This is one of the areas that a more descriptive measurement such as a Foot-Candle can be useful. A Foot-Candle is defined by Wikipedia as "the illuminance cast on a surface by a one-candela source one foot away." It can also be defined as Lumens per square foot. Though this is similar to a Lumen, as alluded to above, there is a key difference, which primarily focuses on the fact that a Lumen is a measurement of the light emitted from a source, while a Foot-Candle is focused on the amount of light that impacts a surface a given distance away from a light source. Though this can seem like splitting hairs, there is actually good reason for doing this, since there can be significant loss of light as it travels to a surface, and, ultimately, to an observer's eye, due to distance, direction and other factors.
Using Foot-Candles to measure levels of illuminance provides an opportunity to compare different light sources in an apples-to-apples manner, even when the two light sources have different characteristics because the focus of the measurement is on the intensity of light at its impact, not at its source. This is particularly important when comparing traditional light sources such as incandescent lights to newer technologies such as LEDs. The reason for this is that the characteristics can be different between two types of lights; for example, lights can emit light in all directions (incandescent), or in a more directional manner (LED), as discussed in more detail in the earlier post that explains How 10,000 Lumens can be brighter than 20,000 Lumens.
Another way to understand the relationship between Lumens and Foot-Candles is to use the metric / International System of Units version of Foot-Candles, which is the measurement Lux. Lux is defined as Lumens per square meter. Therefore, if you already know how many Lumens are emitted by a light source, the Foot-Candles or Lux for that source can be quickly calculated if you know the area over which this light will be shown, in square feet or square meters, respectively. If Foot-Candles are the desired measure, Lux can be converted to Foot-Candles by dividing the Lux by 10.7 to provide the approximate number of Foot-Candles. Whether it is Lux or Foot-Candles, you now have a more precise way of measuring the relative brightness of two different light sources, based on the amount of light that will actually impact a given area. This can be a critical distinction because only light that actually reaches a surface can provide illumination for that surface.
If you'd like find our more about topics related to lighting, check out our other blog entries or click on links for our different light products to find out more about the specific lights. As always, if you have any questions, comment below or reach out to us at 619-777-6LED (6533) or through the email link or contact us page.