---
title: "Natural/artificial lighting, daylight factor numerical"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:daylight-numerical-062p"
source: https://www.bctengineer.com/q/semester_6/building_technology/daylight-numerical-062p
site: BCT Engineering
---

# Natural/artificial lighting, daylight factor numerical

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2062 Poush (old course)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/daylight-numerical-062p
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

What is natural and artificial lighting in the building? Define day light factor with necessary figure and formulae. Calculate the internal illumination of your class room having day light factor 2.0% and outdoor illumination 7000 lux.

## Short answer

Natural lighting uses sun/sky through openings; artificial uses electric lamps. Daylight factor DF = (Ei/Eo)×100%. Internal illumination Ei = DF×Eo/100 = 2.0×7000/100 = 140 lux (below typical 300 lux classroom requirement).

## Answer

#### Natural lighting
Illumination of interior spaces by the light of the sun and sky through windows, skylights and ventilators. It is free, varies with time, and gives good colour rendering. Design uses window area, orientation and light shelves.

#### Artificial lighting
Illumination from man-made sources (incandescent, fluorescent, CFL, LED lamps), used when daylight is insufficient or at night. It is controllable and constant, but it uses energy and costs money.

#### Daylight factor
**Daylight factor (DF)** is the ratio of the indoor illuminance at a point to the simultaneous outdoor illuminance on an unobstructed horizontal surface, expressed as a percentage.

$$
DF = \frac{E_i}{E_o} \times 100\%
$$

```text
 Sky (outdoor Eo)
      \  \  \
   ====[window]====
       \  \
        . point P  (indoor Ei)
```

#### Calculation
Given $DF = 2.0\%$ and $E_o = 7000$ lux.

$$
E_i = \frac{DF \times E_o}{100} = \frac{2.0 \times 7000}{100} = 140\ \text{lux}
$$

**Answer: Internal illumination = 140 lux.** (This is lower than the 300 lux usually needed in a classroom, so supplementary artificial lighting or larger windows would be needed.)

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Natural lighting is:
   A. Light from lamps
   B. Illumination by sun and sky through windows, skylights and ventilators
   C. Only direct sunlight
   D. Light from LEDs
   Answer: B. It is free and varies in time. (Key point: Natural lighting)

2. Daylight factor is:
   A. Ei/Eo as a percentage
   B. Eo/Ei as a percentage
   C. Ei x Eo
   D. Ei - Eo
   Answer: A. DF = Ei/Eo x 100 %. (Key point: DF definition)

3. With DF = 2.0 % and Eo = 7000 lux, indoor illumination is:
   A. 14 lux
   B. 140 lux
   C. 350 lux
   D. 3500 lux
   Answer: B. Ei = 2.0 x 7000/100 = 140 lux. (Key point: Calculation)

4. Why is 140 lux a concern for a classroom?
   A. It exceeds the 300 lux needed
   B. It is below the roughly 300 lux needed, so artificial light or bigger windows are required
   C. Classrooms need no light
   D. It causes glare
   Answer: B. 140 lux is less than the 300 lux usually needed. (Key point: Interpretation)

## Related in Building Technology

- [Daylight factor, thermal factors, natural ventilation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adaylight-thermal-natural-vent) — Building Technology, chapter 1
- [Short note: Daylight factor](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adaylight-factor) — Building Technology, chapter 1
- [First class brickwork in 1:6 mortar numerical](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Amortar-first-class-1to6) — Building Technology, chapter 3
- [Types of window; window for more light](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awindow-types-light) — Building Technology, chapter 6

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- [Industry room 30 × 20 m LED lighting and DB](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustry-room-30x20-led) — Industrial Power Distribution and Illumination, chapter 7, asked 2×
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- [Outdoor lighting purposes and street lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aoutdoor-lighting-street-methods) — Industrial Power Distribution and Illumination, chapter 8, asked 2×