The observed flickering is a problem with lower PWM frequencies. Some drivers have PWM frequencies in the sub 100hz region, and these are visible to the human eye. All my torches I bought with reviews demonstrating a pwm frequency in the khz range, and there is absolutely no flicker visible.
Now back to the lumens output, Im not sure how they have achieved more lumens with a pulsed power source. Maybe the LED responds differently, or is in a different part of its power output curve? Anyway.. ill put out a generic PWM vs Constant current example.
My understanding of driving LEDs with PWM, vs constant current (true constant current) is that the PWM provides less lumens than the constant current due to the decreasing output efficiency with increasing current flow as you tend toward maximum drive current.
Therefore - If you drive an LED at 50% current, It will produce more lumens per watt than an LED driven at 100% drive current. Do we agree with this? Here is a chart from a Cree XP-G datasheet.[IMG]http://i51.tinypic.com/55rymf.jpg[/IMG]
Now If we drive an LED at 350ma current, according to this graph, gives us a reference 100% lumen output.
Now lets drive this LED at a higher drive current, but pulse it to average the same average current. Lets say we drive the LED at 1050mA (3x the drive current). That will give us 260% of the light output, but if we only have it on for 1/3 the time, that will give us an Equivalent drive current of 350mA, but only 87% of the light output. (a 13% loss of output)
Actually thats not the whole story, the Vf of the LED will be higher at a higher drive current too, so there will actually be more than 3x the power(watt) input to the LED at 1050mA. Heres another graph. [IMG]http://i53.tinypic.com/4zw8pi.jpg[/IMG] At 350mA, Vf is 3.0v. At 1050mA, its closer to 3.35v. So if we update the comparison to compare Power input vs Output we get the following.
CC - 350mA, 3.0v = 1.05 Watts => 100% output (gives a ratio of 1.05Watts for 100% output)
PWM - 1050mA, 3.35v (0.333% duty cycle) = 1.1725 Watts => 87% output (_equivalent_ to a ratio of 1.34 Watts for 100% output)
So to put it simply, with PWM, you get a 21% loss of output in this example. (assuming my maths and fundamental understanding is correct)
So with that example, Simple calculations assuming constant Vf, we already see a 21% loss in output for the same power input
So why would we use PWM? PWM is a simpler (read: cheaper) way to control led power. Current control circuits are also more difficult to implement over wide outputs, as in a high output CC driver is fine at 1A drive current, but will be harder to keep operating stable at moonlight levels, the problem being a visible oscillation (or a different type of flicker). The other reason is tint, LEDs are rated for tint at a certain operating current, but many will green shift when driven at lower currents. Driving the LED at full current, but pulsing it will allow the same tint, but permit brightness control.
An example is the Sunwayman V10R I have, that is supposedly a true current controlled circuit, when the brightness is dimmed, the tint visibly becomes more green. My D10 ramping torches also are able to show tint shift when the battery runs low. It is a PWM driver normally, however when the battery runs low the battery may only able to provide say 0.5watts of power, but the PWM part still can change duty cycle. When I ramp the brightness up from minimum, I can see the brightness increase as the power to the LED increases. At this stage the power is pulsed at full current. As I am ramping up the brightness, the PWM duty cycle is increasing toward 100%. However I reach the limit of what the battery can provide about half way up the ramping, and the power to the LED doesnt increase. So as the duty cycle continues to move toward 100%, the battery can no longer deliver the power, and the led drive current must fall proportionally. So what I see is an equivalent of a constant current drive at a lower current level, and therefore I get a visible green tint shift from my torch.
Sorry If that was too confusing, especially the last part.