Illumination: Lux, Lumens & Foot-Candle Converter

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Illumination refers to the amount of light falling on a surface. Measuring illumination is key in cinematography to ensure adequate lighting conditions.

Here’s a converter that lets you easily convert foot-candles to lumens or lux and vice-versa:




Key Terms and Units

Lumens (lm) quantify the total amount of visible light emitted by a source. Unlike Watts, which measures power, Lumens measure brightness.

Lux (lx) is the unit of measurement for illuminance, which describes how much luminous flux (light) is spread over a given area. One Lux equals one Lumen per square meter.

Foot-candles (fc) also measure illuminance in terms of Imperial units. One Foot-candle is equivalent to one Lumen per square foot.

Exposure Value (EV) is a measure of exposure for a given combination of aperture (f-number) and shutter speed. To convert EV to illuminance in lux, you need to consider the ISO sensitivity and use the following formula:

    \[\text{Illuminance (lux)} = \frac{125 \times 2^{\text{EV}}}{\text{ISO}}\]

For example, for EV = 10 and ISO = 100:

    \[\text{Illuminance (lux)} = \frac{125 \times 2^{10}}{100} = \frac{125 \times 1024}{100} = 1280 \text{ lux}\]

Watts (W) measure the power consumption of a light source. It is not a direct measure of the amount of light produced but rather how much electrical energy the light uses.

The formula to calculate Lux is:

    \[\text{Lux} = \frac{\text{Lumens}}{\text{Area in square meters}}\]

For example, if a light source emits 800 Lumens over an area of 10 square meters:

    \[\text{Lux} = \frac{800 \text{ lm}}{10 \text{ m}^2} = 80 \text{ lx}\]

Similarly, the formula to calculate Foot-candles is:

    \[\text{Foot-candles} = \frac{\text{Lumens}}{\text{Area in square feet}}\]

For the same light source emitting 800 Lumens over an area of 100 square feet:

    \[\text{Foot-candles} = \frac{800 \text{ lm}}{100 \text{ ft}^2} = 8 \text{ fc}\]

Since 1 square meter is approximately 10.764 square feet, the conversion between Lux and Foot-candles can be done using these formulas:

    \[1 \text{ fc} = 10.764 \text{ lx}\]

    \[1 \text{ lx} = 0.0929 \text{ fc}\]

Illuminance decreases with the square of the distance from the light source. This is known as the inverse square law:

    \[\text{Illuminance} \propto \frac{1}{\text{Distance}^2}\]

If you double the distance from the light source, the illuminance drops to one-quarter of its original value.

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Author

  • Jan Sørup

    Jan Sørup is a indie filmmaker, videographer and photographer from Denmark. He owns filmdaft.com and the Danish company Apertura, which produces video content for big companies in Denmark and Scandinavia. Jan has a background in music, has drawn webcomics, and is a former lecturer at the University of Copenhagen.

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