The Cairn Optoscan Monochromator has both an entrance and an exit slit, each having an independently settable width. The exit slit width, defines the bandwidth ONLY ASSUMING that the light passed into the monochromator is a beam of zero size. This has a maximum of 30 nm.
In the real world the input light beam however has a nonzero size, and its width is limited by the entrance slit width. The maximum entrance slit width allows any ray of the passband’s center wavelength, within the accepted input beam, to pass the maximum exit slit width, with the rays at the extrema of the entrance width just barely getting through. That is, the maximum entrance slit width is matched to the maximum exit slit width. Since the exit slit width allows a 30 nm spread of the central ray described in the previous paragraph, the maximum entrance slit width is also given as 30 nm.
Fig. 1 shows the family of passband shapes for the case of one slit width set at 30 nm and the other set for sequentially reduced bandwidths. The dotted vertical lines show the Full Width Half Maximum, in all cases 30 nm, although the profile does change. For light-limited fluorescence imaging we have found there to be no practical benefit in setting different widths for the two slits because the improved spectral profile does not fully compensate for the reduction in intensity at the centre wavelength. If the application is not light limited then there could be an argument for having a smaller input slit so that the bandpass characteristics are tighter.
In software, the Optoscan user must set the center wavelength and the entrance and exit slit widths, or use a "bandwidth" parameter to set the two simultaneously. Where this option is available we would recommend it.
Fig. 1
