Measuring the velocity of light is now simple-both conceptually and practically. The compact unit's fast LED emits 20ns pulses of red light at a repeat rate of 40kHz, and its beam splitter allows the light pulses to be emitted both horizontally and vertically with equal intensity, forming a reference beam and a test beam for absolute time and distance measurements. Using triple mirror plates to reflect the beams back on themselves simplifies the optical setup greatly; only a single lens is needed to focus an image of the LED source onto the test beam's distant triple mirror plate.

The returning beams of pulses are deflected by the beam splitter to fall on a photodiode, where the amplified output is displayed on a regular oscilloscope. The clearly visible difference in the arrival times of the reference and test pulses represents the time the test beam pulses took to make the round trip to the distant triple mirror plate. With the reference mirror plate a few centimeters from the LED and the test beam plate 10m away, the time difference is about 65ns, which can be displayed on a regular dual beam oscilloscope. The apparatus also puts out a 10MHz signal for calibrating the oscilloscope timebase.

Relative measurements may also be made without the reference beam by moving the triple mirror or introducing an item of higher refractive index into the beam and noting the change in the pulse arrival time. Requires an oscilloscope, an optical bench with two fixed carriages, and stand for setup.