As we all know light is the visible part of the electromagnetic radiation. The wavelength, which human retina perceives as light, is in the range from 380-400 to 760-780 nm. The shorter wavelength is in the ultraviolet portion of the radiation, and the longer wavelength – in the infrared portion, which is perceived as heat. Nowadays light is considered to have a corpuscular – wave nature. i.e. light on one hand can be considered as an electromagnetic wave propagating in a vacuum at a constant speed , and on the other hand, as a stream of photons – particles which have definite energy, impulse, impulse moment and own mass which equals zero (at rest). Light, unlike sound, can propagate in a vacuum. The environment in which light propagates, affects its speed. The speed at which light propagates in a vacuum is constant and equals 299 792 458 m /s. It is so constant that now even the main unit of measure – the meter is measured by the distance that light travels for a certain time. Thus the speed of light becomes a fundamental physical quantity with absolute value. There are two types of light sources: natural light sources (the stars, lightning, galaxies, auroras, the sun and the moon). Natural sources are also animals and plants which emit light, such as fireflies, some fish, mushrooms and insects (bioluminescence). Some chemical elements under certain conditions also emit light (chemiluminescence) and there are artificial light sources: this is light created artificially by man, such as torches, candles, fire and electric light sources. A laser is the brightest and most powerful light source. It is a source of monochromatic, coherent, directional light. Lasers emit a thin, targeted beam with a constant wavelength (the same color), constant phase and high brightness, and the ordinary incandescent light bulbs are still the most common light source. A bulb, as most people often call the incandescent lamp, is invented by the physicist Thomas Edison and is based on the effect of heating and emitting light from an electric wire with a high resistance when electric current flows through it. Other types of lamps are fluorescent lamps, metal halide lamps, sodium lamps, mercury lamps and LED lamps.
Main characteristics of light
1. It is a luminous flux, measured in lumens (lm), one lumen is equal to the luminous flux emitted by a point – source in a solid angle of 1 steradian, at light intensity of 1 candela. Compared with the consumption in watts (W) it is a key indicator for the energy efficiency of lamps. For example, in a standard bulb with consumption of 100W, the luminous flux is 1300 -1530lm. With energy-saving lamps of the CFL type, 20W equal to 1100lm, i.e. 55lm /W, so it is often said that an energy-saving bulb is 5 times more efficient than the standard one. LED lamps have even greater efficiency in the range of 90 to 160lm /W. Of course, there are models where this ratio is even higher and vice versa.
2. Intensity – light intensity is measured in candela (equal to the light intensity in a given direction, coming from a source of monochromatic radiation with frequency 540•1012 Hz, whose energy intensity of light in this direction is (1/683) W/sr.). This frequency corresponds to the green color to which the human eye is most sensitive. The word candela comes from the Latin word for candle, because it is approximately equal to the intensity of the light emitted by a candle. For this reason, some time ago light intensity was specified in candles. Now this is already obsolete. Thus, for example, a 100W incandescent lamp has light intensity of 100cd, but a 5W LED lamp has light intensity of 30cd.
3. Wavelength – also referred to as period in physics, is the distance between two adjacent peaks (or lowest points) in the sinusoid of the wave. The number of waves per unit time is called frequency. Regarding light, there is ultraviolet light, visible light and infrared light, in a wider range there are radio waves, X-rays and gamma rays.
Figure 1 shows the wavelength as a function of the intensity of some of the most common types of lamps. If we have to analyze the light spectrum in the visible portion of the wavelength of different lamps in reference to the daylight spectrum, we will notice that while in the daylight (sunlight) all lengths have relatively uniform intensity, in incandescent lamps, when the wavelengths are in the ultraviolet and blue spectrum, the light intensity is quite low and increases gradually, and in the red and infrared portion of the spectrum the intensity is even greater than that of daylight. The main disadvantage of a standard bulb is that approximately 95% of the energy consumed by the lamp is emitted as heat, which can be seen in the graph, and only 5% – as visible light. A fluorescent or a luminescent lamp, as shown in Figure 1, has a large amount of spectrum holes and has the weakest parameters of all displayed lamps. A halogen lamp, also known as a tungsten halogen lamp or a quartz iodine lamp, is an incandescent bulb, which has a small amount of halogen added, such as iodine or bromine. The combination of the halogen gas and the incandescent tungsten filament produces a halogen cyclic chemical reaction that precipitates the vaporized tungsten back onto the filament, increasing its life and maintaining the cleanliness of the cover. Its spectrum is very similar to that of sunlight, the only difference being in the blue portion of the spectrum, where the intensity is very low. For this reason the light from halogen lamps seems a bit yellow, that is why there is a peak in the spectrum of this type of bulbs namely in this portion of the spectrum. LED lamps, which we will discuss in more details later, have different spectrum for the different types of LED lamps. The-cold white LED lamp has a spectrum peak of around 450nm wavelength (the blue spectrum), while the warm – white LED lamp has a peak at 550nm wavelength and please note the sharp drop in the blue portion of the spectrum, resulting in warm-white light emitted by this type of LED lamps. The conclusion is that if we combine cool-white and warm-white LED lamps it will strongly resemble the spectrum of sunlight.
