Daylighting & the Nature of Light
What light is, the four quantities that measure it, and the daylight factor.
Lighting design begins with physics. Light is electromagnetic radiation in a narrow visible band; the eye weights it, turning watts into lumens. Learn the four photometric quantities and the two laws that govern how much light lands where — then daylight: the daylight factor, a sky-independent ratio.
Learning objectives
By the end of this lesson, you will be able to — mapped to the course outcomes for Lighting Design:
Describe light as electromagnetic radiation and the visible spectrum.
Define the photometric quantities — flux, intensity, illuminance, luminance — and their units.
Apply the inverse-square and cosine laws to find illuminance from a source.
Explain the daylight factor, its three components and the standard overcast sky.
What light is, and how we measure it
Four quantities — flux (lumen), intensity (candela), illuminance (lux), luminance (cd/m²) — and two laws (inverse-square and cosine) underlie every calculation.[4, 6]
A narrow visible band
Light is electromagnetic radiation; the human eye sees only a narrow band — roughly 380–740 nm, from violet through green to red. The eye's daytime sensitivity (the V(λ) curve) peaks at 555 nm (yellow-green), which is why photometry weights raw radiant watts by this curve to give the lumen-based quantities. Radiometric units measure raw energy (watts); photometric units measure only what the eye sees.[4, 6]
Daylight & the daylight factor
The daylight factor is a sky-independent ratio (indoor ÷ outdoor) under a standard overcast sky, summing the sky, externally reflected and internally reflected components.[3, 4]
A sky-independent ratio
The DAYLIGHT FACTOR (DF) is the ratio of indoor illuminance at a point to the simultaneous unobstructed outdoor illuminance under a standard overcast sky, as a percentage: DF = (E indoor / E outdoor) × 100%. Because it is a RATIO under a fixed sky, it is independent of actual sky brightness, time of day and orientation — that repeatability is the whole point. Roughly: DF ~2% is adequate, ~5% is well-daylit.[3, 4]
At a glance
| Aspect | One | The other |
|---|---|---|
| Light emitted vs landing | Flux (lumen): emitted by the source | Illuminance (lux): landing on a surface |
| Direction vs brightness | Intensity (candela): flux in one direction | Luminance (cd/m²): perceived surface brightness |
| The two laws | Inverse-square: E = I / d² | Cosine: × cos θ for angled incidence |
| Daylight factor | A ratio under a fixed overcast sky | So it is time- and orientation-independent |
| DF components | Sky + externally reflected | + internally reflected (raised by light surfaces) |
Key terms
The total eye-weighted light a source emits — the lamp's rating, in lumens (lm).
Flux per unit solid angle in one direction — the SI base unit (cd = lm/sr).
Light landing on a surface, lm/m² — what a lux meter reads (lx).
The brightness a surface appears to have as light leaves it toward the eye.
E = I / d² — illuminance falls with the square of distance from a point source.
Illuminance reduces by cos θ when light strikes a surface at angle θ from the normal.
Indoor ÷ outdoor illuminance under a standard overcast sky, as a % — sky-independent.
The CIE fixed-gradient sky against which daylighting is designed for the dullest realistic day.
Studio task
For a point 3 m from a 1000 cd source, find the illuminance directly below (inverse-square), then at 45° (add the cosine). Then sketch a room section and mark the three daylight-factor components reaching a desk at the back.
Self-assessment
1. Which photometric quantity does a lux meter measure?
2. By the inverse-square law, doubling the distance from a point source changes the illuminance to —
3. The daylight factor is independent of the actual sky brightness because it is —
Recap
References & further reading
- [1]Gary Gordon, Interior Lighting for Designers (5th ed.). Hoboken: Wiley, 2015.
- [2]Derek Phillips, Lighting Modern Buildings. Oxford: Architectural Press, 2000.
- [3]IS 3646 (Part 1) — Code of Practice for Interior Illumination. Bureau of Indian Standards. https://law.resource.org/pub/in/bis/S05/is.3646.1.1992.pdf
- [4]CIE — photometric quantities, V(λ), the standard overcast sky; and the IES Lighting Handbook (10th ed.). https://cie.co.at/
- [6]M. David Egan & Victor Olgyay, Architectural Lighting (2nd ed.). McGraw-Hill, 2002.
Further reading
- Gary Gordon, Interior Lighting for Designers. Wiley.
- M. David Egan, Architectural Lighting. McGraw-Hill.
- Derek Phillips, Lighting Modern Buildings. Architectural Press.
Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.