A focal-length-to-aperture ratio (<i>F/D</i>) 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 <i>v</i>= 1/λ, spectrograph slit height <i>h</i> and width <i>w</i>, and collecting-optics magnification <i>M. M</i> should be as large as practical. If (<i>vh</i>/14<i>M</i>)<sup>1/2</sup> is less than (1/7) (<i>h/w</i>), the value of <i>F/D</i> for maximum collected scattered flux is (<i>vh</i>/14M)<sup>1/2</sup>. If (<i>vh</i>/14<i>M</i>)<sup>1/2</sup> is greater than (1/7) (<i>h/w</i>), the collected scattered flux is maximum and constant for <i>F/D</i> over the range (<i>vh</i>/14<i>M</i>)<sup>1/2</sup> to <i>vw</i>/2<i>M</i>.
R. L. SCHWIESOW, "Optimum Illumination Geometry for Laser Raman Spectroscopy," J. Opt. Soc. Am. 59, 1285-1287 (1969)