Units luminous intensity

Luminance is the luminous intensity divided by the area of emission of light (Iumens/steradian/m2). This is the power density emitted per unit area. 

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. 

In 1954, the 10th CGPM added the degree Kelvin as the unit of temperature and the candela as the unit of luminous intensity. At the time of the 11th CGPM in 1960, this new system with six base units was formalized with the tide International System of Units. Its abbreviation in all languages is SI, from the French l e Sjstume International d Unitus. 

This unit, most recentiy defined by the 16th CGPM in 1979, replaced the candle and, later, the new candle and a definition of the candela based on the luminous intensity of a specified projected area of a blackbody emitter at the temperature of freezing platinum. 

It is usual these days to express all physical quantities in the system of units referred to as the Systeme International, SI for short. The International Unions of Pure and Applied Physics, and of Pure and Applied Chemistry both recommend SI units. The units are based on the metre, kilogram, second and the ampere as the fundamental units of length, mass, time and electric current. (There are three other fundamental units in SI, the kelvin, mole and candela which are the units of thermodynamic temperature, amount of substance and luminous intensity, respectively.)... 

In this book, we will express our thermodynamic quantities in SI units as much as possible. Thus, length will be expressed in meters (m), mass in kilograms (kg), time in seconds (s), temperature in Kelvins (K), electric current in amperes (A), amount in moles (mol), and luminous intensity in candella (cd). Related units are cubic meters (m3) for volume, Pascals (Pa) for pressure. Joules (J) for energy, and Newtons (N) for force. The gas constant R in SI units has the value of 8.314510 J K l - mol-1, and this is the value we will use almost exclusively in our calculations. 

CANDELA a unit of luminous intensity, defined as 1/60 of the luminous intensity per square centimeter of a black-body radiator operating at the temperature of freezing platinum (1772 °C), Formerly known as a candle. The unit is abbreviated as Cd... 

The most obvious characteristics of laser light are its brightness, its spectral purity, and the directionality of the beam. It is not so obvious how extreme these properties are. It has been pointed out ( 7) that a one-milliwatt He-Ne laser, virtually a toy laser, is 100 times brighter than the surface of the sun in terms of luminous intensity per unit area. Considering only light at the laser wavelength of 632.8 nm, the laser is 2 x 10 times brighter than the sun ... 

Time is one of the so-called base units within the SI system, and so is length. Whereas volume can be expressed in terms of a length (for example, a cube has a volume l3 and side of area l2), we cannot define length in terms of something simpler. Similarly, whereas a velocity is a length per unit time, we cannot express time in terms of something simpler. In fact, just as compounds are made up of elements, so all scientific units are made up from seven base units length, time, mass, temperature, current, amount of material and luminous intensity. 

Table 1.1 summarizes the seven base (or fundamental ) SI physical quantities and their units. The last unit, luminous intensity, will not require our attention any further. 

There are seven base SI units length, time, mass, temperature, current, luminous intensity and amount of material. 

The SI unit of luminous flux, abbreviated hn. One lumen is the amount of luminous flux emitted in a unit solid angle of one steradian by a uniform point source having a luminous intensity of one candela. 2. The interior space of a tubular structure such as an intestine or artery. 

CANDELA (cd). The luminous intensity of 1/6000,000 of a square meter of a radiating cavify at the temperature of freezing platinum (2042 K). (The SI unit of luminous intensity. The unit formerly was called the candle.)... 

Candlepower is the luminous intensity in terms of a standard candle (a candle made of sperm wax, six to the pound, which bums 120grains of wax per hr). It is equal to 1.11 Hefner units Ref Hackh s (1944), 163... 

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. 

The tenth CGPM in 1954 added two more standards when it officially approved both the kelvin for thermodynamic temperature and the candela for luminous intensity. In 1960 the eleventh CGPM renamed its AIKS system of units the International System of Units, and in 1971 the fourteenth CGPM completed the seven-unit system in use today, with the addition of the mole as the unit for the amount of a substance, setting it equal to the gram-molecular weight of a substance. 

A) The SI (Systeme International) units use kilograms, meters, seconds, amperes, kelvin, mole (6.022 x 1023 molecules per gram-mole, and not per kg-mole), and candela for [M], [L], [T], current, absolute temperature, mole, and luminous intensity, respectively. It started from an MKS (m-kg-s) system and included an electrical unit as part of the definition, as first suggested by Giorgi44 in 1904. There is a very slight modification of SI, used in nonlinear optics, confusingly dubbed MKS by its users, but called SI here. 

The candela (cd) is the SI unit of luminous intensity, defined as follows In a given direction, 1 candela of a source emits monochromatic radiation of frequency v = 540 x 1012 hertz and has a radiant intensity in that direction of... 

Finally in the field of light the base unit candela is introduced as unit of luminous intensity. 

SI base units — The Systeme International d Unites (SI) has 7 base units kg (kilogram) for mass M, m (meter) for length L, s (second) for time T, K (kelvin) for temperature , mol (mole) for the amount of a substance N, A (-> ampere) for electric current intensity I, and cd (candela) for luminous intensity /. 

Example radiant intensity 7e, SI unit W sr-1 luminous intensity 7V, SI unit cd photon intensity 7P, SI units s"1 sr"1... 

SI is a mass-length-time-temperature system built from the seven base units of length, mass, time, temperature, amount of substance, electric current, and luminous intensity. In addition two supplementary units of plane phase angle and solid angle are needed to... 

By definition, photometers do not respond to radiation in the infrared or the ultraviolet (Fig. 4-4a). They are light meters in the sense that they mimic human vision that is, they respond to photons in the visible region, similar to the light meter on a camera. A candle is a unit of luminous intensity, originally based on a standard candle or lamp. The current international unit is called a candela (sometimes still referred to as a candle ), which was previously defined as the total light intensity of 1.67 mm2 of a blackbody radiator (one that radiates maximally) at the melting temperature of pure platinum (2042 K). In 1979 the candela was redefined as the luminous intensity of a monochromatic source with a frequency of 5.40 x 1014 cycles s-1 (A, of 555 nm) emitting 0.01840 Js-1 or 0.01840 W (1.464 mW steradian-1, where W is the abbreviation for watt and steradian... 

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 Systeme Internationale d Unites, or SI for short, has gained widespread acceptance in the scientific and engineering community. Two of the base SI units—the ampere for electrical current and the candela for luminous intensity—will not concern us in this book. A third, the kelvin for temperature, will be discussed later. The others are the meter (m) for length, the kilogram (kg) for mass, and the second (s) for time. 

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