Rods are 1000× more sensitive than cones, but see no colour. Cones come in three flavours (S, M, L) tuned to blue, green, and red. The split decides whether 4000K or 6500K is brighter at night (it isn't always 6500K), why peripheral motion startles you, and why football pitches use cool light.
Four ideas to carry into the next specification conversation you have.
Highly sensitive to motion and dim light. Peak sensitivity at 507 nm (blue-green). No colour discrimination.
Concentrated at the fovea. Three types, S (420 nm blue), M (530 nm green), L (560 nm red). Need bright light to work.
Twilight and street lighting fall in the mesopic range, both rods and cones partially active. CIE S 026 mesopic photometry corrects for this.
As light dims, blue objects look brighter than red ones (rod sensitivity shifts blue). Why street lights at cool CCT look brighter at the same lumens.
Two photoreceptor types, completely different jobs.
| Property | Rods | Cones |
|---|---|---|
| Count | 120 million | 6 million |
| Location | Peripheral retina | Central / fovea |
| Peak λ | 507 nm (blue-green) | S 420, M 530, L 560 nm |
| Sensitivity | Very high (1 photon!) | Lower (~1000 photons) |
| Colour | None, monochrome | Trichromatic |
| Speed | Slow (sluggish) | Fast (resolves detail) |
| Function | Night vision, motion | Day vision, colour, detail |
Five projects where ignoring the photoreceptor split costs money or safety.
Most streets sit at 0.1–3 cd/m², mesopic range. CIE S 026 says cooler CCT (4000K+) gives more perceived brightness at the same lumens because rods are still active. Real fuel savings.
FIFA Class IV pitches use 5000–5700K not for aesthetics but because cones see fast-moving white balls better against green grass under cool light.
Driving from bright daylight (10,000 cd/m²) into a 200 cd/m² tunnel triggers rod-cone switching that takes seconds. CIE 88 specifies long graded transition zones.
Emergency exit signs are designed to catch peripheral (rod) vision. Green at high contrast against dark surroundings. The reason ISO 7010 green works in smoke.
Macular degeneration kills central (cone) vision. Peripheral vision is rod-driven and largely preserved. Design for periphery in senior facilities, high contrast paths, low glare.
Our streetlight specifications use CIE S 026 mesopic photometry, meaning fewer fixtures deliver more perceived brightness. Real savings on light pollution and energy.
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