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| Measuring attenuation of light: Limiting acceptance angle of the detector with a lens-aperture system - rejected rays | Parent topic |
Fig. 1a. Rejecting scattered light in a transmissometer by a lens-aperture system. Abbreviations: A - aperture, B - collimated light beam, D - detector with a response insensitive to the direction of radiation within left-hand side hemisphere, L - lens (note the orientation of the lens), pp - principal plane of the lens, S - sample, Vi – elementary scattering volumes of the sample (a short arrow from V1 to V2 represents schematically illumination of volume V2), W - window, θ1/2 - the acceptance angle of the detector, D. See a more detailed description of the optical schematics of this system in the top panel of Fig. 1.
Top panel: Traces of rays scattered by elementary volumes, V1 and V2, of the medium and rejected by aperture A. See traces of the accepted rays in the bottom panel of Fig. 1.
Bottom panel: By adding a second aperture, A', with a diameter equal to that of the beam and coaxial with A, just following the window, W, the contribution of multiple scattering by the parts of the sample located outside the beam-illuminated volume (for example, V2) can be reduced. For example, rays a and b, which reach detector D when aperture A' is removed, are blocked by the latter. Such a reduction, due to reducing the "active area" of detector D from that of the un-apertured lens to the diameter of aperture A', is confirmed by Monte Carlo modeling results (Piskozub J et al 2004) and implies that the beam diameter should be minimized. With the second aperture added, the acceptance angle of the detector becomes a complicated function of the scattering volume position within the sample. However, for sufficiently long sample cells, this is usually the case even without the use of aperture A' for volume elements located far away from the exit window W.
See also a lensless acceptance angle limiting system (Fig. 2).
| CITATION: Swanson N. L., Jonasz M. 2007. Measuring attenuation of light: Limiting acceptance angle of the detector with a lens-aperture system - rejected rays (www.tpdsci.com/Tpc/AtnCfMsAcptAngLimFigLA1.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 21-Nov-2007 Modified: 15-Jan-2008 Peer-reviewed: 22-Dec-2007 |
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