The Inverse Square Law of Illuminance

The Inverse Square Law of Illuminance is a fundamental principle in lighting design and engineering that describes the relationship between the distance of a light source and the level of illuminance (light intensity) on a surface.

The law states that the intensity of light on a surface decreases as the distance between the surface and the light source increases, and this decrease is proportional to the square of the distance between them.

E = I / d2

I = intensity

d = Distance

Where I is the illuminance on the surface in lux (lx), E is the luminous flux emitted by the light source in lumens (lm), and d is the distance between the light source and the surface in meters (m).

Let, ‘S’ be a point source of luminous intensity ‘I’ candela, the luminous flux emitting from source crossing the three parallel plates having areas A A , and A square meters, which are separated by a distances of d, 2d, and 3d from the point source respectively as shown in Fig.

Hence, from Equation, illumination on any surface is inversely proportional to the square of distance between the surface and the source.

This means that if the distance between the light source and the surface is doubled, the illuminance will decrease to one-quarter of its original value. Similarly, if the distance is tripled, the illuminance will decrease to one-ninth of its original value.

The Inverse Square Law of Illuminance is important to consider when designing lighting systems, as it can have a significant impact on the level of illumination achieved in a space. It is also important to note that this law assumes that the light source is a point source, and for sources that are not point sources, other factors may need to be taken into account

The Cosine Law of Illuminance / Lambert’s Cosine Law

Illumination in architecture – home page

FD Architect Community Forum Discussion

• Illumination – introduction
• Basic Terminology and Definitions
• Laws of Illumination The Inverse Square Law of Illuminance Illuminance (E) at any poi
• Laws of Reflection and Refraction
• Visual efficiency & comfort
• Day lighting
• Components of Daylight Factor
• Artificial Lighting Design

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