Electric Energy Utilization Light Light is radiant energy, usually referring to electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. All objects above the temperature of absolute zero (-273.15° Celsius) radiate energy to their surrounding environment. This energy, or radiation, is emitted as electromagnetic waves that travel at the speed of light. Many different types of radiation have been identified. Each of these types is defined by its wavelength. The wavelength of electromagnetic radiation can vary from being infinitely short to infinitely long as shown in figure 4. Visible light has a spectrum between 0.40 to 0.71 µm Figure 4 Various types of electromagnetic radiation as defined by wavelength. Visible light is a form of electromagnetic radiation that can be perceived by our eyes. Light has a wavelength of between 0.40 to 0.71 micro meters (µm).Figure 6f-1 illustrates that various spectral colour bands that make up light. The Sun emits only a portion (44 %) of its radiation in zone. Solar radiation spans a spectrum from approximately 0.1 to 4.0 micro meters. The band from 0.1 to 0.4 micro meters is called ultraviolet radiation. About 7% of the Sun's emission is in this wavelength band. About 48% of the Sun's radiation falls in the region between 0.71 to 4.0 micro meters. This band is called the near (0.71 to 1.5 micro meters) and far infrared (1.5 to 4.0 micro meters). The amount of electromagnetic radiation emitted by a body is directly related to its temperature. If the body is a perfect emitter (black body), the amount of radiation given off is proportional to the 4th power of its temperature as measured in Kelvin units. Illumination and Light Illumination differs from light very much, though generally these terms are used more or less synonymously. Strictly speaking light is the cause and illumination is the result of that 3 Electric Energy Utilization light on surfaces on which it falls. Thus the illumination makes the surface look more or less bright with a certain color and it is this brightness and color which the eye sees and interprets as something useful, or pleasant or otherwise. Light may be produced by passing electric current through filaments as in the incandescent lamps, through arcs between carbon or metal rods, or through suitable gases as in neon and other gas tubes. In some forms of lamps the light is due to fluorescence excited by radiation arising from the passage of electric current through mercury vapor. DEFINITIONS Color The property possessed by an object of producing different sensations on the eye as a result of the way it reflects or emits light. Light It may be defined as electromagnetic radiation that is capable of affecting the sense of sight. Luminous flux It is defined as the energy in the form of light waves radiated per second from a luminous body. Its unit of measurement is the lumen. Example: Suppose the luminous body is an incandescent lamp. The total electrical power input to the lamp is not converted to luminous flux, some of the power lost through conduction, convection and radiation, etc. A fraction of the remaining radiant flux is in the form of light waves lies in between the visual range of wavelength. Lumen It is defined as the luminous flux emitted by a source of one candle power per unit solid angle in all directions. Its unit is luminous flux. Total flux emitted by a source of one candle power is 4π lumens. Lumen = candle power of source × solid angle = CP × ω candela steradians Candle power The CP of a source is defined as the total luminous flux lines emitted by that source in a solid angle for a given direction. 4 Electric Energy Utilization Luminous intensity Luminous intensity in a given direction is defined as the luminous flux emitted by the source per unit solid angle. It is denoted by the symbol I and is measured in candela. Figure 6 Luminous flux emitting from source Let F be the luminous flux crossing a spherical segment of solid angle ‘ω’. Then, Luminous intensity, Luminous Energy Luminous energy is the measure of all the energy received at a particular point or all the energy contained in a particular radiation field. The luminous energy is measured in lumen-seconds and it is denoted by the symbol Qv. Luminous Energy Density Luminous energy density, denoted as Uv, are the amounts of energy per unit volume, measured in either watt-seconds/m3 or lumen-seconds/m3. Plane angle () It is the angle subtended at a point in a plane by two converging lines Solid angle () It is the angle subtended at a point in space by an area 5 Electric Energy Utilization Relation between Solid angle & Plane angle The surface area of the curved surface of the spherical segment ABC = 2rh Illumination (E) Luminous flux received by the surface per unit area. Unit: lux (or) lumen/m2 (or) meter candle (or) foot candle Brightness (L) Luminous intensity per unit surface area of the projected surface in the given direction Lamp Efficiency Ratio of total luminous flux emitting from the source to its electrical power input Unit: lumen/W Coefficient of utilization or utilization factor Ratio of total number of lumens reaching the working plane to the total number of lumens emitting source. Radiant Efficiency Ratio of energy radiated in the form of light to the total energy radiated by the luminous body LAWS OF ILLUMINATION 1. Inverse square law The illumination of a surface is inversely proportional to the square of distance between the surface and a point source 6 Electric Energy Utilization S – point source I – Luminous intensity A1, A2, A3 – area d,2d,3d – distances 2. Lambert’s cosine law The illumination E at any point on a surface is directly proportional to the cosine of the angle between the normal at that point and the line of flux. Assume that the surface is inclined at an angle θ to the lines of flux PQ – the surface area normal to the source and inclined at θ to the vertical axis RS – the surface area normal to the vertical axis and inclined at an angle θ to the source O 7 Electric Energy Utilization 8 .