What is it?: Lumen is a unit of light measurement otherwise known as luminous flux.
We use lumens to compare the total amount of light output from a light emitter. However, lumens isn't everything. In fact, lumens will only tell you a small part of the picture to what you're actually getting and what you're after. We need to know how the lumens are used. Lumens is comparable to the analogy of a car's brake horsepower.
Lumens is measured using a highly specialized light integrating sphere.
What is it?: Lux is a unit of light measurement taking area into account. In other words, light intensity.
We use lux to measure the amount of light output in a given area. One lux is equal to one lumen per square meter. Lux is a good measurement for determining and comparing the brightness of a beam. The beam is spread over a specific area and therefore we use lux to measure light intensity. However, lux is just another number and doesn't provide the complete picture of what we're after. We need to know the beam angle and look at beam shots to fully understand the picture.
Lux is measured using a lux meter.
Beam Angle (FWHM)
What is it?: Beam angle is the angle of which a light is emitted. More specifically, it is the Full Width at Half Maximum.
Since there is no real way to measure the 'edge' of light we measure the beam angle from where the light is at 50% intensity (FWHM). By being able to measure where 50% light intensity ends, it gives us the majority of where the light is used thus representing the beam angle. Smaller beam angles will have an intense hot spot, where as larger beam angles will have a lesser intensity (due to a larger area). Take a look at our beam shots and see if you can gauge where 50% intensity ends, giving you the beam angle.
What is it?: Watt is a unit of power consumed.
More wattage produces more lumens. We can use wattage and lumens to measure efficiency of the light emitter. In the case of LEDs the efficiency is much higher than that of older halogen lights, producing significantly more lumens per watt. However, more power also means more heat is produced.