UPDATED 6/18/2011 - links to LED spectra, in Craig Johnson's site!
Keep in mind that the spectrometer used there runs low in red and especially near-infrared, and will distort the spectra of light sources unless their spectra are narrow or lacking output in/near red and IR. The specific spectrometer model used there is apparently designed to work better at shorter wavelengths including some UV wavelengths.
Infrared, Original Type, GaAs - This was the original commercially successful LED. It is plain old gallium arsenide and emits infrared, with a spectral peak at 950 nm. For all practical purposes, it is completely invisible to the human eye. The voltage drop is typically 1.2 to 1.3 volts. Typical drive current is 50 mA.
High Efficiency Infrared, GaAlAs - This improvement on the infrared LED became available in the early 1980's. Gallium Aluminum Arsenide is used in these for higher efficiency. Another feature is a shorter peak wavelength - typically 880 nm but this varies with the ratio of aluminum to gallium and probably a few other factors. These LEDs have a typical voltage drop around 1.4 volts and a typical drive current of 50 mA. These LEDs are usually slightly visible in a dark room since the spectral output extends to the low 800's of nm which the human eye can usually dimly see.
Original Visible Red, GaAsP on a GaAs Substrate - These LEDs were the original commercially successful visible type. The working chemistry is gallium arsenide phosphide with an arsenic/phosphorus ratio of around 60-40 by number of atoms, on a gallium arsenide substrate. The spectral output typically peaks around 660 nm in a band that is unusually narrow for red LEDs. The color is a pure red. The efficiency is horribly low, a few hundredths to around a tenth of a lumen per watt. Most LED lamps using this technology have a maximum continuous drive current of 50 mA and need 20 mA to be reasonably bright, optimistically working OK at 10 mA. The efficiency is maximized at higher currents of 20 mA and up. Typical voltage drop is 1.6 to 1.75 volts. These dimly glow at 1.5 volts.
Low Current Red, GaP - This was the original high efficiency red LED, which was available as far back as 1976. The chemistry is gallium phosphide doped with zinc oxide. These LEDs are usually impressively efficient (1 to maybe 2 lumens/watt) at low currents of a few mA or less, but are only about 2-3 times as efficient as the original formula red ones at 20 mA. The color varies with current, and is nearly enough pure red at .5-1 mA but more orange at higher currents. If the lamp is not tinted red, the emitted light is usually orange to sometimes slightly yellowish orange at 30 mA. The spectral output is a broad band, nominally peaking at 697 nm and maybe only peaking that far out in the red at really low currents. There is a secondary spectral band in the green peaking around 550-560 nm which is more noticeable at higher currents. Maximum drive current is usually 30 mA, but these LEDs have noticeably nonlinear light output that increases less than proportionately with current above a few mA. Voltage drop is around 1.9 volts.
Super High Brightness Red, GaAlAsP - These things became available in the mid 1980's. Earlier models with opaque substrates were impressive back then with efficiencies of 1-2 lumens/watt. These have since been improved, with transparent substrates and other refinements. Agilent has a similar chemistry which they call "T.S. AlGaAsP". The overall luminous efficacy of the best models is about 9 lumens/watt. The spectrum usually peaks between 650 and 670 nm, but some Agilent models peak slightly shorter. The color is pure red to He-Ne laser red, with a dominant wavelength (wavelength of monochromatic light of matching color) usually in the 640's but anywhere from about 635 to about 650 nm. Efficiency is usually maximized at currents near 20 or 25 mA. Efficiency at low currents of around a mA to a few mA is not impaired as badly as it is in many other types that have efficiency peaking near or over 20 mA. Typical voltage drop at 20 mA is 1.8 to 1.9 volts. Maximum rated drive current is usually 30 mA but sometimes 50 mA. These usually glow dimly at 1.5 volts.
