Luminous intensity, unit measurement

The amount of light emitted by a source is measured by its luminance or by its luminous intensity, which are defined in Figure 18.2. Intrinsic light emission relates to the amount of light emitted per unit area (luminance). Table 18.1 lists approximate luminances for some common light sources. 

On some occasions, protocols may involve SI units of time, electric current, thermodynamic temperature, or luminous intensity. These units are also base units of the SI. Traceability to SI can even refer to realizations of derived SI units, such as those for energy, pressure, and amount of electricity. Solubility per unit pressure may be quoted in (mol/m3)/(m-s2/kg) or in (mol/m3)/ Pa, but should not be written as moTs2/(m2-kg) [5, 20], that is not in reduced form relating to units of quantities not actually measured. 

The most important modem system of units is the SI system, which is based around seven primary units time (second, abbreviated s), length (meter, m), temperature (Kelvin, K), mass (kilogram, kg), amount of substance (mole, mol), current (Amperes, A) and luminous intensity (candela, cd). The candela is mainly important for characterizing radiation sources such as light bulbs. Physical artifacts such as the platinum-iridium bar mentioned above no longer define most of the primary units. Instead, most of the definitions rely on fundamental physical properties, which are more readily reproduced. For example, the second is defined in terms of the frequency of microwave radiation that causes atoms of the isotope cesium-133 to absorb energy. This frequency is defined to be 9,192,631,770 cycles per second (Hertz) —in other words, an instrument which counts 9,192,631,770 cycles of this wave will have measured exactly one second. Commercially available cesium clocks use this principle, and are accurate to a few parts in 1014. 

These terms refer to the radiant or luminous intensity of source projected onto a surface, as shown in Figure 8. The intensity is divided by the area of a source projection on the surface. The radiometric SI units of measurements are watts per steradian per square meter (W sr m ) or watts per steradian per square centimeter... 

The source to photodetector distance should always be noted as part of any illuminance or irradiance measurement, as demonstrated in Figure 13. The inverse square law states that as distance increases, the signal decreases inversely proportional to the square of the distance. This is true for a point source relationship where the source to detector distance is at least five times greater than the largest source dimension. So in effect, if the distance is halved, the signal strength per unit area is quadrupled and vice versa. Luminance and radiance measurements are independent of distance because the detection area and source intensity vary in direct proportion with distance. 

The luminous intensity of light used in above equation is the illumination on the unit surface which is placed at a right angle to the direction of the emitter at a distance of 1 meter. To obtain the value, measure the illumination at some distance from the emitter using a photometer (the photoelectric cell type or photocell type is popular), and calculated from the follov/ing equation ... 

The units of light intensity are so numerous that we often become confused. But we have only one which we can measure, it is the luminosity, which is the same as the above luminous intensity of light. Other units are derived from the luminosity. The new unit of luminosity is the candela and 1 candle power equals 1.0l8 candelas. 1 candela means... 

Luminance is the luminous intensity per unit area of a source of light or of an illumination. It is measured in candles per square meter. 

The International System of Measurement (SI for Systeme International d Unith), a modern elaboration of the original metric system, was set up in I960. It was developed to provide a very organized, precise, and practical system of measurement that everyone in the world could use. The SI system is constructed using seven base units, from which all other units are derived (Table 1.1). The chemist is not usually interested in electric currents or luminous intensity, so only the first five of the base units on Table 1.1 will appear in this text. The meaning of mole, the base unit for amount of substance, is explained in Chapter 9. Until then, we will use the first four base units meter (m), kilogram (kg), second (s), and kelvin (K). 

The International System of Units, abbreviated as SI (from the French name Le Systeme International d Unites), was established in 1960 by the 11th General Conference on Weights and Measures (CGPM) as the modern metric system of measurement. The core of the Si is the seven base units for the physical quantities length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. These base units are ... 

Symbol I. The luminous intensity of any surface in a given direction per unit projected area of the surface, viewed from that direction. It is given by the equation L = d//(dAcos6), where Its the luminous intensity and 6 is the angle between the line of sight and the normal to the surface area A being considered. It is measured in candela per square metre. 

Under the common system, it was defined as the flux on one square foot of a sphere, one foot in radius, with a light source of one candle at the center that radiates uniformly in all directions. Under SI, luminous flux is measured in lumen units (symbol Im) and has as its formula cd x sr, which are the SI Base Units of candela and stera-dian for a solid angle. Thus, the lumen is the luminous flux emitted in a solid angle of one lumen nniformly distributed in a solid angle of one steradian by a point source having a nniform intensity of one candela. See also LUX. 

Candle (unit of luminous intensity) n. Candle power is a measure of intensity of a source of light as compared with a standard candle. 

The history of the base unit candela is as follows. Before 1948, the units for photometric measurements were be based on flame or incandescent-filament standards. They were replaced initially by the new eandle based on the luminance of a Planckian radiator (a black-body radiator) at the temperature of freezing platinum. This modification was ratified in 1948 by the 9th CGPM, which also adopted the new international name for the base unit of luminous intensity, the candela, and its symbol cd. The 13th CGPM gave an amended version of the 1948 definition in 1967. 

The International System of Units (SI) is the internationally accepted standard system of measurement in use throughout the world. The units of the SI are ascribed to seven fundamental physical properties and two supplementary properties. These are length, mass, time, electric current, thermodynamic temperature, the amount of a substance, luminous intensity, and the magnitude of plane and solid angles. 

Measurement is the act of quantifying a physical property, an effect, or some aspect of them. Seven fundamental properties are recognized in measurements length, mass, time, electric current, thermodynamic temperature, amount of a substance, and luminous intensity. In addition, two supplementary or abstract fundamental properties are defined plane and solid angles. The base units for the seven fundamental properties can be manipulated to produce derived units for other quantities that are the effect of combinations of these fundamental properties. For instance, a Newton is a derived unit measuring force and weight, and a square meter is a derived unit used to measure area. 

Candela (cd) is the unit of luminous intensity, i.e. the measure describing the power of a light source to emit light. 

The SI system of measurement is used in science. It has seven base units the meter (length), kilogram (mass), second (time), kelvin (temperature), mole (amount of substance), ampere (electric current), and candela (luminous intensity). 

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