Quantities and Units

Luminous Flux and Intensity

Luminous flux, Phi, is measured in lumens (lm). Luminous intensity, I, is flux per unit solid angle, measured in candela (cd = lm/sr):

I = dPhi / dOmega
Phi = integral over solid angle of I(direction) dOmega

An intensity sample is not itself a quantity of lumens. Integrating a sampled distribution requires the solid angle represented by each sample.

Illuminance

Illuminance E is incident luminous flux per receiving area, in lux (lx = lm/m2). For a point-source, far-field approximation on a plane:

E = I(direction) * max(0, cos(theta)) / r^2

Here r is the source-to-receiver distance in metres and theta is the angle between the receiving normal and the direction toward the source. This assumes an unobstructed path, no intervening attenuation and sufficient distance for the point-source approximation. It is not a near-field model of an extended luminaire. A zero distance is outside this model.

See Rendered Illuminance View for plane orientations and display settings.

Luminance

Luminance L is intensity per projected emitting area, in cd/m2. For a directional average over a luminaire:

L_average(direction) = I(direction) / A_projected(direction)

A physical, positive projected area is required. This average is not an image of local surface luminance; a point geometry cannot provide a meaningful finite emitting area. Geometry and orientation therefore affect luminance-based glare calculations even with unchanged intensity.

See Luminance Table and UGR Table.

Normalization and Units

Lamp flux describes the source contribution; luminaire flux is the integrated output distribution. Lamp count and per-lamp flux also differ from total lamp flux. State which denominator is used for an efficiency or utilization ratio.

Radiometric power in watts is not interchangeable with photometric flux in lumens. A physically justified conversion requires spectral information and photopic weighting, not a replacement of the unit label.