Definition
Optical devices that align light rays into parallel paths so that a projected image appears to be focused at infinity rather than at the surface of the display itself. In a Head-Up Display (HUD), collimators allow the pilot to view symbology overlaid on the outside world without refocusing the eyes between the instrument image and distant objects.
Plain English
A lens system that makes the HUD picture look like it is far away in the distance, so the pilot's eyes can stay focused outside the cockpit and still read the display at the same time.
Context Anchor
Seen in HUD descriptions, especially when explaining why the displayed flight information can appear over the outside scene.
Derivation
From Latin 'collimare,' a misreading of 'collineare,' meaning 'to align' or 'to make straight.' The word literally means 'to make light rays line up,' which is exactly what the device does -- it straightens diverging light into parallel rays.
Why Pilots Care
Allows pilots to read HUD information without changing eye focus from the outside environment, reducing workload and improving awareness during critical phases of flight.
Grounding Statement
In a HUD, the collimator is part of what makes nearby display light seem to come from far out in front of the aircraft.
Intuition Check
A collimator is not just a screen or projector. Its job is to shape the light so the HUD image appears at a distant focus.
Example Sentence 1
The HUD's collimators project the airspeed and altitude symbology so it appears to float out at the runway, letting the pilot keep eyes outside during the approach.
Example Sentence 2
Maintenance checks verify that the collimators keep HUD imagery aligned at infinity so the pilot does not need to refocus.