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Architecture Terminology Lighting

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Light is a form of electromagnetic radiation. It is similar in nature and
behaviour to radio waves at one end of the frequency spectrum and
X-rays at the other.


Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye.


Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2). A non-SI term for the same unit is the nit. The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.

Lambertian reflectance

Lambertian reflectance is the property that defines an ideal "matte" or diffusely reflecting surface. The apparent brightness of a Lambertian surface to an observer is the same regardless of the observer's angle of view.


Daylight factor

In architecture, a daylight factor is the ratio of the light level inside a structure to the light level outside the structure. It is defined as: DF = (Ei / Eo) x 100%


SC – Sky Component

ERC – Exterior Reflectance Component

IRC – Interior Reflectance Component


Daylighting is generally broken into two categories: side-lighting through windows in walls, and top-lighting through skylights in roofs and clerestory windows very high up on walls.


Glare is a result of excessive contrast, or of light coming from the wrong direction. The contrast between the bright outside environment viewed through a window and the darkness of the interior space creates glare.


Luminous intensity

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on theluminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

candela (cd), a measurement of the magnitude
of luminance or light reflected from a surface, i.e. cd/m2


Luminous flux

Luminous flux is the quantity of the energy of the light emitted per second in all directions. The unit of luminous flux is lumen (lm). One lumen is the luminous flux of the uniform point light source that has luminous intensity of 1 candela and is contained in one unit of spatial angle (or 1 steradian).

lumen (lm), a measurement of the visible light energy emitted


The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre.

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Luminosity Function

A luminosity function or luminous efficiency function describes the average spectral sensitivity of human visual perception of brightness. It is based on subjective judgments of which of a pair of different-colored lights is brighter, to describe relative sensitivity to light of different wavelengths.

Spectral Sensitivity

Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.


Illuminance † Lumens per square metre (lm/m2) or lux (lx), a measure
of the light falling on a surface.

Illumination produced from a light source perpendicular to the surface:
E I d 2
E illumination on surface (Iux)
I Illumination intensity from source (candela or cd)
d distance from light source to surface (metre or m).


Efficiency of lamps in lumens per watt (lm/W). Luminous
efficacy Luminous flux output Electrical power input.

Glare index

A numerical comparison ranging from about 10 for
shaded light to about 30 for an exposed lamp. Calculated by
considering the light source size, location, luminances and effect of
its surroundings.

Luminous efficacy

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source.

Waldram diagram

A graphical ready-reckoner used to estimate the vertical sky component.


vertical sky component

A measure of the amount of skylight incident at a point on a vertical plane in relation to the unobstructed skylight incident on the horizontal plane.


Daylight harvesting.

Artificial lighting energy use can be reduced by simply installing fewer electric lights where daylight is present or by automatically dimming/switching off electric lights in response to the presence of daylight – a process known as daylight harvesting.


Light tubes

Light tubes or light pipes are physical structures used for transmitting or distributing natural or artificial light for the purpose of illumination, and are examples of optical waveguides.

In their application to daylighting, they are also often called tubular daylighting devices, sun pipes, sun scopes, or daylight pipes. Light pipes may be divided into two broad categories: hollow structures that contain the light with a total external reflection, and transparent solids that contain the light by total internal reflection. The principles governing the flow of light through these devices are those of nonimaging optics.


Prism lighting

Prism lighting is the use of prisms to improve the distribution of light in a space. It is usually used to distribute daylight, and is a form of anidolic lighting.Prism lighting was popular from its introduction in the 1890s through to the 1930s, when cheap electric lights were commonplace and prism lighting became unfashionable.

Anidolic lighting prisms

Pavement lights (UK), vault lights (US), floor lights, or sidewalk prisms are flat-topped walk-on skylights, usually set into pavement (sidewalks) or floors to let sunlight into the space below. They often use anidolic lighting prisms to throw the light sideways under the building. They declined in popularity with the advent of cheap electric lighting.

Standard overcast sky

A completely overcast sky whose luminance is defined by the Commission Internationale de l’Eclairage (CIE), used in tests of the adequacy of lighting. The light from a CIE standard overcast sky is non-directional in relation to the points of the compass. It is brightest at the zenith (vertically overhead) and gets duller towards the horizon (to 1/3 of the value at the zenith).


Average daylight factor

The ratio of the daylight incident on a horizontal plane 0.85m above the floor of a room (the working plane) as a percentage of the outdoor illuminance incident on a horizontal plane from an unobstructed standard overcast sky.

Diffuse sky radiation

Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. Also called sky radiation, diffuse skylight, or just skylight, it is the reason for the color changes of the sky.



In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.


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