M77781A
MS257
™ USB/RS232
MONOCHROMATOR AND SPECTROGRAPH
15
Minimizing Stray Light
Stray light is unwanted light which interferes with the light being measured or transmitted. This
results in additional noise to light measurements. At best, this results in incorrect measurements.
At worst, the signal cannot be measured. Stray light can occur for a number of reasons:
Incorrect setup
The most significant sources of stray light are light leakage. Light could enter the MS257 through
misplaced covers, as well as slit holders and detectors without adequate sealing. Make sure that
all covers, detectors and flanges are well seated and screwed down.
Overfilling
lf the F number of the source is lower than the F number of the monochromator this produces
what is referred to as overfilling. The light will spill over the sides of the entrance mirror and will
reflect around the housing resulting in stray light. The MS257 is designed to prevent overfilling.
But even so, the light has already entered the housing and must be absorbed by the baffles. It is
better to match the source F number to the monochromator's F number and prevent overfilling.
Reflections from walls
Light is dispersed by the grating; the selected wavelengths strike the exit mirror and are imaged
at the exit focal plane. Other wavelengths, zero order 'white' light, and other orders of the
diffracted light strike interior walls and must be absorbed before they bounce around and emerge
at the exit as stray light at the detector. The MS257 makes use of light traps and baffles as well
as black paint in order to minimize the amount of light which can eventually find its way to the
detector.
Gratings
Gratings are not perfect reflectors, so a certain amount of incident light is scattered and
contributes to stray light. In fact, gratings are a major source of stray light since they are within
the optical path. Imperfect rulings or ion etched blazed lines contribute to stray light. Often the
major source of stray light is dust - and sometimes even fingerprints. Keep the gratings clean by
minimizing dust and occasionally blowing them clean with dry clean air or nitrogen.
Mirrors
Mirrors gradually accumulate dust, and can have imperfections. These surfaces scale light and
add to stray light. If the bevels of the mirrors are not adequately masked, these also add to the
scatter. Often, the major source of stray light is dust - and sometimes even fingerprints. Keep the
mirrors clean by minimizing dust and occasionally blowing them clean with dry clean air or
nitrogen.
Detectors
The detector has never been considered a source of stray light, since with monochromators there
is a very small exit slit through which light would have to reenter once it has reflected off the
detector surface. Spectrographs however have wide flat focal plane detectors which readily
reflect light. The shiny surface of a silicon diode array can reflect as much as 20% of the light back
into the spectrograph. The stray light can be extremely significant in certain areas of the spectrum,
particularly if light from relatively high intensity wavelengths are finally reimaged onto areas of the
detector which are recording low intensity wavelengths. The MS257 has the focal plane angled in
such a way as to direct light reflected from the detector away from the mirrors and grating and into
'safe' baffles.
