A range of cut-off filter solutions that allow stray light to be checked at a range of wavelengths from 200 nm to 390 nm. Each liquid filter is permanently sealed by heat fusion into a 10 mm high quality far UV quartz cell and a 5 mm path length cell (in the conventional 10 mm format) of the same liquid. The 5mm cell is used to provide a differential reference to the measurement of the 10 mm cell (see explanation below). Starna alkali halide stray light Certified Reference Materials are prepared in accordance with ASTM International E-387.

The peak wavelength is different on each instrument due to their different stray light characteristics, but in all three cases the absorbance maximum is greater than 0.7A, so all these instruments satisfy the USP stray light requirement.

For comparison purposes, the equivalence of the two test methods is indicated in the table:

Several of these filters are recommended for instrument qualification by Pharmacopoeias and other standardisation bodies, including the US and European Pharmacopoeias. To comply with these specifications, when measured against a water blank at the recommended wavelength the measured absorbance should be not less than given in the table:

Glass cut-off filters that allow stray light to be checked, nominally at 280 nm,360 nm and 450 nm. Note that variations from melt to melt of the glass can cause variation in these wavelengths, up to ± 10 nm,  so each Starna filter is individually certified. Sliding window covers are provided to protect the surface from damage when not in use.

Stray light, also called Stray Radiant Energy or Power, is any light reaching the detector that is outside the Spectral Band Width selected for analysis by the monochromator. It can be due to optical imperfections or stray reflections within the monochromator itself or to light leaks or other effects in the rest of the optical system. As the detector cannot discriminate between the analytical wavelength and the stray light, the stray light contributes to the detector signal and introduces an error in the measured absorption. The stray light is not absorbed even at high concentrations of the absorbing species, so its effect is a negative deviation from the linear relationship between concentration and absorbance (the Beer-Lambert law) on which most quantitative determinations are based.

Stray light is wavelength and instrument dependant. It can be present at any wavelength but is most noticeable when the energy throughput of the system at the analytical wavelength is relatively low, and any stray light will be comparatively more significant. At these wavelengths, any deterioration in the instrument optics or light source will exaggerate the apparent stray light, so it is desirable to check it on a regular basis, as it is an excellent way of monitoring the condition of the instrument optics.

The usual way of assessing stray light is to measure, at the desired analytical wavelength, a sample that totally absorbs the radiation at that wavelength, but transmits at all other wavelengths. Any light detected by the instrument is then stray light. 

Practically, the usual method is to use cut-off filters that cut off all light near the analytical wavelength and transmit at all higher wavelengths. The certified wavelength is that at which the spectrum transitions 2.0 A.  Below this wavelength, within the indicated usable range, any indication of light transmission must be stray light.