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| Measuring attenuation of light: Limiting acceptance angle of the detector with a two-aperture system | Parent topic |
Fig. 2. A system of two coaxial apertures of the same radius can be used to limit the acceptance angle in a light attenuation measurement. Abbreviations: AW and AD apertures, both of radius r, B - collimated light beam, D – detector with a response insensitive to the direction of radiation from within the left-hand hemisphere, S – sample, Vi – elementary scattering volumes of the sample. For simplicity, we assume that the refractive indices of the media on the sample and detector sides of the window W are the same, otherwise refraction at the window needs to be taken into account. Note that the acceptance angle, θ, of the detector for a scattering volume inside the beam-illuminated part of the sample is defined each by the distance of the volume from aperture AD and the diameter of aperture AD. For volume V1, this angle is approximately equal to atan(2r / D1) ~ 2r / D1 [radians]. The acceptance angle is graphically depicted by the outermost rays emanating from a scattering volume which can reach the detector. This volume, as any other elementary volume, scatters light in all other directions too but that light is prevented by the apertures from reaching the detector. For scattering volumes located outside the beam (for example, V2), i.e. for multiple-scattered light, the detector acceptance angle is determined by the diameters of both apertures and the distance, D, between them, in addition to the effect of the position of the scattering volume. The acceptance angle for those elementary volumes is generally smaller than that for scattering volumes inside the beam. It follows that AW reduces the contribution of the multiple-scattered light (Swanson NL et al 1999). A short arrow from V1 to V2 represents schematically illumination of volume V2. In reality, V2 receives illumination from the entire sample.
See also a lens-based system limiting the acceptance angle (Fig. 1).
| CITATION: Swanson N. L., Jonasz M. 2007. Measuring attenuation of light: Limiting acceptance angle of the detector with a two-aperture system (www.tpdsci.com/Tpc/AtnCfMsAcptAngLimFigTA.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 21-Nov-2007 Modified: 20-Jan-2008 Peer-reviewed: 22-Dec-2007 |
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