The Luminous flux (Lumens) is the total light emitted by a lamp regardless of the direction of emission.
Mtbr measures actual Lumen output of bike lights by going to special laboratories that have an integrating sphere to measure actual Lumen output. We use the FL-1 standard and measure the Lumen output at the 30 second mark after turning the light on.
In addition, we run the lights for 3 minutes and graph the output over time. A powerful fan is directed at the side of the light to allow airflow and simulate riding conditions. The lights typically decreases in brightness as the light heats up, but every light reacts differently as you can see in the graphs. In general, a light head with less metal heat sinks will get hot and decrease in brightness more than other lights.
It is also interesting to note what our measured output is, in comparison to the manufacturer's claimed output. We of course appreciate manufacturers who deliver according to their Lumen claims, and some even deliver more real output than what they claim.
Click on the graphs below to enlarge.