THE LIGHTING REFERENCE
LIGHTING · SCIENCE · PRACTICE

Calculate, then question the assumptions.

Four transparent teaching tools for units, efficacy, geometry and a preliminary area estimate.

Primary-source research · Original explanations · Reviewed October 4, 2026
01

Use a model within its boundaries

These calculators expose the equations and inputs. They demonstrate relationships; they do not establish compliance or produce a complete lighting design. Consult the photometry and design references before interpreting a result.

02

Keep the inputs defensible

A correct equation with unjustified inputs can still give an unreliable decision. Document where each value came from, the exact product configuration, and which phenomena the calculation excludes.

Illuminance conversion

1 fc = 10.7639104167 lx. No design target implied.

Luminous efficacy

Output ÷ electrical input. Both values must describe the same system boundary.

Ideal point-source illuminance

E = I cos(θ) / d². Far-field, small-source approximation; excludes interreflection.

Average illuminance estimate

Eavg = N × lumens × utilization × light-loss factor / area. Illustrative factors must be replaced with justified project values. This does not evaluate uniformity or glare.

PRIMARY SOURCES

Check the evidence.

Public definitions and catalog scope support the cited explanations. Full normative criteria remain in the applicable official document.

  1. 01
    Source recordlux ↗
  2. 02
    Source recordluminous intensity ↗

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