Stray light

Unwanted light that reaches the detector without following the intended spectral or imaging path.

As an already concentrated sample becomes more concentrated, its absorbance sometimes stops rising proportionally and gradually levels off. One important cause is that transmitted light at the target wavelength has become very weak while light that should not enter the channel still reaches the detector.

How unwanted light enters the detected signal

Stray light arises from scattering, reflections, higher diffraction orders of gratings, imperfect imaging or external light leakage. In a spectroscopic system, radiation outside the intended bandwidth that reaches the detector adds to the intended transmitted light. The recorded signal then no longer represents only the selected wavelength region.

Stray light is not a fixed background identical at every wavelength. Source output, monochromator transmission, detector response and sample absorption at other wavelengths all affect its contribution to the final result.

Why high absorbance is most sensitive

When absorption is weak, the intended transmitted light is much greater than the stray component, which contributes only a limited fraction. As absorbance rises, the target light decreases exponentially and stray light becomes a rapidly increasing fraction of the total signal. The instrument incorrectly treats this extra light as transmission through the sample.

Typical consequences are overestimated apparent transmittance, depressed absorption peaks and a negative deviation in the absorbance–concentration relationship, limiting photometric linearity and dynamic range at high absorbance. For concentrated nucleic acids or other strongly absorbing samples, dilution or a shorter path is usually more reliable than accepting a plateaued reading.

One stray-light value cannot describe all performance

ASTM's opaque-filter method requires the test wavelength, blocking material, spectral bandwidth and calculation method to be specified. The result primarily describes the stray radiant power ratio under that combination of conditions; it cannot be extrapolated directly to every wavelength and sample independently of the method.

Lower stray light does not establish better wavelength accuracy, photometric accuracy, signal-to-noise ratio or baseline stability. These require separate evaluations. An unusual high-absorbance result can also arise from the liquid column, Blank, scattering or detector response; a flattened curve alone does not establish a unique cause.

Understanding stray light requires comparing the relative amounts of intended transmitted light and unwanted light, while keeping test conclusions within clearly defined wavelength and method conditions.