MCS260B
CORNERSTONE 260B MONOCHROMATORS
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8.2 F NUMBER MATCHING
The Cornerstone 260B monochromator is an F/3.9 instrument. Without matching the F/# of the
incoming light beam, stray light increases, and throughput of desired signal suffers. To illustrate this,
the photos below show a fiber optic cable placed at the entrance of a monochromator. Fibers are
typically F/2. The Oriel model 77529 may be used to match the F/# to the monochromator.
Figure 25: Mismatched F Numbers Resulting in Stray Light.
Figure 26: F Number Matcher Used with Fiber Optic Cable.
Filling the acceptance cone means focusing the beam on the slit at the correct F/number. Focusing
at F3.9 results in an overfill of the grating along vertical and horizontal diameters and underfill of the
corners. If input beam is focus at lower F/#, light is lost as it misses the grating and reflects inside
the monochromator as stray light. All the light strikes the grating if you focus at a higher F/#, but the
image of the radiant element on the slit is larger than it needs to be., with system efficiency loss.
The F/# for monochromator input is the lens or mirror focal length divided by the beam diameter.
