What are the IESNA cutoff classifications?

The Illuminating Engineering Society of North America (IESNA) defines several outdoor luminaire cutoff classifications, each with different photometric criteria. For these classifications, two relevant zones are defined with respect to the nadir of a luminaire (the nadir is defined as the angle that points directly downward, or 0°, from the luminaire). One zone applies to angles at or above 80° above nadir, and the second zone covers all angles at or above 90° above nadir, or above the horizontal plane of the luminaire (see Figure 11). Light emitted in the 80° to 90° zone is more likely to contribute to glare, and light emitted above the horizontal is more likely to contribute to sky glow. The four IESNA classifications are defined as follows (IESNA 2000):

• Full cutoff—The luminous intensity (in candelas) at or above an angle of 90° above nadir is zero, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 10% of the luminous flux (in lumens) of the lamp or lamps in the luminaire.
• Cutoff—The luminous intensity (in candelas) at or above an angle of 90° above nadir does not numerically exceed 2.5% of the luminous flux (in lumens) of the lamp or lamps in the luminaire, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 10% of the luminous flux (in lumens) of the lamp or lamps in the luminaire.
• Semicutoff—The luminous intensity (in candelas) at or above an angle of 90° above nadir does not numerically exceed 5% of the luminous flux (in lumens) of the lamp or lamps in the luminaire, and the luminous intensity (in candelas) at or above a vertical angle of 80° above nadir does not numerically exceed 20% of the luminous flux (in lumens) of the lamp or lamps in the luminaire.
• Noncutoff—There is no candela limitation in the zone above maximum candela.

Figure 11. Angles referenced by the IESNA cutoff classifications

Source: Adapted from Bullough 2002 © IESNA, used with permission