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 LEDs & Colour

LEDs can be made to emit light in a range of colours. These are not simply for decorative use, but have a range of practical applications, too. Here is a brief guide to the most common colours available and the practical uses to which they are most often put.

Click here for a quick guide to the colours of the visible spectrum and their respective wavelengths.

WHITE light allows perfect colour distinction, but can spoil dark-adapted vision, which then requires a period of re-adjustment, once the light source is extinguished.
RED light is traditionally used for preserving night vision. Red light does not cause your pupils to contract so much and as a result your eyes do not need to re-adjust to the darkness once a red light is turned off. Red light is also used as a "safe" colour in monochrome photographic processes and does not spoil film which is being developed.
YELLOW light offers some benefits of both red and white lights. While providing reasonable colour distinction, it allows a reasonable level night vision to be maintained, if it is at a low enough level. Another benefit of yellow light is reduced reflection and glare when reading, leading to reduced eye strain over longer time periods.
GREEN light can also be used to preserves night vision, but with the improved ability to distinguish colours - particularly useful for reading a map or chart at night. It is also less easily detected by night vision equipment, but more easily detected by the human eye, at lower brightness levels than a red light.
BLUE can also be used to read maps at night and is often preferred by military personnel, as it increases the level of contrast. It is also the classic backstage working light colour for theatres and shows. Blue LED light has the same effect as UV light on most fluorescent materials.
BLUE-GREEN brings similar night vision benefits to the green light and the blue light, but with improved colour distinction. Some users prefer the blue-green light for this reason.
INFRA-RED is for use in conjunction with night vision equipment. Otherwise invisible to the human eye.
ULTRA-VIOLET is most commonly used to detect forged banknotes, which glow under UV light. Some UV LED torches have become popular at clubs and parties, where they are used to make fluorescent materials shine brightly.

 

WARNING:
Ultra Violet light can cause serious damage to the eyes.
Never direct the beam of a UV light source into the eyes
of any living creature.

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The colour of light and its wavelength
The colour of the light we see is determined by its wavelength, which in turn is often used to denote the colour of light. The wavelengths associated with light are tiny and measured in nanometres (nm) - billionths of a metre. LEDs produce light that is almost coherent, meaning that it is almost all on one wavelength, giving very pure colours. Here is an approximate guide to the colours of light and their wavelengths.

MID INFRA-RED:
4600nm - 1600nm - Invisible
LOWER INFRA-RED:
1300nm - 870nm - Invisible
850nm - 810nm - Nearly invisible, very dull red
NEAR INFRA-RED:
780nm - Very dim cherry red when viewed directly
770nm - Deep cherry red when viewed directly
740nm - Deep cherry red
RED:
700nm - Deep red
660nm - Red
645nm - Bright red
630nm - "He-Ne Laser" red
620nm - Orange-red
ORANGE:
615nm - Reddish orange
610nm - Orange
605nm - Amber
YELLOW:
590nm - "Sodium" yellow
585nm - Yellow
575nm - Lemon yellow / greenish
GREEN:
570nm - Blueish green
565nm - Blue-green
555nm - Blueish lime green
550nm - Emerald green
525nm - Pure green
BLUE-GREEN:
505nm - Greenish blue / turquoise
500nm - Greenish cyan
495nm - Sky blue
BLUE:
475nm - Azure blue
470nm - 460nm Bright blue
450nm - Pure blue
BLUE-VIOLET:
444nm - Deep blue
430nm - Bluish-violet
VIOLET:
405nm - Pure violet
400nm - Deep violet
NEAR ULTRA-VIOLET:
395nm - Deep purple with a slight red tinge
ULTRA-VIOLET (UV/A):
370nm - Nearly invisible. May appear a very dim purple when viewed through Woods Glass.

WARNING:
Ultra Violet light can cause serious damage to the eyes.
Never direct the beam of a UV light source into the eyes
of any living creature.


WHITE:
White LEDs come in a wide range of shades from yellowish to purplish white. The most noticeable colour cast is generally in the centre of the beam and is usually slightly blue, surrounded by "pure" white. The size and noticability of the blue centre is often a guide to the quality of the LED: the whiter the beam, the better the quality.

