DBG: Lights for bat work: LED's
Modern LED's provide the best current solution to the need for portable lighting.
They have the highest efficency of any modern light source; which means your torch gives more light, and puts less drain on the battery. They provide a compact source of light, so its easy to focus; and unlike other sources the light is directional, so there is less light wasted. All this adds up to make LEDs the best choice of light source for most applications - and especially for portable lighting.
Development of white light LED's
Visible light LEDs first became useful in the early 1970's as indicators. Available only in red, orange and yellow they were too dim to be useful for illumination. Almost a decade later green LEDs became available; but it was only in the mid '90's that "superbright" blue LEDs were available. By adding a yellow phosphor to the capsule a white light was produced.
This chart shows how light sources have develped over the years. In 1999 LED efficiency surpassed that of conventional incandescent bulbs, and in 2006 Cree demonstrated white light LEDs with a better efficiency than fluorescent tubes. LED's overtook discharge lamps in 2007, and now have by far the highest efficiency of any light source.
In 2014 Cree demonstrated an LED producing a staggering 300 lumens per watt, while leds currently in production are achieving up to 200 lumens per watt. (lm/W)
(Jan 2015) New from Cree in 2014, the best (V6) of their XP-L range of LED's provides 560 lm at 3.15W - 180 lumens per watt, and a massive 1200 lumens for only 10W in.
By comparison our "low energy" CFL light bulbs deliver 60 lm/W, and quartz-halogen headlight bulbs a puny 20 lm/W.
A lumen is the standard unit for measuring total light output, and a candle produces about 13 lumens.
Candlepower is a measure of the intensity of the brightest part of the beam.
A 5W LED bulb produces the same amount of light (450 lumens) as a 40W incandescent or 13W CFL lamp.
EXAMPLES OF CURRENT LED's used in portable lighting (all white light)
5mm Nichia LEDs (5-10 Lumens ea)
Some low power flashlights use a cluster of these (often 9) with 3 AA cells to provide a compact and cheap torch for general use.
Surface mount SMD LED's are used in groups in lights for automotive use etc.
COB (chip on board) LED used for broad area illumination. This one has 60 led chips on a substrate, the phosphor covers all the chips.
It measures 80*20mm, gives 3000lm @ 27W = 110 lm/W
|CREE P2 (67-73 Lumens @ 350ma)
CREE P3 (73-80 Lumens @ 350ma)
CREE P4 (80-87 Lumens @ 350ma)
Luxeon K2 PWC4 (85 Lumens @ 350ma) shown here >
CREE Q4 (100-107 Lumens @ 350ma)
SSC P4 U Bin (91-118 Lumens @ 350ma) shown here >
CREE Q5 (107-114 Lumens @ 350ma)
|CREE MC-E K Bin (370-430 Lumens @ 350ma per core)
CREE MC-E M Bin (430-490 Lumens @ 350ma per core)
|SSC P7 B Bin (570-700 Lumens @ 2800ma)
SSC P7 C Bin (700-800 Lumens @ 2800ma)
SSC buy chips from CREE and apply their own phosphor and packaging which impacts upon light output and heat transfer.
CREE XM-L T6 (1000lm @ 3A)
Cuurently the best LED for use in flashlights.
CREE XP-L V6
1200 lumens at 10W from a single die
Limit on luminous efficiency of light sources
1 lumen is defined as 1lm = 1.46mW of radiant power at 555nm wavelength. This means that the maximum theoretical efficiency is 685 lm/W at 555nm (monochromatic green light). The theoretical efficiency limit for a red LED = 530 lm/W A white LED is usually made by combining a yellow phosphor with a blue LED. The practical efficiency limit for a white LED was believed to be about 250 lm/W. The efficiency is improved if less phosphor is used; this is why a cool white LED is brighter than a warm white. By comparison the maximum ideal efficiency of a thermal light source is 95 lm /w and highest practical efficiency limit is about 50 lm/w.
Light output of LED's
The output in lumens of a power LED is often measured at a current of 350mA. This is not the maximum available output; but as more current is passed through the LED more heat is generated and this makes it difficult to take steady measurements at a particular temperature. At a current of 350mA the forward voltage across the LED is about 3.3V so the power is just over 1W. These graphs are taken from data for a CREE MC-E LED but are typical. The light you get out depends on the power being upplied to the LED.
Getting rid of heat
If we assume a luminous efficiency of 100 lm/w then only 15% of the incoming energy is being produced as light. (That's pretty good!) However it still means that 85% of the input energy is used to produce unwanted heat. An LED running at 7W wastes 6W as heat. The MC-E has a thermal resistance of 3 deg C per watt, so 6W results in an 18 degree rise above pill temperature - and possibly 40 degrees above ambient. Its important to keep LED's cool, because they become less efficent as they get hotter. Also their lifetime is reduced if they get too hot.