Ocean Optics Collimation tester - Manual (Page 2)

Collimation Tester Instructions
2 CT-10000-000-05-201401
3. The maximum sensitivity of the collimation tester is achieved when the reflected angle is
108°, but sensitivity is reduced by only 1% at 90°, so the angle is not critical. Two overlapping
images will be seen on the screen, one from each face of the collimation tester. Fringes
(parallel patterns of light and dark) will be seen in the overlap region.
4. Adjust the lens spacing until the fringes are parallel to the shadow of the cursor wire of the
tester. The beam is now collimated.
Tips
If the fringes are not straight, check to see that the lenses of the beam expander are
centered, square to the beam and with the proper face towards the collimated light (if
these adjustments are appropriate). Any residual fringe curvature or wiggles indicates
aberrations or errors that cannot be focused out.
If no fringes are seen in the initial setup, it may be that the beam is so far from
collimation that the fringes are too fine to be seen. In that case, reduce the reflected
beam angle to 45°or even 30°, and move the screen accordingly. This reduces the
sensitivity of the tester. Make the initial adjustment for collimation and then return to
90° for the final tuning.
USEFUL RANGE, APERTURE, WAVELENGTH
Each size of collimation tester is designed to have 5 – 6 horizontal fringes across the aperture when
illuminated by a parallel beam. In theory,
to have at least one fringe to set against, the smallest beam
that can be tested is about 1/5 full aperture. In practice, it is found that the fringe slope changes from
positive to negative when going through collimation and that the fringe spacing decreases away from
collimation so that smaller aperture beams can be collimated by finding the center position between
two defocused positions. When testing smaller apertures, it may also be necessary to reduce the
reflected beam angle to get an appropriate amount of overlap of the two beams. The fringes only
appear in the overlap area.
The choice of 5 – 6 fringes across the aperture is a trade-off between sensitivity and utility. For
sensitivity, it is desirable to have only one full fringe over the aperture. The actual design allows an
individual tester to be usable over a range of beam sizes of at least 5 to 1. If a collimation tester is to
be used at a specific wavelength for a single aperture with critical demands on collimation, you can
either select a larger aperture such that one fringe will be seen across your aperture or a custom design
can be made for optimum sensitivity. Collimation testers are usable in wavelength over the
transmission range of the glass, that is, from about 350 nm to about 2500 nm. To observe the fringes in
the UV, a fluorescent screen may be necessary while in the IR, an image converter is suitable out to
1200 nm and an IR phosphor card can be used further out.