Measurement of Daylight

MEASUREMENT OF DAYLIGHT

Luminous flux and intensity

The amount of light being given off by a particular source, in all directions is called luminous flux (or "luminous power"). It is a measure of the total perceived power of light and it is measured in lumens.

Radiant flux is a related measure that quantifies the total power of the light radiation from a source. It is not just visible light but also infrared and ultraviolet light and it is measured in watts.

Illuminance

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface.

It is wavelength-weighted by the luminosity function to correlate with human brightness perception.

Illuminance is the measure of light currently used by most performance indicators to determine daylight availability in the interior.

The amount of light falling on a surface is "illuminance", and it is measured in lux (lumen/m²) (Fig. 3.10)

Illuminance levels can be measured with a luxmeter as shown in figure 3.11.

Typical Illuminance Values

Minimum Levels for Tasks and Activities

Luminance

It 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 (emitted or reflected from a particular area) a given solid angle. (Fig. 3.12)

The SI unit for luminance is candela per square metre (cd/m²)

Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view.

Luminance is thus an indicator of how bright the surface will appear.

In this case, the solid angle of interest is the solid angle subtended by the eye's pupil. Luminance is used in the video industry to characterize the brightness of displays.

A typical computer display emits between 50 and 300 cd/m2. The sun has luminance of about 1.6× 109 cd/m²

Luminance levels can be measured with a luminance meter, shown in fig. 3.13. It can also be measured using high dynamic range (HDR) imaging techniques together with a digital camera and luminance mapping software (e.g. Photolux).

Luminance levels can be predicted through the use of computer simulation with recognized and validated software.