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Artificial Light Sources

The development of artificial light based on electric energy sources began at the end of the 19th and early 20th centuries.  In developing artificial lights, the technological effort is aimed at producing light that will resemble natural daylight (sunlight).  Artificial light is measured in two ways – the specific frequency range of the source and the strength of illumination, measured in lumens.  Artificial light can be classified into three main sources, in accordance with its developmental generation and the technology that enabled its existence.

  

Evening sunlight at the seaside

Evening sunlight at the seaside

 

First generation – bulbs based on heating of a wire filament (incandescent lamps) or an arc

A standard modern incandescent lamp is composed of a glass bulb containing a coil of metal wire, such as tungsten (W), in a vacuum.  Incandescent lamps emit non-ionizing radiation in the visible light range and do not usually emit radiation in the ultraviolet (UV) range, except in extreme conditions of very high power. Additionally this type of lamp also emits invisible infra-red non-ionizing radiation which is felt as heat.

 

The halogen lamp is a type of incandescent lamp filled with a halogen gas such as iodine (I) or bromine (Br).  It emits visible light and a little UV light.  As UV is ionizing radiation  that may cause, with prolonged exposure, burns and skin cancer, prolonged close exposure to halogen lamps should be avoided.

 

The carbon arc lamp comprises two tubular carbon (C) electrodes connected to an electric voltage source in the open air.  This lamp emits visible light and also UV radiation.  Because of its high intensity it is forbidden to look at its light source directly.

    

Quality incandescent light bulb, after Edison

Quality incandescent light bulb, after Edison

 

 

Second Generation – Gas discharge lamps

Fluorescent lighting is based on electric discharge of a gas (mercury) leading to emission of high-energy photons (usually UV), that impact the fluorescent coating of the bulb, producing visible light.

 

The type of radiation emitted by fluorescent tubes and compact fluorescent lamps (CFL) includes, in addition to visible light, a small amount of UV radiation in the UVA range (315-380 nanometers), and even shorter wavelengths (higher energy) in the UVC range.

 

Exposure to UV radiation from CFL lamps containing mercury may be reduced by distancing the lamps from the user by 30 cm or more.

  

Compact fluorescent lamp (CFL)

                                                                                                                          
 
 Compact fluorescent lamp (CFL)

 

Third generation – Light-Emitting Diode (LED) lamps

A light-emitting diode (LED) consists of a semiconductor  that has undergone doping.  An electric current passing through the diode excites the atoms to high energies.  When the atoms return to lower energy levels, energy in the form of photons in the visible light range is released.

 

LED lamps emit visible light and do not emit UV radiation.

 
 

Modern LED lamps

Modern LED lamps

 

 

 

The first experimental attempts to produce artificial light from electricity began some 40 years before Thomas Edison registered a patent in 1879 on the incandescent light bulb he developed.  Other scientists, including William Sawyer, Joseph Swan and Albon Man were involved in the development of the incandescent electric light bulb, and even registered patents on different versions of the light bulb during the same period of time.

 

Thomas Edison, inventor of the incandescent light bulb – wax effigy

Thomas Edison, inventor of the incandescent light bulb – wax effigy

 

Artificial electric lighting completely transformed human life.  It enabled activity after dark, changed the daily timetables of workers, and expanded the hours of leisure and social activity.  It provided a tremendous impetus to improving and expanding sources of energy in view of the ever-increasing demand, and consequently to the economic development of many countries on a global scale.

 

Details on the development of artificial light, including the incandescent bulb, fluorescent lighting and LED lighting may be found on the US Department of Energy website.

 

Sources of artificial light are of three main types: incandescent or arc lamps, lamps based on discharge of gas, and light-emitting diodes.  The quality of light produced by each of these sources is characterized by its effect on the sense of sight in the human eye, and it is measured in one of two ways – the specific range of frequencies of each light source, and the strength of illumination.  The strength of illumination is measured in lumen units – the intensity of visible light as perceived by the human eye.

 

 

Light bulbs of different generations

Light bulbs of different generations

 

 

 

 

 

 

 

 

   

   

Standard incandescent lamps, halogen lamps, arc lamps and special materials                           

Incandescent lamps

A standard modern incandescent lamp is composed of a glass bulb containing a metal wire filament such as tungsten (W) in a vacuum.  The wire filament has high electrical resistance.  When an electric current passes through this wire filament, collisions among the electrons and atoms of the wire lead to incandescence of the wire (i.e. make it glow) resulting in light production.  Because over time the incandescence leads to evaporation of the atoms of the wire filament, in order to prolong the lifetime of the lamp and to preserve the quality of light, the lamp is sometimes filled with a noble gas (for example, argon), which reduces the evaporation of the wire filament.

