Illumination Engineering Module I A Beginning Definition-Light • Light may be defined as electromagnetic radiation that is capable of affecting the sense of sight. Introduction- Illumination Engineering • Light • Electromagnetic radiation • Wavelength in the range from about 4,000 (violet) to about 7,000 (red) angstroms • Perceived by the normal unaided human eye • The lighting systems • A source employing any physical phenomenon • Incandescence, Electrolumniescence or Flourescence. • Some control scheme • Luminaire Lighting Engineers or Illumination Engineers • Efficacy • Illuminance per Watt of energy consumed. • Reduce energy consumption yet have efficient Illumination to enhance productivity • Modern electronic controls together with energy efficient lamps. • An integrated system for the complete electrical system of a building. Necessity of Illumination • Light is a natural phenomenon, very vital for existence • Humans depend on Light for all activities. • Artificial light enables extended activity period • Vision - the most important sense accounting for 80% information acquisition for humans • Information may be acquired through sun/moon light (direct/ reflected) or by using artificial light (closest to natural light). Teichmuller’s definition for lighting. “We say the lighting is good, when our eyes can clearly and pleasantly perceive the things around us”. • Artificial light should be Functional and pleasant both Physiologically and Psychologically. • Often achieved employing multiple sources. • Sources should be economic and energy efficient. • All sources today employ electrical energy. Properties of Light • Rectilinear propagation: Light travels in straight lines. • Reflection: Light striking a smooth surface turns back into the original medium. • Refraction: Light bends when entering a transparent medium. The Nature of Light • Physicists have studied light for centuries, finding that it sometimes behaves as a particle and sometimes as a wave. Actually, both are correct! Photons and Light Rays • Light may be thought of as little bundles of waves emitted in discrete packets called photons The wave treatment uses rays to show the direction of advancing wave fronts. Light Rays and Shadows • A geometric analysis may be made of shadows by tracing light rays from a point light source: The dimensions of the shadow can be found by using geometry and known distances. • The diameter of the ball is 4 cm and it is located 20 cm from the point light source. If the screen is 80 cm from the source, what is the diameter of the shadow? The ratio of shadow to the source is same as that of ball to source. Radiation • Light is the Radiant Energy that provides visual sensation. • Similar to radiant heat • But has different frequencies and wavelengths. • Visible Light – spans from 180nm to 700nm wavelength. • Human Eye responds from 380 (violet) to 700nm (red). • Necessary for us to understand the suitability of various types of sources of light. • Artificial sources are made to produce radiation close to this. • Artificial sources employed • Incandescent lamps which depend on temperature of the filaments giving a continuous spectrum • Gas or discharge lamps giving a discontinuous – Line spectrum / Band spectrum. Physical Processes Employed in the artificial sources • 1. Incandescence Thermo luminescence is by definition radiation at high temperature. The sources employing this process are Incandescent Lamp, Gas Lamp, (flames and in oil Lamps and wax candles). They lead to a continuous spectrum of radiation. 2. Luminescence Luminescence Electro luminescence by definition Chemical or Electrical Action on gases or vapour radiation. Here color of radiation depends on the material employed. Usually this process leads to Line or Band Spectrum. 3. Fluorescence Fluorescence is a process in which radiation is absorbed at one wavelength and radiated at another wavelength eg: UV impinging on Uranium – Fluorescent oils. This re radiation makes the light radiated visible. • 4. Phosphorescence Phosphorescence is a process when energy is absorbed at some time and radiated later as glow. Examples of this process are Luminous paints that contain calcium sulfide that lead to Phosphorescence. They produce light Radiation after exposure to light. • Good efficient lighting is obtained by combining Luminescence and Fluorescence. • Fluorescent lamp is Luminescent source of low luminous value activating Fluorescent surfaces which lead to visible radiation. • Intensity depends on gas or vapor involved and phosphor material. • The temperature of the material play a role in radiation. • Light – Radiant energy that provides visual sensation • Human eye can sense – 380nm (violet) to 700nm (red) • Maximal relative energy content of sunlight • Maximal luminosity of human eye • At 550nm (corresponding to green light) • Artificial light sources Incandescent Lamp Gas Discharge Lamp • Physical Processes employed for artificial lighting • Incandescence • Luminescence • Fluorescence • Phosphorescence • Good efficient lighting obtained by combining luminescence & fluorescence. Radiation Temperature • Radiation Temperature of the materials follow Steafan Boltzman’s Law: W = kT4 …………………….