Where lumens measure the amount of light a light source emits, lux tells how much that light source illuminates an object or workspace that is one square meter in area and placed one meter away from the light source. Lux meters, or light meters, determine the amount of available light in either luxes or in foot-candles (illuminance of one square foot area, located one foot from a light source). Lux meters contain a central unit housing the computerised electronics that operate the meter a light sensor and a digital or analogue display.
Determine the distance from the light source to the work plane in both feet and meters. Measure the work area being lighted, calculating one square foot and one square metre area at the centre of the work plane---where the illuminance is brightest. Record these on a piece of paper.
Switch on the light meter, allowing the display to zero out. Next, remove the cover from the light sensor. The lux meter is ready to measure lux at any distance from the light source.
Take the light meter to the centre of the work area, lining up the light sensor with the light source. First, hold the light sensor horizontal to the light and record either the lux or foot-candles. Next, hold the light sensor at a 45-degree angle to the light source and record the results. Finally, hold the light sensor vertical to the light source and record those numbers.
To test the light meter's accuracy, place the light half the distance from the work plane and repeat the three angle measurements above. According to the inverse square law, the readings should be double those of the original work plane. Now place the light source twice the distance from the work plane. The readings should be half of the original readings.
A little straightforward math and a calculator converts the light meter results to other measurements. For instance, multiply foot-candles (Fc) by 10.76 to find lux and multiply lux by 0.0929 to get foot-candles. One lumen per square meter is equivalent to one lux while one lumen per square foot is equal to one foot candle.
Things you need
- Light meter/lux meter
- Lighted work plane
- Imperial measuring tape
- Metric measuring tape