Daylight factors are expressed as the percentage of natural light falling on a work surface compared to that which would have fallen on a completely unobstructed horizontal surface under exactly the same sky conditions. To deal with these highly variable sky conditions, many building codes and design briefs use daylight factors as the design criteria instead of illuminance on the working plane. This makes it highly variable - sometimes changing quite substantially in a matter of only a few minutes. Sky luminance is, in turn, affected by the position of the Sun as well as the amount, location and opacity of clouds within the visible sky dome. Illuminance levels deal with the actual amount of light hitting a surface and are therefore significantly affected by variations in the amount of light given off by the sky. This process establishes the spatial distribution of ambient light levels within the model, from which it can generate any number of forward ray-traced images as set up by the user. It does this iteratively, continuously calculating and updating the estimated light levels at each point on each surface until a minimal difference threshold is met. RADIANCE works by resolving the radiant exchange of energy between light sources and surfaces within the model by dividing each surface into a series of sample points and generating spherical rays from each point to determine which other surfaces, light sources or sky sections are 'visible' from that point. Illuminance values are displayed in _Lux _and are usually much more important to a designer as almost all building regulations and international standards specify the minimum Lux values required for different environments and the tasks undertaken within them. These are based on the amount of light falling on objects and therefore does not represent surface colours and reflectances - though light levels on each surface are affected by the colour and reflectance of the surfaces that surround them. RADIANCE can also generate illuminance images.

For a more detailed description of lighting units, see the Square One wiki topic on Light Measurements.

The measure of luminance is the cd/m 2 or its imperial equivalent the Nit. Luminance is based on the amount of light reflected off objects, so such images display the colour and reflectance of each visible surface. These are the same as we perceive when looking at a scene or when using a camera.

Like most lighting tools, RADIANCE defaults to generating luminance images.

How to use ecotect software