Not 20/120 lumens, but 20/120mA. Per the Optisolis datasheet, the output is approximately 8 lumens and 42 lumens. However, the high setting appears much brighter than a light rated at 45 lumens. By contrast, my original battery drainers are in the 8+ lumen range (depending on LED), but will light a room sufficient to navigate. That is using a "dead" cell which continues to put out a declining light until it drops below the 0.6V range. The lanterns will not go as low on high, but do a really good job of draining cells on the low setting.
The lanterns were purpose built for someone living off grid, using only solar to charge cells. They have AA NiMH and Li-Ion cells. The emphasis is on extended runtime with a respectable brightness. As with anything, there has to be a compromise. I visually selected the high and low settings after testing a lot of current sense resistors in circuit. In testing, I gauged whether I could read or do other tasks efficiently. The 200mA setting was only marginally brighter than the 120mA setting. But, it made the LEDs heat up in short order, as the heatsinking ability of the copper surface area is limited.
That is what I get from reading too fast, lol. 8 and 40+ lumens are still respectable outputs 8 for area lighting and 40 should be enough to work and read by as we used to do more with 15-30 incan lumens. I've personally found around 10 lumens a good enough amount of light for a room I recently purchased some cheap 3AA LED tap lights from WM that use a chip LED warm white color and are rated at 10 lumens and with diffused light they well light up a room in the dark. 40 lumens is pretty darn bright for a small lantern. 200ma is getting pretty high current for depleted cells for sure. I'm not as much interested in something like this to drain batteries but rather a way to use any battery at hand for light the lithium ion support is what makes this like a Popeil's pocket lanterman