Bicycle Measures of light output
Three units are used to measure light output. Manufacturers do not always quote the most appropriate figure—for example, watts are commonly quoted, but wattage alone is a poor measure since (a) it reports the consumption of power rather than the output of light, and (b) lamp optics will significantly impact the portion of the light which is delivered where it is needed, for example concentrated in a spot beam or dispersed as in running lights. Candelas, measuring the intensity of a beam, are more appropriate when the aim is to illuminate brightly a small spot, while lumens, measuring the entire production of light, are more relevant to the purposes of broad beams or non-directional running lights.
Watts
The watt (W) is the unit of power, and is usually quoted for the electrical power input, not the light power output. Electrical power is the product of voltage and current (watts = volts × amperes). Input power is only useful when comparing lights of similar technologies. A 3 W halogen dynamo headlight will light the road up about the same as a rechargeable light of around 7–10 W, but the rechargeable usually outputs much more light to the sides, which is useful on trails (although wide and narrow beam versions are available). A 3 W LED is somewhat brighter than a halogen lamp but generally less well focused. HID lights put out large amounts of light and are often quoted in terms like “80 W halogen equivalent”.
Candelas
The candela (cd) is the SI unit of luminous intensity, that is power per unit solid angle, weighted according to the sensitivity of the human eye to various colours of light. A typical candle produces light with about 1 candela of luminous intensity. A lamp can produce higher luminous intensity either by producing more light, or by focusing it tighter. The luminous intensity of a light depends on many factors, including the colour of the light and the eye’s sensitivity to that colour, the optics involved, reflector and lens. Despite its complexity, it is a more useful measure than watts, because it defines how much usable light is shed in a given place: a dynamo headlight designed for road use and focused for seeing the road makes more efficient use of the power of the lamp than lights using rotationally symmetrical optics.
Lumens
The lumen (lm) is the SI unit for luminous flux, the total amount of light emitted by a source, weighted according to the sensitivity of the human eye to various colours of light. Lumens per watt is a common measure of the efficiency of a light source. The luminous flux is of less value for bicycle lighting due to the importance of directionality. Luminous intensity is much more useful, but lumens per watt is a handy way to compare the output of otherwise similar lights.