Illumination of a surface: \(E=\frac{I \cos \theta}{d^{2}}\)
The illumination \(E\) of a surface by a light source is a measure of the
luminous flux per unit area that reaches the surface. The value of \(E\) [in
lumens (lm)
per square foot] is given by the formula shown, where \(d\) is the distance from
the light source (in feet), \(I\) is the intensity of the light [in candelas
(cd)], and \(\theta\) is the angle the light source makes with the vertical. For
reading a book, an illumination \(E\) of at least \(18 \mathrm{~lm} /
\mathrm{ft}^{2}\) is recommended. Assuming the open book is lying on a
horizontal surface, how far away should a light source be placed if it has an
intensity of \(90 \mathrm{~cd}\) (about \(75 \mathrm{~W}\) ) and the light flux
makes an \text { angle of } 65^{\circ} \text { with the book's surface (i.e.,
} \theta=25^{\circ} \text { )? }