Today we'll talk about artificial light sources. They were invented by people to illuminate spaces where the sun is not present or when it has gone away. In order to understand what characteristics are important for artificial light sources, we need to consider the spectrum of the Sun. The Sun is a highly red-hot body that emits a continuous spectrum over a wide spectral range. In this case, its energy is mainly spent on heat, heating. Only the part of the spectral range that our eye can see is used for lighting. This range is limited to a certain wavelength from blue, purple to red, resulting in 400 to 700 nanometers. It is this spectral range that people are interested in when talking about artificial light sources. This figure is related to the eye sensitivity curve, which has a maximum of 400 to 700 nanometres in the green range and an edge of 400 to 700 nanometres.
Let's remember the structure of the eye: it has cones and sticks. The cones are of three types and are responsible for determining a particular color. Their maximum is in the blue, green and red ranges. If they get blue, green, red light even with a narrow wavelength, but the same intensity, this light is perceived by us as white. Therefore it is easy to deceive a person: it is enough to take three narrow spectra of blue, green and red range, and the eye will perceive it as white. It is widely used in displays, when we have these three colors to create the whole palette and eventually a white image. But this is not always good for light sources. It was once thought that you could really deceive the eye like that and use the three colors to create white light. But now there is an understanding that this is not very good.
Initially, people used very simple light sources. It all started with a fire, rays, candles: they glow with hot carbon, which emits light in a wide enough range. But these light sources were uncomfortable. They emitted more heat than light. They were replaced by gas lanterns, kerosene lamps, which also used the combustion process. Later, arc lamps were invented - these are lamps in which an electric current passes through and discharges. Such lamps found their wide application in the XX century, and still some of them are used as very powerful light sources. The materials they use were originally carbon, but later they were replaced by refractory metals. An incandescent lamp was invented and is still in use. It has a very wide spectrum of light in the visible range and with a large contribution of the infrared range, that is, it is very warm. Because of this, most of the energy is converted to heat and the efficiency of such a lamp is low.
As more efficient lamps, fluorescent lamps are now widely used. They are also classified as discharge lamps. They generate current between the electrodes in a sparse atmosphere and gases glow. The most popular of these are mercury lamps. They include mercury vapour, which emits light mainly in ultraviolet, 254 nanometres, but there are even longer wavelength bands. In the early 20th century, inorganic phosphors were used to surround the discharge lamp and convert ultraviolet light into a visible spectrum. In general, our eye perceives this as a spectrum of the visible range. Fluorescent lamps, which are mercury vapour discharge lamps, have been widely used in all establishments since the mid-20th century. The actual spectrum of such lamps is not close to solar.
The colour rendering index (CRI) parameter is used to determine how close the spectrum of a particular radiation source is to that of the sun. It shows how integral the spectrum of your source is overlapped by the Sun's spectrum. The more it overlaps, the closer it is to 100%. If you take the spectrum of daylight, its color rendering index will be more than 95%, about 97-98%. The color rendering index of conventional fluorescent lamps is about 70%. This is determined by the selection of those phosphors that convert the ultraviolet spectrum of mercury glow. There are more expensive lamps with a higher color rendering index, they use more fluorescence. Luminophores in this case are inorganic phosphors.
Now it is increasingly said that it is necessary to use more energy-efficient lamps, so the fluorescent lamps have been replaced by LED lamps - light-emitting diode (LED). These are semiconductor devices based on spur-of-the-zone semiconductors. Typically, semiconductors are either A3B5 or A2B4, such as gallium arsenide, indium phosphide or zinc selenide. From an environmental point of view, these are not the most pleasant elements, but the advantage of such LED lamps is that they are very energy-efficient due to the fact that they have almost all the energy is spent on the fact that the visible photons are lit up. That is, they are much less heated than conventional lamps n