亮度是表示眼睛从某一方向所看到物体发射光的强度。

Brightness is said from a certain direction eyes that see objects launching light intensity.

youdao

我们都知道如今大部分渲染器是后向的不是前向的。从摄影机发射光线，命中物体后采样。

As we know, almost the all ray-tracers are based on Backward Ray-Tracing, cast rays from camera, sample the lights, not emit photon from light sources.

youdao