The daytime running lights have been the standard here in Sweden since long before I was born. I knew that the rest of the world would come around eventually ; )
PWM in only visible if it is done with a low frequency, some cars does this but not all. The only reasons to choose a low frequency is in order to use bad switching transistors or to avoid the need to filter the PWM signal. It is the reason those manafont XM-L drop-ins use such a low PWM frequency, it is in order to avoid using an expensive switching transistor. Higher PWM frequency means a higher number of "switches" per unit of time which means that the switching transistor is going to spend more time in its dynamic range which in turn means that there is going to be more losses in the transistor which in turn means more heat and possibly a burnt out transistor.
PWM above 100Hz is hard to detect if there is no motion, if there is motion then it is very easy to spot. Take a day when it rains or snows, light up the falling droplets or flakes and check the trails left on your retinas by them. If they are continuous, then the PWM uses a high frequency and is not as easily detectable. If the trails shows up as streaks of dots or a trail of short lines, then you have a case of low frequency PWM. I never use my low PWM frequency lights when it snows since I find the "strobe"-phenomenon it creates incredibly annoying.
Exactly as SPAMBOT said, Low frequency PWM is used to keep the costs down. Higher frequency switching needs faster transistors. Faster high power transistors are more expensive!
REgarding the "free lumens" on the graph, I believe it represents the light output for the period that the LED is on. It maybe doesnt count the time when the LED is off.
The IMPORTANT figure there is the lumen output efficiency. the quoted lumens efficiency is still lower at 18lumens per watt vs 20 lm per watt.. (thats actually really really low anyway comapared to an XM-L!! which gets more like 130 lumens per watt)
Interesting point and I agree. Light meters probably work somewhat like sound level meters, and there would be an lowpass RC filter providing a time constant of something like 200 - 500 ms.
Agreed that the efficiencies are important, but we must compare apples to apples.
The 18 lu/W value refers to operation at 22.5 A while the 20 lu/W refers to operation at 13.3 A.
Here's the same diagram with some reference lines added so we can compare the efficiencies under the same conditions.
The power input at 15 A is given by 15 A x 2.4 V which gives 36 W for both cases.
The efficiency with continuous current is 700 lu / 36 W x 100 giving 19.4 lu/W while the efficiency with pulsed light is about 850/36 giving 23.6 lu/W, almost 22% higher.
If the same order of improvement occurs with your XM-L you'd get more like 158 lu/W!!
I hear what your saying, in that the lumens "output" at the "same" 13A is higher in the pulsed condition on the Graph. This is correct, the graph clearly shows that the pulsed state is far more efficient, producing more "output". Reading this graph would make it easy to assume that the pulsed output produces more light, however I believe this isnt the whole truth. Seems too good to be true.
My point is that the that the datasheet shows that the lumens efficiency is LOWER with the 22.5A Pulsed 50%, than the 13A constant, consistent with what is expected, a 10% efficiency drop. So im not sure how the graph works...
