Here is a short text on measurement and characterisation of light to help when selecting a bulb or lighting system for a bicycle. This is bit scientific but might be good background detail to have in mind when comparing various lighting systems.The most important point when choosing a light is how bright it is and this will be determined by the power of the light source. However; the focus of the light and likewise the distance to the subject being illuminated affect lamp intensity on the target.How bright is a light?Light output is called "Luminous flux" and it is measured in Lumens. This characteristic is also sometimes called radiance since it describes the brightness of the light at source and therefore the light output. In a given system, this number is governed by the power supply and the lamp design and can be considered a constant. A lamp with a higher Lumens number will therefore be brighter.But this is not the whole story because how the light is focused can make a huge difference to the lamp performance.It stands to reason that the more you focus the light onto a given spot the brighter that spot will become; so the light appears to get brighter but of course the bulb is still giving out the same amount of light (Lumens), it is the distribution that is different.To quantify the focused light performance, a different term called Luminous intensity is used, measured in Candelas. If we imagine a focused light beam in the shape of a narrow cone, the Candela number will tell you the intensity of light in that cone. The narrower the angle, the higher the Candela for a given light source. This fact means that Candela numbers are the same regardless of distance to the light source.
Yawn - How can we summerise this?