Glossary - NO

nanometer, nm† a unit of wavelength equal to 10−9 m. See the Appendix for conversion values.

narrow-angle diffusion that in which flux is scattered at angles near the direction that the flux would take by regular reflection or transmission. See wide-angle diffusion.

narrow-angle luminaire a luminaire that concentrates the light within a cone of a comparatively small solid angle. See wide-angle luminaire.

national standard of light† a primary standard of light that has been adopted as a national standard. See primary standards of light.

navigation lights† an alternative term for *position lights*.

navigation light system a set of aircraft aeronautical lights provided to indicate the position and direction of motion of an aircraft to pilots of other aircraft or to ground observers.

near infrared† the region of the electromagnetic spectrum from 770 to 1400 nm.

near ultraviolet† the region of the electromagnetic spectrum from 300 to 380 nm.
night the hours between the end of evening civil twilight and the beginning of morning civil twilight. Note Civil twilight ends in the evening when the center of the sun's disk is 6° below the horizon, and begins in the morning when the center of the sun's disk is 6° below the horizon.

nit, nt† a unit of luminance equal to 1 cd/m2. Note the candela per square meter (cd/m2) is the SI unit of luminance.

nonrecoverable light loss factors (initial or maintained) factors that give the fractional light loss that cannot be recovered by cleaning or lamp replacement. Comprised of those components that account for the lamps operating at other than their rated luminous value. This factor is applied to lighting calculations irrespective of the age of the lighting system.

normal ac power† power supplied to a facility during non-emergency situations. It is usually supplied by a local electric utility.

normal lighting† permanently installed task and corridor electric lighting normally for use when the premises are occupied.

numerical display (digital display)† an electrically operated display of digits. Tungsten filaments, gas discharges, light-emitting diodes, liquid crystals, projected numerals, illuminated numbers, and other principles of operation can be used.

O object color† the color of the light reflected or transmitted by an object when illuminated by a standard light source, such as CIE source A, B, C, or D65. See *standard source* and *perceived object color.* obstruction beacon† See *hazard or obstruction beacon*.

obstruction lights aeronautical ground lights provided to indicate obstructions.

occulting light a rhythmic light in which the periods of light are clearly longer than the periods of darkness.

opaque† impenetrable to light; not able to transmit, or not transmitting light.

orientation the relation of a building with respect to compass directions.

Ostwald color system a system of describing colors in terms of color content, white content, and black content. It is usually exemplified by color charts in triangular form with full color, white, and black mixtures at the apices providing a gray scale of white and black mixtures, and parallel scales of constant white content as these grays are mixed with varying proportions of the full color. Each chart represents a constant dominant wavelength (called hue), and the colors lying on a line parallel to the gray scale represent constant purity (called shadow series).

overcast sky one that has 100% cloud cover; the sun is not visible.

overhang the distance between a vertical line passing through a specified point (often the photometric center) of a luminaire and the curb or edge of a roadway.

ozone-producing radiation UV energy of wavelength shorter than 220 nm that decomposes oxygen, O2, thereby producing ozone, O3. Some UV sources generate energy at 184.9 nm, which is particularly effective in producing ozone.P panel (open) face exit sign† a transilluminated sign where both the exit legend and background are translucent.