Displacement Follower User’s Manual
Version 1.4, April 19, 2004
IM1008
Page 12
Figure 12
The minimum target required for vertical tracking along the vertical tracking axis.
Focusing and System Noise
Once you have selected a lens system selected and placed an illuminated target in front of the tracking head, open the lens
aperture as wide as possible. Focus the target by moving either the head or the target to the proper position along the optical
axis.
The system locks onto a contrast ratio that is determined by registering the amount of light coming from both the light and
dark portions of the target. This referencing of light levels is described in
Operating Procedures
, page 16. In general, using
the brightest DC light source available produces the best signal-to-noise ratio at the output. A high intensity light aids in
“washing out,” and thus preventing the tracker from picking up, the 60 Hz modulation from any AC room lights. With
intense light, the lens aperture ordinarily can be stopped down until the appropriate light-level reading is achieved. Stopping
down the lens increases the signal-to-noise on the output at the rate of 3 dB per f-stop. Stopping down the lens also gives
rise to a better depth of field, which can give a sharper focus for targets that move about the optical axis.
If changing the lens aperture does not produce appropriate light-level readings, you must adjust the high voltage to the
photo tube.
High Voltage Adjustments
When the system is used for very small or for relatively large ranges of measurement, adjustment of the lens aperture is
often not sufficient to produce appropriate light levels. Such cases require adjustment to the high-voltage photo tube. For
low light conditions, increase the high voltage; for high light conditions, decrease it or place neutral density filters at the
lens system. Since much of the system’s noise results from the high voltage supply needed to operate the photo tube, the
lower the high voltage can be set, the better.
To adjust the high voltage while reading light levels, simply insert a small screwdriver into the hole located on the back
panel of the control unit. It is often advantageous to make this adjustment with the lens aperture set to something other than
wide open or completely closed. Then, future deviations in lighting can be handles with aperture adjustments instead of
additional high voltage changes.