All discussions made so far were based on the assumption that the luminous intensity or the candle power from a source is uniformly distributed over the surrounding surface.
But none of the practical type of lamp provides light uniformly distributed in all directions because of its unsymmetrical shape.
It is often necessary to know the distribution of light in various directions to ascertain how the candle power of a light source varies in different directions.
The luminous intensity in all the directions can be represented by polar curves.
If the luminous intensity in a horizontal plane passing through the lamp is plotted against angular position, a curve known as horizontal polar curve is obtained.
If the luminous intensity in a vertical plane is plotted against the angular position, a curve known as vertical polar curve is obtained.
The typical polar curves for an ordinary filament lamp
The polar curves are used to determine the mean horizontal candle power (M.H.C.P.) and mean spherical candle power (M.S.C.P.).
These are also used to determine the actual illumination of a surface by employing the candle power in that particular direction as read from the vertical polar curve in illumination calculations.
The mean horizontal candle power of a lamp can be determined from the horizontal polar curve by taking the mean value of the candle powers in a horizontal direction.
