Light has a major affect on how a person perceives or experiences a space. Control of light is one of the skills that every architect must master. The science of light and lighting is an incredibly complex field; however, we will try to simplify lighting down to its core architectural concepts.
What is Light?
Light is defined as the portion of the electromagnetic spectrum that a human is able to perceive. This visible radiation generally comes from the sun and is actually quite a small portion of the electromagnetic spectrum. The human visible spectrum falls in the wavelength range of 370 nanometers to 800 nanometers.
Electromagnetic waves that have a shorter wavelength than light are ultraviolet while those that have a longer wavelength are infrared. While our eyes are not able to see these kinds of waves, we are able to see or feel their effects. Ultraviolet radiation will tan our skin while infrared light can be felt as heat.
White light is effectively an even distribution of the different wavelengths of visible light. In architecture, the standard is considered daylight at noon during the month of June. Northern light, while generally the most consistent, has more light at blue end of the spectrum. Late afternoon light has more radiation in the red wavelength.
A colored surface reflects light in the wavelength that matches its color and absorbs the other wavelengths of color. For instance, a red surface reflects mostly red light and absorbs most of the other color light. This is also true of transparent or translucent materials - red glass transmits red light and absorbs the other colors of light.
Luminous Flux is the rate at which a light source emits light. Measured in lumens, luminous flux is a measurement of the amount of light coming from a source.
Luminous Intensity is a measure of the light intensity and takes into account the luminous flux (amount of light) and the angle of distribution (amount of coverage). For instance, a spot light and a flood light may both emit 1000 lumens, but the spot light has a much higher intensity because the light is focused into a smaller cone. Luminous intensity is measured in candelas or candlepower.
Illuminance is a measure of the amount of light that falls on a specific surface area and is measured in lux (metric) or foot candles (imperial). A 1000 lumen spot light will illuminate a small area much more than a 1000 lumen flood light because more of the spot light's lumens are hitting the small surface. As a rough guide, 1 foot candle (FC) = 10 lux.
Luminance is the measure of an object's brightness or the amount of light that is coming off an illuminated surface. The unit of measurement is the foot-lambert. The brightness of an object has to do with the amount of light that hits it and the amount of light that is reflected back.
Incidence is the light hitting a surface. The angle, from normal, of the light is referred to as the Angle of Incidence.
Reflection is the return of light after hitting a surface. The angle, from normal, of the light on its return path is the Angle of Reflection. The Law of Reflection dictates that when light hits a smooth surface the angle of incidence is equal to the angle of reflection.
Diffusion occurs when a surface is irregular, which creates a scattered reflection of light. It also occurs when a translucent object scatters light as it passed through. When this happens, the reflected or dispersed light is considered diffuse.
Absorption happens when a material only reflects or returns a portion of the light that hits it. Even a mirror is not perfectly reflective, so some of the light radiation is absorbed by the material. The measure is the absorptance.