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.
Decreasing the distance to the subject has a similar effect to reducing the angle, because as you move the subject towards the light source, the area illuminated by the light becomes smaller and so subject appears brighter. The quantification of this characteristic is called LUX. Lux is the measurement of actual light available at a given distance. One lux equals one lumen incident per square meter of illuminated surface area, in other words : Lux = Lumens/square meter.
If we agree that the lumens from a particular light source are fixed, then we can see from the formula that the Lux increases as the area is decreased. Halve the area where the light falls and you get double the Lux on the target. This is the reason a single bulb might be enough to illuminate a small bedroom, but inadequate in a factory warehouse; the increase in area on the factory floor makes the Lux value much lower, even though the Lumens are the same.
The difference between Lux and Candela is that lux measures the illumination of a surface, instead of that of an angle. The result is that the distance to the surface becomes an important factor: the more distant the surface is from the light source, the less well it is illuminated.
Yawn - How can we sumerise this?
Let's get back to something we understand - bicycles.
When selecting a lighting system more Lumens means more light which is a good thing. More Candela is also preferable but be aware that this might just be because the light is more focused rather than brighter. This may be what you want or you may prefer more of a floodlight which will have a lower Candela. Lux is a less useful number in a mobile application because the distance to the target will vary and may not be known like it is in a static scenario. However it could be considered a guide to how visible a rider is from their light since it indicates how bright the light appears at a distance.
Hopefully all this helps rather than hinders.
Of course how visible an object is when illuminated depends on it's reflectance, absorption, scatter, refraction and some other factors, but let's leave that for another time.
Cup of tea anyone?