Question

Which of the following is the correct order for the path of light in a HUD?

(a) CRT -> mirror -> collimating lens -> combiner glass -> pilot

(b) CRT -> collimating lens -> mirror -> combiner glass -> pilot

(c) CRT -> mirror -> combiner glass -> collimating lens -> pilot

(d) Collimating lens -> CRT -> mirror -> combiner glass -> pilot

The question was posed to me in an internship interview.

My question is taken from Head Up Displays topic in chapter Displays and Man-Machine Interaction of Avionics

Answer 1

Correct answer is (a) CRT -> mirror -> collimating lens -> combiner glass -> pilot

The best explanation: The symbols and flight parameters are generated and magnified by the CRT (Cathode Ray Tube) and is reflected almost 90° by a mirror. The reflected rays are passed through a collimating lens which produces a parallel ray of light with focus at infinity. The collimated rays are then displayed in a combiner glass which combines the outside world picture with the flight parameters generated to be viewed by the pilot.