Stray light is light in an optical system, which was not intended in the design. The light may be from the intended source, but follow paths other than intended, or it may be from a source other than the intended source. This light will often set a working limit on the dynamic range of the system; it limits the signal-to-noise ratio or contrast ratio, by limiting how dark the system can be. Ocular straylight is stray light in the human eye. (Wikipedia)

Stray light (false light) is light being detected but does not belong to the bandwidth of the chosen measuring wavelength. The effect of stray light is caused by light scattering, diffraction or malfunction of the instrument. Stray light causes a decrease in the measurable absorbance range and reduces the linear. (Hellma-Analytics)