Ultrasonic frequency

The frequency at which an ultrasonic sensor operates.

Ultraviolet

Any radiant energy within the wavelength range 100 to 400 nanometers is considered ultraviolet radiation (1 nanometer = 1 billionth of a meter, or 1 X 10^-9 m).

Uniformity

The degree of variation of illuminance over a given plane. Greater uniformity means less variation of illuminance. The uniformity ratio of illuminance is a measure of that variation expressed as either the ratio of the minimum to the maximum illuminance or the ratio of the minimum to the average illuminance.

Uplight

Light directed upward at greater than 90° above nadir. The source of uplight can be from a combination of direct uplight and reflected light.

Venting

Holes in the reflector assembly of a downlight.

Vertical illuminance

The average density of luminous flux incident on a vertical surface, measured in footcandles (fc) or lux (lx). One fc equals 10.76 lx.

Visual performance

The quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy.

Voltage drop

The difference between the voltages at the transmitting and receiving ends of a feeder, main, or service.

Voltage regulation

The change in output voltage that occurs when the load (at a specified power factor) is reduced from rated value to zero, with the primary impressed terminal voltage maintained constant.

Wall-washing

The practice of illuminating vertical surfaces, such as walls. Wall-washer luminaries are designed to illuminate vertical surfaces.

Warm-up time

The time it takes for a lamp to produce 90% of its initial light output when it is started, unless otherwise indicated.

Wavelength

The distance between two corresponding points of a given wave. Wavelengths of light are measured in nanometers (1 nanometer = 1 billionth of a meter, or 1 X 10^-9 m)

Weight

The weight of a luminaire plus ballast (except for certain track luminaires with separately mounted ballasts, when the weight is that of the lamp and track head only). For modular compact fluorescent lamp (CFL) ballasts, the weight of the ballast without a lamp. For self-ballasted CFLs, "weight" indicates the total product weight.

Zenith

In the lighting discipline, zenith is the angle pointing directly upward from the luminaire, or 180°. Zenith is opposite nadir. In astronomical usage, zenith is the highest point in the sky, directly above the observation point.