Daylight Metrics Explained

The daylight factor (DF) is the oldest daylight metric and still the most quoted. It is simply the indoor illuminance at a point expressed as a percentage of the unobstructed outdoor illuminance, both measured under a standard CIE overcast sky. A DF of 2% means the point receives 2% of whatever is falling outside. Rules of thumb followed: 2% for a 'daylit' room, 5% for a bright one.

Its appeal is that it is a single, stable, geometry-only number - it depends on window size, room depth and surface reflectances, nothing else. But look at what it throws away. Because the reference sky is always overcast, DF has no sun in it at all. That means it cannot tell a north-facing room from a south-facing one, cannot see the difference between Chennai and Copenhagen, and cannot register a single hour of the year. A design that scores a lovely DF can still bake in afternoon glare or sit dark for half the working day. DF answers 'how does this geometry gather a grey sky?' - a real question, but rarely the one you actually have.

Where DF still earns its keep: very fast early massing comparisons, and codes that are written around it (parts of the UK and India's residential guidance still reference DF). Just never mistake a good DF for a well-daylit building.

The metric that replaced DF for serious work is spatial daylight autonomy, written sDA 300/50%. Read the numbers literally: it is the percentage of the floor area that receives at least 300 lux for at least 50% of the occupied hours across a full year, simulated against the location's real weather file (EPW). Where DF gives one grey ratio, sDA runs an annual climate-based model, drops a sensor grid across the floor (typically 0.5 m spacing at desk height, ~0.76 m), and asks of every point: was it bright enough, often enough?

A point that clears 300 lux for half the year 'passes'; sDA is the share of grid points that pass. IES LM-83 defines the method, and LEED v4/v4.1 awards its daylight credit largely on it: roughly, sDA >= 55% earns a point and >= 75% earns more. The beauty is that the number means something a client understands - '75% of this floor is reliably daylit through the working year' - and it is directly comparable between options. Deepen the room and sDA falls; add a clerestory and it rises. That responsiveness is exactly what DF lacked.

Worked read: imagine a 10 m-deep open office. Near the south glazing every point clears 300 lux almost always; by the back wall points sit dark most of the day. If 41 of 60 grid points pass, sDA = 68% - daylit, but short of the 75% exemplary target, telling you the back third needs help (a lightshelf, a second aperture, or lighter finishes).

sDA rewards brightness, so on its own it would push you toward ever-bigger, sunnier windows - straight into glare and overheating. Its essential partner is annual sunlight exposure, ASE 1000,250: the percentage of the floor that receives at least 1000 lux of _direct_ sunlight for more than 250 occupied hours a year, with shades and blinds modelled open. It is a deliberately pessimistic 'how much of this space gets blasted by sun?' check.

High ASE is a warning flag for glare, fading and localised overheating. LM-83 and LEED treat ASE <= 10% as the goal; above that you are told to justify it or add shading. The two metrics are meant to be read together: you want high sDA and low ASE at the same time, and the tension between them is the whole daylighting design problem. A big south window pushes sDA up and ASE up; a well-sized overhang or brise-soleil can hold sDA while pulling ASE back down. That is precisely the kind of trade-off simulation is for - and why Module 6 pairs so tightly with this one.

In hot-dominated Indian climates ASE deserves extra weight: unshaded east and west glass can blow past the 250-hour threshold easily, punishing both comfort and the cooling load you will model in Module 4.

These metrics are not only a LEED formality - they map onto how you actually reason about daylight in Indian practice, and onto the codes you will meet in Module 9. India's residential energy code, the Eco Niwas Samhita, and green-rating systems like GRIHA and IGBC all reward daylight and penalise over-glazing, and they increasingly expect climate-based evidence rather than a bare daylight factor. NBC 2016 (published as SP 7:2016) and older guidance still lean on the daylight factor for residential adequacy, so you will often report both - a DF for code familiarity and sDA/ASE for the real design story. Knowing which metric a given authority wants, and never overstating what it proves, is part of the craft; statutory daylight compliance always defers to the relevant code and authority.

Climate changes which metric bites hardest. In the hot-dry zone (Jaipur, Ahmedabad) and composite zone (Delhi) the sun is strong and clear for much of the year, so ASE is usually the binding constraint - unshaded glass sails past the 250-hour direct-sun threshold and drives both glare and the cooling load you model in Module 4. In the warm-humid zone (Chennai, Mumbai) diffuse light is plentiful but overcast monsoon months pull annual illuminance down, so achieving sDA while still shading the harsh hours is the balancing act. In temperate Bangalore the job is gentler. The point is that the same target (sDA >= 55%, ASE <= 10%) demands very different sections in different zones - deep overhangs and smaller apertures in hot-dry, generous but well-shaded glazing in warm-humid. Reading the metric against the climate, not as a universal rule, is exactly the judgement this course is training.

sDA and ASE reduce the year to pass/fail counts. Useful daylight illuminance (UDI), developed by Nabil and Mardaljevic, keeps more of the story by sorting every occupied hour at every point into bins: below ~100 lux is too dark (daylight fell short), roughly 100-3000 lux is useful daylight that displaces electric light without glare, and above ~3000 lux is too bright (probable glare and heat). Some schemes split the useful band further (e.g. 100-300 supplementary, 300-3000 autonomous).

The payoff is nuance. Two rooms can share an sDA yet behave very differently - one comfortably useful most of the time, another swinging between gloom and glare. UDI surfaces that by reporting, for each point, the fraction of hours in each bin. A great daylit space maximises 'useful' hours while minimising both 'too dark' and 'too bright'. It maps neatly onto how a room actually feels through a day and is a favourite of researchers and careful practitioners, even though LEED formalised sDA/ASE rather than UDI.

A practical note on UDI's upper bound: the 3000-lux ceiling is a rule of thumb for a probable-glare level, not a hard glare metric - genuine glare is judged by luminance and DGP in Lesson 5.4, not by illuminance bins. Treat 'too bright' UDI hours as a flag that a point is likely uncomfortable and worth a proper glare check, rather than as proof of it.

All three - sDA, ASE, UDI - are climate-based daylight metrics (CBDM): they only exist because we now simulate against a full annual sky rather than one overcast snapshot. The next lesson opens up the engine (Radiance) that makes them computable; Lesson 5.3 explains the annual-sky method itself.