A solid phase or liquid reference standard of known optical and physical characteristics, intended to be used to verify and/or optimise the performance of an optical instrument (e.g., spectrophotometer, nephelometer, microplate reader). It is typically presented as a solid/sealed cuvette or a multiple-well test plate and may include filters or a light source emitted at specified wavelengths. Verification/optimization functions may include optical density (absorbance/scattering), fluorescent or luminescent accuracy, linearity, repeatability and/or limit of detection readings, and the ability to verify optical alignment and precision of the instrument or analyser. This is a reusable device.