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Standards of Illumination. -The intensity of illumination is meas­ured by the amount of light which falls on unit area of a surface. The amount of energy received by unit surface cannot be easily deter­mined in absolute measure. The eye is the most sensitive means of detecting light, but it does not give a quantitative measure of it. By means of the eye it is possible, however, to make an accurate compar­ison of two intensities of illumination.

To compare sources of illumination, a standard source of illumina­tion is necessary. The choice of these standards is more or less arbi­trary. There are a number of such standards in use. The British stand­ard candle is defined to be a candle made of spermaceti, weighing six to the pound and burning at the rate of 120 grains per hr. This standard does not have a sufficiently constant illuminating power to make it of scientific value. Its illuminating power changes with atmos­pheric conditions and with the conditions under which it is burned.

Candle Power. - The candle power of a lamp is a specification of its illuminating power in terms of some standard candle. For example, the illuminating power of an incandescent lamp may be thirty times that of a standard candle and is then said to be a 30-cp lamp. But the illuminating power of a light varies according to the direction from which it is observed. It becomes necessary, therefore, to measure the average candle power in a given plane. The average illuminating power in the horizontal plane is called the mean horizontal candle power. The mean spherical candle power denotes the average illumination by a source of light from all directions in space.

The Foot-candle. - A Foot-candle is the intensity of illumination upon a surface at a point which is 1 ft distant from a source of 1 candle, the surface being perpendicular to the light rays at that point.

The Lumen. - The unit used to denote quantity or amount of light is the lumen. A lumen is the amount of light falling on a surface that has an area of 1 sq ft when every point of the surface is 1 ft from a point source of light of 1 candle [2, С. 104 - 108].

3.1.4 Note to text 3.1.3:

· a 30-cp lamp - лампа мощностью в 30 свечей (ср - candle power)

Texts Magnetism and Electricity

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