Artificial Lighting
The sources, the lumen-method calculation, and glare.
When daylight runs out, the lamp takes over. This unit surveys the artificial sources — from the wasteful incandescent to the LED that has superseded most of them — then the core skill: the lumen method that sizes a scheme, and the glare a designer must control.
Learning objectives
By the end of this lesson, you will be able to — mapped to the course outcomes for Lighting Design:
Compare the artificial light sources by efficacy, colour temperature, CRI and life.
Apply the lumen method to find the average illuminance or the number of luminaires needed.
Explain the utilisation factor, maintenance factor and room index.
Distinguish discomfort from disability glare and the means to control them.
The sources
Compare the sources by efficacy, colour temperature, CRI and life — and remember CCT and CRI are independent.[1, 4]
| Source | Efficacy (lm/W) | Colour | CRI | Life |
|---|---|---|---|---|
| Incandescent (GLS) | 8–15 | ~2700 K | 100 | ~1,000 h |
| Tungsten halogen | 16–24 | ~3000 K | 100 | 2,000–4,000 h |
| Compact fluorescent (CFL) | 46–75 | 2700–6500 K | 80–85 | 8,000–15,000 h |
| Fluorescent tube (T5/T8) | 60–104 | 2700–6500 K | 70–90 | 10,000–20,000 h |
| Metal halide | 65–115 | 3000–4500 K | 60–95 | 6,000–20,000 h |
| High-pressure sodium | 85–150 | ~2000 K | 20–25 | 24,000 h+ |
| LED (current) | 80–200+ | 1800–6500 K (tunable) | 80–95+ | 25,000–50,000 h+ |
LED figures are a 2026 snapshot and keep improving — quote ranges with a date.
From the filament to the LED
Incandescent (8–15 lm/W, CRI 100, ~1,000 h) wastes most energy as heat; halogen is a crisper version. Fluorescent and CFL (46–104 lm/W) brought efficiency; sodium (HPS up to 150 lm/W but CRI ~20 — the amber street lamp) and metal-halide are the HID family. LED (today ~80–200+ lm/W, tunable colour, 25,000–50,000+ h) has now superseded most of them on efficacy, life and control. FLAG: lamp figures are version-volatile — quote ranges, with a date.[1, 4]
The lumen method & glare
The lumen method E = (N·n·Φ·UF·MF)/A sizes a scheme; UF comes from the room index and MF allows for ageing and dirt. Then control glare — discomfort and disability.[3, 4]
Lumen method · how many luminaires?
Luminaires needed
10
N = (E · A) / (Φ · UF · MF) = 10.0 → round up
That delivers ≈ 400 lux maintained (target 400).
An average over the working plane — use point-by-point methods for non-uniform schemes, and read UF from the room index.
E = (N·n·Φ·UF·MF) / A
The standard average-illuminance design formula: E = (N × n × Φ × UF × MF) / A, where E is the maintained illuminance (lux), N the number of luminaires, n the lamps per luminaire, Φ the lumens per lamp, UF the utilisation factor, MF the maintenance factor and A the floor area (m²). Rearranged to size a scheme: N = (E × A) / (n × Φ × UF × MF). It gives an AVERAGE level over the working plane — use point-by-point methods for non-uniform schemes.[3, 1]
At a glance
| Aspect | One | The other |
|---|---|---|
| Efficacy | Incandescent: 8–15 lm/W (mostly heat) | LED: 80–200+ lm/W (and long life) |
| Colour temperature | Warm = LOW Kelvin (2700 K, amber) | Cool = HIGH Kelvin (6500 K, blue) |
| CCT vs CRI | CCT: the hue of the white | CRI: colour fidelity — independent of CCT |
| Lumen-method factors | UF: fraction reaching the plane (room/luminaire) | MF: ageing + dirt (~0.6–0.8) |
| Glare | Discomfort: annoys (UGR ≤ 19 in offices) | Disability: actually reduces what you see |
Key terms
Lumens produced per electrical watt (lm/W) — the efficiency of a light source.
The hue of white light in Kelvin — warm is LOW (2700 K), cool is HIGH (6500 K).
Colour Rendering Index (0–100) — colour fidelity vs a reference; independent of CCT.
E = (N·n·Φ·UF·MF)/A — the average-illuminance design calculation.
The fraction of lamp lumens reaching the working plane (room shape, reflectance, luminaire).
Allowance for lamp ageing and dirt (~0.6–0.8) — design to the maintained level.
K = (L·W) / [Hm·(L+W)] — selects the utilisation factor from a table.
The measure of discomfort glare; offices are held to UGR ≤ 19.
Studio task
Use the calculator above to light a 40 m² classroom to 300 lux, then to a drawing office at 600 lux — note how the luminaire count changes. Then pick a source for each and justify it by efficacy and CRI.
Self-assessment
1. In the lumen method E = (N·n·Φ·UF·MF)/A, the maintenance factor (MF) accounts for —
2. On the Kelvin scale, a 'warm' light corresponds to —
3. Discomfort glare differs from disability glare because discomfort glare —
Recap
References & further reading
- [1]Gary Gordon, Interior Lighting for Designers (5th ed.). Wiley, 2015.
- [3]IS 3646 (Part 1) — Code of Practice for Interior Illumination (recommended lux & glare). BIS. https://law.resource.org/pub/in/bis/S05/is.3646.1.1992.pdf
- [4]IES — The Lighting Handbook (10th ed.); CIE — UGR. Illuminating Engineering Society. https://www.ies.org/
Further reading
- Gary Gordon, Interior Lighting for Designers. Wiley.
- Mark Karlen, James Benya & Christina Spangler, Lighting Design Basics. Wiley.
- IES, The Lighting Handbook. Illuminating Engineering Society.
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.