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LEDs

Overview

A diode is an electronic component that lets curent through in one direction only.

A light emitting diode (LED) is a special kind of diode that emits light when current passes through it. Like other diodes, LEDs also let current through in one direction only, so if they are connected the wrong way they do not emit light. Ordinary incandescent light bulbs will light up when current passes through them in either direction (as long as enough current is flowing to heat them up enough to glow).

Most LEDs emit just a single colour but there are also white LEDs which emit a broad spectrum of light. White LEDs are normally made from a blue LED covered with a yellow phosphor. The blue light makes the phosphor glow brightly, similarly to how UV light makes fluorescent colours glow. White LEDs are the brightest LEDs, and are even brighter than the blue LEDs that are used to make them. This is because:

  1. A lot of time and money has been spent developing white LEDs so they can be used as a light source.
     
  2. Our eyes are not very sensitive to blue light and the phosphor converts some of the blue light to light of colours our eyes are more sensitive to.

The voltage an LED needs to light up depends on what colour light it emits. Higher energy photons like blue and UV need more voltage to produce than IR and red photons. IR, red, and amber LEDs need about 1.6-2.2 V, while green and blue LEDs need about 2.5-3.7 V, and UV LEDs 3.1-4.4 V.

The most common LEDs come in a 5 mm diameter resin housing (or less commonly 3 mm). They are inexpensively available in:

Also available are orange-red (sometimes called sunset red), PC amber (PC = phosphor converted), yellow, green, cyan, royal blue, violet, pink and purple. Most of these LEDs are monochromatic, which means they emit just a single wavelength (colour) of light, but white, pink and PC amber LEDs are made using a blue LED with a phosphor. PC amber LEDs can be run at higher power than monochromatic amber LEDs, so they can produce more light, but are a little less efficient. Purple LEDs are either made using a blue LED with a red phosphor or with blue and red LEDs together.

LED Beam Shape

Most 5 mm LEDs have a domed end which focuses their light in a, say, 20° beam. It means they can be used to cast a directional light as-is, without any extra reflector. The domed tip can be chopped off (and filed smooth) if a wider beam is wanted.

Some 5 mm LEDs are much shorter, which makes them handy for Christmas lights as the rounded tip doesn't focus light, so they are not so directional. Others come with a flat top, or even with a conical indentation in the tip which help spread the light in a wider beam.

The brightness of 5 mm LEDs are rated using millicandela, which is a unit of light intensity.

A "naked" power LED (LEDs designed to work at 350 mA or more) is not very directional, so is good for car interior lighting if used behind a diffuser, but very unpleasant and inconsiderate for other people when used in headlamps because of the glare and the intensity of the light source (the amount of light coming from such a small area). To make a beam from a power LED, a reflector, total internal reflection optic, or lens is needed.

  • Reflectors have the most spill – the light around the central beam that comes directly from the light source. This means they are good for general work.
     
  • A total internal reflection optic is a carefully shaped lump of plastic that takes advantage of simple physics to bounce light forwards. (You can see the same reflection thing happening in a swimming pool on the underside of the water surface when the angle between your sight line and the water surface is small – the underside of the water surface looks reflective.) They offer good beams with little spill. Wide angle and elliptical optics are available.
     
  • An aspherical lens offers the narrowest beams and thus the best throw. However, if the power LED is positioned up against the back of the lens a nice, even, wide beam will result. An LED with a narrower beam angle will result in more of the LED's light focused forward.

White LED Tint

Power LEDs normally have better (whiter) colour than 5 mm LEDs. It can be hard to find 5 mm LEDs that don't have a noticeable blue tint.

LEDs, and power LEDs in particular, are available in a choice of cool white, neutral white and warm white tints. The different tints are produced by slightly different compositions of the yellow phosphor that is added to a blue LED to produce white light. They each have their advantages.

  • Cool white LEDs have a slightly blue tint. They are the most efficient (ie, brightest for a given drive power), the most commonly available, and may be slightly cheaper than the other tints. They provide the most lumens per dollar.
     
