Types of light sources

Light sources, i.e. optical radiation, are objects in which various forms of energy are converted into energy of electromagnetic radiation in the visible region of the electromagnetic spectrum. We divide light sources into own and non-owned. Bodies that emit radiation themselves are called intrinsic sources, while bodies that reflect light are called non-intrinsic sources. Intrinsic sources can be further divided, depending on the way they emit light, into natural and artificial sources (see below).

Color temperature

Color temperature (or chromaticity temperature ) characterizes the spectrum of white light and is measured in kelvins. Light of a certain color temperature has the color of thermal radiation emitted by a black body heated to this temperature. We consider yellow, orange and red to be warm colors, while shades of blue and purple are cold.

Color temperature (K)	Source
1 900	Candle
2 200	Sodium-vapor lamp
2 700	Tungsten light bulb
2 700 – 3 500	Warm white LED bulb
3 000	Halogen bulb
3 000 – 4 000	Sunrise and sunset
4 000 – 5 000	Fluorescent lamp
5 000	Direct sunlight
6 000	Cloudy sky
6 000 – 6 500	Cold white LED bulb
10 000 and above	Clear midday sky

Natural resources

Sun, stars and moon

Sunlight

Sunlight is electromagnetic radiation , which is produced by nuclear transformations in the interior of the Sun and reaches the surface through flux, absorption and emission. The temperature of the Sun's surface reaches 5,778 K. The intensity of electromagnetic radiation depends on the distance from its source. Light that hits the Earth's surface is filtered by the Earth's atmosphere.

The solar constant (also solar constant or solar/solar irradiance) is the flux of solar energy passing through an area of ​​1 m², perpendicular to the direction of the rays, per 1 s at the mean distance of the Earth from the Sun, measured outside the Earth's atmosphere. The constant includes the entire spectrum of solar radiation, not just visible light. The most precisely measured value during solar minimum is 1360.8 ± 0.5 W/m.

The flow of solar energy to the Earth's surface is called insolation and is dependent on the height of the Sun, i.e. latitude, part of the day and part of the year on Earth. The insolation symbol is Is, the unit is W/m².

In the atmosphere, the insolation value is lower than the solar constant value due to losses to the surroundings, while outside the atmosphere these values ​​would be the same.

Moonlight

The moon itself does not emit light radiation. We explain its glow as a reflection of sunlight. Polarized light is produced by reflection. The moon is therefore a non-proprietary light source. The color temperature of the moon reaches 4100 K.

Starlight

As in the case of the Sun star, nuclear transformations occur in other stars, which, among other things, lead to the emission of electromagnetic energy into space. The color of a star's light is determined by the frequency of its radiation and depends on the temperature of the star's outer layers. Starspots are areas of the surface with a lower temperature than the average value. Star spots on the Sun are called sunspots. The electromagnetic radiation of the stars covers the entire electromagnetic spectrum, from the wavelengths of radio waves to gamma radiation, i.e. not only visible light.

Fire

Fire is a form of combustion. It is a combination of heat and light that are released during the exothermic oxidation of flammable gases released from the fuel. The heat and light is created by the flames moving over the fuel. A flame is a visible area of ​​burning gases or vapors, solid components of the source, e.g. candle wick, wood, also play a significant role in it. The color and luminosity of the flame is typical of the substance being burned, which is used in the analysis of an unknown substance in spectroscopy . A fire is ignited when a combustible substance is exposed to heat or another source of energy. It is sustained by the heat it produces, and with some exceptions (e.g. hydrogen can be burned in chlorine to form HCl) it goes out when there is a lack of fuel and oxygen or a lack of heat (heat, not temperature, which is why fire burns even in winter).

Lava is molten rock that has come to the surface as a result of a volcanic eruption. Magma is the underground equivalent of lava. The principle of releasing light from lava is the same as in the case of fire.

Aurora

The aurora borealis is a uniform name for light phenomena that occur in the atmosphere at heights from 80 to 1000 km, but most often in the ionosphere about 100 km above the ground. The ionosphere is a region of high concentration of ions and free electrons.

During an eruption in sunspot regions on the Sun's surface, a cloud of particles is released. This is called the solar wind and is made up of protons, electrons and alpha particles. A cloud moves through space, and if it encounters Earth's magnetic field, most of the particles are reflected back into space, while some are captured and spiraled toward Earth's magnetic poles. Solar wind particles collide with molecules in the atmosphere and electrons are excited and de-excited. During this event, electromagnetic radiation is emitted, which is in the visible spectrum.

Lightning

Lightning is produced during a storm as a strong electrostatic discharge, which is accompanied by the emission of light (excitation and subsequent de-excitation of surrounding electrons, mainly nitrogen).

Organisms

Some organisms are capable of bioluminescence, which is a special type of chemoluminescence in which light is emitted using an oxidation reaction in the organism (oxidation of luciferin in the presence of luciferase). The reaction is very efficient, approximately 95% of the energy is converted into light (comparison: the efficiency of a conventional light bulb is 10%).

Animals

Fireflies are a family with very little morphological differences - the length and intensity of the light they emit helps them recognize a partner of the same genus. In addition, some beetle larvae can glow, this is probably a protective coloration.

The Nobel Prize was even awarded for the discovery of the green fluorescent protein of the jellyfish Aequorea victoria , and British scientists were then able to modify the substance to search for cancerous growths and thus aid difficult diagnosis. Animals of the deep sea also very often glow. The sea bream has a bioluminescent organ on its dorsal fin, which attracts smaller fish to it, which it then catches between its sharp teeth.

