$\"\"$

Illuminance $\"\"$ in foot candles on a surface that is $\"\"$ feet from a source of light with intensity $\"\"$ candelas is,

$\"\"$ .

Where $\"\"$ is the measured angle between the direction of light and a line perpendicular to the surface being illuminated.

Consider $\"\"$ .

Divide the numerator and denominator by $\"\"$ .

$\"\"$ .

Trigonometric ratios:

$\"\"$ and $\"\"$ .

Therefore, $\"\"$ .

$\"\"$

Verified that $\"\"$ .