Stray light is defined as the detected light of any wavelength that lies outside the bandwidth of the selected wavelength. In the presence of stray light of intensity Is, the equation for transmittance (T) becomes T = (I + Is)/(I0 + Is) where I0 is the incident light intensity and I is the transmitted light intensity. Clearly, the lower the value of I, the more dominant becomes the stray light term and so can cause errors in the quantification of low fluorescence signals or at high levels of absorbance.
Reference: Owen T (1996) Fundamentals of modern UV-visible spectroscopy. A Primer. Hewlett Packard, 142pp.