( 1 ) k ≈ 5.71x10-12 Its Boltzman’s constant Or radiation constant Say Ambient Temperature is T0 W = k ( T4 – T04 ) watts/cm2 ................(2) Thus energy radiated depends on the 4th Power of temperature. So efficiency is high at high temperatures. Wien’s displacement law • Relationship between the temperature of a blackbody (an ideal substance that emits and absorbs all frequencies of light) and the wavelength at which it emits the most light. • When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. • The product of the peak wavelength and the temperature is found to be a constant. • Useful for determining the temperatures of hot radiant objects • For a determination of the temperature of any radiant object whose temperature is far above that of its surroundings. • λm.T = a ( constant ) …………………( 3 ) T In °k • λm In μm → corresponds to wavelength where radiation is a maxima. • a = 2960 for a perfect black body = 2630 for platinum Colour Temperature • Color temperature is the temperature at which complete radiator must be operated to match the color of luminous source. • Blue sky -25000°K., • Fluorescent Lamp - 4500°K., • Candle flame - 2000°K. Eye and Vision • Eye acquires > 80% information acquired by human. • An Eye comprises of Iris, Focusing Lens and Retina. • It Resembles – a Camera in general structure and action. • • • • The eye communicates through optic nerves located on the retina. They are a system of double nerves called Rods and Cones. Rods are responsible for Dim light / Night vision and Cones are mainly concentrated around or at Fovea and are responsible for form/color sensitivity. • As a result vision is of two types; • (i) Photopic and • (ii) Scotopic • Photopic vision • Involving cone cells and is used for discrimination of fine details for critical observation. • They are densely packed and transmits sharp images. • The cone cells have low sensitivity below 0.01 ft lamberts and cease to function < 0.001 ft lamberts. • 1 lambert is 1/π candles / m2 • 1 ft lambert is 1/π candles / ft2 • Scotopic vision • Involving Rods • Takes over when brightness < 0.01 ft lambert. • This vision has no color discrimination ability. • Most images have gray appearance and are viewed as silhouettes lacking sharp details. • Eyes have good ability to change from one to other. • This shift in Luminosity and ability of eye to adjust is known as Purkinjee effect • Eye functions under varying illumination levels by a change in pupil size together with change in Retinal Nerve System (i.e. cones/ rods) • Upon increase of intensity of illumination by a decrease in Pupil size producing clearer images with greater and fine details. • Pupil diameter varies in the range of 1.2 – 2 mm. Eyes are error free and accommodate very well. • Eye is Unconscious to variations in natural light. • Thus human eye is achromatic with a dispersive power little greater than water. • For near vision eyes easily focus for blue and tires to focus for red • For far vision eye easily focus for Red and strains to focus blue. Luminosity of Eye • Maxima sensitivity of • cone cells – 550nm (yellowish green hue) • rod cells – 507nm (bluish green) • Good lighting • Prevention of defective vision • Optimization of resources • Improving conditions of visibility • Visibility depends on – (Observer Issues) • size / details of object • level / quality of illumination • contrast / color • available time • Visibility depends on – (Observer Issues) • efficacy of individual • one’s eye defects • optical / physical fatigue • distraction • Causes of fatigue – • Rotating source • focusing on the source of glare • reading double impression • after a days work pupil is dilated • A nights rest offsets fatigue due to a days work • Visibility reduces due to eye defects and fatigue • Eye defects caused due to – • Aging • Use • Abuse • Good illumination looks for producing clear and quick images • Eye tasks - for long duration requiring increased illumination • More exacting the task, more illumination is required • Apart from quantity, quality is also important • Illumination affects Physiology and Psychology also • Natural Illumination conditions need to