A reference solution or soluble substance 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, real-time PCR instrument). Verification/optimization functions may include optical density (absorbance/scattering), fluorescent/luminescent accuracy and compensation, linearity at specified wavelengths and/or stray light interference. It may be used to verify pipetting/dispensing/aspiration/diluting functions of liquid-handling instruments, using spectrophotometry/nephelometry to determine concentration/residual volume. Solutions may be single or short-term use.