IV. CUTOFF

Beyond distribution and range, luminaires are defined by how well they control or cut off light at selected vertical angles. Typically this reference point is referred to as zero degrees vertical or "nadir". Designs without significant cutoff characteristics distribute light in zones unlikely to contribute to useful visibility, contribute to light pollution, and are inefficient.

Figure 6: Definitions of Cutoff Luminaires

Definitions
The definition of cutoff is based on what proportion of a luminaire's output is being distributed at 80° and 90° above nadir. See figure 6 for graphic examples of some of the luminaire cutoff types defined below. (Extracted from IES Publication RP33-99 (2/99))

Noncutoff
A luminaire's light distribution is designated as noncutoff when there is no limitation of illumination in any zone.

Full Cutoff
A luminaire's light distribution is designated as full cutoff when the candela at 90° above nadir is 0 and less than 10% of rated lamp lumens at 80° above nadir.

Cutoff
A luminaire's light distribution is designated as cutoff when the candela at 90° above nadir is less than 2.5% of rated lamp lumens, and less than 10% of rated lamp lumens at 80° above nadir.

Semicutoff
A luminaire's light distribution is designated as semicutoff when the candela at 90° above nadir is less than 5% of rated lamp lumens, and less than 20% of rated lamp lumens at 80° above nadir.

Example:
A luminaire with tested data showing a total of 16,000 Rated Lamp Lumens has a candela tabulation that produces 18 candela at 90° (<2.5% of Rated Lumens) and 55 candela at 80° (<10% of Rated Lumens). These values fall within the defined ranges shown in Figure 6, classifying this as a cutoff luminaire.