Light

Photometry Intensity Depends on the total amount of light and on the solid angle covered. Inverse square law A three-dimensional, spherical wave will expand in spherical surfaces of increasing radius, so the energy of the wave is spread out over the spherical surface as shown in the figure . The area A of a sphere of radius r is given by the formula A=4 pi r2, so when the radius of the sphere is increased t twice its original raduis, the area is four times the original area. The same amount of energy is therefore spread out over four times the original area, so the intensity in this case, the power per unit area, is only 1/4 of the original intensity. All types of waves oby this law when they are emitted and allowed to travel freely away from the source with no focusing or other means of confinement. Example for the typical behavior of the intensity according to the inverse square law. Distance Intensity 1 m 1w/m2 2m 1/4 3 m 1/9 . . . . D 1/D2 Intensity Power / unit area [units cd = candles] Luminance Intensity per unit area [units candles/m2] Examples: A small globe and a large globe of 50 W have both the same intensity. A small globe of 50 W and a large globe of 300 W can have the same luminance. Try it! Measuring Light Intensity

Ch. Elster
Aug 26 14:27:03 EDT 2020