BS ISO 19986:2020 Lasers and laser-related equipment — Test method for angle resolved scattering. 6 Test method 6.1 Principle The fundamental operating principle of the instrument for ARS measurements (see Figure 2) is based on measuring the radiant power scattered from the specimen as a function of the scattering angles using a detector with a small but finite aperture that is scanned along a defined trajectory on or within the full scattering sphere about the specimen. The simplest type of measurement is a detector scan with the plane of incidence (in-plane scan). The angle of incidence is kept constant during a single measurement. The ARS is either measured by rotating the detector with respect to the specimen and the illumination system, or, alternatively, the detector is fixed and the illumination system and the specimen are rotated with respect to the detector. 6.2 Measurement arrangement and test equipment 6.2.1 General The instrument employed for the determination of ARS is divided into four functional sections which are described in detail in Figure 2.If radiant power fluctuations or drifts of the radiation source of more than 5 % are expected over the time scale comprising calibration and measurements, an additional reference channel shall be implemented into the setup shown in Figure 2. The back reflection of a tilted superpolished substrate of a material transparent in the relevant spectral region of the laser and positioned before the spatial filter as well as an appropriate detector, preferably similar to the actual scatter detector, should be used for this purpose. 6.2.2 Environmental conditions The instrument should be located in a clean room environment or under a laminar flow system to prevent scattering from dust particles. Some applications, in particular in the UV range, require operation in vacuum or specific gas atmospheres to avoid absorption of radiation in air or to suppress Rayleigh scattering from air molecules. The environmental conditions during measurements shall be documented. 6.2.3 Radiation source Excellent beam quality and sufficiently high incident radiant power are essential for ARS measurements of optical components. Therefore, lasers, laser diodes or narrow or broad band sources may be used as radiation sources. The source used shall be documented. The temporal radiant power variation of the radiation source shall be measured and documented. This comprises long-term drifts as well as short-term fluctuations. Drifts and fluctuations smaller than 5 % are below the overall uncertainty of measurement and can be...
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