The PVHD was well received by pilots that tested it in helicopters as well as fixed wing aircraft. It was flown in F4 and A10, as well as helicopters. Initial production, however, was for the SR-71 Blackbird as an aid when refueling in the air.
The initial concept demonstration was done in Canadian military laboratories and later development was undertaken by Varian Canada in Georgetown, Ontario. In 1981, Varian sold the project to Garrett Manufacturing in Rexdale, Ontario.
In actual use, the pilot initially sets the brightness of the line so that it just disappears when looking at it with his central vision. When the line does move due to an aircraft attitude change, the peripheral vision, being more sensitive to movement, picks up the movement and the brain subconsciously registers the information, and makes use of it.
In all variants, the aircraft gyro system provides pitch and roll information for the processor, which drives the projection system to keep the line parallel to the earth horizon. The subliminal effect on the pilot's peripheral vision aids the him in retaining attitude awareness and quickly correcting the onset of the aircraft deviating from the desired attitude.
A later production version used a microprocessor to sample and process the pitch/roll gyro information and a NeNe laser as a light source for the projector. The projector consisted of X and Y axis galvanometers to scan the line across the cockpit at more than 30 times per second in the form of a vector scanned display. This type of projection technology is now commonly used in laser light shows.
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