Editing Ultraviolet–visible spectroscopy (section)

====Stray light====
{{See also|Stray light}}
Stray light<ref>{{Cite web |date=2015-06-12 |title=What is Stray light and how it is monitored? |url=https://lab-training.com/what-is-stray-light-and-how-it-is-monitored/ |access-date= |language=en-US}}</ref> in a UV spectrophotometer is any light that reaches its detector that is not of the wavelength selected by the monochromator. This can be caused, for instance, by scattering of light within the instrument, or by reflections from optical surfaces.

Stray light can cause significant errors in absorbance measurements, especially at high absorbances, because the stray light will be added to the signal detected by the detector, even though it is not part of the actually selected wavelength. The result is that the measured and reported absorbance will be lower than the actual absorbance of the sample.

The stray light is an important factor, as it determines the ''purity'' of the light used for the analysis. The most important factor affecting it is the [[Monochromator#Stray light|''stray light'' level of the monochromator]].<ref name="dev" />

Typically a detector used in a UV-VIS spectrophotometer is broadband; it responds to all the light that reaches it. If a significant amount of the light passed through the sample contains wavelengths that have much lower extinction coefficients than the nominal one, the instrument will report an incorrectly low absorbance. Any instrument will reach a point where an increase in sample concentration will not result in an increase in the reported absorbance, because the detector is simply responding to the stray light. In practice the concentration of the sample or the optical path length must be adjusted to place the unknown absorbance within a range that is valid for the instrument. Sometimes an empirical calibration function is developed, using known concentrations of the sample, to allow measurements into the region where the instrument is becoming non-linear.

As a rough guide, an instrument with a single monochromator would typically have a stray light level corresponding to about 3 Absorbance Units (AU), which would make measurements above about 2 AU problematic. A more complex instrument with a [[Monochromator#Double monochromators|double monochromator]] would have a stray light level corresponding to about 6 AU, which would therefore allow measuring a much wider absorbance range.