DBG: Simple science 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.

Energy supply

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 energy source needs to be compact and light, and the most important parameter here is the energy density. This is measured in Watt-hours per gram - the higher the better.

First look at the values for common batteries - first rechargeable, then single-use. (updated Feb 2015)

Type Size Volts Capacity Energy Price weight Price p/Wh E density
    V Ah Wh p g   Wh / g
 Li Ion  18650  3.6  2.4  8.64  250  45  29  0.2
 NiMH  HR6 (AA)  1.2  2.6  3.12  150  27  48  0.16
 Lead-acid  6V 4Ah  6  4  24  600  800  25  0.03
                 
 Zinc-Carbon  R6 (AA)  1.5  1.1  1.7  20  18  12  0.10
 Alkaline  LR6 (AA)  1.5  2.6  3.2  40  25  12  0.13
 Alkaline  LR03 (AAA)  1.5  0.85  1.3  25  12  19  0.11

 

The clear winner on all counts is the Li-Ion rechargeable cell. 

However lets compare this with common fuels.

Fuel Cost per kg Density Price Energy density
   (p)  (g / litre)  (p / Wh )  Wh / g
hydrogen  320    0.008  40
hydrocarbons - petrol, diesel, paraffin etc and gases - butane, propane  190  737  0.016  12
alcohols - ethanol, methanol  190  785  0.032  6

 

By comparison with batteries, hydrocarbons and alcohols are a very economical, portable and compact energy source. Even the alcohols have an energy density 30 times that of the best batteries!

Optics

All torches use a reflector, and in some cases a lens to focus the beam.

The reflector is the most important part of the optical arrangement of a torch.  It gathers the light from the light source and forms 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.

To get a good beam you need a compact light source.

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Casing and controls

The casing provides protection for the components; keeps the optics correctly aligned; acts as 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.

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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.

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Light source

Main choices for a light source are:  Source lm/W
Flames.  Candles and wick lamps are still very widely used for lighting where mains electricity is not available.

The main source of light is hot carbon particles in the flame. These are caused by incomplete combustion of the fuel, so efficiency is very low.
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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.  These mantles exhibit "candoluminescence" - they emit light at shorter wavelengths (i.e. bluer) than their temperature would suggest. xyz 1
Heated filament. Incandescent 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.  xyz 12
Gas-filled bulbs These are often filled with a mixture of nitrogen and argon.  The higher pressure 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.
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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. A typical 55W lamp (H7) emits 1400 lm or 1600lm with xenon fill.
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Compact Fluorescent Lamp This uses a gas discharge in a folded or wound tube to excite a white phosphor. Its a large light source, suitable for lighting, but can not be focussed into a beam. xyz 60
High Intensity Discharge (HID) Arc lamps use high voltages to set up an arc discharge between two electrodes in a chamber filled with a gas and containing metal salts.  The arc heats up and excites the gas and evaporating  the salts and soon a plasma is formed.  This emits light at many wavelengths.

The envelope that contains the gas confines the plasma 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 as good as incandescent lamps.  They take a few seconds to warm up and can be damaged by turning on and off frequently. A typical 35W lamp emits 2800 lumens
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Light Emitting Diodes LED's Unlike incandecent lamps which create light from a hot source, the active element of an LED needs to be kept cool, and this can limit the useful light output. For use in flashlights the diode is mounted on a baseplate "star". This is often made of a ceramic material that is a good conductor of heat. The electronic circuit that controls the current to the LED is mounted underneath. xyz

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