Unit VLighting Design

Vision, Interior & Exterior

The eye that judges the light, how we measure it, and lighting the night.

≈ 35 min + studio task

In the end, the eye is the judge. This unit covers the physiology of vision — rods and cones, the three regimes, adaptation, and why the eye reads contrast, not absolute light — then how we measure light, the cultural dimension, and lighting the exterior without polluting the night sky.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Lighting Design:

CO5 · Understand

Explain the physiology of vision — rods, cones, the three regimes and adaptation.

CO5 · Understand

Explain why the eye reads luminance contrast, not absolute light level.

CO5 · Apply

Use the lux meter and the units of light measurement.

CO6 · Analyse

Discuss exterior/façade lighting, the cultural dimension and light pollution.

Rods, cones, contrast

The physiology of vision

Rods (night, colourless) and cones (day, colour) give three regimes; and the eye reads luminance contrast, not absolute lux — so contrast and uniformity matter as much as quantity.^{[4, 6]}

DiagramThe eye with rods and cones, and the three vision regimes — photopic, scotopic and mesopic

The two photoreceptors

The retina has two photoreceptors: RODS (~120 million; very sensitive, for night and peripheral vision, but no colour) and CONES (~6 million; colour and fine detail, but needing more light). This gives three regimes: PHOTOPIC (day, above ~3 cd/m²) — cones dominate, full colour and acuity, peak sensitivity 555 nm; SCOTOPIC (night, very dim) — rods only, colourless, peak 507 nm; and MESOPIC (twilight, street lighting) — both active. As light falls, peak sensitivity shifts toward blue-green (the Purkinje shift) and reds darken first.^{[4, 6]}

DiagramLuminance contrast — a brighter task against a darker background; the eye reads the difference, not the absolute level

Lux meters & dark skies

Measurement, exterior & the night

Measure with the lux meter; light the exterior for meaning and safety — but control light pollution with full-cut-off fixtures, warmer colour and shielding.^{[4, 2]}

DiagramA bare fixture spilling light as sky glow and trespass versus a full-cut-off fixture sending all its light downward

The lux meter

The field instrument is the LUX METER (illuminance meter) — a cosine-corrected, V(λ)-filtered photocell reading ILLUMINANCE in lux on a plane. A LUMINANCE meter reads surface brightness (cd/m²) over a small angle; lab instruments (the goniophotometer, the integrating sphere) measure a source's full intensity distribution and total flux. The units recur: lumen (flux), candela (intensity), lux (illuminance), cd/m² (luminance).^[4]

The vision facts

At a glance

Aspect	One	The other
Photoreceptor	Rods: sensitive, night, no colour	Cones: colour & detail, need more light
Regime	Photopic (day) / scotopic (night)	Mesopic (twilight, street lighting)
What the eye reads	Not absolute lux —	Luminance CONTRAST (task vs background)
Measurement	Lux meter: illuminance on a plane	Luminance meter: surface brightness (cd/m²)
Exterior care	Light the building after dark	…without sky glow, trespass or glare

Vocabulary

Key terms

Rods

~120 million retinal cells — very sensitive, for night/peripheral vision, but colourless.

Cones

~6 million retinal cells — colour and fine detail, needing more light.

Photopic / scotopic / mesopic

Day (cones), night (rods, colourless) and twilight (both) vision regimes.

Adaptation

The eye adjusting to the light level — fast to brighten, slow (~30 min) to dark-adapt.

Luminance contrast

The difference between task and background brightness — what the eye actually reads.

Lux meter

A cosine-corrected, eye-weighted photocell measuring illuminance in lux on a plane.

Light pollution

Wasted exterior light — sky glow, light trespass, glare and clutter.

Full-cut-off fixture

An exterior luminaire emitting no light above horizontal — controls sky glow and trespass.

Apply it

Studio task

Take a lux meter to three spaces and record the level; then judge each not by the number but by the contrast and comfort. Finally, sketch a façade-lighting idea that lights the building, not the sky.

Check your understanding

Self-assessment

1. In dim (scotopic) conditions, vision is —

2. The eye judges legibility chiefly by —

3. A 'full-cut-off' exterior fixture helps by —

In a nutshell

Recap

The retina's rods (night, colourless) and cones (day, colour) give three regimes — photopic, scotopic and mesopic — with slow dark adaptation.

The eye reads luminance CONTRAST, not absolute lux — so contrast and uniformity matter as much as quantity; more lux is not always better.

Measure with the lux meter (illuminance) and luminance meter (brightness); the units are lumen, candela, lux and cd/m².

Light the exterior — façade, landscape, street — for meaning and safety, but control light pollution with full-cut-off fixtures, warmer colour and shielding.

The evidence

References & further reading

[1]Gary Gordon, Interior Lighting for Designers (5th ed.). Wiley, 2015.
[2]Derek Phillips, Lighting Modern Buildings. Architectural Press, 2000.
[4]CIE — V(λ), vision and photometry; the IES Lighting Handbook (10th ed.). https://cie.co.at/
[6]M. David Egan & Victor Olgyay, Architectural Lighting (2nd ed.). McGraw-Hill, 2002; and the International Dark-Sky Association (light pollution).