LEDs

overview | beam shape | white led tint | power 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. LEDs still let current through in one direction only.

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. They are inexpensively available in infra-red, red, amber, green, blue, ultra-violet, and white, and less commonly available in orange-red (sometimes called sunset red), orange, yellow, yellow-green, cyan, violet, pink and purple. Most of these LEDs emit just a single wavelength (colour) of light, but white and pink LEDs are made using a phosphor. 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 come with a flat top, which makes them handy for Christmas lights as they are not so directional.

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

A "naked" power LED is not very directional, so is good for car interior lighting. 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.) 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. The Cree XR-E is the best LED for using with an aspherical lens because it has a slightly narrower beam angle than other LEDs.

White LED Tint

Power LEDs - LEDs designed to work at 350 mA or more - 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 harder to find. They illuminate folliage much better than cool white LEDs do.
   
• 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 better. They are the hardest to find.

Common Power LEDs

Cree is one manufacturer of power LEDs and in my opinion one of the better ones. Their name has become almost synonymous with high power LEDs. For ease of use most of the LEDs linked below are mounted on 20mm star heatsinks, but smaller round heatsinks are also available, or bare emitters without a heatsink board. They all need heatsinking, especially if run above 350 mA. Tint is cool white unless otherwise mentioned.

For drivers to run power LEDs see the LED Driver List.