Section 6: Selected Applications & Techniques

With oil at the correct level, sufficient beam energy was transmitted across
the boundary between the housing wall and the oil to attenuate the reflected
beam so that multiple back reflections were all contained in the first gate (Fig.
60b). The lack of oil at the correct level allowed the multiple back reflections
to spill over into the second gate (Fig. 60c). Thus, the test was a fail-safe test
that signaled "no test" (no signal in the first gate), "go" (signals in the first gate
only), and "no go" (signals in both gates).