# Question about lumens, leds and lasers



## irkuck (Apr 9, 2009)

The question is how the lumens given for describing the light output of power LEDs are related to the output light power of lasers.

Assume I have power LED with 1000 lumen output radiating at 555 nm. What will be the equivalent power of laser? I found formulas which say it will be 1000lumen/683(lumen/Watt) = 1.46 W. (the 555 nm is selected becuase then the luminous efficacy factor is equal to 1).

For white light this becomes bit more complicated since one has to take red, green, blue components separately
and calculate the equivalent laser power.

But the laser powers required seem to be quite high. Are these calculations OK?




I have several fibre optic ceiling lights. They are supplied from projectors which have 40-50 W MR16 halogen spot with reflector. Light input to the fiber optic system has about 9 mm diameter.

I figured out I can substitute the halogen lamp with high-power LED diode with about 1000 lumen output, e.g. Osram Ostar. I think this should have advantage over the halogen since the LED emitter has diameter almost matching the input to the fiber optic so if it is close to the entrance of light almost all light wil enter the fibers.

But here is the problem I am not able to answer. Assume that instead of LED I would use and RGB laser system. I found that such systems are available, they combine red, green, blue laser beams with half-transparent mirrors into a single white light beam. Now, if I would expand the white beam to the diameter of fiber optic light input (9mm), what would be the power of laser beam equivalent to the 1000 lumens rom power LED?

The trouble is that light output of lasers is given in mW,
and LEDs in lumens.


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## LukeA (Apr 9, 2009)

White light is 242.5lm/W. Maybe 600lm are making it into the fixture, so that's roughly 2.5W of output power.


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## Benson (Apr 9, 2009)

irkuck said:


> The question is how the lumens given for describing the light output of power LEDs are related to the output light power of lasers.
> 
> Assume I have power LED with 1000 lumen output radiating at 555 nm. What will be the equivalent power of laser? I found formulas which say it will be 1000lumen/683(lumen/Watt) = 1.46 W. (the 555 nm is selected becuase then the luminous efficacy factor is equal to 1).
> 
> ...


For 555nm, yes. The optical power to illuminate a room _is_ quite high; or rather, the lasers you're accustomed to are quite dim. They make a very bright spot (high lux), but don't have much total output.

To put the numbers in perspective, a D-bin P7 LED making 800-900 lumens of white uses about 10W, so 1.46W output to match these levels is not surprising at all. As LukeA said, the conversion for white light is around 240 lumens/watt, so the white P7 is really giving 3-4W of output. If you're looking for tunable color, but don't need the characteristics of laser light (extremely narrow-band or collimated light), perhaps you'd be better off with an RGB LED setup?


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## irkuck (Apr 10, 2009)

Thank you guys for your kind answers. I understand now the relation between lumens and light power. I also realized that application I am dreaming is depending on precise white light distribution across the beam. Hence the RGB beams should be collimated to have same diameter and mixed using half-transparent mirrors to produce perfect white. As light sources there can be used RGB power LEDs or lasers. LEDs are so much cheaper that can be considered free comparing to lasers but lasers have lower divergence. But my first step should to supply system with high power white LED source to check
the required light level.


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