 

Incandescent lamps are manufactured in a variety of sizes, voltages and electric powers.  The standard basic form of the incandescent lamp is considered as energy-wasteful because its efficiency is only 4%. Over the years more economical sources of light were developed, such as the halogen lamp, the fluorescent lamp, the LED lamp  and others.  In recent years, with increasing awareness of the need to conserve energy, both for economic reasons and to protect the environment, many countries have prohibited or reduced the use of incandescent lamps.

 

 

Old style incandescent lamps using carbon filaments

Old style incandescent lamps 
using carbon filaments 

  

Modern incandescent lamps using tungsten filaments

Modern incandescent lamps using tungsten filaments

 

Type of radiation emitted by incandescent lamps

Standard modern incandescent lamps produce non-ionizing radiation in the visible light range and do not usually emit radiation in the ultraviolet (UV) range, except in extreme conditions of very high power. The radiation range depends on the material of the metal filament in the lamp. Additionally this type of lamp also emits invisible infra-red non-ionizing radiation which is felt as heat.

 

Halogen lamps

The halogen lamp is a type of incandescent lamp generally containing a tungsten (W) filament and filled with a halogen gas such as iodine (I) or bromine (Br).  The halogen gas reduces the amount of tungsten gas vapor emitted by the filament, thus prolonging the life of the lamp and improving the quality of the light.  The lamp may therefore operate at a higher temperature than a standard incandescent lamp filled with noble gas, thus increasing its efficiency (by as much as 10% or more) and providing better illumination and effective use of electrical energy. This feature allows for the use of relatively smaller halogen lamps, integrated into compact lighting systems, such as floodlights.

 

Standard halogen lamps

Standard halogen lamps

 

Halogen spotlights in a meeting room

Halogen spotlights in a meeting room

 

Halogen lamp for courtyard

Halogen lamp for courtyard

 

Halogen floodlights in sport stadium

Halogen floodlights in sport stadium

 

 

Type of radiation emitted by Halogen lamps

Halogen lamps produce visible light. A small amount of the radiation emitted is in the UV range.  Because some of the radiation in this range is ionizing (like some of the sun's light) and may cause burns or skin cancer with prolonged exposure, such exposure in proximity to halogen lamps should be avoided.

 

To reduce the potential for exposure to UV radiation the quartz of the glass bulb is usually mixed with a minimal amount of UV-absorbing material (a process known as 'doping') or by optical thickening.  Another approach is to encase the halogen lamp in a glass case that reduces the risk of burns.

 

Halogen lamps that emit UVB radiation are used intentionally for scientific or medical purposes, such as dental treatments.

 

Occasionally, halogen lamps explode when reaching high temperatures.  One of the risks in using them is the danger of fires when they are located close to inflammable materials, such as in the roofs of wooden houses.

 

The USA Government Archives contain Recommendations for the use of Halogen lamps, in the section dealing with questions and answers relating to various safety issues.

 

 

Carbon arc lamps

The carbon arc lamp comprises two tubular carbon (C) electrodes connected to an electric voltage source, usually in the open air, with an electric current passing between them, restricted by means of a mechanism known as a ballast.  The electric voltage creates a state of plasma in the air that enables the passage of an electric current accompanied by emission of light. The carbon arc lamp was invented in the early 19th century and its commercial use began in 1870 for street lighting and big buildings.  The carbon arc lamp produces better light than the incandescent lamp, but the life of the carbon electrodes is shorter and requires appropriate care.  Today its use is restricted to equipment such as floodlights, stage lighting and hand-held torches.

 

Arc of light passing between two carbon coils

Arc of light passing between two carbon coils

 

Type of radiation emitted by carbon arc lamp

This lamp emits visible light and also UV radiation.  Because of its high intensity it is forbidden to look at its light source directly.

 

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Fluorescent lamps, compact fluorescent lamps (CFL) and light-emitting diode (LED) lamps 

What is fluorescence?

In visible light fluorescent illumination is usually produced in one of two ways:

  • A process whereby energy-bearing photons, of frequencies higher than visible light (such as UV radiation), are absorbed by certain fluorescent materials, rapidly producing spontaneous emission of photons at visible frequencies (usually within about 10 nanoseconds).
  • Collision between energy-bearing electrons leads to excitation of electrons or molecules of certain fluorescent materials.  The excited electrons spontaneously return to lower energy levels, a process accompanied by the emission of photons in the visible light range.
 

