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.The off-axis point where the intensity of the LED is half its on-axisintensity is referred to as theta one-half (q ½).Thus, twice the q ½ valuerepresents the LED s full viewing angle.Figure 2.9 illustrates the on-axis luminous intensity value (lv) withrespect to its theta one-half point.Note that the intensity is normallyobtained through the use of a photometer and that light is visiblebeyond the q ½ point.Photometer1v (0 degrees)¸ 1/2q 1/2 = 1v (0 degrees)YourLEDFigure 2.9 Measuring luminous intensity and the theta one-half point of an LED. FUNDAMENTALS OF LIGHT 432.3.6 Current and Voltage ConsiderationsThe luminous intensity of an LED is approximately proportional tothe amount of current supplied to the device.Thus, the greater thecurrent, the higher is the intensity.However, design limits will resultin an upper boundary on both current and light intensity.Most LEDs are designed to operate at a current of 20 mA.Whenconsidering an LED for a specific application, you need to considerthe operating current of the LED in comparison to the amount ofheat that can be tolerated by the application also.For example, LEDsthat are designed to operate at 12 V would have a greater heat dissipa-tion than LEDs designed to operate at 6 V.Because heat dissipationis a significant factor in the life of an LED, it s important to considerthe current rating of the LED and its applied and forward voltage ifyour application requires a long-life LED.In addition, it s also important to consider the location of the LEDas this will govern its ability to dissipate heat.For example, althoughmost LEDs used as indicators in monitors, televisions, and varioustypes of toys do not reside under a plastic panel, in certain instancesproducts are designed to include this type of panel as a protectivemeasure against glare, dirt, dust, or other elements.In this situation,the plastic panel will adversely affect the ability of the LED to dis-sipate the generated heat.2.3.7 Lumens, Candelas, Millicandelas, and Other TermsWhen comparing LEDs with incandescent lighting, it s importantto note that a direct comparison of the two is not possible.This issueresults from the fact that the amount of light emitted from an LEDis specified by the measurement at a single point, known as the on-axis luminous intensity value (lv).This measurement of an LED s lumi-nous intensity is not directly compatible with the light produced byincandescent lightbulbs, with the latter using spherical candlepoweras a mechanism to denote the quantity of light emitted by the light-bulb.Thus, to obtain a solid appreciation for the relationship betweenlumens and candela, let s turn our attention to the manner in whichthese units are related. 44 INTRODUCTION TO LED TECHNOLOGY/APPLICATIONS2.3.7.1 Lumens A lumen represents a unit of light output, which isequal to 1/60 × À of the light emitted by one square centimeter of anideal blackbody surface at the melting point of platinum, with À equalto the constant 2.14159 &.Lumens were defined in terms of the 1931official photopic function that was modified in 1988, which essen-tially examines the quantity of light produced at every wavelengththat is present.One watt of light at any single wavelength representsa lumen (unit is lm), which is mathematically 681 times the officialphotopic function of the wavelength.2.3.7.1.1 Watts as a Measurement Tool To alleviate some potentialconfusion concerning watt numbers, let s differentiate between thewatt number used for light output and the unit used for measuringelectrical power.When using the term watt for measuring electricalpower, it represents volt times the amperage (ampere) in a direct cur-rent (dc) circuit, that is,watt = volt × ampereIf we are referring to a single-phase ac circuit, thenwatt = volt × ampere × power factorwhere power factor represents the ratio of real power to apparentpower and is a number between 0 and 1.In comparison, the wattnumber used to express light output, as previously noted, is math-ematically 681 times the official photopic function of the wavelength.To simplify this, manufacturers use the watt number as a mechanismto compare light output to an incandescent lightbulb output.Then,a 60 W light output is equal to a 60 W incandescent lightbulb.Tofurther muddy the water, often two watt numbers are used in a prod-uct specification, one for the electrical power consumption whereasthe other is used to specify light output.Fortunately, reading the fineprint on the package usually alleviates the potential confusion.Returning our attention to the standard 100 W lightbulb, we canobtain knowledge of different lighting terms.The standard 100 W,120 V incandescent lightbulb is typically rated for 750 hr and emits1710 lm.The lumens rating represents the unit of luminous flux andis equal to the light radiated by a source of 1 cd intensity radiatingequally in all directions.Thus, in some ways, we can actually think of FUNDAMENTALS OF LIGHT 45a 1710 lm incandescent lightbulb as providing the light of 1710 can-dles, because a candela can be thought of being loosely equivalent to aburning candle.Lumens are used to measure and compare lightbulbsas stand-alone light sources.You can usually examine the packagingof a lightbulb and obtain its rated life in terms of hours of operation,its wattage, its voltage, and amount of light it provides in lumens.2.3.7.2 Lumens per Watt and Lux Two terms that require our atten-tion are lumens per watt and lux.The term lumens per watt providesan indication of the efficiency of a lightbulb, including its efficiency inconverting electrical energy into light.In comparison, lux is used tomeasure the light intensity produced by a lighting fixture and is mea-sured in lumens per square meter.Thus, the higher the lux reading,the more light the fixture provides over a given area.2.3.7.3 Watt Dissipation If you examine the packaging on an LED,you can usually find information concerning the number of watts thedevice will dissipate.If this information is not displayed on the pack-age, you can easily compute the value by determining the forwardvoltage (Vf) and milliamp (mA) rating of the LED.In some cases,both Vf and mA are listed on the package, with watts dissipated curi-ously missing.Thus, let s review how we can compute the watts dis-sipated by an LED
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