Daylight Factor Calculator

The daylight factor is the ratio of indoor illuminance at a point to the simultaneous unobstructed outdoor horizontal illuminance, expressed as a percentage. It is deliberately defined under the CIE standard overcast sky, whose luminance is three times brighter at the zenith than at the horizon and is rotationally symmetric, so the result is independent of orientation, date and time of day. That independence is the whole point: it turns daylighting into a single geometric property of the room. The average daylight factor formula divides net glazed area times visible transmittance times maintenance factor by the floor area times one minus the square of mean surface reflectance. The reflectance term captures interior inter-reflection, which is why a room finished in white returns a materially higher figure than the same room in dark timber. The result decomposes into a sky component, an externally reflected component from facing buildings and ground, and an internally reflected component from room surfaces. Practical thresholds under BS 8206-2 are roughly 2 percent average for kitchens, 1.5 percent for living rooms and 1 percent for bedrooms, with 2 to 5 percent typical for offices. Two caveats matter. Because the metric assumes overcast sky, it systematically undervalues sunny climates and cannot flag glare or overheating. And it ignores distribution, so the depth check, room depth no greater than about 2.5 times window head height for single-sided rooms, remains a necessary companion test.

The daylight factor originated in Britain around the turn of the twentieth century, in the practical problem of proving whether a new building had unlawfully obstructed a neighbour's light. Alexander Pelham Trotter, an electrical engineer and later an electrical adviser to the Board of Trade, is generally credited with proposing the ratio of interior to exterior illuminance as an objective measure in the 1890s, since expressing indoor light as a fraction of whatever the sky happened to be delivering removed the need to agree on absolute conditions. In the 1920s Percy Waldram built this into a working method for rights-of-light litigation with the Waldram diagram, a graphical projection of visible sky from a point that let surveyors read off the sky component directly. The reference sky itself was formalised later. Parry Moon and Domina Eberle Spencer published their mathematical description of overcast sky luminance distribution in 1942, and the CIE adopted it as the standard overcast sky in 1955, giving the daylight factor a defined and reproducible basis. The Building Research Station, later the Building Research Establishment, then issued daylight protractors and design tables that made hand calculation routine for architects, and the method passed into British Standard 8206-2 and CIBSE Lighting Guide LG10. It remains embedded in planning practice and rights-of-light assessment, even as annual climate-based simulation has displaced it for energy and comfort work.