Cycles of all sorts

There are three units used in the measuring of light output. Manufacturers are in the business of selling product, so do not always quote the most appropriate figure - for example, watts is commonly used, but wattage alone is a poor measure since the light optics will have a significant impact on the proportion of that power which is delivered where you want it.

Watts

The watt (W) is the unit of electrical) power, the product of voltage and current (watts = volts x amperes). This is only really useful when comparing lights of similar technologies, and even then only in a limited sense. A 3 W halogen dynamo headlight will light the road up about the same as a rechargeable light set of around 7–10 W, but the rechargeable will put out much more light to the sides, which is useful on trails (although wide and narrow beam versions are available, so even this is a generalisation). 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 "80W halogen equivalent".

Candela

The candela (cd) is the SI unit of luminous intensity. Formally, it is the luminous intensity in a given direction of monochromatic light source of frequency 540×10¹² Hz having a radiant intensity in that direction of 1/683 watt per steradian. The measured intensity of a given light is thus dependent on many factors, including the colour of the light and the eye's sensitivity to that colour, the optics involved, reflector and lens. This is a more useful measure than watts, despite its complexity, because it defines how much usable light is shed in a given place: a dynamo headlight, which is designed for road use and is focused to place light where it is needed 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 a derived unit for luminous flux, formally the luminous flux emitted into unit solid angle by a non-directional point source having a luminous intensity of 1 candela. Solid angle is expressed in steradians (sr); the solid angle subtended by a surface at a given distance is defined as the surface area divided by the distance squared - imagine a sphere of 1m radius, place a 1cd light source in the middle, and the luminous flux expresses how much light (1 lumen) will fall on each square metre of the inner surface of the sphere; i.e. 1 cd = 1 lm/sr. Lumens per watt is a common measure of the efficiency of a light source. Clearly the luminous flux of a light source is of little value in isolation; the light will be changed by the optics. Luminous intensity is a much more useful measure, although lumens per watt is a handy way of comparing the output of otherwise similar lights.