The fluorescent lamp

Fluorescent lighting in the form of a lamp or tube is based on electric discharge of ionized gas (plasma). Fluorescent lamps and tubes contain noble gases (e.g. argon (Ar), neon (Ne), krypton (Kr) or xenon (Xe)) or compounds of these, as well as substances such as mercury (Hg), sodium (Na), and compounds of metals and halogenic substances (metal halides).  In addition, the lamp or tube is coated internally with fluorescent material.

 

The process of lighting takes place as follows:

  1. The electrical connection provides the starter, the electrodes heat up, and thermal emission of electrons occurs.
  2. The electric voltage between the electrodes accelerates the thermal electrons, leading to direct excitation or ionization of the atoms of mercury (Hg) or other substances inside the lamp or tube.  In addition, indirect ionization (Pening ionization) occurs as the atoms of the noble gas (or gas mixture) in the tube, excited by the thermal electrons (in the electric field), transfer energy by colliding with the mercury atoms.  This energy leads to excitation or ionization of the mercury atoms, and the number of excited mercury atoms reaches higher energy levels.
  3. The mercury atoms that undergo excitation to high energy levels spontaneously return to lower energy levels.  This is accompanied by emission of UV radiation, especially in the 253.7 and 185 nanometer wavelengths (and in other materials in accordance with the wavelengths appropriate to their electronic properties).
  4. The UV radiation produced in the tube causes excitation of the atoms of the inner fluorescent layer coating of the tube which leads to spontaneous emission of visible light.

 

The perceived colors of the emitted light depends on the atomic properties of the  tube's fluorescent surface coating and is perceived as different colors, such as 'white' light, 'warm' light, etc. according to the fluorescent material.

 

 
 

Fluorescent light with electronic starter

Fluorescent light with electronic starter

 

Typical fluorescent lamp

Typical fluorescent lamp

 

Fluorescent lighting in subway station

Fluorescent lighting in subway station

 

Fluorescent lighting in an office

Fluorescent lighting in an office

 

Type of radiation emitted by fluorescent lamps and tubes

Each fluorescent light has spectral lines (specific light frequencies) characteristic of the mixture of gases and coating materials used in the lamp or tube. Ultraviolet radiation of these spectral lines is generally in the UVA range (380-420 nanometers).  Sometimes fluorescent lights emit shorter wavelengths, in the UVB and UVC ranges, if, for example,   mercury vapor is used that is not absorbed in the coating layer of the fluorescent lamp.  Emission of UV radiation of short wavelengths can be avoided by coating the lamp or tube with a layer of photoluminescent material, which absorbs the UV radiation and allows the emission of less energetic photons.

 

The following are details of some of the spectral lines of fluorescent light emitted by photoluminescent materials.  The figures are in nanometers (nm):

Mercury (Hg): 436.6, 546.5, 577.7, 580.2

LAP (LaPO4:Ce3+,Tb3+) , CAT (CeMgAl11O19:Tb3+): 587.7, 542.4, 577.7, 580.2, 580.0

Yttrium Europium Oxide, YEO (Eu+3:Y2O3):  587.6, 593.4, 599.7, 611.6

Argon (Ar): 760.0, 811.0

 

The coating layer is not always uniform, or it may be cracked or damaged, enabling a leakage of UV radiation.  To reduce exposure one should remain at a distance of a few dozen centimeters (or even several meters) from the fluorescent source.  In addition, one should purchase items only from a known manufacturer, of Standard Mark quality, and replace worn out lamps from time to time.

 

Compact Fluorescent Lamp (CFL)

A CFL is a fluorescent lamp of reduced size. This type of lamp produces lighting of similar intensity to that of incandescent lamps.  The functioning of the CFL is similar to that of regular fluorescent lamps: a CFL usually contains a mixture of gases and mercury vapor.  The electrons of the mercury atoms undergo thermal excitation to high energy levels, and emit UV light when they return to lower energy levels.  The UV light produced in the lamp causes excitation of the atoms of the inner fluorescent layer coating of the lamp  which leads to spontaneous emission of visible light and heats up other components in the lamp, such as the glass.  Although light produced in this fashion has a different spectrum to that produced by the incandescent lamp, the use of  several special phosphorescent materials alters the light spectrum emitted by CFLs to resemble the light emitted by incandescent lamps.  Coating the lamp with a layer of phosphorescent materials prevents the emission of short wavelength UV radiation by photoluminescence, which absorbs the UV radiation and enables the emission of less energetic photons (i.e. of longer wavelengths with lower frequencies).  However, UV radiation may trickle out when the coating layer is of poor quality, or cracked or inadequately manufactured, or damaged over time.