High Efficiency Red, Orange Red, and Orange, GaAsP on GaP substrate - This was the first non-low-current high-efficiency red LED. The working chemistry is gallium arsenide phosphide, with an arsenic/phosphorus ratio around 40-60 on a gallium phosphide substrate. The GaP substrate is transparent to the emitted light and GaAs is not. This is one reason why these LEDs are more efficient than the original formula red ones. The color is normally orange-red with a dominant wavelength around 620 nm. A slight variation of this has a dominant wavelength usually around 610-615 nm and is considered orange. The peak wavelength is usually around 630 nm. Typical drive current is 5 to 20 mA and maximum current is usually 30 mA. Typical voltage drop at 20 mA is around 1.9 volts.
Ultrabright Orange-Red, Orange, Yellow, and Green, InGaAlP - The indium gallium aluminum phosphide LEDs came out in the early 1990's. These are usually either red-orange with a dominant wavelength around 610-617 nm, or yellow or "amber" with a dominant wavelength around 590 nm. Dominant wavelength can be as low as the 550's of nm ("pure green" or "emerald green", yellowish green but less yellow than usual for LEDs) and as high as the low 630's (He-Ne laser red). The most efficient ones are usually orange to orange-red. Overall luminous efficacy for orange and orange-red ones is anywhere from 7 to 28 lumens/watt, with most made after 2000 achieving at least 12 lumens/watt. Yellow ones are mostly a little less efficient, 4 to 15 lumens/watt with most made after 2000 achieving at least 8 lumens/watt. Green ones are less efficient still (mostly 3-4 to about 8 lumens/watt) but more efficient than GaP and GaAlP green LEDs. With the exception of some more-red models, these LEDs have significantly reduced efficiency at low currents of a few mA or less. Typical drive current is 5 to 20 mA. Maximum drive current is usually 30 mA in 3 and 5 mm "bullet" style units. Typical voltage drop at 20 mA is around 1.9 to 2.3 volts depending on the specific variation.
2-chip yellow GaP - These were the first yellow LEDs that I saw in Radio Shack in 1976. The LED lamps had a GaP low-current-red and a GaP green chip in parallel. The color varied greatly with current - reddish orange at .2 mA to a yellow that was not orangish at 30 mA. The efficiency was a few tenths of a lumen per watt, but maybe a little higher at lower currents where the red chip worked best.
Silicon Carbide Yellow - These were an early yellow LED with low efficiency. Someone who I have had contact with, Craig Johnson, has gotten samples of this type of LED in his hands! These are horribly inefficient and were made in Russia around 1973-1975. More info in Craig Johnson's LED Museum - 1970's Exhibits. and my Yellow SiC LED Page.
UPDATE - Someone e-mailed me saying that someone had a yellow-glowing silicon carbide semiconductor way back in 1907! The guy who supposedly had a glowing semiconductor back then was Henry Joseph Round. A bit more info in this minor collection of newspaper biographies. Even more info on Round's 1907 discovery and its confirmation by Lossev in 1923 HERE in the web site of Lumex, an LED manufacturer.
Yellow GaAsP - These were the first yellow LEDs to see really widespread use. The working chemistry is gallium arsenide phosphide, with a low arsenic/phosphorus ratio of maybe 20/80 to 10/90, on a gallium phosphide substrate. The color is usually orangish yellow ("amber") with a dominant wavelength around 590 nm or in the upper 580's. The efficiency is disappointing, maybe a couple tenths of a lumen per watt. Typical drive current is 20 mA and maximum drive current is usually 30 mA. Typical voltage drop at 20 mA is about 2 volts. Efficiency is usually maximized at currents near or over 20 mA.
High Efficiency Yellow GaAlAsP - This improved yellow which appeared in the mid or late 1980's was still not impressively efficient, with overall luminous efficacy around roughly .5 lumen per watt. See above for info on InGaAsP LEDs which include the only really efficient yellow ones that I know of. Besides the somewhat higher efficiency, the characteristics of GaAlAsP yellow LEDs are like those of the GaAsP yellow ones.
