More about torches

A torch comprises four main parts: the energy supply, the source of light, the optics to create a beam, and the casing and controls.

Lets start by looking at alternatives for the energy supply.  Most obviously we have batteries, with clockwork, butane gas, flammable liquids, and others as "fringe" sources. 

The clear winner on all counts is the Li-Ion.  However lets compare this with energy stored in flammable liquids and gases.

By comparison with batteries, hydrocarbons and alcohols are a very economical, portable and compact energy source.

Optics

The reflector is the most important part of the optical arrangement of a torch.  It gathers the light from the light source and focuses it into a parallel beam.  If this is made accurately, with the light source positioned in exactly the right place, the beam will show an image of the light source.  Any errors will result in distortion and softening of the beam. There are two main types of reflector surface - they can be mirror-like "specular" or textured "orange peel".  The orange peel type give a smoother change from the "hotspot" - the brightest part of the beam.  It also evens out slight imperfections in production.  Orange peel is by far the most popular kind of reflector. Some of the light from the source goes forward and misses the reflector.  This provides "spill" - a wide, dim circle of light which is useful as it lights up the area just in front of you and lets you see your immediate surroundings. The narrowest beam you can get is determined by the ratio between the source and the reflector diameters - small source, big reflector, tight beam.

Casing and controls

The casing provides protection for the components; keeps the optics correctly aligned; acts a a heat sink to remove heat from the light source; and completes the return circuit for the current. A high power LED is normally mounted on a board which is supported by a metal "pill", and this often accommodates the control circuit also.  Its important that there is good thermal contact from the LED to the body of the torch, or the LED will overheat and fail.
Most LED torches provide only one control - a switch (usually a tail switch).  Some of these are not very good quality and do not provide a sufficiently good contact especially when high currents are being drawn.		Many torches allow you to step through off-low-full which is an advantage.

Light source

Main choices for a light source are:		Efficiency
Flames.  Candles and wick lamps are still very widely used for lighting where mains electricity is not available. The source of light is hot carbon particles in the flame. These are caused by incomplete combustion of the fuel, so efficiency is very low.		0.1 lumens per watt
Gas mantles.   These are made from ceramic materials that are able to resist heat well.  Fuel - paraffin or gas is burnt to heat the mantle.  Because the combustion is much more complete the efficiency can be much higher.		1 lumen per watt
Traditional filament bulbs work by passing a current through the filament which causes it to heat up.  For best efficiency the temperature should be as high as possible; however at high temperatures the tungsten evaporates and gets deposited as a black layer on the glass.  Eventually the filament becomes too thin and breaks.		12 lumens per watt
Gas-filled bulbs: bulbs are often filled with a mixture of nitrogen and argon.  This reduces the evaporation of tungsten from the filament. However a gas fill does conduct heat from the filament and this heat loss reduces efficiency. Where maximum efficiency is important a more expensive gas such as krypton or xenon may be used.  These are better at reducing evaporation, and also conduct heat less.		15 lumens per watt
Tungsten - Halogen bulbs: by adding a small amount of a halogen (often Iodine) the tungsten on the capsule is returned to the filament.  This needs the capsule to be hot (200C) so quartz is often used instead of glass. A gas fill as above can also be used to further improve efficiency.		20 lumens per watt
Arc lamps (HID) use high voltages to set up an arc discharge between two electrodes.  The discharge excites the gas molecules which causes them to emit light at specific wavelengths. The envelope that contains the gas confines the gas into a small spherical shape ideal for focussing.  An outer envelope filters out ultra-violet light. Because these lights emit at set wavelengths the colour rendering is not very good.  They take time to warm up and can be damaged by turning on and off frequently.		80 lumens per watt
LED's (see below)		135 lumens per watt
Sources: Users Guide to Off-Grid Energy Solutions  The Great Internet Light Bulb Book

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.  The graphs below are taken from data for a CREE XR-E LED but are typical.

forward voltage of an LED as a function of current	light output as a function of current
As the current increases the voltage also rises.  At 350mA Vf is about 3.2V.  At 1000mA Vf is 3.6V so 3.6W is being used.	The CREE XR-E Q5 is rated 107 lm at 350mA.  Its maximum rated current is 1A so from the graph we can see that almost 200 lm is achievable.
light output of LED vs temperature	Getting rid of heat
	If we assume a luminous efficiency of 100lm/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.

Limit on luminous efficiency of light sources

1lumen 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 is 234 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.

LIST OF LED's

http://www.dealextreme.com/forums/Forums.dx/Forum.-209~threadid.218591 thefezman Tuesday, November 25, 2008 3:39 PM

5mm Generic LEDs (0.5-5 Lumens ea) 5mm Nichia LEDs (5-10 Lumens ea)
Any Generic "High Power" LED (10-70 Lumens, varies wildly) Luxeon I (40 Lumens @ 350ma) Nichia IO Moon (45 Lumens @ 350ma) Luxeon III (80 Lumens @ 1000ma) Luxeon V (87-120 Lumens @ 700ma)	Luxeon K2
CREE P2 (67-73 Lumens @ 350ma) CREE P3 (73-80 Lumens @ 350ma) Luxeon Rebel 80 (80 Lumens @ 350ma) CREE P4 (80-87 Lumens @ 350ma) Luxeon K2 PWC4 (85 Lumens @ 350ma, 250lm at 1500ma)	CREE XR-E P4
SSC P4 T Bin (70-91 Lumens @ 350ma) CREE Q2 (87-94 Lumens @ 350ma) CREE Q3 (94-100 Lumens @ 350ma) Luxeon Rebel (100 Lumens @ 350ma) Edison Optics KLC8 (100 Lumens @ 350ma) CREE Q4 (100-107 Lumens @ 350ma) SSC P4 U Bin (91-118 Lumens @ 350ma) CREE Q5 (107-114 Lumens @ 350ma)	SSC P4
Osram Golden Dragon Plus Ultra White (120 Lumens @ 350ma) Osram Diamond Dragon  (280 Lumens @ 1.4A) CREE R2 (114-120 Lumens @ 350ma) CREE MC-E K Bin (370-430 Lumens @ 350ma per core) CREE MC-E M Bin (430-490 Lumens @ 350ma per core)	CREE MC-E (4 cores)
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.

NOTE: The Cree MC-E has four cores and the rating is for total output with all cores running at 350mA per core.  You can (apparently) wire 'em up as 2s2p to draw lower current higher voltage.  When running at the max rated 700mA per core they produce about 175% of spec output i.e. 490 * 1.7 = 800lm  - same as SSC.