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