The candela (cd) is the SI unit of luminous intensity: luminous flux per unit solid angle in a specified direction. A candela figure tells you about the light directed along that part of the beam.
Candela, lumens and lux
Lumens describe luminous flux; candelas describe its directional concentration; lux describes illuminance at a receiving surface. Narrow optics can produce a high centre-beam candela value by concentrating output. That number alone does not describe beam width, edge quality or total useful coverage.
Practical example
For a point-source approximation and a receiving surface perpendicular to the ray, illuminance in lux equals candela divided by distance in metres squared. An illustrative intensity of 4,000 cd gives about 1,000 lx at 2 m and 250 lx at 4 m. This calculation assumes the same direction and output.
Reading a fixture report
Check whether the number is peak intensity, centre-beam intensity or a value at another angle. Use the intensity distribution and beam/field angles alongside it. Close distances, large sources, oblique surfaces and particular optical systems can require a fuller model or direct measurement.
Equipment examples
The ARRI Orbiter uses interchangeable optics. Compare the data for the fitted optic rather than treating the bare fixture name as a complete photometric specification.
Related terms
Lumen · Lux · Beam Angle · Photometrics
References
NIST: luminous intensity and the candela · Konica Minolta: photometric quantities