1. Light and Color
The wavelength, λ, of light determines its “color”
2. Describe light by a spectrum (光谱)
“Intensity” of light at each wavelength
A graph of “intensity” vs. wavelength
We care about wavelengths in the visible spectrum: between the infra-red (700nm) and the ultra-violet (400nm)
3. Examples
White and Grey
Helium Neon Laser(激光)
Normal Daylight
Tungsten Lightbulb
4. 个人理解：
一条光线 可以用 上面的 SPD 图表来表示，SPD图表描述了 光线的每一个波长上光子数目的 情况，SPD图表可以转换为 XYZ，XYZ可以转为了RGB，所以，就有了一条光线对应视觉中的什么颜色。
5. Radiometric Quantities
a. Flux: Total in some domain
b. Irradiance: Per unit area
c. Intensity: Per unit angle
d. Radiance: Per unit angle per unit projected area
Basically, all the latter are differential(微分) quantities that have no meaning(没有意义) until you integrate(积分) them over some domain Like you have to integrate speed to get distance traveled
5.1 Radiant Flux
a. Total amount of energy passing through a surface or region of space per unit time
Typically denoted by 
Also called power
Measured in watts (W) or joules/second (J/s)
b. Typically used for a light’s total output
You talk about a 60W light bulb
5.2 Irradiance
Irradiance is the power arriving at a surface, per unit area on the surface
Denoted E in PBR, or sometimes Ir or sometimes I
Units: Wm-2
5.3 Irradiance to Flux
Integrate over area:
Hence, we can also say (
5.4 Measuring Angle
The solid angle subtended by an object from a point P is the area of the projection of the object onto the unit sphere centered at P
Measured in steradians, sr
Definition is analogous to projected angle in 2D
If I’m at P, and I look out, solid angle tells me how much of my view is filled with an object
5.5 Intensity
Flux density per unit solid angle:
Only meaningful for point light sources (otherwise we would need some concept of the area of the light)
5.6 Radiance
Flux density per unit area perpendicular to the direction of travel, per unit solid angle:
Units Wm-2sr-1, power per unit area per unit solid angle
Radiance is the fundamental measurement
All others can be computed from it via integrals over area and/or directions
Radiance is constant along lines
No r2 falloff, the per unit solid angle takes care of it
5.7 Irradiance from Radiance
Integrate radiance over directions in the upper hemisphere:
cos term deals with projected solid angle. θ is angle between w and n (the normal)