The use of "flat" focusing devices, such as Fresnel lenses and holographic optical elements, for the passive optical separation of fluorescence and scattered light is evaluated theoretically and experimentally. Although "flat" lenses do not focus incident light isochronically, and should therefore have different spatial focusing characteristics for very short pulses (scattered light) and pseudo-continuous-wave signals (fluorescence), the optical quality of the flat lenses tested is insufficient to achieve such discrimination. Additionally, if the coherence length of the fluorescence is less than the difference in optical pathlength from extreme positions on the flat lens, fluorescence cannot be considered to be a pseudo-cw signal; this limitation constrains the potential applications of such instrumentation.
Mary K. Carroll, Richard M. Miller, Richard A. Keller, and Gary M. Hieftje, "On the Feasibility of Passive Optical Separation of Scatter and Fluorescence," Appl. Spectrosc. 46, 442-447 (1992)