LESSON 9.3 — Daylighting

A. Standard Map

B. Mechanism in Words

1. Define the metric: Daylight levels change continuously. DF normalises interior illuminance against the simultaneous exterior sky illuminance, yielding a stable geometric ratio for a given room. The ratio holds approximately constant across varying sky brightness as long as the sky distribution type does not change.

2. Decompose DF into three sources: Light reaches any interior point via three distinct paths — directly from visible sky (SC), after reflection from external surfaces (ERC), and after bouncing off internal surfaces (IRC). Each component dominates at a different location in the room. SC is largest near the window; IRC increasingly dominates deep in the room where no sky is directly visible.

3. Check adequacy against IS 3646 targets: Once DF is calculated or measured, compare it to IS 3646 minimum values for the occupancy type. If the DF is below the minimum, the design must widen or raise windows, use high-reflectance interior finishes, introduce secondary daylighting devices (light shelf, clerestory), or supplement with artificial lighting.

4. Select the right daylighting device for depth: Side windows are effective to ~5 m; light shelves extend useful daylight to 5–7 m by bouncing light onto the ceiling; clerestories serve 5–8 m in single-storey halls; atria and sun pipes carry daylight to zones unreachable by perimeter windows.

5. Manage glare as a parallel requirement: Maximising DF at a point can create glare if uncontrolled direct sunlight or bright sky patches enter the line of sight. Glare control — external louvres, internal blinds, frit glass, deep reveals, light shelves — must be designed concurrently, not as an afterthought.

6. Distinguish DF from dynamic metrics: DF is an instantaneous, static ratio under an overcast sky. It does not represent the percentage of hours above a threshold illuminance (that is daylight autonomy, a climate-based metric). These are distinct — high DF does not guarantee high daylight autonomy in monsoon-heavy climates.

7. Supplement where DF is unavoidable low: Core zones, basement levels, and north-facing single-aspect spaces will have low DF regardless of design effort. Apply the lumen method to size artificial lighting for these zones, targeting the IS 3646 maintained illuminance level.

C. Core Concept Explanations

C1. Daylight Factor — Definition, Formula, and Design Sky

Daylight Factor (DF) expresses interior illuminance as a percentage of exterior simultaneous illuminance under a standard overcast (CIE) sky:

DF (%) = (E_i / E_o) × 100

• E_i = illuminance at the interior reference point (lux)
• E_o = simultaneous exterior horizontal illuminance from an unobstructed sky (lux)

Design sky for India: IS 3646 and Indian daylighting practice use E_o = 8,000 lux for an overcast sky condition. Some references use 10,000–15,000 lux for a clear sky — the question will always specify which value to use.

Inverse formula for illuminance from DF:

E_i = DF × E_o / 100

Example: DF = 2.5%, E_o = 8,000 lux → E_i = 2.5 × 8,000 / 100 = 200 lux

DF components — additive:

DF = SC + ERC + IRC

Design implications:
– Increasing ceiling and wall reflectance raises IRC — most cost-effective way to improve DF at the rear of a room.
– Raising the window head height increases SC reach deeper into the room (sky visible from further back).
– ERC is largely fixed by the external context and cannot be easily designed in.

C2. Target DF by Space Type — NBC / IS Practice

IS 3646 specifies both minimum DF and recommended maintained illuminance. The two are related but independently specified — a space may meet the lux target under artificial light without meeting the DF requirement for daylighting adequacy.

C3. Window-to-Floor Area Ratio — Recommended Ranges

The Window-to-Floor Area Ratio (WFR) is the ratio of glazed opening area to floor area of the room. Unlike WWR (window-to-wall), WFR directly links aperture size to the volume of space it must illuminate.

NBC 2016 guidance:
– Habitable rooms (residential): minimum WFR = 1/10 (10%) of floor area for light and ventilation.
– Schools and institutional buildings: WFR ≥ 1/6 to 1/5 of floor area recommended.
– Deep commercial floor plates: WFR required on perimeter only; core zones must rely on artificial light or secondary daylight devices.

Why DF is non-linear with WFR:

Doubling window area does not double DF at all points in the room. The relationship is non-linear for three reasons:
1. SC diminishes with depth faster than linearly — the solid angle subtended by the sky shrinks rapidly with distance from the window.
2. Obstruction effects — as windows are enlarged, the frame-to-glass ratio, glazing transmittance, and external obstruction angle all interact non-linearly.
3. IRC contribution plateaus — once walls and ceiling are well-lit by daylight near the window, additional window area adds diminishing IRC increment deep in the room.

Practical consequence: A 20% WFR does not guarantee twice the DF of a 10% WFR at the rear of a 12 m deep room. The depth-to-height ratio of the room has more influence on rear-desk DF than window width alone. Room depth ≤ 2.5 × ceiling height is the rule of thumb for effective daylighting with side windows.

