Stray Light

Polychromatic radiation that enters a spectrometer goes through a number of optical elements before reaching the detector. Since all elements that comprise the spectrometer are not perfect, the radiation that reach the detector and outlines the operational spectral range can be contaminated with stray light.

Stray light is a critical parameter for a spectrometer and can be defined as all radiation of undesired wavelengths that reach and trigger a signal at the detector. Stray light can be an issue throughout the entire spectral range of a spectrophotometer, however it becomes very strong when we approach the UV range (190-350 nm). 

When using UV/Vis spectroscopy, stray light introduces an error in the measured absorbance value. The absorbance readings begin to decrease due to the increase in stray light and leads to a negative deviation from the Beer-Lambert’s law. Stray light can be caused by ambient light scattering, high intensity light sources, scattered light from imperfect optical elements (quality of the mirrors and diffraction gratings) or reflections of the non-optical elements of the spectrometer.

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Irregularities of the reflecting surfaces are among the most common types of imperfections in optical elements. The process by which each these elements are produced has a strong influence in generating low stray light levels in the spectrometer. Holographic gratings generate significantly less stray light than ruled gratings, because the process used to produce these types of gratings is not subject to mechanical irregularities. In spectroscopic applications such as Raman spectroscopy, where signal-to-noise is critical, holographic grating are generally selected.

In order to strongly reduce the stray light, Sarspec engineers have simultaneously optimized the optical and mechanical designs of the spectrometers. By using a symmetrical Czerny-Turner configuration and high quality optical and non-optical elements, it is possible to reduce stray light levels to less than 0.1%, over the entire dynamic range of the Sarspec spectrometer.