Radiance, lux and lumens of light measurement

Light measurement - Radiance

Radiance describes the amount of light falling radially - in rays outward - from the light measurement, and falling at an angle on the illuminated object. So it characterizes both the light emitted from the light source and the light reflected by the illuminated object. Radiance is measured in watts per steradian per square meter (W x sr^-1 x m^-2). Radiance is used as an indicator of how much of the light source's power will be reflected by the illuminated object when an optical device like a camera is aimed at the objet from any angle.

When lighting a stage set, a movie scene, or a photographic scene, light meters can determine if the available light or the current lighting set up is sufficient. Lighting can be adjusted up or down to produce the best camera shots or the best viewing capability for theater audiences. Measurements are taken at the light measurement and at the illuminate object. Additional measurements are taken at the camera lens or at different tiers of audience seating as an indicator of perceived lighting, what the eye would see in photos or film or what the audience will see in a live performance setting.

Light measurement - Lux

Lux is a unit of measure that gauges how the human eye perceives the intensity of light falling on an object it hits or passes through the object being lit. Faces and other opaque objects that reflect light have a measurement of 1 Lux, which equals 1 lumen (lm) per square meter. Light from the full moon produces a lux of 0.1, whereas light from full sunlight generates 10,000 lux.

Light measurement - Lumens / Seeable Lumens

Lumens describe the quantity of light put out by the light source. The color of the light source (sunlight puts out amber or yellow light measurement, indoor lighting appears blue) influences the seeable lumens or the way the eyes identify the color of the object being illuminated. Yellow light is detected by the cones in the eye and the blue light is detected by the rods. When using a light meter to measure light falling on the illuminated object, one lumen (lm) is equivalent to one candela (cd) and one lumen second equals 0.001464 joules or about 10,000lux. Human eyes have a maximum sensitivity of 680 lumens, which is roughly equivalent to 1 watt (w).