Intensity, luminous

The set-up consists of a photometer, a tunnel of 1.5 m diameter with an exhaust fan and a stand to mount the flare. First of all, the photometer is calibrated using a standard quartz tungsten halogen lamp of 1000 watts kept at a distance of 1 foot (0.305 m) from the detector. The flare mounted on a stand is positioned in the 

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Luminous intensity candela cd Luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 X 10 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. 

The number of lumens indicates the total amount of power but gives no indication of the density" of that power. This last measure is given by the 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 luminance relates to the luminous intensity radiated from an entire surface of a light source ... 

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. 

The brightness and lifetime limits of a formulation are determined by its light capacity, in (Imh)/L, which is defined as the integral of luminous intensity I, in lumens, with respect to reaction time /in hours for one Hter of formulation (237) ... 

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.)... 

Flares. A flare compn is described in Refs 15 16 contg powd Mg, Na perchlorate, and a binder which can be cast and cured, and which has a luminous intensity of 50,500 candle-secs/g Incendiaries. A mixt of 1 OOg of Na perchlorate and 58.7g of powd Al has a Qr of 2751cal/g and can be used as a filler for incendiary bombs (Ref 6). A proc is described in Ref 14 for coating Na perchlorate with Al, and the use of the coated material in incendiaries Photo flash Compositions. Mixts of 80% Ca metal and 20% Na perchlorate and 80% Ca, 10% Na perchlorate, and 10% Na nitrate have been examined as expl photoflash compns for both sea level and high altitude use (Ref 13) Refs 1) Gmelin, System No 21 (1928), 408 Supp Part 1 (1964), 180 2) H.H. Willard ... 

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 ... 

At the centre of the cloud is the young stellar object destined to become the Sun. It accounts for approximately 99.9 per cent of the mass of the nebula and there are various examples of this in the heavens, including the classic pre-main sequence T-Tauri star. The star continues to evolve, blowing off bipolar jets (see Figure 4.5) and beginning a solar wind of particles. Of course, the star does not reach its full luminous intensity and the best theories suggest that the Sun was some 30 per cent less luminous when the Earth began to form. 

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. 

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