Principle of measurement
The measuring principle of distribution and radiation method is based on the spherical coordinate system, as shown in picture 1.
Picture 1 spherical coordinate system
Through measuring the irradiance of the sphere surface, if the light source is taken as the origin of the spherical coordinate, θ is taken as the polar angle, φ is taken as the azimuth angle, E（θ，φ）is taken as the luminance produced by the light source in any position（θ，φ）of the assumed spherical surface. The polar angle scanned by the detector is from 0 to 2π, the azimuth angle scanned is from 0 to π, and then the radiant flux produced by the light source is:
Changeable angle radiation method
There must be one motor system that can rotate around the angle direction as shown in picture 1, in order to realize the irradiance measurement of each point on the spherical surface. There are usually the following four kinds of system:
a) The lamp does not move, the detector moves around the measured lamp;
b) The detector does not move, the light source moved around the luminescence center in. θ and φ direction;
c) The combined motion of the detector and the light source, for example, when the detector moves around θ angle, the light source moves in the direction of φ angle;
d) The movement of the retroreflector ( the probe is fixed) equates to the movement of the probe.
The above four kinds of system have the corresponding features. For example, for the light source of larger measuring size, if using the probe movement method, the movement arm length of the detector will become very long; the laboratory space should be big enough. For measuring the light source of sensitive flash point direction, if using the light source movement method, the radiant flux will fluctuate in the process of measurement. Therefore, the fluctuation of the radiant flux must be revised. If using the retroreflector movement method, the mirror surface should have high reflectance for the ultraviolet ray; the requirement of making the retroreflector is very high.
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