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Standard Candle

In early days, candle was taken as a standard unit of the illuminating power of a source. A standard candle is one, which is made of sperm wax weighing ½ of a pound, 7/8 inch in diameter and which burns at the rate of 120 grains per hour. The normal height of the wick being 4.5 cm. If a source gives light 60 times the light given out by a standard candle, its power is taken as 60 candle power. The power of all sources of light is calculated in comparison with a standard candle. But depending upon the conditions i.e. change in the shape of the wick and size of the flame during different seasons, such a standard unit cannot be used scientific work. It was found that the flame of a candle was not of constant brightness in spite of careful specifications.

With the advent of gas lighting, a need was felt for a more reliable standard. The Vernon-Harcourt pentane lamp burns pentane vapour and air mixture under specified conditions and gives light of about ten times the original candle. With the progress of science, in the year 1948 an international unit of light was adopted.

The primary standard of light is defined in terms of black body radiations at a definite temperature. A small hole constituting the black body is at the end of cylinder of fused thorium oxide has a higher melting point than platinum.

Platinum is melted by the heat produced by eddy currents included in it by a coil carrying a high frequency electric current. When allowed to cool, platinum remains at its freezing point for a short time. During this time, the brightness of the hole in the cylinder defines the standard source. Originally the unit of illuminating power of sources (luminous intensity) was taken as candle power. The unit now used is Candela (It is Latin word for candle) or International Candle. One candle is actually equal to 0.982 times the original candle. International standard candle or candela is defined as 1/60 of the light (luminosity) coming out of a hole 1 sq cm in area in a hollow cavity acting as a black body radiator maintained at the freezing point temperature of platinum (1773˚C).

Now a days, the illuminating power of a source of light viz. glow lamps, incandescent lamps, electric bulbs, etc. is given in terms of the above standard unit.

Secondary units: As primary standards cannot be used and prepared readily and as they require a lot of technical skill and precision, for practical purposes electric bulbs having tungsten filaments are very carefully compared with primary standards. These electric bulbs having a known value of illuminating power are available and they are worked at the specified current and voltage.

Therefore, in order to find the illuminating power of any source, it is generally compared with these secondary standard electric lamps.

Luminous Flux: The amount of light i.e. visible radiant energy which flows from a source or illuminating surface in one second is known as luminous flux. (It is only that part of the total radiation, which is visible and can affect the eye.)

Lumen: It is the unit of luminous flux. It is defined as the luminous flux per unit solid angle due to a point source of one international candle power. Let there be a point source of light of one international candle power. Draw and imaginary sphere of radius r with the source as the centre.

Suppose the total flux = F

Total solid angle = 4π steradians

∴ One lumen = F/4π

F = 4π lumens.

Note: Lumen is also defined as the flow of light energy per second through 1 sq metre of a surface of one metre radius, when a source of one international candle power is placed at the centre of curvature.