Are blue LEDs really blue?

[dave]

A little ot but I though maybe of interest. Just to ask if blue LED's actually are blue? i.e. emit blue light.

This advert is selling "Blue LEDs" but the description says "Clear Led" - & "blue lens". Is it that the lens filters white light to emit blue (rather than the semiconductor emitting blue)?

Wonder if these blue leds we see in lots of kit these days really are blue.

[Scott M]

Yes, the light frequency emitted is down to the chemistry of them. White LEDs filtered to blue would be more expensive to make and, bearing in mind their weird spectra, wildly innefficient.

[dave]

Ah Ok thanks - I thought perhaps the opposite was true in an attempt to make them cheaper :-)

[The Natural Philosopher]

LEDS tend to be monochromatic: white LEDS are either emitting a series of spectral lines in the actual LED, or have more than one emitter on in the things.

Needless to say they don't match the spectrum of a hot body like a sun or a filament.

This is informative:

"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."

[Man at B&Q]

Neither,, did you read what you quoted below?

Some manufacturers now add other coloured LEDs, alongside the white, in their "bulbs" in order to get a better colour balance.

MBQ

[The Natural Philosopher]

below indicates they have more than one emitter. A phosphor or phosphors.

[Harry Bloomfield]

dave formulated the question :

the light from the chip really is blue and the lens is clear. There was the basic red, amber and green, then much later came the development of blue. Very much later came white, but it was still tinged with blue. Now you can get a much better white and even a fairly convincing warm white LED.

[polygonum]

the laptop I am using at this very moment there are several blue LEDs. One looks like a standard "pure" blue. The other three are towards turquoise - a very much more gentle and attractive a colour. But definitely not monochromatic.

[Andy Burns]

Oh, that's /so/ last year DAAHling, my previous laptop had $LOTS of blue LEDs, my latest netbook has all white LEDs...

[polygonum]

:-)

I thought white was the year before? N'est pas?

Also got a sort-of tangeriney colour instead of orange. Again quite nice.

[Adam Aglionby]

Cyan is far end of colour binning for Blue LEDs, some makers `blue` binned LEDs almost looked green :-(

Bit more detail , Craig Johnson`s LED Museum: