Hi, just started cycling about 2 weeks ago. Noob rider. Silent reader too. But experienced light builder.
About the light you all discussing. About those (I love diy light stuff)cree t6 smd(or u guys call it led) is rated 5v 5watt smd, depend on manufacturer, you can overdrive(overpower) it to 6/7watt(usually used for strobing) or some underdrive(underpower) to 4watt to reduce heat.
Their 10watt rate is around 400lm. Measured 10cm away from bare bulb (no reflector or lens).
What you see in the ground luminated from the light is not the measurement of the lumens. Depeending on the reflector/lens build for the t6, it affect how the light is concentrated or spread wide... Spread light always is softer than concentrated (those of you have zoomable Torchlight would understand) so lumens doesn't determine your illumination, reflector and lens plays a major factor too. That why branded like topeak, lynnze and cateye even at lower rated lumens seems brighter is due to the lens/reflector designs
The usb mini light someone posted few page back, the dual t6 is fired around 8watt(4 watt each) at 5v consuming 1.6A... yes, 5V 1.6A for several reasons, pumping 10watt will consume 2A from your power bank running for 15min will kill your powerbank circuit . If your power bank doesn't support 2A(or it's 1A PB) it might instantly turn off or work at lower wattage. I would say the is mini light fires about 700-800 actual lumens.
My personal advise, Get a good long life light that last min 3hrs, low power last 5hrs . Lumens is not really a big deal if you ride on well lit road, the light is for ppl to see you. And avoid dark roads at all times. You don't want to get robbed of your bike.
I agree with you, the lens is a major factor. It will make a big impact on how the light is distributed on the path. Some poorly design light have the light concentrated on a small spot. They can be very bright and can shoot out 100-200ft but this kind of light is useless for night MTBiking. A good light for MTBiking is one that is gives you evenly disperse light from 5-30ft in front of where you are going.
Road biking at night are different from riding MTB at completely dark trail. Road bike can get away with decent light as the road is not so dark and are flat and smooth. MTBiking at trails with technical downhill requires really good and bright light.