Definition
In the context of the eye, the diaphragm is the iris — the colored, adjustable structure that opens and closes around the pupil to control how much light reaches the retina. It functions like an aperture, expanding in low light and contracting in bright light.
Plain English
The part of the eye that gets bigger or smaller to let in more or less light, the same way a camera lens opens wider in the dark and closes down in bright sun.
Context Anchor
Seen in night vision discussions when comparing the eye to a camera and explaining how the eye adjusts to changing light.
Derivation
From the Greek 'diaphragma,' meaning 'partition' or 'barrier across.' In anatomy and optics, it refers to any structure that divides a space and regulates what passes through — in the eye, what passes through is light.
Why Pilots Care
Rapid and complete adjustment of the eye's diaphragm is required for dark adaptation; any interference delays or limits night vision and raises the risk of spatial disorientation.
Analogy
Think of the aperture on a camera lens. It opens wide in dim light to gather as much as possible, and narrows in bright light to avoid being overwhelmed. The eye's diaphragm does exactly the same job.
Intuition Check
Diaphragm does not mean the breathing muscle below your lungs here. In this context, it means a light-control part that changes the size of an opening.
Example Sentence 1
When the pilot stepped from the bright ramp into the darkened hangar, the diaphragm of each eye widened to let in more light.
Example Sentence 2
A sudden bright flash from an oncoming aircraft caused the diaphragm to constrict, temporarily wiping out the pilot's ability to see the runway environment.