Lighting

Variables of sunlight

Sunlight at a given time can be defined by the following properties: spectrum, direction, intensity.

• Spectrum concerns the the range and irradiance of different EM wavelengths that make up solar radiation reaching the Earth’s surface. Some are harmful like ultraviolet rays that can damage human tissues, while visible rays are the ones we use to see colours, and infrared rays that are mainly responsible for heating of the surroundings.
• The Earth rotates around its own tilted axis once everyday, and orbits the Sun once every year. Hence, depending on the time of day and date, the distance and direction from the Sun at a particular location can vary.
• Intensity is the measure of energy transferred by radiation per unit area. This can vary by cloud cover, direction, and scattering in the atmosphere. Radiation from the Sun can be split into direct and diffuse radiation, where diffuse is the light received by reflections and scattering in places that don’t directly receive any light. The total intensity of light at a point can be measured using a Solarimeter.

The $altitude$ and $azimuth$ angles can be used to determine the position and incidence of the Sun from a particular building or facade.

• The altitude angle is between the vertical and horizontal plane, measured from the horizontal, and is the angle between the plane and incoming ray of light.
• The azimuth angle can be  is taken from the North point to the Sun’s position on 2D, top-down floor-view kind of plan, essentially a bearing towards the Sun. 

Daylight

Daylight availability can vary over time of day and cloud cover. Light factor can be measured in Candela, Lumens, or Lux, and these units apply to daylight too. The total daylight factor at a location is the sum of all the components of daylight: direct or sky (SC), externally reflected (ERC), and internally reflected (IRC).

There are standards in place for recommended light levels (daylight factors) in indoor spaces as lighting is essential for human comfort and wellbeing, and time orientation (circadian rhythm). There must be enough light to carry out tasks and feel good, while also limiting the amount of annoying glare that would disturb inhabitants.