Rods, cones and the night blind spot
Retina cells:
| Cell | Gives | Light | Where |
|---|---|---|---|
| Cones | Color, fine detail | Need bright light | Crowd into fovea: small central pit, where most "looking" happens |
| Rods | Movement, dim-light detection; no color | Far more light-sensitive than cones once fully dark-adapted | None in fovea (all cones) |
Dark adaptation:
- Takes at least 20 to 30 minutes (TC AIM RPA 3.2.6.2(b)).
- Bright light destroys it: keep cockpit lights low at night (TC AIM AIR 3.5; PHAK ch.17).
Night blind spot:
- Low light: fovea (all cones, no rods) goes dead at center of vision.
- Faint object at night disappears if looked at straight on (TC AIM AIR 3.5).
- Night also loses acuity, color; may lose depth perception, size judgment (PHAK ch.17).
Off-center viewing (fixes the blind spot): look to the side of the object, not at it. Image falls on rod-rich retina outside the fovea.
| Item | Rule | Source |
|---|---|---|
| Offset | 10 to 15 degrees to one side of object | TC AIM AIR 3.5 |
| Fade time | Held image fades after 2 to 3 seconds | PHAK ch.17 |
| Shift | To a new off-center point every few seconds (rule of thumb), to keep rods refreshing | - |
Diet:
- Missed or postponed meals lower blood sugar, impairing night flight performance (PHAK ch.17).
- Low blood sugar may cause stomach contractions, distraction, breakdown in habit pattern, shortened attention span (PHAK ch.17).
- Too little vitamin A impairs night vision (TC AIM RPA 3.2.6.2). Extra vitamin A does not raise it above normal.
Trap: staring straight at a faint light or aircraft at night. Fovea is all cones, no rods: target fades. Holding an off-center gaze fades it too.
References
- PHAK ch.17, Night Vision (Night Blind Spot, Hypoglycemia and Nutritional Deficiency)
- PHAK ch.17, Night Vision (Scanning Techniques)
- TC AIM AIR 3.5
- TC AIM RPA 3.2.6.2