Ultraviolet radiation / 紫外辐射
Optical radiation for which the wavelengths are shorter than those for visible radiation.
波长小于可见辐射波长的光辐射。

Uniform-chromaticity-scale (UCS) diagram / 均匀色度标尺（UCS）图
A twodimensional diagram in which the coordinates are defined with the intention of making equal distances represent as nearly as possible equal steps of color discrimination for color stimuli of the same luminance throughout the diagram.
二维坐标图。其坐标定义使整个色度图上的坐标间距尽可能与同一亮度的颜色刺激的色差步长相等。

Upper [lower] (luminous) flux / 上部[下部]（光）通量
Of a source. The luminous flux emitted above [below] a horizontal plane passing through the source.
对光源而言。高于[低于]通过光源的水平面而发射的光通量。
Note: It is essential that the plane be specified in every case.
注 重要的是该平面应在每种情况下特别指定。

Uniformity ratio of illuminance / 照度均匀比
On a given plane. A measure of the variation of illuminance over the plane expressed as either
1. The ratio of the minimum to the maximum illuminance.
2. The ratio of the minimum to the average illuminance.
对给定平面而言。照度在平面上的变化的量度，有两种表示方式
1.照度最小值与照度最大值之比。
2.照度最小值与照度平均之值比。
Note: In some countries, the reciprocal of these ratios is used, characterized by values greater than unity.
注 :一些国家使用以上比值的倒数，其值大于1。

Upward [downward] light output ratio (luminaire efficiency)
The ratio of the flux emitted above [below] a horizontal plane passing through the luminaire to the total bare lamp flux.

Utilance (U) / 光通利用率（U）
Ratio of the utilized flux to the luminous flux leaving the luminaires.
被利用的光通与离开灯具的光通量之比。

Utilisation factor / 照明利用率
Ratio of the utilized flux to the luminous flux emitted by the lamps.

Utilized flux
Luminous flux received on the reference surface under consideration.