Green LEDs, GaP - The first green LEDs were made with gallium phosphide. Most of these are nitrogen-doped for maximum efficiency along with a very yellowish shade of green (dominant wavelength usually 565-570 nm). Efficiency of these is usually a couple tenths of a lumen per watt. Efficiency is usually maximized at currents near or over 20 mA, but a few more efficient ones have efficiency peaking around 15 mA at about half a lumen per watt. Typical voltage drop at 20 mA is about 2.1 volts. Maximum drive current is usually 30 mA.
High Efficiency Green LEDs, GaAlP - The gallium aluminum phosphide green LEDs have characteristics similar to those of the nitrogen-doped gallium phosphide ones, except the overall luminous efficacy is higher - around 1 lumen/watt.
"Pure Green" LEDs, GaP - These are yellowish green, but less yellow than usual with dominant wavelength in the 550's of nm. Hewlett Packard refers to this color as "emerald green". Overall luminous efficacy is horribly low, generally under .1 lumen per watt. (InGaAsP LEDs of this color get around a lumen per watt.) Efficiency is maximized at currents near or over 20 mA. Maximum drive current is usually 30 mA.
InGaN (indium gallium nitride) ultrabright blue and green LEDs:
Most of these are a slightly turquoisish blue (dominant wavelength around 470 nm), a slightly whitish green (dominant wavelength around 525-527 nm), or "traffic signal" bluish green. These are also made in deeper shades of blue, blue-violet and UV and sometimes lime green.
Overall luminous efficacy of the Nichia blue ones was about 4 lumens per watt at 20 mA in 1997 and has since improved to about 7-8 lumens/watt in 2001.
The overall luminous efficacy of Nichia's green ones was around 13 lumens/watt at 20 mA in 1997 and has improved to about 35 lumens/watt in 2001.
White LEDs are blue LED chips covered with a phosphor that absorbs some of the blue light and fluoresces with a broad spectral output ranging from mid-green to mid-red. The overall luminous efficacy of Nichia's units in 1997 was approx. 7.5 lumens/watt but has since increased to 15-20 lumens per watt by 2002 and with a few models achieving at least 30 lumens/watt in 2006. Lumileds is now producing units achieving 35 lumens/watt and ones based on Cree blue chips may soon achieve 60-plus lumens/watt.
The spectrum of these white LEDs consists of the LED band in the mid-blue plus the phosphor band from mid-green to mid-red. The spectrum runs low in far red and blue-green, high in mid-blue, low in violet-blue and really low in the violet. The color rendering index is 85 according to Nichia and other manufacturers mostly claim a more conservative figure of 70 that I think is easily exceeded.
Most white LEDs have a higher color temperature in the range of 5000 to 8000 Kelvin - icy cold pure white to bluish white.
There are now "warm white" LEDs with color temperature around 3500 K, which have the phosphor converting more of the blue light from the LED chip into yellowish light. Nichia has taken this to an extreme with yellow LEDs that are blue ones with phosphor. The color of those is an only slightly whitish non-orangish yellow with dominant wavelength in the upper 570's nm.
With the exception of Cree, the manufacturers use a sapphire substrate to
deposit the active semiconductor layers on. Since sapphire is nonconductive,
two leads have to be atached to the chip.
This gives non-Cree GaN LED chips an unusual appearance. Many have a light emitting area resembling a "square dumbbell" or "double diamond" or two squares merged together at the corners.
Cree uses a silicon carbide substrate which is conductive. They only need to attach a top lead to the chip so their LEDs have a chip apearance like that of most non-GaN LEDs.
InGaN LEDs have a maximum drive current of optimistically 30 mA, although heatsinkable high power models with maximum current of 350 mA and sometimes more are now common.
Typical voltage drop at 20 mA is about 3.4-3.8 volts. These LEDs are unusually intolerant of voltages and currents in excess of their ratings and are considered static sensitive.
Wideband GaN Blue - This was the first ultrabright blue LED, pioneered by Nichia in the mid 1990's. It is not quite as efficient as the narrowband blue - maybe around 3 lumens/watt. Its efficiency seems nearly enough constant as current is varied from a few mA to 30 mA. The color is a slightly whitish blue. The spectrum has significant content through the violet, blue, and green regions. The peak wavelength is typically 450 nm. Maximum drive current is optimistically 30 mA. These LEDs are unusually intolerant of voltages and currents in excess of their ratings and are considered static sensitive.
