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 relationship between concentration and absorbance of the instrument.

Stray light can be a problem at any wavelength, but if energy throughput of an instrument decreases, as when moving into the UV range, the influence of increasing stray light will more and more affect the measured values.

To measure stray light, filters are needed which would ideally absorb all light of the wavelength at which the measurement is to be performed and transmit higher wavelengths (so called cut-off filters). Hellma stray light filters do not allow light with wavelengths lower than a certain wavelength to pass (cut-off wavelength). Therefore any indication of light transmission below the cut-off wavelength must be stray light.

According to European Pharmacopeia (5th revision) checking for stray light is described as follows:

„Stray light may be detected at a given wavelength with suitable filters or solutions: for example the absorbance of a 12 g/l solution of potassium chloride R in a 1 cm cell increases steeply between 220 nm and 200 nm and is greater than 2.0 at 198 nm when compared with water as compensation liquid. Suitable certified reference materials may also be used.“

To measure the amount of stray light realistically, a filter with a cut-off wavelength as near as possible to the measuring wavelength should be chosen. The following table shows the stray light filters offered by Hellma. They are suitable for stray light measurements at cut-off wavelengths of 200 nm, 259 nm and 385 nm or lower.