A focal-length-to-aperture ratio (F/D) for the illuminating laser beam in a scattering experiment where the scattered light is collected at right angles to the laser beam has been determined. This ratio allows a maximum amount of scattered light to be collected and depends on illuminating wave number ν = 1/λ, spectrograph slit height h and width w, and collecting-optics magnification M. M should be as large as practical. If is less than (1/7) (h/w), the value of F/D for maximum collected scattered flux is . If is greater than (1/7) (h/w), the collected scattered flux is maximum and constant for F/D over the range to νw/2M.
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