欢迎纠错：
理解 为什么可以用 RGB 来表示 Radiance
a. 在PBRT 中有写到：
Radiometry provides a set of ideas and mathematical tools to describe light propagation
and reflection.
意思就是，辐射度量学 提供了 一套思想和数学工具 来描述 光的传播与放射。
b..
在 <Real-Time-Rendering> P108 里面中写到
Radiance (symbolized as L in equations) can be thought of as the measure of the brightness and color of a single ray of light.
意思就是， Radiance 测量的 就是 一条光线 的亮度和颜色。
P210 里面看到
Light is perceived in the visible band, from 380 to 780 nanometers (nm). Light from a given direction consists of a set of photons in some distribution of wavelengths. This distribution is called the light’s spectrum.
意思就是，一条光线 可以用 spectrum 来表示。spectrum个人理解也就是PBRT的SPD图表。
c
PBRT的SPD 是可以转换为 XYZ，XYZ 可以转换为RGB。
所以，得到，RGB-> XYZ ->SPD -> Radiance 测量的结果
（假设在做测量一条光线的实验，得到的 其实就是一张SPD图表，也就是说，Radiance 测量的数据，就是 一张SPD图表）
d.
有另外一些文章也给出来一些思路
To integrate the spectral data of a light source (the technique is the same for materials), we need to sum up all the data from the spectrum multiplied by the distance between two consecutive(连贯) samples. The resulting number(积分的结果) defines the power of the light source (which can either be expressed in Joules(焦耳) or in Watts(瓦特)). This concept if important because many renderers don't deal with spectrum data directly but relies on the user to specify the power or intensity of light sources. Being able to compute this power can be useful to set our light sources with physically meaningful data (note that from this spectrum we can also compute an XYZ value which we then convert to RGB). Mathematically, this integration takes the following form:
Where Se(λ) is the emission spectrum of the light which is also called (in radiometry) the light spectral intensity.
（个人理解就是，从 spectrum (SPD) 可以计算出 XYZ，可以转换为RGB，然而，上面的公式也说了，spectrum 的 积分可以得出Power，也就是 Radiant Flux ，但是我个人理解，spectrum 记录的是一条光线的数据（SPD图表数据计算出来的只会是特定一种颜色），那就是说，上面的公式的其实得到的就是一条光线的 Power值，其实也就是 Radiance （L））