Two methods for the qualification of Stray Light are described in ASTM E387. The Specified Wavelength method is accepted by pharmacopoeias including EP and USP for instrument qualification, although their detailed requirements are slightly different. The alternative Filter Ratio or Mielenz method is accepted by the USP and is more convenient to use when measuring very low stray light levels.
Stray Light Cut-off Filters – Specified Wavelength Method
A range of liquid cut-off filters that allows stray light to be checked at wavelengths from 200 nm to 390 nm.
Stray Light Cut-off Filters – Filter Ratio Method
These Reference Materials are described in the US Pharmacopeia (Chapter <857> to qualify the Stray Light (or Stray Radiant Energy) of ultraviolet spectrophotometers. A range of materials covers wavelengths from 190 nm to 385 nm
Stray Light Glasses
These filters are a convenient routine check for stray light between 220 nm and 450 nm using the Specified Wavelength method. They are not specifically cited in the pharmacopoeias.