Lumen Calculator

Lumen, Lux, Candela, and Footcandle Conversions

Lighting specifications use several related units. Lumens measure total luminous flux emitted by a source. Lux measures illuminance received per unit area. Candelas measure luminous intensity in a given direction. Footcandles are an imperial illuminance unit.

Key Relationships

$$E = \frac{\Phi}{A}$$ $$I = \frac{\Phi}{\Omega}$$ $$\Omega = 2\pi\left[1 - \cos\left(\frac{\theta}{2}\right)\right]$$

Here $\Phi$ is lumens, $E$ is lux, $A$ is area in m², $I$ is candelas, $\Omega$ is solid angle in steradians, and $\theta$ is the apex angle. One footcandle equals $10.764\,\text{lx}$.

Practical Notes

Use area when converting between lumens and lux or footcandles. Use apex angle when converting between lumens and candelas for directional sources such as spotlights and LEDs with a defined beam angle.

Frequently Asked Questions

What is the difference between lumens and lux?

Lumens describe total light output from a source. Lux describes how much of that light falls on a surface per square meter.

How do I convert lux to lumens?

Multiply lux by area in square meters: $\Phi = E \times A$.

What is the apex angle?

It is the full angle between the axis of peak intensity and the direction where intensity drops to 50% of the maximum. It defines the beam spread for directional lamps.

How do footcandles relate to lux?

Multiply footcandles by 10.764 to get lux, or divide lux by 10.764 to get footcandles.

When is solid angle 4π steradians?

For a source that radiates uniformly in all directions, such as an ideal isotropic point source, $\Omega = 4\pi\,\text{sr}$.

Report

Lumen, Lux, Candela, and Footcandle Conversions

Key Relationships

Practical Notes

Frequently Asked Questions

Related Tools

Recent Visits