Figure (22-E1) shows a small diffused plane source S placed over a horizontal table-top at a distance of 24 m with its plane parallel to the table-top. The illuminance at the point A directly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 18 m from A.
In triangle ABS,
AS =2.4 m
AB = 1.8 m.
Hence BS = √(2.42+1.82) = √9 = 3 m.
Let, ,
Illuminance at A = 25 lux.
Let the intensity along SA =.
Since it is a small extended source, from Lambert's cosine law, the intensity along SB:
I
.
The angle between SB and normal at B is also θ.
Hence the illuminance at B,