NOTE - 450 broadband bright blue LEDs have a nice trick - brief pulses
of high current at a low duty cycle will bring up a secondary
"superluminescent" spectral feature in the very near UV at around 380-390
nm. Other LED types that I have tested do nothing like this.
More details on this in my LED Blacklight Page.
NOTE 7/8/2000 - the Radio Shack 900-8005 is not a 450 nm LED as described in their "commercial catalog", but a 470 nm blue - apparently a Nichia NSPB-500S.
Silicon Carbide Blue - These are the original commercially successful blue LEDs pioneered by Cree in the early 1990's. I have been advised that Siemens dabbled into the blue LED game as far back as the 1970's and produced working models, but my impression from seeing commercially successful products is that Cree is the first significantly commercially successful producer. The color is turquoise blue at 10-20 mA and more aqua at lower currents. The efficiency is only around a tenth of a lumen per watt at 20 mA, but higher at lower currents. The spectrum is broad, like that of wideband 450 nm GaN blue LEDs but with less violet. Maximum drive current is optimistically 50 mA and usually quite safely 30 mA. The maximum chip temperature of silicon carbide old type blue LEDs is 150 degrees C, higher than that of most other LEDs.
Violetish Blue GaN on a SiC substrate (Cree "standard blue") - These blue LEDs are not as efficient as other GaN types, but they are impressive. Even though the overall luminous efficacy is only around half a lumen per watt, the intense slightly violetish, slightly whitish blue color gets attention. The spectrum is very broadband but is extra-strong in the mid-blue to violet, peaking around 428-430 nm. According to Cree the dominant wavelength is 466 nm. The efficiency does not vary much with current from a few mA to 20 mA, but higher currents extremely slightly favor a deeper blue color. Maximum drive current is very optimistically 30 mA. I think typical drive current should be 10 mA. The voltage drop at 20 mA is anywhere from 3.8 to about 5 volts, and generally higher in earlier models.
UV LEDs - Nichia produces two models with a UV chip (at least mostly GaN in the working layers) with a strong and narrower-than-usual band peaking around 370-375 nm. There is some weak broadband visible output (The ones I have seen glow a dim violetish white) and you may want to filter these if you are using them as blacklights. Typical operating current is 10 mA, max is 15 mA, and typical voltage drop at 10 mA is 3.9 volts. These cost around $20 (US$) last time I checked. More info is in my web files:
my Blacklight LED File
UV and "near-UV" LEDs with peak longer peak wavelengths up to 405 nm are now common. Ones with epoxy bodies tend to have very limited life expectancy, generally merely hundreds of hours to optimistically 2,000 hours according to some manufacturers. This is due to the epoxy in closer proximity to the LED chip degrading.
Toyoda Gosei now manufactures a 385 nm UV LED chip. Cree and Uniroyal manufacture "UV" LED chips with peak wavelength around 400 nm.
Pink and Purple LEDs: Pink LEDs consist of a blue LED chip with one or two phosphor layers over the chip. The first phosphor layer may be a yellow-glowing one, a technique normally used to make white LEDs. A phosphor layer producing red, orange or pink light is added afterwards. Some pink LEDs may be white ones with a pigment or dye rather than a phosphor.
Some pink LEDs have run into trouble:
* Some are blue LEDs merely painted with fluorescent paint or even fluorescent fingernail polish which can wear off.
* One model was a white one with a pink phosphor or dye added which faded noticeably after only a few hours of use.
ETG manufactures a purple LED, which is a deep blue LED with an orange-glowing phosphor added over the chip. The overall color is pinkish purple. They also make a similar one that is more pink.
Pink LEDs and the ETG Purple are described in greater detail, with photos, in Craig Johnson's Pink LED Page.
Links to other LED-related files in this site:
My LED main page.
My bright and efficient LED page. Includes suppliers!
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