  • Neutral white LEDs are the closest to a pure white without any strong tint and are almost as efficient as cool white LEDs but are a little harder to find. They illuminate folliage much better than cool white LEDs do, making them look green not blue.
     
  • Warm white LEDs have a phosphor which produces more red light than the other tints do, and so have a colour similar to incandescent bulbs. They are noticeably less efficient than cool and neutral tint LEDs but are generally regarded as illuminating foliage the best. They are the hardest to find of the white tinted power LEDs, so may provide the fewest options.

Common Power LEDs

Cree is one of the best manufacturers of power LEDs. Their name has become almost synonymous with high power LEDs.

For ease of use in torches most of the LEDs linked below are mounted on 16mm star heatsinks, but larger and smaller heatsinks are also available, or bare emitters without a heatsink board. They all need heatsinking, especially if run above 350 mA, because the hotter an LED is run the less efficient it is. The newest LEDs are rated at 85°C, which gives a better idea of how much light they'll make when in use than the old 25°C rating does.

This table gives links to a few example LEDs. The LED's tint is cool white unless otherwise mentioned. Beam angle is FWHM (full width, half maximum) – the width of the beam where the intensity drops to half the maximum intensity (ie, the intensity in the middle of the beam).

Some of the less popular and older power LEDs have been moved to a separate Old Power LEDs page.

LED Type Description/Features
(beam figures are full width half maximum)
Common Efficacy Bins
(with linked examples)
Lumens at 350mA Lumens at Maximum Current Maximum Current
Cree XR-E

This is probably the power LED which really got things going. It was the first one to really have good efficiency.

90° beam, aluminium ring around the LED.
Warm white versions available, and various sizes and shapes of base board.

An 8° optic used to be available that clipped onto the LED – very handy.

Green (P)
(520-535 nm)
67 103 700mA
Blue (K)
Blue (K)
(465-485 nm)
31 67 1A
P4 warm white 81 178
Q3 94 207
Q5 WH (5000-5700 K) 107 235
R2 WC (6350-7000 K) 114 251
Cree XP-E2 White and coloured versions available (in theory).
110° beam (white, PC amber).
130° beam (amber, red).
135° beam (blue, green).
1mm2 emitter area.
All colours have the same 1A maximum current rating.
Lumen rating at 85°C.
P3 (Red, 620-630 nm) copper base 73.9 192 1A
R4 (1C cool white) 130 283
Cree XT-E 115° beam.
Lumen rating at 85°C.
Quite inexpensive.
R5 (unknown bin)
R5 neutral white (2B, 4B)
139 429 1.5A
Cree XP-G 125° beam.
2mm2 emitter area.
R4 neutral white (4B) 130 433 1.5A
R5 (unknown bin) 139 463
Cree XP-G2 115° beam.
2mm2 emitter area.
Lumen rating at 85°C.
R4 cool white (1A)
R4 neutral white (5A2) copper base
130
(147 at 25°C)
429
(486 at 25°C)
1.5A
R5 cool white (1A) 139
(158 at 25°C)
458
(520 at 25°C)
S2 (2B)
S2 (2B) copper base
148
(168 at 25°C)
488
(553 at 25°C)
Cree XP-G3 125° beam.
2mm2 emitter area.
Lumen rating at 85°C.
S3 warm white (7B) copper base 156
(170 at 25°C)
666
(740 at 25°C)
2.0A
S4 neutral white (5C) copper base
S4 neutral white (3A) copper base
164
(179 at 25°C)
700
(778 at 25°C)
LED Type Description/Features
(beam figures are full width half maximum)
Common Efficacy Bins
(with linked examples)
Lumens at 350mA Lumens at Maximum Current Maximum Current
Cree XM-L

125° beam.
4mm2 emitter area.
Can have 4 separate dice that are different colours.

Luminous flux bins are actually determined at 700mA.
Note that the Cree datasheet only mentions warm white up to T3 bin, neutral white up to T5 bin, and cool white up to U2 bin.

