ECE532 Biomedical Optics © 1998 Steven L. Jacques, Scott A. Prahl Oregon Graduate Institute

# EXAMPLE: Intensity of a flashlight

A cheap flashlight projects a beam that is 200 mW to create a 10-cm-diameter spot on a wall 25 cm meters away. The face of the flashlight has a 3-cm diameter. What is the radiant intensity of beam?

• The geometry of the problem is:
• The diameter of the spot on the wall is 10 cm. The diameter of the face of the flashlight is 3 cm. The apparent source of the flashlight is located at a virtual position x [cm] behind the lamp and parabolic reflector of the flashlight. The value of x is solved and equals 10.7 cm:

• The distance from virtual position of source to wall is R = 25 + 10.7 = 35.7 cm. A sphere with radius R would have a total surface area of Asphere = 4π(35.7 cm)2 = 16016 cm2.
• The circular area of the spot on the wall is Aspot = π(5 cm)2 = 78.5 cm2.
• The solid angle of the cone of light radiating from the flashlight equal (4π [sr]) Aspot/Asphere:

• The radiant intensity is P/Ω: