Illumination

Lighting the Surface: Understanding Illuminance

Illuminance is a measure of how much luminous flux—the perceived power of light—is spread over a given area. In simpler terms, it is the intensity of light falling on a surface. It is a crucial concept in lighting design, photography, occupational safety, and horticulture, as it directly relates to how brightly we perceive a surface to be lit. Illuminance is distinct from luminance, which is the light *reflected from* or *emitted by* the surface and is a measure of its perceived brightness. Illuminance is concerned only with the light arriving at the surface.

For example, imagine reading a book. The illuminance is the amount of light from a lamp that actually hits the page. The page's brightness, which depends on how much light the white paper reflects, is its luminance. The SI unit for illuminance is the Lux, which is defined as one lumen per square meter. An older Imperial unit, the foot-candle, is still widely used in the United States. This converter allows for easy translation between these two units, which is essential for architects, lighting designers, and engineers who need to ensure that spaces like offices, schools, and public areas are lit to safe and comfortable levels according to established standards.

Relevant Formulas and Concepts

• Definition of Illuminance (Optics/Photometry): Illuminance (Eᵥ) is defined as the luminous flux (Φᵥ) incident on a surface divided by the area (A) of that surface: Eᵥ = Φᵥ / A.
• Inverse-Square Law (Physics): For a point source of light, the illuminance on a surface decreases with the square of the distance (r) from the source. Eᵥ ∝ Iᵥ / r², where Iᵥ is the luminous intensity of the source. This means if you double the distance from a light source, the illumination on a surface drops to one-quarter of its original value.
• Lambert's Cosine Law (Physics): Illuminance is maximal when the light strikes the surface perpendicularly. If the light strikes at an angle (θ), the illuminance is reduced by a factor of cos(θ): Eᵥ ∝ cos(θ). This is why the sun feels less intense in the morning and evening.

A Deep Dive into Common Illumination Units

• Lux (lx): The SI unit of illuminance. One lux is equal to one lumen of light spread over an area of one square meter (1 lx = 1 lm/m²). It is the global standard for specifying lighting levels in architecture and engineering. For reference, direct sunlight is over 100,000 lux, an overcast day is about 1,000 lux, a typical office is 300-500 lux, and full moonlight is less than 1 lux.
• Foot-candle (fc): An Imperial and US Customary unit of illuminance. One foot-candle is equal to one lumen of light spread over an area of one square foot. Since one square meter is about 10.764 square feet, 1 foot-candle is equal to 10.764 lux. It is still commonly used in the US for lighting design specifications, particularly in building codes and theatrical lighting.