In the world of portable lighting, which includes bicycle lighting, two (often confusing) measures of light have become references by which manufacturers rate (and market) the brightness of their products. A lumens rating refers to the total amount of light that a light source such as an LED produces. Lux by contrast is a measure of the proportion of the total light that actually makes it to a predetermined area at a set distance. The former is much more commonly used in the US and the latter in Europe especially in Germany. Less often used except in automotive lighting (particularly in the US) is the measure of candelas, which is more difficult to explain but essentially measures the "force" of light at a certain distance from the source. 

A useful way to understand lux is imagine a small room lit with a standard 100 watt incandescent light bulb (which normally produces roughly 1700 lumens). The room and its walls are lit with a concentration on the floor immediatly below the lamp. A person inside the room switches on a 5 watt incandescent flashlight producing 25 lumens and shines it also on the floor thus producing a highly visible (i.e. brighter) but much narrowly confined circle of light. The 25 lumen flashlight produces many more lux within the narrow circle of light on the floor than the 1700 lumen light bulb! If the person holding the flashlight lowers it closer to the floor, the lux rating of the increasingly smaller circle of light will increase incrementally, but its lumens rating will not change.

SourceWikimedia Commons. Contributor: United States Army Description: ID 080416-A-6524C-004. Two U.S. Army soldiers focus on the door revealed in the beam of a flashlight as they prepare to breach the door during a search for weapons caches in Baghdad, Iraq, on April 16, 2008. The soldiers are from Echo Company, 2nd Squadron, 2nd Stryker Cavalry Regiment.

A lumens rating describes how much total light is produced by a light source whereas, a lux rating describes how much light makes it to a pre-determined target area. 

Another way to think about the measures is to use water as a proxy for light. Imagine a garden hose that discharges a constant volume of water per second. The “volume of water per second” is an analog for the “lumens” rating of a light source. Then imagine that the nozzle of the hose is set to produce a narrow stream of water over a long distance and then adjusting the nozzle so that it produces a spray unable to project water very far at all, but wetting a much larger circular area. In each instance, the total volume of water discharged per second remains the same. When set at narrow, the lux rating within a “predefined area” is very high at the beginning of the stream and declines relatively slowly as the point of measure moves away from the nozzle and the stream of water broadens. When the hose is set on diffuse, the lux rating within the predefined area is much lower at the same distance from the nozzle and declines rapidly as the point of measure moves away from the nozzle. The term "beam angle" is a concept paralleling how narrowly the stream of water leaving the nozzle is focused. Notice again that the volume of water leaving the nozzle is the same at all settings meaning that the lumens rating would remain the same in all cases.

At the same time, the lux rating measuring the volume of water actually hitting a predetermined area could vary dramatically. If the narrow stream were hitting only a small portion of the "predefined area", the actual lux rating would be an average of the total volume of water captured by the entire area – including those areas not directly impacted by water. But, the lux rating at the same distance would be much higher if the "predetermined area" were to include only the smaller area directly impacted by the stream or spray for that matter. Would both measures capture the same amount of lumens?
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Source: Wikimedia Commons. Contributor: Queenslander (Brisbane, Qld. : 1866-1939).

A candela is a unit of measure for light common in the automotive lighting industry but less often seen in the bicycle lighting field particularly in advertising. The metric was originally based on the amount of light produced by a candle but its base value and method of application have changed significantly since. Understanding candelas is not necessarily difficult if you think of a light source in the same manner as the water/light hose example above. Unlike a lux rating which measures the “amount” of light received within a predetermined area at a set distance, a candela rating (measured in cds) represents the “force” of light emitted in a specific direction from a source. It is thus a function of the “lumens emitted” from, and the “beam angle” in, a predetermined direction from a light source. The “total” candelas emitted in all directions from a light source will be equal the total lumens it produces (think the law of the “conservation of energy” from physics 101). Like a lux measurement, a candela rating from a light can vary significantly depending on the precise direction from the light source being measured whether behind, above or in front. Polar examples would be a laser emitting a perfectly focused beam of light and a bare light bulb hanging from a ceiling emitting light homogenously in all directions - both having identical lumens ratings. Another example would be the case of a flashlight beam pattern incorporating a hot center and a broader but less bright halo around the hot center, the candela rating will be much higher in the precise directions leading to the hot spot at the center of the beam pattern than in the directions leading to the halo. The utility of measuring the “force” of light emitted in a predetermined direction is that it provides a basis for assessing how far light from a source will reach in a certain direction - much like estimating how far the stream of water from a hose will reach. 


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