C4. Daylight Devices — Light Shelves, Clerestories, Sun Pipes, Atrium Wells

Light shelf — dual benefit clarification:
The external portion shades the lower window zone from direct sun (reduces glare). The internal portion reflects high-angle sky light onto the ceiling (extends IRC reach). Both effects operate simultaneously and are the primary reason light shelves are preferred over simple tinted glass for deep offices in tropical climates.

Atrium W:H ratio:
– W:H ≥ 1:1 (wide, shallow atrium): floors near the base still receive useful diffuse sky light.
– W:H < 0.3:1 (narrow, deep atrium): lower floors receive very little daylight; the atrium functions mainly as a circulation void.
– Light-coloured atrium walls (reflectance ≥ 0.5) multiply effective reach by increasing IRC contribution to lower floors.

C5. Glare — Disability vs Discomfort; Control Strategies

Glare occurs when a source in the visual field is significantly brighter than the adaptation level of the eye, degrading visual performance or causing discomfort.

Two types:

Glare metrics (awareness):
– Daylight Glare Index (DGI): Used for daylighting; scale 16 (just perceptible) to 28 (intolerable).
– Unified Glare Rating (UGR): Used for artificial lighting; scale 10–30; maximum 19 for offices.

Glare sources in daylighted spaces:
– Unshielded bright sky visible from a seated position.
– Direct sunlight patches on desks.
– High-contrast transition between a bright window and dark adjacent wall.
– Computer screen reflections of bright windows.

Glare control strategies — ranked by preference:

C6. IS 3646 Illuminance Table — Recommended Maintained Levels

IS 3646 (Parts 1–3) — Code of Practice for Interior Illumination — specifies recommended maintained illuminance for building occupancies. These are the artificial + daylight combined levels that must be maintained at the working plane throughout the service life of the lighting installation.

Key exam values to memorise:

D. Worked Numericals and Parameter Tables

D1. Worked NAT — DF Calculation with IS 3646 Adequacy Check

Problem: An office room in Mumbai is 10 m deep, 6 m wide, 3 m floor-to-ceiling height, with a single window on one short wall (2.4 m wide × 1.8 m high, sill at 0.9 m above floor). At a workstation located 7 m from the window, the following components are measured: SC = 0.8%, ERC = 0.2%, IRC = 1.4%. Design sky illuminance = 8,000 lux.

(a) Calculate the Daylight Factor at the 7 m workstation.
(b) Calculate the interior illuminance at that point under design sky.
(c) Does this meet the IS 3646 minimum DF for a general office? If not, state one design change that would increase IRC.
(d) What maintained illuminance (lux) would need to be provided by supplementary artificial lighting if the actual maintained illuminance target is 300 lux?

Solution:

(a) DF calculation:

DF = SC + ERC + IRC = 0.8 + 0.2 + 1.4 = 2.4%

(b) Interior illuminance under design sky:

E_i = DF × E_o / 100 = 2.4 × 8,000 / 100 = 192 lux

(c) IS 3646 adequacy check:

IS 3646 minimum DF for general office = 2%

Calculated DF = 2.4% ≥ 2% → DF requirement is satisfied.

However, illuminance of 192 lux is below the IS 3646 recommended maintained illuminance of 300 lux for offices. The room meets the daylighting ratio requirement but not the absolute illuminance target under design sky.

Design change to increase IRC: Paint ceiling and upper walls in high-reflectance finish (reflectance ≥ 0.70). IRC is directly proportional to the mean surface reflectance of the room. Increasing reflectance from a typical 0.4–0.5 to 0.7–0.8 can increase IRC by 40–80% at deep workstations.

(d) Supplementary artificial lighting required:

Required illuminance: 300 lux
Daylight-provided illuminance: 192 lux
Supplementary artificial lighting needed: 300 − 192 = 108 lux

D2. DF Component Behaviour — Reference Table

Note: Values are indicative ranges for a standard side-window room; actual values depend on window size, room geometry, and surface reflectances.

E. Common Confusions

• DF = percentage of time above threshold: DF is an instantaneous ratio under an overcast sky condition — it says nothing about the temporal distribution of daylight. “Daylight autonomy” (the percentage of occupied hours when daylight alone meets the illuminance target) is a separate dynamic metric requiring climate data and simulation.

• Higher WWR always improves DF at all points: Enlarging windows raises SC near the window but the IRC contribution deep in the room is limited by surface reflectances, room depth, and ceiling height, not just window area. Doubling WWR rarely doubles rear-of-room DF.

• Lux = lumens: Lux (lm/m²) is illuminance — flux per unit area at the receiving surface. Lumens (lm) is the total luminous flux emitted by a source. A 1,000 lm lamp spread over 10 m² gives 100 lux; the same lamp focused on 1 m² gives 1,000 lux. The quantities are different and not interchangeable.

• Light shelf increases total visible light in the room: A light shelf does not increase total light — it redirects it. The lower window zone is shaded (SC reduced there), while high-angle sky light is bounced to the ceiling (IRC increased deep in the room). The net effect redistributes light; the total may be similar or slightly reduced.