be reproduced. • Artificial Illumination characteristics are influenced by • The physical characteristics of room or object or illuminating equipment. • Color finish of walls or ceiling etc. • Factors governing illumination quality : • glare • diffusion • direction / focus • composition • distribution • Illumination requirement for equal visibility - at least 100 ft candles or more. ft candles - a non-SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot • Functioning of eye may be assessed by the Visual acuity, • Refers to your ability to discern the shapes and details of the things you see. • Visual acuity is dependent on optical and neural factors, i.e., (1) The sharpness of the retinal focus within the eye (2) The health and functioning of the retina (3) The sensitivity of the interpretative area of the brain Visual Acuity Vs Illumination Contrast Sensitivity Vs Illumination Nervous Muscular Tension Vs Illumination Frequency of Blinking Vs Illumination Convergence rate Vs Illumination • Apart from illumination, visibility is talked in terms of • Visual acuity, • Visual efficiency • Visual speed • Visual health. • Acuity is the ability to distinguish details depending upon: • Brightness of the object, • Characteristics of light entering the eye • Contrast details. Variation of visual acuity with background brightness A foot-lambert or footlambert (fL, sometimes fl or ft-L) is a unit of luminance in United States customary units and some other unit systems. A foot-lambert equals 1/π or 0.3183 candela per square foot, or 3.426 candela per square meter (the corresponding SI unit). Acuity variation with surrounding brightness • Age Vs. Vision • reduction of visual activity • decrease in size & elasticity of pupil • decrease in flexibility of optic lens • leading to higher illumination requirement • Monochromatic light & acuity : • distinct images on retina • details are distinguished well Monochromatic light is light (optical radiation) where the optical spectrum contains only a single optical frequency. • Color sensation by eye has a lag which depends on : • presentation & cessation of stimulus • rate of rise / fall (different for various colors) • simultaneous colors & combination of colors • Combination of different colors reduces acuity which is known as Chromatic Aberration Laws of Illumination Terms used in Illumination 1. Light 2. Luminous flux 3. Lumen 4. Plane angle 5. Solid angle 6. Steradian 7. Candle power 8. Luminous intensity reduction factor 9. Glare 10.Lamp efficiency • Light That part of radiant energy from a hot body which produced the visual sensation on human eye is called light. • Luminous Flux The total quantity of radiant energy per second responsible for visual sensation from a luminous body is called Luminous Flux. It is represented as F of Ø and measured in lumens. • Lumen It is the unit of luminous flux. One lumen is defined as the luminous flux emitted per unit solid angle from a point source of one candle power. • Solid Angle The angle subtended by the partial surface area of a sphere at its centre is called as solid angle. It is measured in steradians and equal to the ratio of area of the surface to the square of radius of sphere, ω = area of surface/ square of radius = A/ r 2 steradians • Steradian • The unit of solid angle. One steradian is defined as the solid angle that is subtended at the centre of a sphere by its surface having area equal to radius square, • Candle Power • The light radiating capacity of a source is called its candle power • The number of lumens given out by a source per unit solid angle in a given direction is called its candle power. • It is denoted by C.P. Total flux emitted = CP X solid angle • Luminous Intensity • Luminous intensity in any particular direction is the luminous flux emitted by the source per unit solid angle in that direction. • It is denoted by I and its unit is candela or candle power (CP) . • Luminous intensity of source in a particular direction, I = φ / ω • Reduction Factor • Reduction factor of a source of light is the ratio of its mean spherical candle power to its mean horizontal candle power. Reduction factor = MSCP/ MHCP • MEAN SPHERICAL CANDLE-POWER • Mean Spherical Candle Power, a unit of measure that represents the average output of a light source measured in all directions (360°) MSCP= F/4π • MEAN HORIZONTAL CANDLE-POWER • The average value of the candle-power of a light source in all directions in a horizontal plane through the source, • Illumination • When light falls on a surface, it becomes visible, the phenomenon is called as illumination. • It is defined as luminous flux falling on a surface per unit area. It is denoted by E and