Stray light, or false light, is one of the effects in optical spectroscopy that are absolutely unwanted but (almost) unavoidable. The name “stray light” describes uncontrolled, vagabond light. It comprises scattered light, uncontrolled reflections, over-illumination, and the overlay of spectral orders. The origin of stray light and its impact on the desired signal is complex and diversified; the same is true for the prevention and reduction. For example, the analysis of an LED will create rather little stray light compared to illumination with strong, broadband light. The response of the receiver, detector, or experiment at the different wavelengths (available as desired and undesired signals) is also important. Consequently, stray light must be viewed application-dependent, which is why an entire chapter is dedicated to the issue; the impact of stray light is treated separately in this book’s companion volume.
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