An Ellipsoidal light is supplied with a certain size lens or lenses that
determines the field angle, anywhere from five to ninety degrees. Field
angle is the angle of the beam of light where it reaches 10% of the
intensity of the center of the beam. Most manufacturers now use field
angle to indicate the fixture's spread typically in this series (5°,
10°, 19°, 26°, 36°, 50°, 75°, 90°).