Some very accurately made corner cube reflectors send the light back in a path that is so parallel that the light receiver must actually be placed inside the light transmitter to properly detect the light being returned. Figure 4c corner cube reflectors have a wide variety of applications. Several highly accurate corner cube arrays were left on the moon during some of the Apollo moon missions in the early 1970s. Scientists have been using powerful lasers and specially modified telescopes to bounce light off of the reflectors. By measuring the time the light pulses take to make the round trip from the earth, to the moon and back, the distance can be measured down to inches.
Some systems have effective ranges of several miles.
Remember, light travels about one foot in one nanosecond, so for a round trip of 10,000 feet would cause a pulse delay of 10,000 nanoseconds or 10 microseconds. Some alarm systems also use the retro-reflective technique. Pulsed light is bounced off a distant plastic reflector and is collected by a nearby light receiver. Objects moving between the light transmitter and the reflector break the established light path, setting off the alarm. Some industrial systems also use the technique to monitor products moving down a production line.