4. Color temperature – also called spectrophotometric or colorimetric temperature, characterizes the emission intensity of the light source as a function of the wavelength in the optical range. According to Planck’s formula, the color temperature is defined as the temperature of an absolutely black body, emitting radiation of the same color tone as the specified emission. It characterizes the relative portion of a definite color emission in a visible light source. It is used in colorimetry, astrophysics (the study of the distribution of energy in the stars spectrum) and is measured in Kelvin. One Kelvin degree is equal to one Celsius degree, but both scales are offset from one another (° C = K – 273,15). Color temperature characterizes the spectral composition of the light source.
To make it more clear we will give examples: the standard paraffin candle has a color temperature of 1900 K, incandescent lamps – 2700-3000 K , halogen lamps – 3500 K, lamps for illuminating the shop-windows of meat stores (they have increased intensity in the red portion of the spectrum) – 3890 K, fluorescent lamps – 4500-6500 K, daylight (average) – 5200 K, daylight on a cloudy day – 6500 K, aquarium lamps (they have actinium blue tint) – 10 000 K, a clear blue sky in winter – 15 000 K, blue sky in polar latitudes – 20 000 K.
5. Illumination – it characterizes the relation between the luminous flux, falling on a surface and its area. It is measured in lux (lx) and is equal to 1 lumen per square meter. The amount of light, reflected from the surface, is called brightness.
How to interpret what is written on the packing of lamps.
1. Light output in lumens. If you divide those lumens by the wattage of the light bulb (lumens/watt) you will get its effectiveness. As a base you can use an incandescent bulb: 100W correspond to 1300-1530 lumens, 75W – 920-1060 lumens, 60W – 700-810 lumens, 40W – 410-470 lumens, 25W – 220 – 250 lumens.
2. Lamp durability. It is measured in hours. Standard bulbs function about 1000h. LED lamps have duration of over 15 000 h. Energy-saving lamps also have a long life, but it depends on their commutation frequency. By frequent switching their durability is greatly reduced.
3. Color of light. This is the color temperature which we already explained in detail. Standard lamps emit only warm-white light, while LED lamps color temperature can be selected between 2700 K warm white, 5500 – 6500K cool white or 4000K neutral – which is closest to the sun. Best efficiency is obtained by cold-white light.
4. Number of commutations. This information is particularly important for energy – saving bulbs. Do not install standard compact fluorescent bulbs (3000-6000 commutations) at places such as toilets or corridors, equipped with motion sensors, where they will be turned on and off frequently, i.e. more than 3 times a day. LED lamps have no problem with switching.
5. Time to reach maximum brightness or time required to heat up the lamp. Standard compact fluorescent (energy-saving) lamps heat up relatively more slowly than conventional lamps and LED lamps.
6. Adjusting the intensity of illumination, lamp dimmability. Always make sure the lamp is dimmable if you plan to use it in a system with adjustable lighting. Most standard energy-saving lamps are not dimmable, but there are exceptions.
7. Operating temperature. If the lamp is used for outdoor lighting or in higher mountainous areas in winter, the luminous flux can be reduced at low temperatures. In general, energy-saving lamps are more sensitive to changes in temperature and humidity.
8. Sizes. It is important to check the size of the lamp if you need to put it at a specific place with small dimensions. Normally, LED lamps are the most compact ones of all lamps.
