How the Brightness of Safety Lights is Measured

Understanding Measurements (Units) of Brightness

When asked how “bright” a light is, the answer is rather subjective. You might think the atrium of a movie theater is bright until you step outside into the sunlight. Take the lights in your home: if both your kitchen and your bathroom are perfectly lit, and each is illuminated by a single light bulb, are they equally bright? What about a flashlight versus a laser pointer? Though brightness is difficult to quantify, there are certain units of measurement that help us accurately and objectively consider the amount of light produced by a source. The three most common units are Lumens, Candela, and Candlepower. This article will define each term and discuss their relationship with one another.

Lumens
Lumens (L) is a SI unit of measurement that represents the total amount of visible light emitted by a source. It considers the intensity of the light (candela) in relation to the space in which it fills. Simply put, the higher the lumen value of a light source is, the greater the area that will be illuminated by it. Lumens is useful when comparing lights to provide general visibility or to illuminate a particular area. 

Candela
Candela (cd) refers to the intensity of a single beam of light in a particular direction. Unlike lumen, which measures how much is illuminated by the light source, candela measures how far away that light can be seen. Candela values are particularly useful when purchasing focused light products such as overhead project lamps, spotlights, warning lights and beacons. Candela values can be manipulated by focusing all of the light output into a concentrated area, such as spotlights, or by spreading it over a larger area, such a photography softboxes. Simply blocking or obscuring part of a light source, however, will not change the candela value. 

Candlepower 
Candlepower (cp) is an obsolete unit of measurement for luminous intensity (brightness). One candlepower measured the intensity or brightness emitted by a light compared to one standardized candle. Candlepower was replaced by the nearly equivalent SI unit, Candela, in 1948. Today, candlepower and candela are used synonymously since 1 candlepower is equivalent to 0.981 candela. 

Lumens vs. Candela
Technically, 1 candela is equivalent to 12.57 lumens, but using that comparison at face value would be misleading. While both Lumens and Candelas are used to quantify brightness, the units actually measure two different aspects of luminous intensity. Lumens measures overall light output while candela measures light intensity in a singular direction. When shopping for a safety light or signal, it is important to understand which unit is most relevant to your application. For example, if you want a light to spread out over a large area or effectively light the area around it, pay attention to lumens. On the other hand, if you want your light to be seen from far away or projected in a concentrated area, you most likely want to pay attention to the candela value of the light. A standard incandescent light bulb may have a high lumens rating but a small candela value, conversely, a spotlight would have a low lumens value and a high candela rating. 

Candela vs. Candlepower
Candlepower and Candela are used synonymously nowadays. Candlepower was the standard used when candles were the primary light source. However, as technology advanced, engineers found the international unit, Candela, to be more useful in their work. Since the base values were nearly identical, scientists and engineers agreed in 1948 that candela would be the new unit of measurement for luminous intensity. 

SAE Class
In a way, SAE Class measures brightness, but it is not a unit of measurement in and of itself. Rather, it is a certification system created by the Society of Automobile Engineers to classify warning lights on vehicles. SAE Class uses candela as the unit of measurement in their ratings with Class 1 as the highest candela values and Class 3 as the lowest. SAE Class is relevant when searching for safety vehicle lights for authorized use.