Are the IESNA cutoff classifications a good indicator of direct uplight?

Except for the full cutoff designation, the Illuminating Engineering Society of North America (IESNA) cutoff classifications are not a good indicator of direct uplight, because glare control was the original reason they were developed. A luminaire that has the IESNA full cutoff classification does not have any light going directly upward from the luminaire and will not, if mounted correctly, emit light directly into the sky. The direct uplight from a cutoff luminaire can vary from 0% to 16% of the light output of the lamp(s) in the luminaire, and the uplight from a semicutoff luminaire can vary from 0% to 31% of the lamp light output.

However, careful consideration of these classification definitions can be very important when evaluating outdoor luminaires for their potential to cause light trespass or glare. Please read the following sentence carefully:

The definitions are given in terms of luminous intensity (in candelas), but the values are made in reference to luminous flux of the light source (in lumens).

Casually skimming these definitions could lead to the assumption that for a cutoff luminaire, no more than 10% of the lamp luminous flux is emitted between 80° and 90° from nadir, or that no more than 2.5% of the lamp luminous flux is emitted above 90° from nadir. In fact, neither of these assumptions is correct. The following is a technical discussion of these discrepancies.

Consider the hypothetical luminous intensity distribution [shown in Figure 12] for a luminaire equipped with a 1000-lumen lamp. The luminous intensity at any angle between and including 80° and 90° from nadir is 100 candelas, which is 10% of the numerical value of the lamp lumens. The luminous intensity 90° from nadir and at any angle above is 25 candelas, which is 2.5% of the numerical value of the lamp lumens. [The distribution of the remaining 73% of lamp lumens at angles below 80° from nadir does not affect the cutoff classification of the luminaire and, thus, is not shown in Figure 12.] This hypothetical luminaire can be classified as a cutoff luminaire. Interestingly, a luminaire with this distribution would emit 11% of the lamp lumens between 80° and 90° from nadir, and nearly 16% of the lamp lumens above 90° from nadir. (Bullough 2002, reprinted with permission from IESNA)

Figure 12. Hypothetical luminous intensity distribution at 80°–180° for a cutoff luminaire containing a 1000-lumen lamp (units are in candelas)

Source: Bullough 2002, © IESNA, used with permission

The quantity of lumens emitted above 90° from nadir is calculated by multiplying the maximum allowable intensity value, 25 candelas, by the solid angle over which they are emitted, 2π steradians, totaling 157 lumens or 16% of the 1000 lumens emitted by the lamp. For the percentage of lamp lumens in the glare zone, the solid angle from 80° to 90° is approximately 1 steradian. Multiplying the solid angle, 1 steradian, by the maximum allowable intensity value, 100 candelas, the total allowable lumens in the glare zone is 109 lumens or 11% of the 1000 lumens emitted by the lamp.

Conversely, consider another hypothetical luminous intensity distribution [shown in Figure 13], also for a luminaire equipped with a 1000-lumen lamp. The luminous intensity at one angle between 80° and 90° from nadir is 125 candelas, exceeding 10% (100 candelas) of the numerical value of the lamp lumens. The luminous intensity at one angle above 90° from nadir is 40 candelas, exceeding 2.5% (25 candelas) of the numerical value of the lamp lumens. Such a luminaire would emit only 3% of the lamp lumens between 80° and 90° from nadir, and about 1% of the lamp lumens above 90° from nadir. [As in the example above, the distribution of the remaining 96% of lamp lumens at angles below 80° from nadir does not impact the luminaire's cutoff classification and, thus, is not shown in Figure 13.] Because of its luminous intensity values, this luminaire cannot be classified as a cutoff luminaire even though it emits significantly less light upward than the [previously shown] hypothetical luminaire [Figure 12]. Indeed, even if this same luminaire were fully shielded so that it emitted no light above 90°, it could not be considered a full cutoff or even a cutoff luminaire, because of its higher-than permitted intensity at one angle between 80° and 90°. (Bullough 2002, reprinted with permission from IESNA)

Figure 13. Hypothetical luminous intensity distribution at 80°–180° for a luminaire containing a 1000-lumen lamp failing to meet the cutoff classification (units are in candelas)

Source: Bullough 2002, © IESNA, used with permission

Certainly, these luminous intensity distribution examples represent extreme cases. They do, however, serve to emphasize the caution that is required when interpreting the various cutoff classifications. If, for example, one is concerned about minimizing direct uplight from a luminaire, it is not necessarily true that a cutoff luminaire will emit a smaller proportion of its luminous flux upward than a semicutoff (or even a noncutoff) luminaire, even if the luminaires are equipped with the same lamp. A full cutoff luminaire, on the other hand, will never emit direct uplight.

The best way to estimate the luminous flux emitted directly upward by a particular luminaire is to consult a zonal luminous flux summary prepared by the luminaire's manufacturer.

Continuing with the logic above and generalizing for each cutoff classification, ranges of lamp lumen percentage are shown in Table 5 in terms of uplight and lamp lumens in the glare zone. The direct uplight of a luminaire that has the IESNA semicutoff classification theoretically can vary from 0% to 31% of the total lamp lumens and the lamp lumens in the glare zone can vary from 0% to 22%.