These comparisons lead to the conclusion that at present LED lamps are the most economical ones, have the best light spectrum, have the possibility of selecting color temperature, they are compact, with long durability, they heat up fastest of all lamps and can save you a lot of money.
LED lamps (light-emitting diode lamps) are produced on the basis of light-emitting diodes (LED). LED lamps have long durability and high electrical efficiency (very high light output), which is several times higher than that of incandescent lamps and significantly higher than most fluorescent lamps. Using some chips they can achieve emission of over 150 lumens per watt.
Like incandescent bulbs and unlike most fluorescent lamps (e.g. pipe and CFL lamps) LED light, when turned on, reaches full brightness in some milliseconds, without the need for warm-up time. The durability of fluorescent lighting is reduced by frequent commutations. The initial cost of a LED lamp is usually higher than that of conventional lamps and is commensurate with the cost of energy saving lamps (CFL) but along with the continuing research in the field of organic LEDs (OLED) and polymer light-emitting diodes (PLED), the price per lumen of luminous flux of LED lamps rapidly lowers. Their fast increasing distribution contributes to this.
Some LED lamps are produced to be directly compatible with the incandescent lamp socket or fluorescent lamps, and when we replace standard fluorescent tubes with LED tubes in lighting facilities we need to remove the throttle, the starter and all electronics and submit pure 220 volts at each end of the LED cigar. The packaging of a LED lamp may show the amount of lumens, the power consumption in watts, the color temperature in kelvins or a description (e.g. „warm white“), the durability in hours, and sometimes the equivalent power of an incandescent bulb.
LEDs do not emit light in all directions, and their characteristics, related to the direction of light, are considered in their design, that is why there are lamps with a forward directed luminous flux and lamps with a flux that spreads in all directions (in this case the lamp looks like a cob).
The luminous flux of a single LED is smaller than that of an incandescent lamp and of compact fluorescent lamps, so in most cases more LEDs are used in the design of the lamp. Nowadays more powerful versions of the LED lamps are getting affordable. LEDs are adversely influenced by high temperatures, so LED lamps which have high power usually include elements designed to dissipate heat such as radiators and cooling fins.
LED lamps for general usage normally emit white light. The LEDs emit light in that tape of wavelengths whose color is typical for the energy group of the semiconductor material which is used to make the LED. In order to emit white light, diodes are required either to mix the light of red, green and blue LEDs or to convert the light, which is emitted from one of the other colors, by means of phosphor.
White LED lamps have longer durability and a higher efficiency (more light for the same amount of electricity) than other types of lighting. LED sources are compact, which gives flexibility in designing light fixtures and excellent control over the distribution of light by small reflectors or lenses. Due to the small size of LEDs, the control of the spatial distribution of light is extremely flexible and the light and the spatial distribution of the LED emission can be controlled without loss of efficiency.
LED lamps are made to replace incandescent bulbs with a screw base or compact fluorescent bulbs (energy saving CFL). These lamps are produced with standard lamp sockets and shapes, such as the screw base of the conventional bulb, the MR16 shape with a double-pin base, or a GU5.3 screw base (a double-pin cap) or GU10 ( bayonet fitting) and are made compatible with the voltage supplied to the sockets. They include schemes for adjusting the AC power and converting the voltage to a suitable value.
Some models of LED bulbs are compatible with dimmers that are used for incandescent lamps.
Sometimes LED lamps have as a feature the direction of light. This makes them usable for artistic directional lighting. LED lamps are also used for illuminating paintings, castles and churches, for highlighting façade ornaments, etc. The price of LED lamps has reduced sensitively during the past years and at the moment they cost 10 euro, while their previous price was more than 50 euro. LED bulbs have higher output and are more efficient than compact energy-saving fluorescent bulbs and their durability is more than 30 000 hours, although this durability decreases when working at a higher temperature than specified. Incandescent bulbs have durability of 1,000 hours and compact fluorescents lamps – about 8,000 hours.
LED bulbs keep the intensity of their light output till they stop functioning. Normally their intensity may fall by less than 10% after 6000 hours of functioning, and in the worst case –by no more than 15 %. LED lamps come in a variety of color properties. The purchase price is higher than that of most lamps but greater efficiency can lower the total cost of ownership (the purchase price plus the cost of electricity and replacement of bulbs).
It is absolutely possible that LED lamps become the main light source and replace completely incandescent lamps.