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 laboratory 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 optical alignment and precision of the instrument/analyser. This is a reusable device.