• Glare is always caused by high illuminance: High illuminance alone does not cause glare. Glare is caused by high luminance contrast — a bright source against a much darker surround. A 500 lux classroom lit uniformly with diffuse overhead lighting is glare-free; a 200 lux room with an unshaded bright window can produce severe discomfort glare.

• Frit glass eliminates glare: Frit reduces peak luminance by diffusing the bright sky patch, but it does not eliminate glare — it softens and spreads it. For full glare control, angular cutoff (louvres, deep reveals) is more reliable than diffusion alone.

F. Exam Traps

G. Answer-Writing Cues

H. PYQ Linkage Note

I. Mini-Check — Lesson 9.3

Q1. (NAT) A library reading room is 12 m deep, 8 m wide, with clerestory windows on one long wall. At a reading table 9 m from the window wall, the components are: SC = 0.4%, ERC = 0.1%, IRC = 1.6%. Design sky illuminance = 8,000 lux. Calculate: (a) the Daylight Factor at the reading table, and (b) the interior illuminance under design sky.

Answer:

(a) DF = SC + ERC + IRC = 0.4 + 0.1 + 1.6 = 2.1%

(b) E_i = DF × E_o / 100 = 2.1 × 8,000 / 100 = 168 lux

IS 3646 recommends 500 lux for library reading areas. The daylight level of 168 lux is well below target — artificial supplementary lighting of at least 332 lux is required. The DF of 2.1% meets the general office minimum of 2%, but library reading is a more demanding task.

Q2. (MSQ) Which of the following statements about the Daylight Factor (DF) are correct? Select ALL that apply.

(A) DF is a ratio of interior to exterior illuminance expressed as a percentage under an overcast sky condition.
(B) A high DF guarantees that a room will achieve its target illuminance for the majority of occupied hours throughout the year.
(C) The Sky Component (SC) is typically the dominant contributor to DF at locations more than 8 m from a window.
(D) DF = SC + ERC + IRC, and all three components are additive.
(E) In India, the design overcast sky illuminance commonly used with IS 3646 is 8,000 lux.

Answer: (A), (D), (E)

(B) is wrong — high DF does not guarantee temporal adequacy; daylight autonomy requires dynamic climate simulation. (C) is wrong — beyond 6–8 m, SC approaches zero and IRC dominates, not SC.)

Q3. (MCQ) An architect installs a light shelf at window head height (2.4 m sill, 2.8 m head, light shelf at 2.2 m) in a deep open-plan office. What is the primary effect on daylighting distribution in the room?

(A) Total luminous flux entering the room increases because the shelf reflects additional external light inward.
(B) The lower window zone receives more direct sky light because the shelf blocks heat from the upper portion.
(C) High-angle sky light is redirected onto the ceiling, increasing IRC in the deep zone, while direct sun is shaded from the lower window, reducing glare near the window.
(D) The light shelf replaces the need for artificial lighting in the deep zone.

Answer: (C)

A light shelf redirects (does not add) light — the shelf reflects high-angle sky light from above onto the ceiling, boosting IRC in the rear of the room. Simultaneously it shades the lower window zone from direct sun, reducing glare. It does not increase total flux (A is wrong), it does not affect heat (B is wrong), and it cannot eliminate the need for artificial light in a 12 m deep room (D is wrong).

Q4. (MCQ) Which of the following correctly matches a space type with its IS 3646 recommended maintained illuminance?

(A) General office — 500 lux; hospital ward — 100 lux; corridor — 300 lux
(B) General office — 300 lux; hospital ward — 300 lux; corridor — 100 lux
(C) General office — 750 lux; hospital ward — 300 lux; corridor — 200 lux
(D) General office — 300 lux; hospital ward — 500 lux; corridor — 150 lux

Answer: (B)

IS 3646: general office = 300 lux; hospital ward (general) = 300 lux; corridor = 100 lux. Option (A) reverses ward and corridor values. Option (C) uses 750 lux which is the drawing office standard. Option (D) assigns 500 lux to hospital ward which is the examination room level, not the general ward.

Q5. (MCQ) A south-facing open-plan office in Chennai has a fully glazed curtain wall (WWR ≈ 70%) with no external shading. Occupants consistently pull down roller blinds during working hours despite sufficient daylight. What is the most likely primary reason?

(A) The office has insufficient lux levels — the glazing area is too small for adequate daylighting.
(B) Discomfort glare from unshielded bright sky and direct sunlight entering the line of sight of screen users makes the bright glazing intolerable despite high illuminance.
(C) The Daylight Factor is below the IS 3646 minimum of 2% for offices.
(D) The building does not comply with ECBC WWR limits, causing structural overheating.

Answer: (B)

Roller blinds are closed not because there is too little light (WWR 70% in Chennai provides abundant daylight) but because uncontrolled direct sun and bright sky patches produce severe discomfort and disability glare at screen workstations. High WWR without glare control defeats the daylighting intent — occupants blank out glazing entirely, adding artificial lighting load. The fix is external shading (louvres, brise-soleil) or a light shelf that shades the lower zone while admitting diffuse sky light.