Various optics are available that loosely clip onto the LED (some tweaking may be needed to allow for solder blobs; glue is required for long term installation):

T4 warm white (6A, 7C) 123 781 3A
T5 (1C) 133 846
T6 neutral white
(3C, [T5?] 5C)
T6 warm white
([T3?] 7C)
144 911
U2 (0D, 0R)
U2 neutral white ([T5?] 3C)
155 976
U3 (1A) 165 1040
T6 EZW 6V warm white
(7C?)
183 715 2A
Cree XM-L2

125° beam.
4mm2 emitter area.
Lumen rating at 85°C.

Luminous flux bins are actually determined at 700mA.

Various optics are available that loosely clip onto the LED (some tweaking may be needed to allow for solder blobs; glue is required for long term installation):

T4 warm white (6C, 7B)
T4 warm white (7B2 80 CRI, 7B) copper base
126
(143 at 25°C)
789
(898 at 25°C)
3A
T5 (5A2, 5B1, 5D, 5D3)
T5 (5D) copper base
136
(155 at 25°C)
855
(973 at 25°C)
T6 (0D)
T6 neutral white (3B, 4C, 4D);
T6 neutral white (3C, 4C, 4D) copper base
147
(167 at 25°C)
920
(1048 at 25°C)
U2 (1A)
U2 neutral white ([T5?] 4C)
U2 (2C) copper base
157
(179 at 25°C)
986
(1123 at 25°C)
Cree XP-L

125° beam.
4mm2 emitter area.
Lumen rating at 85°C.

Luminous flux bins are determined at 1050mA.

A version without a dome is available, the XP-L HI (for High Intensity).

U5 (7A3, 7A warm white) 131
(149 at 25°C)
809
(919 at 25°C)
3A
U6 XP-L HI (7C warm white on copper board) 140
(159 at 25°C)
874
(990 at 25°C)
V3 (5D neutral white 153
(174 at 25°C)
944
(1073 at 25°C)
V4 (3D, 3D neutral white) 161
(183 at 25°C)
989
(1124 at 25°C)
V5 (2A, 2A cool white) 168
(191 at 25°C)
1034
(1175 at 25°C)
LED Type Description/Features
(beam figures are full width half maximum)
Common Efficacy Bins
(with linked examples)
Lumens at 350mA Lumens at Maximum Current Maximum Current
Cree XHP35

12V operating voltage.
125° beam.
Lumen rating at 85°C.

A version without a dome is available, the XHP35 HI (for High Intensity) with a 115° beam.

E2 590 1408 1050mA
E4 635 1516
Cree XHP50

6V and 12V versions.
120° beam.
Lumen rating at 85°C.

Luminous flux bins are determined at 1400mA (for 6V) or 700mA (for 12V).

J4 1120 2072 3A (for 6V)
1.5A (for 12V)
Cree XHP50.2

2nd generation XHP50, better thermal stability than 1st gen.

6V and 12V versions.
120° beam.
Lumen rating at 85°C.

Luminous flux bins are determined at 1400mA (for 6V) or 700mA (for 12V).

J4 1120 2045 3A (for 6V)
1.5A (for 12V)
Cree XHP70

6V and 12V versions.
120° beam.
Lumen rating at 85°C.

Luminous flux bins are determined at 2100mA (for 6V) or 1050mA (for 12V).

N4 1710 3270 4.8A (for 6V)
2.4A (for 12V)
Cree XHP70.2

2nd generation XHP70, better thermal stability than 1st gen.

6V and 12V versions.
125° beam.
Lumen rating at 85°C.

Luminous flux bins are determined at 2100mA (for 6V) or 1050mA (for 12V).

N4 (7A warm white 6V) 1710 3358 4.8A (for 6V)
2.4A (for 12V)
P2 (5C neutral white 6V, 25 mm disc, 20 mm star) 1830 3594
LED Type Description/Features
(beam figures are full width half maximum)
Common Efficacy Bins
(with linked examples)
Lumens at 350mA Lumens at Maximum Current Maximum Current

For drivers to run power LEDs see the LED Driver List. For more lumen output figures see Cree Product Characterization Tool.