The Science of Illuminance

Illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.

Introduction

Illuminance quantifies how much light falls on a surface, which is crucial for lighting design, photography, and ensuring adequate visibility in various environments.

History

The study of illuminance developed with photometry in the 18th century. The lux unit was established as part of the metric system, and modern illuminance measurements became standardized with the development of photometric standards.

Key Units

Lux (lx)

The SI unit of illuminance, equal to one lumen per square meter.

Foot-candle (fc)

An imperial unit equal to one lumen per square foot, approximately 10.76 lux.

Phot (ph)

A CGS unit equal to 10,000 lux, rarely used today.

Applications

  • Architectural and interior lighting design
  • Photography and cinematography
  • Workplace safety and ergonomics
  • Plant growth and horticulture
  • Street and outdoor lighting standards

Illuminance Calculation

E = Φ/A = I·cos(θ)/r²

Illuminance equals luminous flux divided by area, or luminous intensity times cosine of angle divided by distance squared.