 

The energy consumption of a CFL is about a third or a quarter (or even less) that of a similar incandescent lamp, so that CFLs are considered energy-conserving.  CFLs are designed in various forms, such as coil, folded tube, etc., in attempts to maximize the amount of light while minimizing the lamp's volume.

 

 

Energy-conserving CFL

          

נורות CFL - גווני הארה שונים  

Energy-conserving CFL    

 

Type of radiation emitted by CFLs:

The radiation spectrum emitted by CFLs includes, in addition to visible light, a small amount of UV radiation.  The spectral lines of UV radiation depend on the use of phosphorus compounds in the CFL.  They include spectral lines in the range of UVA (380-420 nanometers) and lines with even shorter wavelengths (higher energy) in the UVC range.

 

Possible effects of radiation of fluorescent light from CFLs

The risks entailed in the use of CFLs are low, and involve possible exposure to UV radiation from CFLs containing mercury vapor, or exposure to the mercury from broken lamps.  Exposure to UV radiation from mercury-containing CFLs may be reduced by having them at a reasonable distance from the user.  According to the United States Food and Drug Administration (FDA), a safety distance of more than 1 ft (about 30 cm) reduces the risk of exposure to UV radiation from CFLs.  Also, one should avoid touching broken CFLs or remaining in their vicinity; pieces of broken CFLs should be sealed and stowed away, and discarded at a suitable collection or recycling site.

 

Further details on the safe use of CFLs may be found on the Website of the US Food and Drug Administration (FDA)

 

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Light Emitting Diode (LED) lamps

A diode allows an electric current to travel in a single direction.  A diode is created at the intersection between two types of materials (semiconductors).  When an alternating current is applied to the diode in what is termed a forward bias (direction), the current passes through the diode, while in a reversed bias (direction) the current does not travel through the diode.  The diode also functions when a direct electric current passes through the diode along its own bias (direction).

 

A light-emitting diode (LED) is composed of a semiconductor that has undergone doping.  When an electric current passes through the diode, the atoms of the semiconductor are excited (move up) to high energy levels.  When the atoms revert spontaneously to lower levels they emit photons in the wavelengths of visible light.  The color of light obtained depends on the properties of the materials composing the diode.  Light-emitting diodes do not emit light in all directions.  They must therefore be combined in such a way as to enable this function.  The diodes are integrated into a structure resembling a regular incandescent lamp.  As in such a lamp, LEDs produce strong lighting instantly.  In contrast, a fluorescent lamp does not light up immediately.  The quality of lighting of LED lamps and their output fade over time, although over the years the quality of LED lamps has improved.  LED lamps have a long lifetime (25,000-100,000 hours) and they are energy-efficient.   Today, LED lamps emitting white light are gradually replacing incandescent lamps and CFLs.  Since about a quarter of electricity consumption is for lighting, the move to LED lighting will have a tremendous impact on global energy conservation.

 

 

 
 

Display of LED and LCD screens

Display of LED and LCD screens

 

A variety of energy-saving LED lamps

A variety of energy-saving LED lamps

 

Sign illuminated by LED

Sign illuminated by LED

 

Advantages of LED lamps

  • High level of brightness and power
  • High efficiency
  • Use of low-voltage currents
  • Low heat emission
  • High reliability (physical stability)
  • No UV radiation!
  • Long lifetime
  • Easily programmed and controlled as part of an electronic array (for example in computer and TV displays)

 

Type of radiation emitted by LED lamps

LED lamps produce light in the visible range and do not emit UV radiation.

 

The LED lamp is of tremendous importance, and it is used in a considerable variety of fields.  It is sufficient to mention that LED TV screens have replaced the cathode tube screens that were bulky and costly in production and consumption of electric power.  Humanity is greatly indebted to the inventors of the blue LED lamp that subsequently led to the breakthrough in developing the white LED lamp.

 

In recognition of the importance of the invention and development of the blue LED lamp, the Royal Swedish Academy of Sciences awarded the 2014 Nobel Prize for Physics to three Japanese scientists: Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura.  The blue LED posed a challenge for three decades.  The Japanese scientists reached their breakthrough when they developed the blue P-N diode intersect from a material called gallium nitride (GaN).  While red and green LED lamps have been known since the 1960s, without the blue light it would not have been possible to manufacture lamps giving white light, produced by the blending of the three colors.

 

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References

 

 28.2.16