output vs. input

[papasan]

i'm curious if someone has a chart or a link to one that shows lumen/watt output versus miliamp input. i'm curious as to how much efficiencey really goes up or down as the mA input changes.

i'm mostly curious about nichia and luxeon whites.

 

[Jonathan]

I don't know if the Lumen/Watt chart exists, but the information needed to produce one is in the datasheets.

All of the Lumileds datasheets (which includes Superflux and Luxeon) include the following: charts of light output versus current at a junction temperature of 25C, charts of forward voltage versus current at 25C, charts of light output versus junction temperature at max current, and information about the change forward voltage with junction temperature. You would need to make some assumptions about your heat sink, to calculate junction temperature, and you could calculate the numbers.

In a nutshell, just from looking at the charts for the 5W luxeon: On page 6 there is a chart of luminous output versus current, and it clearly has a hump at around 400mA, but it doesn't intersect with zero, which suggests that efficiency goes up (in terms of lumen/mA) until the current gets quite low, perhaps peaking at 50 to 100 mA, perhaps lower. The forward voltage drop goes down pretty rapidly as current decreases, so I wouldn't expect a voltage drop of less than about 6V. Finally, the junction will be quite cool for anything less than a watt into the device, which will help efficiency quite a bit.

-Jon

 

[Steve-at-Springboard]

FYI, the maximum efficiency possible is 683 lumens per watt (if I remember correctly. That means that all electrical energy is converted to light, not possible at this point. According to my source, the king-of-the-hill is the low-pressure sodium lights at around 150-170 lumens/watt. The Luxeons are promising 50 lumens/watt at some point so we are still turning 90% or more of our precious battery power into heat! Room for improvement!

 

[Jonathan]

This is not quite correct.

A Lumen is a unit of light flux which includes the spectral sensitivity characteristics of the human eye. This means that 1 watt of photons of the proper color of green would be 683 lumen. However 1 watt of photons of a particular color of red might only be 50 lumen, and 1 watt of photons of IR would be _0_ lumen.

If one had a perfectly efficient _white_ light source, meaning that all of the electrical energy poured in gets converted to photons of a mix of colors so that the output appears white, then the _performance_ would be something like 200-300 lumen per watt. (The range depends on the fact that different mixes of colors could be called 'white'.)

Still _lots_ of room for improvement, but not all of that performance deficiency is electricity being turned into heat. Some of it is electricity being turned into light that we don't see very well.

-Jon

P.S. A sample of the LS5W was reported at 45 lu/watt _at low power_ (about 0.3W going into the 5W rated device).