 

The phenomenon of electroluminescence was first observed in a piece of Silicon Carbide (SiC), in 1907 by Henry Joseph Round. The yellow light emitted by it was too dim to be of practical use and difficulties in working with Silicon Carbide meant that research was abandoned. Further experiments were carried out in Germany in the late 1920s by Bernhard Gudden and Robert Wichard Pohl, using phosphor materials made from Zinc Sulphide doped with Copper (ZnS:Cu), although once again, the low level of light produced meant that no in depth research was carried out. In 1936 George Destriau published a report on the emission of light by Zinc Sulphide (ZnS) powders, following the application of an electric current and is widely credited with having invented the term "electroluminescence".

British experiments into electroluminescence, using the semiconductor Gallium Arsenide (GaAs) in the 1950s led to the first "modern" Light Emitting Diode (LED), which appeared in the early 1960s. It is said that early experimental laboratory LEDs needed to sit in liquid nitrogen while operating and considerable effort was required to make the breakthroughs needed to create devices that would function efficiently at room temperature. The first commercial LEDs were only able to produce invisible, infra red light, but still quickly found their way into sensing and photo-electric applications.
The first visible (red) light LEDs were produced in the late 1960s, using Gallium Arsenide Phosphide (GaAsP) on a GaAs substrate. Changing to a Gallium Phosphide (GaP) substrate led to an increase in efficiency, making for brighter red LEDs and allowing the colour orange to be produced.
By the mid 1970's Gallium Phosphide (GaP) was itself being used as the light emitter and was soon producing a pale green light. LEDs using dual GaP chips (one in red and one in green) were able to emit yellow light. Yellow LEDs were also made in Russia using Silicon Carbide at around this time, although they were very inefficient compared to their Western counterparts, which were producing purer green light by the end of the decade.
The use of Gallium Aluminium Arsenide Phosphide (GaAlAsP) LEDs in the early to mid 1980s brought the first generation of superbright LEDs, first in red, then yellow and finally green. By the early 1990's ultrabright LEDs using Indium Gallium Aluminium Phosphide (InGaAlP) to produce orange-red, orange, yellow and green light had become available.
The first significant blue LEDs also appeared at the start of the 1990's, once again using Silicon Carbide - a throwback to the earliest semiconductor light sources, although like their yellow Russian ancestors the light output was very dim by today's standards. Ultrabright blue Gallium Nitride (GaN) LEDs arrived in the mid 1990s, with Indium Gallium Nitride (InGaN) LEDs producing high-intensity green and blue shortly thereafter.
The ultrabright blue chips became the basis of white LEDs, in which the light emitting chip is coated with fluorescent phosphors. These phosphors absorb the blue light from the chip and then re-emit it as white light. This same technique has been used to produce virtually any colour of visible light and today there are LEDs on the market which can produce previously "exotic" colours, such as aqua and pink.

Scientifically minded readers may have realised by now that the history of LEDs has been a long, slow "crawl up the spectrum", starting with infra-red. Indeed, the most recently developed LEDs emit not just pure violet, but genuine ultra-violet "black" light. How much further up the spectrum LEDs can "go" is a matter of speculation, but who knows ? it may one day even be possible to produce LEDs which emit X-rays.
However, the story of LEDs has not just been about colour, but brightness too. Like computers, LEDs are following their own kind of "Moore's Law", becoming roughly twice as powerful (bright) around every eighteen months. Early LEDs were only bright enough to be used as indicators, or in the displays of early calculators and digital watches. More recently they have been starting to appear in higher brightness applications and will continue to do so for some time to come. For instance: all American traffic signals will have been replaced with LEDs by late 2005; the automotive industry has sworn to banish all incandescent bulbs from cars by the end of the decade, replacing them with LEDs - even in headlights. Most of the large video screens seen at outdoor events use many thousands of LEDs to produce video pictures. Very soon, LEDs will be bright enough to light our homes, offices and even our streets as well. The extreme energy efficiency of LEDs means that solar charged batteries can power LED units by night, bringing light to the Third World and other areas with no mains electricity.
The once humble Light Emitting Diode has truly come of age and is now making the jump from mere indicator to true... ILLUMINATOR !

For further news, information and articles on LED technology, see the LED Lighting: News & Articles section of our Links page.