical media, the material becomes birefringent, meaning light travels at different speeds along different axes. The brewster quantifies this effect's strength for a given material.
The stress-optical coefficient links applied stress directly to induced birefringence through the relation Δn = C × σ, where Δn is the difference in refractive indices and σ is the applied stress in pascals. For optical crown glass, typical values lie around 3 to 5 B. Polycarbonate and acrylic, common in engineering optics, sit closer to 15 to 25 B, making them more prone to stress-induced optical distortion. This property matters in precision applications where any warping of the optical path introduces measurement error or image degradation.
The unit is named after David Brewster, the 19th-century Scottish physicist who investigated polarization and optical properties of stressed materials. His work on light polarization at specific angles (Brewster's angle) established foundations for understanding how materials interact with light under external forces. The brewster unit carries his name to honor this contribution to optical physics.
Where stress-optical effects matter
Optical windows in high-pressure vessels, laser optics subjected to thermal gradients, and precision measurement windows in industrial equipment must account for stress-induced birefringence. Epoxy or acrylic windows in pressure chambers experience radial and hoop stresses that alter their refractive index; if the change is uncontrolled, it introduces focus errors or wavefront distortion. Precision spectrometers and interferometers are particularly sensitive because even nanometer-scale changes in optical path length propagate into significant measurement uncertainty.
Manufacturers and optical engineers use the brewster to predict how much birefringence will develop under specified loading conditions. By knowing a material's stress-optical coefficient in brewsters, they can calculate the induced phase retardation and determine whether the material remains acceptable for the application or whether stress-relieving, coating, or material substitution is required. This unit is less common in everyday industrial vocabulary than in specialist optical design work, appearing primarily in materials specifications, optical simulation software, and stress-analysis reports for precision systems.