Bioluminescence is also used by many bacteria, for example Vibrio fischeri lives on some cephalopods, which can then be confused with the reflections of moonlight on the surface.

Plants

The green-yellow primrose of the feathered gorse is glowing.

Mushrooms

Mushrooms are also capable of bioluminescence, but they only glow when they are doing well. Common sedge, which grows in Europe as well, was observed in North America, emitting a green light. Other representatives are the olive mushroom ("Jack-O-Lantern"), the common boletus and several dozen other species. The evolutionary significance is not entirely clear.

Artificial resources

Heat sources

Thermal sources have relatively low efficiency in the visible region of the electromagnetic spectrum due to the need for the substance emitting the light energy to be heated to a high temperature.

Light bulbs

A light bulb consists of a glass bulb filled with a vacuum or inert gas and a red-hot tungsten filament that is placed in the center of the bulb. Excimers are formed around the fiber, the subsequent decay of which produces light radiation. A mixture of inert gas and halogen (most often iodine) inside the cell increases the light efficiency of the bulb. Such bulbs are called halogen bulbs. Light bulbs are used, for example, in lighting and laboratory instrumentation.

The main disadvantage of light bulbs is the low efficiency of converting electrical energy into light – most of the energy (90%) is converted into heat.

Luminiscence

Substances in which luminescence occurs are called luminophores. Their emission spectrum is discrete, i.e. substances with a continuous energy spectrum do not exhibit luminescence, e.g. metals. Luminophores can therefore be gases, liquids or solids, and their gases through which an electric current passes. Luminophores have the ability to convert absorbed energy of various kinds into electromagnetic radiation in the visible region.

The luminophore particle (electron) is first excited from the ground to an excited energy state. The difference by which the energy of the electron increases has the value h*f, where f is the frequency of the absorbed radiation. By transferring energy to another particle or transitioning to a lower energy level, the particle returns to its basic energy state (deexcitation) and emits a luminescence quantum of energy equal to h*f. No specific temperature is required for luminescence and it occurs even at low temperatures, which is why it is also called cold light.

Depending on the form of excitation energy, we recognize photoluminescence (Photoluminescence can be further divided into fluorescence and phosphorescence according to the time for which the luminescence persists after the excitation energy ceases to act. Fluorescence occurs only when the luminescent substance is irradiated. Phosphorescence usually persists longer.) , radioluminescence, cathodoluminescence, chemiluminescence (chemiluminescence arising in a living organism is referred to as bioluminescence - see above) etc.

Luminescence is widely used in practice. In the construction of luminescent lamps and gas lasers, luminescence in vapors of metals and noble gases at reduced pressure is used. In scintillation detectors and dye lasers, we find the luminescence of liquids, especially solutions of organic dyes. Crystalline luminophores, so-called crystal phosphors, are used in television screens. Luminescence is also used in the marking of cell structures for the evaluation of morphological and functional changes, where the luminescence ability of certain substances is used to highlight a certain structure.

Fluorescent diodes

Luminescent diodes are based on the principle of the release of charge carriers and the subsequent creation of excited states when the electric current PN passes through the transition in the conductive direction.

Discharge lamps

Discharge lamps fall into the category of luminescent light sources. They consist of a quartz tube filled with gas or metal vapors and electrodes at both ends. An electrical voltage is applied to the electrodes, causing the free electrons inside the tube to move from the cathode to the anode, and the impacts excite and ionize other gas atoms. When electrons subsequently transition from an excited to a ground energy state, they emit electromagnetic radiation.

Discharge lamps can be divided according to the type of discharge into arc, spark, glow and electrodeless, according to the type of environment into gas and vapor, and according to the time mode into continuous and impulse. The most common uses are high-pressure discharge lamps, for example in optical devices, solar simulators or in dermatology. Discharge lamps generally have more uses due to their high efficiency.

Fluorescent lights

A fluorescent lamp is a type of low-pressure discharge lamp, which consists of its own fluorescent body, usually a glass cylinder with barium electrodes. It contains mercury vapor and argon, and a luminescent layer on the walls. The electric field accelerates the electrons flowing from the cathode to the anode, which are then able to knock other electrons out of the substance, resulting in avalanche ionization. During the discharge, electromagnetic waves with a wavelength corresponding to UV radiation are generated in the fluorescent lamp. Electrons from the luminescent layer acquire the energy of UV radiation (excite), pass through a non-radiative transition to a lower level, from which energy is emitted - in the form of visible light. Fluorescent lights glow discontinuously, dark and light parts alternate in the tube.

Fluorescent lamps have a higher color temperature than incandescent lamps - that's why the light from fluorescent lamps is white rather than yellow, we perceive it as "cold light".

Lasers

Lasers differ from previous light sources in that they emit light in one wavelength - they are monochromatic. The laser beam is linearly polarized (ie it only oscillates in one plane). The waves are coherent, in phase. Thanks to these properties, the laser beams show only minimal divergence. They are used in medicine, in industry (cutting, drilling), in military technology, in laser printers or as a pointer.

Links

References

• BENEŠ, Jiří, et al. Základy lékařské biofyziky. 1. edition. Praha : Univerzita Karlova, 2005. 196 pp. ISBN 80-246-1009-4.

• NAVRÁTIL, Leoš – ROSINA, Jozef, et al. Medicínská biofyzika. 1 (dotisk 2013) edition. Praha. 2005. 524 pp. ISBN 978-80-247-1152-2.