You’d look at these handy chart BLFers have made 
:
oh
there it is
i was wrong
looks like it just keeps going down, more watts == less lumens/watt
i was thinking it would be shaped like a bell, somewhere in the middle would be a sweet spot
but no
ok thanks
so this dude that wants to get the flashlight with the max lumens per watt, how would anyone advise him?
wle
Is it efficient to flood an entire room with light when I’m trying to focus on one object in the room?
good question
is it?
maybe the flashlight can do eyeball tracking and only put the light where you are trying to look
it could also sense (brain scan) when you are actually using peripheral vision, then make the beam all floody 
wle
You can estimate your design using OEM tools. Available to download from manufacturer’s website. But almost all of those official simulation tools only rated to OEM’s minimum and maximum range. You have to test or extrapolate yourself for current below and above OEM rated.
Theoretically, you will get the most efficiency when you drive any LED with as low as possible current (put consistent binned CCT/CRI/tint etc… aside). You can achieve this by using as much as possible LEDs at as low as possible current per emitter.
Unfortunately that’s NOT the case. There are few limiters:
A. Cost.
B. Design.
C. Space. related to above point
D. Optic compatibility for directional applications.
E. Driver operating range and efficiency limit.
[Clemence]
As you can see from the chart above, lm/W varies depending on how hard the LEDs are driven. Unless a flashlight only has a single mode, the dimmer mode will usually be more efficient, with higher lm/W.
“Photopic luminous efficacy of radiation has a maximum possible value of 683 lm/W, for the case of monochromatic light at a wavelength of approximately 555 nm (green).”
https://en.wikipedia.org/wiki/Luminous_efficacy
Lumens are defined as visible light, and the human eye is most sensitive to green light. In theory, a green-only LED will be much more efficient than a high CRI white light.
Useful advice cannot be given until we know more. What is he trying to achieve? How much light does he need? I assume 1 lumen of green light at a very high lm/W isn’t the goal.
The driver has a high effect on efficiency. If you run one or multiple LEDs with a Vf of 3.0V at the chosen current with a fully charged 4.2V cell, you are wasting 30% of the power as heat before even reaching the LEDs if you use a linear driver without PWM. The overall efficiency wil be even worse if you are using PWM as the LEDs will be run at higher current.
Still, linear drivers are the most popular because of price, compact size and firmware availability, despite their lower efficiency.
Unclosed quotation mark error! 
yeah i got the impression he isn;t all that technical - he just thought that would have a simple answer, not a science project
it;s here most efencent small led flashlight
wle
I think there is a point at very low power levels where efficiency is poor, so if the graph went down to the 0.1 or 1 lumen level, you could see a bit of a bell shape.
I would ask him why he cares about that metric when most lights are SO close to one another in most output ranges
he is only asking about lumens, which tells me he is a beginner
most lumens per watt comes from the greenest, coolest color temperature, lowest CRI LEDs
I suggest he expand his horizons to consider the Quality of the light, not just the Quantity
things like, how close is the color of the LED to Sunlight, including is the light High CRI, and is it free of Green Tint!
Cause myself, I have no interest in more lumens of green tint in cool white color temperature, and that makes red look brown and skin look dead. Even though that is what has the most lumens per watt.
sometimes beginners ask questions that show they are not aware of more subtle, and more important variables, than just how bright a light is.
so answering his question, may be a bad idea, because the question is not based on a broad enough understanding of the variables involved.
My suggestion is to get him to start by deciding what battery size he wants to use
then how heavy and large a light he wants
then how much he cares about the color of the light
it helps if he defines the application for the light
what I would use on a rifle to shoot coyotes is totally different than what I would use in my house, to find something under the couch
courses for horses
Jon, even a tint snobs would care about max efficiency depending on the applications. For example, in my survival kit, I highly prefer a Zuper efficient flashlight with whatever color it comes with for the longest run time possible.
We don’t have enough data of what he’s trying to achieve. So, it’s better not to assume anything yet.
[Clemence]
The simplest answer is usually the best. But, as others have said, there’s a bit of info missing. To get the “most lumens per watt” you first have to know at what light level it will be used. You want the driver to be optimized at exactly the level you wish to use, so there is no need to throttle it with PWM. And then, you want to use the most efficient LED that is capable of that level of output. Some higher output capable emitters are more efficient at low levels than the lower output emitters, but not in all cases. Also, as others have said, all emitters start at their most efficient and get worse with more power, so using multiple emitters will always be more efficient than using just one of the same emitter for the same light output.
I think his point is that lumens/watt by itself is a very “odd” request. “Most lumens/watt at around 100 lumens for a whole battery drain cycle” or something similar makes more sense.
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/lumpow.html
For 100% luminous efficiency, you need a pure 555nm green light source, like a laser, due to the sensitivity of the human eye which is how lumens are measured.
The efficacy is 683lm/W which is where all the energy is being converted into radiant flux.
Since it is a pure 555nm light, the radiant flux is equal to the luminous flux.
A better metric might be “lumen-hours” - lumens multiplied by how long you get them for. For a given LED, cell and flashlight form factor, the best light is the one that gives you the most lumen-hours in the mode you expect to use most often. That way, you’re accounting for all efficiency factors like LEDs, drivers, spring resistances, cell models and whatnot.
Unfortunately, manufacturers don’t generally supply that information, but some flashlight reviewers do runtime graphs of output over time, and you can figure it out from those - you’re basically working out the area under the line. You also get the bonus of being able to see how well-regulated the light output is - flatter lines are better regulated.
“lumen hours” varies with the battery/cell though.
That’s true.
Reviewers who put in the kind of work it takes to get runtime charts will make sure to use decent cells, and you can multiply / divide to account for capacity differences, but yes, there’s still room for variation.
The holy grail is to get the same reviewer using the same cell to review all of the lights you’re interested in Selfbuilt on CPF was the one whose reviews first showed me how remarkably useful that could be.