The illumination upon a surface varies inversely as the square of the
distance of the surface from the source. Thus if the illumination at a
surface one meter from the source
is X units, then the illumination at 2 meters will be X/4 at 3
meters will be X/9 and so on.
Strictly the inverse square law operates only when the light rays are from a point source and are incident normally upon the surface.
Thus illumination in lamberts/metre2 on a normal place
= Candle power / ( Distance in meters )2
However, illumination from a uniform diffusing area such as indirectly lighted ceiling is independent of distance, provided the distance concerned is small in relation to the size of the source.
Eh= En cos θ ( I cos θ ) / D2
where
Eh = illumination on a horizontal plane,
En = illumination due to light normally incident,
θ = the angle of incidence,
D = distance from the source.
Strictly the inverse square law operates only when the light rays are from a point source and are incident normally upon the surface.
Thus illumination in lamberts/metre2 on a normal place
= Candle power / ( Distance in meters )2
However, illumination from a uniform diffusing area such as indirectly lighted ceiling is independent of distance, provided the distance concerned is small in relation to the size of the source.
Electric Illumination > Cosine law:
The illumination received on a surface is proportional to the cosine of the angle between the direction of the incident light rays and the normal to the surface at the point of incidence. This is mainly due to the reduction of the projected area as the angle of incidence increases. ThusEh= En cos θ ( I cos θ ) / D2
where
Eh = illumination on a horizontal plane,
En = illumination due to light normally incident,
θ = the angle of incidence,
D = distance from the source.
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