Page 162 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 155

tracing the possible rays to your instrument and your detector. For a free-space-
optical system where direct sunlight and diffuse sky radiation are disturbing,
placing simple opaque “snouts” on receiver input-windows can be very effec-
tive, either with or without additional collimating optics (Fig. 7.14). Their effec-
tiveness depends on the acceptance solid angle, and hence is improved by
increasing the aspect ratio, length to width, of the tube. Small angles give better
protection, but can make the required pointing accuracy intolerable. Some
designs have used thin metal “potato-cutter” structures to give simultaneously
a large aperture and small acceptance angle in a compact structure. A low-
reflectance coating or internal apertures inside the snout will reduce light
transmission by multiple scatter. Painting the inside of the tunnels black is sur-
prisingly ineffective, mainly because even matte black paints are highly reflec-
tive at grazing incidence. For outdoor applications snouts also offer some
protection against rain, although drain-holes should then be provided. In dusty
environments the use of long aspect ratio tunnels can protect against dust build-
up. In extreme environments a flow of clean air or water pumped through and
out of the snout will be needed. Even electrostatic deposition electrodes may be
called for.
In the design of high sensitivity dark-field measurements, for example in a
compact turbidimeter (Fig. 7.15), absorbing, rather than simply reflecting the
unwanted light is a big advantage. This application requires great care if good
performance is to be obtained. You will want to use a high powered source for
best sensitivity, although the ultimate performance is likely to be defined by the
fraction of stray light, rather than the sensitivity of the photoreceiver. Try to
avoid any light impinging of the cuvette corners, using thin, accurately sized
apertures; they are points of strong scattering. Any light incident on the silica
walls out of the beam-paths should be absorbed in coatings. These are very
helpful, but difficult to maintain. The only way to minimize the power reflected

Simple "snout" Snout + collimation

"Potato-cutter" Apertures

Air-supply
Sharp-edged
apertures
Figure 7.14 Even simple snouts and apertures can greatly improve the performance of
receivers subjected to intense ambient light.

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