---
title: "Short note: Daylight factor"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:daylight-factor"
source: https://www.bctengineer.com/q/semester_6/building_technology/daylight-factor
site: BCT Engineering
---

# Short note: Daylight factor

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2068 Baisakh (old course), 4 marks
- Video: [Lighting Calculations: Daylight Factor](https://www.youtube.com/watch?v=F0IwXvwqI4g) — Silkensolutions
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/daylight-factor
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Write a short note on day light factor.

## Short answer

Daylight factor (DF) is the ratio of indoor illuminance to simultaneous outdoor illuminance under overcast sky, expressed as percentage. Components: sky component (SC), externally reflected component (ERC), internally reflected component (IRC). Recommended values: 2% for houses, 3-5% for offices/classrooms, 5-10% for fine work. DF decreases with distance from window.

## Answer

**Daylight factor (DF)** is the ratio of the illuminance at a point on a working plane inside a room, due to skylight (excluding direct sunlight), to the simultaneous illuminance on a horizontal plane outdoors from an unobstructed sky, expressed as a percentage.

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

where $E_i$ is the indoor illuminance (lux) and $E_o$ is the outdoor illuminance (lux).

#### Components
$DF = SC + ERC + IRC$
- **Sky component (SC)**: direct light from the sky through the window.
- **Externally reflected component (ERC)**: light reflected from outside surfaces such as nearby buildings.
- **Internally reflected component (IRC)**: light reflected from the room surfaces.

#### Points
- It is a measure of the adequacy of daylight, and for a given point it stays constant under an overcast sky.
- It falls with distance from the window; points near the window have higher values.
- Recommended values: about 2 % for houses, 3 to 5 % for classrooms and offices, and 5 to 10 % for drawing rooms and workshops with fine work.
- Factors: window size and position, type of glass, obstructions, and the colours of room finishes.

## Check yourself

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

1. Daylight factor is the ratio of:
   A. Outdoor to indoor illuminance
   B. Indoor illuminance at a point to simultaneous outdoor unobstructed horizontal illuminance, in %
   C. Window area to floor area
   D. Direct sunlight to sky light
   Answer: B. DF = Ei/Eo x 100 %, using sky light only. (Key point: Definition)

2. DF is the sum of:
   A. SC + ERC + IRC
   B. SC + ERC only
   C. Direct sun + sky
   D. IRC - ERC
   Answer: A. Sky component + externally reflected + internally reflected components. (Key point: Components)

3. How does DF vary in the room?
   A. Constant everywhere
   B. Higher near the window, falls with distance
   C. Higher far from the window
   D. Depends only on time of day
   Answer: B. Points near the window receive more sky light, so DF falls with distance. (Key point: Variation)

4. Recommended DF for classrooms and offices is about:
   A. 0.5 %
   B. 2 %
   C. 3 to 5 %
   D. 20 %
   Answer: C. Classrooms and offices need 3 to 5 %; houses about 2 %. (Key point: Recommended values)

## 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
- [Natural/artificial lighting, daylight factor numerical](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adaylight-numerical-062p) — Building Technology, chapter 1

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