measured in lumen per square meter or metercandle. E = ะค / A lux • Lux • One meter candle or lux is defined as the illumination produced by a uniform source of one CP on the inner surface of a sphere of radius one meter. • Glare • In the human eye, the opening of pupil is controlled by the intensity of light received by the eye. If the eye is exposed to a very bright source of light, the pupil of the eye contracts automatically in order to reduce the amount of light admitted and prevent damage to the retina. This effect is called glare. • Glare is defined as the brightness within the field of vision so as to cause discomfort and interference in vision. • Lamp Efficiency • It is defined as the visible radiations emitted by it in lumens per watt. • Usually, the light sources do not radiate energy only in the visible spectrum. The radiant energy is also accompanied with infrared and ultra violet radiations. • Sun light produces majority of radiations in the visible spectrum. The tungsten lamp produces small radiations so its efficiency is very poor. • Reflection Factor • Whole of the light incident on a reflecting surface is not reflected. Some portion of it is absorbed by the surface. • The ratio of the reflected light to the incident light is called reflection factor. Law of Illumination • The illumination on a surface depends upon the luminous intensity, distance between the source and surface and the direction of rays of light. • It is governed by following laws : 1. Inverse square law 2. Lambert’s cosine law Inverse Square Law • Intensity of Illumination produced by a point source varies inversely as square of the distance from the source. • Inverse Square Law • It states that the illumination of a surface is inversely proportional to the square of the distance of the surface from the source. E α I/d2 • As light radiates from a point source, the intensity of Light (I) is inversely proportional to the square of the distance (d) from the source • As intensity is the power per unit area (W/m 2), it naturally decreses with the square of the distance as the size of the radiative spherical wavefront increses with distance. • Illumination of a surface is inversely proportional to the square of the distance between the surface and the light source provided that the distance between the surface and the source is sufficiently large so that the source can be regarded as a point source. Lambert’s Cosine Law • This law states that the illumination on any surface is proportional to the cosine of angle between the direction of the incident flux and perpendicular to the area. • E= I Cos α /d 2 For this scheme , shows the variation of Illuminance on the floor. It is well known that Illuminance is maximum under the lamp at ‘B’. • Sometime surface is not normal to the direction of Light.But, inclined by some angle. • According to this law, Illumination at any point on a surface is proportional to the cosine of the angle between the normal at that point and the direction of luminous flux. This – tells us the variation of Illuminance on arbitrary surface inclined at an angle of α. As shown in Fig Fechner's law • • • • • Weber–Fechner law Psychophysics, Weber's law and Fechner's law physical stimulus and the perceived change Weber's law- human response to a physical stimulus in a quantitative fashion • In physiology, a stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to detect external stimuli, so that an appropriate reaction can be made, is called sensitivity. Fechner's law • Fechner noticed in his own studies that different individuals have different sensitivity to certain stimuli. • For example, the ability to perceive differences in light intensity could be related to how good that individual's vision is. • He also noted that the human sensitivity to stimuli changes depends on which sense is affected • He used this to formulate another version of Weber's law that he named the "measurement formula". • Fechner's law states that the subjective sensation is proportional to the logarithm of the stimulus intensity. • The relationship between stimulus and perception is logarithmic. • • This logarithmic relationship means that if a stimulus varies as a geometric progression (i.e., multiplied by a fixed factor), the corresponding perception is altered in an arithmetic progression (i.e., in additive constant amounts). • Weber in 1830 found that I – Stimulus (Intensity) produces dI – Least perceptible increment affecting sense organs. Then the ratio dI / I=Constant Under fixed – 1) Fatigue 2) Attention and 3) Expectation. • Thus we have sensitivity given by the equation • Here I0 is the threshold intensity. This is known as Frechner’s Law.
