UV FAQ：Stray Light

What’s stray light?

Stray light (or false 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, by limiting how dark the system can be.

Stray light is the light that appear in where it should not appear in UV-VIS System. In principle, a specific wavelength bandwidth emitted from monochromator should arrive at detector 100%, but it is not possible in the real UV-VIS optical system. There is no perfect UV-VIS system that is no stray light. But Higher UV-VIS product, stray light could be less.

Stray light can be caused by diffraction and scattering, the aging of optical parts, or the instrument may need to troubleshooting. 

How stray light affects the result and data?

Stray light is very important and key specification for UV-VIS Spectrophotometer, it is the main source of analysis error, and directly restrict the upper limit of sample concentration, it is more important for high concentration sample analysis.

Stray light can make test result against Lambert-Beer's Law, when stray light is absorbed by sample, test value will be more than actual value, if not, test values will be less than actual value.

The lower stray light, the lower analysis error, but the lower stray light, the higher price. For same level and similar price UV-VIS spectrophotometer, the lower stray light, the better performance.

According to Lambert-Beer's Law, the relational model between absorbance with stray light is:

A=-lg(T+S)+lg(1+s)

A: Absorbance

T: Transmittance

S: Stray light

For example:

When T=0%, S=0%; A=-lg0.1=1

When T=10%, S=1%; A=-lg (0.1+0.01)+lg1.01=0.9629

From this example, when transmittance is 10%, stray light is 1%, the absorbance will decrease from 1A to 0.9629A.

How to test stray light?

Fig.1 Stray light test method diagram

Cut off filter method is popular test method for stray light, use solid filter or liquid standard solution filter to test stray light.  These liquid stray light filters are still in use today and are recommended by many organizations such as European Pharmacopeia.

Solid filters’ repeatability is the same as the liquid filters and there are no dangerous chemicals to deal with and easy to carry with.

Test method and standard filter selection are shown as Fig.1, 200nm stray light: Potassium Chloride Cut off filter, 220nm stray light: Sodium Iodide Cut off filter, 340nm stray light: Sodium Nitrite Cut off filter.