The light is emitted in all the directions from source and it is not limited to a particular plane, now you describe plane angle and solid angle of the light. Plane angle : The plane angle may be defined as below When two straight lines laying on the same plane and meet a point O they will be the angle between the lines at a meeting point. This angle is known as the plane angle. In this figure, It is shown the plane angle. AOB is the plane angle. And the Plane angle is represented in radians.
Solid angle: The solid angle has defined an angle Which is subtended at a point in space by an area. In case of plan angle the area is enclosed by two lines, but in case of solid angle, the area is enclosed by new videos lines laying on this Surface and meeting at a point stop the solid angle is shown in the figure.The major of solid angle is given below.
From a point of light source Which is radiated out energy in all directions. Let, The point source as Centre and any distance R as radius, imagine a spherical surface to be constructed and the lines of flux Will cross area a forming a cone.
Since the surface area of the sphere is πR2, And the total solid angle subtended by the points in the View of the equation is given as