True white light contains all the colours of the rainbow, VIBGYOR -Violet, Indigo, Blue, Green, Yellow, Orange, Red. Each light colour has a specific frequency and wavelength. We can specify the light colour by either frequency or wavelength because product of frequency (f) and wavelength (w) is a constant, speed of light (c)
c = w X f
So higher frequency means shorter the wavelength. Lower frequency means longer the wavelength.
Our eyes can see wavelengths ranging from about 380 to 740 nanometers (nm) or alternately frequencies ranging from 430 to 770 Terahertz. Violet is the highest frequency (shortest wavelength) while Red is the lowest frequency (longest wavelength) which our eyes can see.
Although we commonly say sunlight (or the rainbow produced by it) has seven colours, if you look at the rainbow you know it is a continuous range or spectrum of light colours. This is because sun is thermal analog light source which produces a continuous range of wavelengths because of its high temperature. The below graph shows its spectrum. The black parts are the wavelengths that human eye cannot see.
[Public Domain Image, image source: Christopher S. Baird, data source: American Society for Testing and Materials Terrestrial Reference. Taken from https://wtamu.edu/~cbaird/sq/2013/07/03/what-is-the-color-of-the-sun/]
Incandendescent bulb is also a thermal analog source of light and hence produces a continuous spectrum of white light. Below graph shows its spectrum. The black parts are once again wavelengths that human eye cannot see.
ThreePhaseAC [CC BY-SA]
The spectrum for the bulb is similar in shape to the sun. The peak for bulb is at a longer wavelength (lower frequency) than the sun because bulb is at a lower temperature and hence produces lesser energy photons of light.
Now we come to LEDs. Unlike the sun and incandescent bulb LEDs or light emitting diodes are not a thermal analog light source. They are digital, non-thermal light source made of semiconductor material and based on quantum physics. The semiconductor body of the LED is divided into two regions, p-type (positive) and n-type (negative) semiconductor in contact with each other. The n-type region has extra free electrons while p-type region has missing electrons, called electron holes or just holes. Where the two regions touch each other, a p-n junction is formed. This page explains well how a p-n junction is formed and works https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/pn_junc/pn_junc.htm This youtube video also explains it in a simplified way.
When forward bias is applied across the p-n junction of the LED, that is, when positive terminal of a DC source like a battery is connected to the p region and its negative terminal is connected to the n region of the LED, electrons and holes start moving across the p-n junction and combining with each other. Thus current starts flowing through the LED. Free electrons in the n region are at a higher energy level compared to the electrons that have combined with the holes. This energy difference E is emitted as a photon of light of frequency f given by the equation
E = h x f
Here h is Planck's constant.
Since the free electrons are at nearly equal energies and electrons combined with holes are at nearly equal energies, the E varies only a little around its average value. So the frequencies of light produced are also around a specific colour frequency. So we cannot have white light (which has many colours) just from the LED.
So the solution they found is to use one or more phosphors along with basic LED. Phosphors are substances that absorb particular (typically higher) light frequencies and as a result emit other (typically lower) frequencies. As mentioned in Wikipedia and many other sources, typically blue LED is used with yellow phosphor to produce white light.
- https://en.wikipedia.org/wiki/Phosphor#White_LEDs
- https://ieeexplore.ieee.org/document/4147275
- http://www.photonstartechnology.com/learn/how_leds_produce_white_light
Of course other LED colours and other phosphors are also possible but we will focus on this method because it is the one most commonly used in commercial LEDs and seems to be used in the street LEDs I have seen because they have a bluish colour.
The below graph shows power spectrum of standard white LED, a tricolour fluorescent lamp and an incandescent bulb. Although they may appear similar white to the human eye, LED has significantly more blue peak between 400 and 490 nanometers wavelength. The other peak is from greenish to reddish with most output in yellow due to yellow phosphor. The blue light can cause sleep problems, bad health and eyesight damage as we will see in subsequent articles.
[Image taken from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4734149/]
But if sunlight and all white light contains blue light, why is blue light from LEDs a problem? This technical article gives some interesting hints though I have not been able to search for medical papers or articles regarding the same yet. https://www.electronicsweekly.com/blogs/led-luminaries/seoul-led-attempts-sun-spectrum-emulation-2017-11/
It suggests that eyes have evolved to protect against even or uniform spectrum of the sun using partial spectrum sensors and so eye's natural protection mechanisms get confused by lumpy spectrum such as LED with a big blue peak. Therefore natural protection mechanisms like pupil contraction, squinting and closure do not kick in when blue intensity is high but other colour intensities are low. As a result more blue light reaches retina from spectra with high blue peaks such as LED. And this blue light is harmful (proved in medical research papers) as we will see in subsequent articles.