Brewster (unit)

The Brewster, symbolized as BU, is a unit used in photoelasticity to measure stress-induced birefringence in a material. Specifically, it quantifies the stress required to produce a relative retardation of one wavelength (typically 589.3 nm, the sodium D line) per centimeter of optical path length.

In simpler terms, a Brewster is a measure of how much stress needs to be applied to a material to change the color of light passing through it by a certain amount, related to the wavelength of the light and the thickness of the material.

The relationship between stress (σ), Brewster number (BU), thickness (t), and wavelength (λ) can be expressed as:

Retardation = σ * t * (BU/λ)

Where:

• Retardation is the phase difference between the two orthogonal components of light.
• σ is the stress applied to the material.
• t is the thickness of the material.
• λ is the wavelength of light.

Therefore, 1 Brewster is the stress that produces a retardation of one wavelength per centimeter path length.

The Brewster unit is commonly employed in engineering and materials science for the non-destructive testing and analysis of stress distributions in transparent materials like plastics and glass. The isochromatic fringes observed in photoelastic experiments are directly related to the stress distribution expressed in Brewsters.

Different materials have different Brewster constants. A higher Brewster constant means that less stress is required to produce a given amount of birefringence. The constant is temperature-dependent, and needs to be carefully calibrated for accurate measurements.