White LEDs?

the colour of purity.
Most of my friends who are reading this will have the same perception of White Light as taught to us in School(CBSE). That white light is the light obtained by adding wavelengths of Blue, Green and Red. This is a huge misconception and not entirely true. I feel betrayed sometimes, when I learn of things which were taught to us wrongly at school. This is criminal and reflects the "study-to-fetch-marks" attitude of our education system, but well thats another story.

Colour is a generic term for three of its characteristics. We call the sky blue, and even the ballpoint pen ink is blue. But certainly they are different. So what makes up colour, it comprises of three attributes.(HSB model= hue, saturation, brightness model)

1. Hue: This is what we normally call 'colour' and christen as red, blue green, orange etc. It represents the shade of the object. It is represented on a scale of 1-360 as follows(known as the HSB/HSL encodings):
2. Saturation: This attribute, in lucid terms, is what we know as the "contrast" of an image, or rather, the density of the hue. The lesser is the saturation, the more faded the image is, and hence the word "desaturation".
3. Brightness: Last but not the least. The term is self explanatory. Also called "Value" of the colour, varies from 0-100%.

NOTE: 0 Hue= pure red
0 Saturation=Gray, fuzzy image
0 Brightness= dark or a black image

So what does one do if one wants white light? Do we mix the three Red, Blue and Green?
Well, the preceding discussion reveals that mixing "red" "blue" and "green" just take care of the HUE. What about the other two attributes of colour?
White light is supposed to have NO Hue(note 'no' hue is different from 'zero' hue which is red) and 100% brightness.

The impetus for me to write this article was the Latest IEEE-Spectrum magazine issue, in which I read about a certain Pete Vukusic (Univ. of Exeter), who explains that white light is actually a result of random scattering of light within an object. This takes care of no hue, 100% brightness and the contrast or saturation as well. Quoting from Wikipedia.com,

the color white results when transparent fibers, particles, or droplets are in a transparent matrix of a substantially different refractive index. Examples include classic "white" substances such as sugar, foam, pure sand or snow, cotton, clouds, milk, etc. Crystal boundaries and imperfections can also make otherwise transparent materials white, as in the case of milky quartz or the microcrystalline structure of a seashell.

So I think it should be very much clear to a layman now. That light needs to be scattered in a medium, due to drastically varying Refractive Index to get white light.

The latest IEEE-Spectrum issue, talks about the perfect white that exists in nature- and that is the light reflected from the shell of an insect- a beatle called CYPHOCHILUS. Scattering light to obtain white is done by the clouds in ~ 1Km thickness, by milk in ~ 1cm. But the Cyphochilus does it in-hold your breath- 5 micrometers! Such is the extent of scattering from its shell which conjures up the finest White existing in nature. The scientist mentioned before- Mr. Vukusic, found through an electron microscope that the shell of the Cyphochilus is composed of an entirely random network of entangled filaments of cuticle, of the order of 250 nanometers in cross section. Since this highly random arrangement scatters all wavelengths of light, and orderly arrangement in some beetle families scatters only selected wavelengths, and results in imparting colour to shell- the wavelength which is not scattered.

This is the Cyphochilus - looks cute dont it?

What does this have to do with white LEDs? Well, the scales of the Cyphochilus have important applications in producing white LEDs. If a synthetic form of this creature's shell be developed and used for the casing for an LED, the scattered light would be white. This could revolutionaise the household lighting systems, as LEDs are very low wattage devices and look pretty too. Only disadvantage being that since the light has to be scattered, these White LEDs cannot be used for uni-directional light.

White LEDs are of course made today, but with a different procedure. Quoting again from Wikipedia.com-: (its a bit hi tech, i suggest the easy-going dont read it!)

Most "white" LEDs in production today are based on an InGaN-GaN structure, and emit blue light of wavelengths between 450 nm – 470 nm blue GaN. These GaN-based, InGaN-active-layer LEDs are covered by a yellowish phosphor coating usually made of cerium-doped yttrium aluminum garnet (Ce³⁺:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce³⁺:YAG. The single crystal form of Ce³⁺:YAG is actually considered a scintillator rather than a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white".

Cheers