Lighting Calculation Metrics

ElumTools can compute a number of different lighting Metrics. For most calculations performed in General Use or Emergency modes the default will be Illuminance. Daylighting calculations can optionally use Daylight Factor and Horticultural calculations will default to PPFD. Other available metrics will be optional when using the Add (or Edit) Calculation Points commands.

Illuminance (E) - Incident light

The Illuminance metric measures the light incident on a surface or workplane - measured in Footcandles or Lux (lumens per unit area). Illuminance measurements may be made in Global Illumination mode or Direct Calculation mode (See Two Calculation Methods). Depending on the calculation mode selected and model, Illuminance measurements may show direct and interreflected contributions.

Exitance (M) - Reflected light

The Exitance metric measures the light reflected off a surface - measured in lumens per square foot or lumens per square meter. Exitance measurements are only available in Global Illumination mode. The Exitance measurements may show direct and interreflected contributions depending on the model. Exitance can only be calculated for points coincident with surfaces.

Luminance (L) - Reflected light

The diffuse Luminance metric measures the light reflected off a surface. It is measured in candelas per square meter. Diffuse Luminance measurements are only available in Global Illumination mode. The Luminance measurements include both direct and interreflected contributions incident on the measured surface depending on the model. Luminance can only be calculated for points coincident with surfaces.

Daylight Factor (DF) - Daylight Mode only

The ratio of illuminance at an interior calculation point, to the illuminance produced by an unobstructed, overcast sky. Measurements may depict direct contribution from the sky and interreflected contributions from the model. Only available for Daylight Mode.

Unified Glare Rating (UGR) - Discomfort Glare metric for electric lighting

The Unified Glare Rating metric (UGR) measures discomfort glare from electric lighting in an interior environment. The calculation considers the ratio of luminaire luminance and position compared to the background luminance of the environment. The result is given as a UGR value between 10 and 30, where a high value indicates significant discomfort glare and a low value indicates little discomfort glare. UGR measurements are calculated according to CIE 117-1995 and are only available with the Global Illumination calculation mode. See Appendix F for more information.

Photosynthetic Photon Flux Density (PPFD) - Horticulture lighting metric

Measurement of radiant energy in the photosynthetically active radiation (PAR) spectrum. See Horticulture Help Topic and Appendix G.

Cylindrical Illuminance (Ez)

Cylindrical illuminance is the average illuminance, from all directions, orthogonal to the meter aiming direction. Typically used as a measure of vertical illuminance incident on a person’s face. This is also used in combination with horizontal illuminance to produce Modeling Effect (below).

Modeling Effect

Modeling Effect is the ratio between cylindrical illuminance and horizontal illuminance at the same point: Ez/E.