We have seen that difficulties in falling asleep may occur due to improper use of electronic devices. Their screens, composed of DEL, show a harmful blue light, that we will study.
Light is composed of rays of different wavelengths. While we perceive it white, visible light runs from violet to red, its spectrum ranging from a wavelength from 380 nm to 780 nm.
What is a LED?
The led, light-emitting diode, is a component of electronics and Photonics (role in optical signals).
These light sources have invaded our environment, because of their low consumption of energy and long life expectancy. The LED will represent 75% of the lighting in 2020. They are found for lighting our homes, public places, in the streets and motorways. They are even used in computer monitors and televisions, smartphones…
Whence the blue light?
Some of these LEDs produce a cold white light or blue light also called HEV light, high visual energy. This part of the spectrum of white light is emitted by the Sun at low intensity and so did not affect our health. However, LEDs emit blue light in a way much stronger than UV.
It is composed of two parts: the turquoise blue light, which is essential for our body and purple blue light which seems to be harmful to humans.
In addition, the blue light produced by LEDs is busy more energy than natural light. Artificial light generates intensities that can be up to 1000 times higher than the accepted visual comfort threshold.
How does a led?
The most economical LED manufacturing method to associate a diode emitting a wavelength short (in blue) to a phosphor (substance that emits light, if crossed by energy) yellow, to produce white light:
The LED emits a signal when it is crossed by an electric current. She doesn’t let the current pass in one direction: from the anode (+) to the cathode (-), which are its two terminals.
The semiconductor is the engine of the LED. It consists of a stack of layers, called bands of energy. When an electron passes from the band of conduction in the valence band, a photon is emitted. This electron crosses so the band of forbidden energy between two energy levels and recombines with a hole. The emission of a photon is possible thanks to a hole-electron recombination. Thus emitted photon has energy.
When an appropriate voltage is applied to the drivers, the electrons are able to recombine with the holes of electrons inside the device, releasing energy in the form of photons.
An electron hole is the absence of an electron in the valence band, which normally would be filled without the hole in a semiconductor formed bands.
Despite their many advantages, LEDs present risks related to:
- A spectral imbalance of the LED (which corresponds to a high proportion of blue light in white LEDs)
- To very strong intensities emitted by these elements of reduced size.
The photochemical effect risks on health are associated with the blue light from the LED. They are often caused by a little intense but very repeated exposure. We will see later how blue light disturbs our sleep, and more generally our agency.
Photochemistry is a branch of chemistry that involving the study of the chemical effects of light (in the broad sense, from infrared to ultraviolet)
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