A lux meter is a device for measuring brightness, specifically, the intensity with which the brightness appears to the human eye. This is different than measurements of the actual light energy produced by or reflected from an object or light source. The lux is a unit of measurement of brightness, or more accurately, illuminance. It ultimately derives from the candela, the standard unit of measurement for the power of light. A candela is a fixed amount, roughly equivalent to the brightness of one candle.
While the candela is a unit of energy, it has an equivalent unit known as the lumen, which measures the same light in terms of its perception by the human eye. One lumen is equivalent to the light produced in one direction from a light source rated at one candela. The lux takes into account the surface area over which this light is spread, which affects how bright it appears. One lux equals one lumen of light spread across a surface one square meter.
A lux meter works by using a photo cell to capture light. The meter then converts this light to an electrical current, and measuring this current allows the device to calculate the lux value of the light it captured.
The most common use of this type of meter is in photography and video filming. By measuring the light in luxes, photographers can adjust their shutter speed and depth of field to get the best picture quality. The device can also be very useful for filming outdoor scenes of television programs or movies as it allows adjustments to make sure scenes filmed in different light levels have a consistent brightness on screen.
To a very limited extent, it is possible to use a camera as a makeshift lux meter. This works because some cameras measure light and automatically adjust the exposure time appropriately. Using a formula, a photographer can then reverse the calculation and convert the stated exposure time into light levels. This method has several limitations, including the fact that some light is absorbed as it passes through the camera lens, so the calculation must be adjusted accordingly. The method can be useful in situations where a separate meter is impractical, however, such as with delicate objects in a museum that can’t easily be reached without disruption.
Another common use of a lux meter is in meeting health and safety regulations. It can be used to check whether a room is bright enough to meet any rules designed to protect workers from suffering damage to their eyesight, for example. The meter takes into account the size of the room in a way that simply measuring the intensity of the light source in lumens would not.