Range independence of signal in variable focus remote Raman spectrometry.

Sensitivity often restricts remote Raman spectrometers to operate at ranges inside their very large hyper-focal distances. Operation against a daylit background then requires minimizing the system etendue. This can be done by focusing the system at the range of interest; the depth of field is then etendue limited and varies with the square of the range. For the usual case of depth of field limited sampling depth, this cancels the inverse square range effect on signal strength, making the signal range independent (if atmospheric absorption is neglected). This concept is proven analytically and experimentally.