Starting with a xenon source that supplies prime UV performance, we
mount the bulb vertically, because horizontal mounting leads to sagging,
instability, and shorter arc-life. The xenon source is focused onto the
entrance-slit of the excitation monochromator with an elliptical mirror.
Besides ensuring efficient collection, the reflective surface keeps all
wavelengths focused on the slit, unlike lenses (with chromatic aberrations
that make them optimally efficient only at one wavelength).