PX282 - C5 - definitions

consider ray of light:
- has a specific intensity, $I_{λ}$
  - specific because it is in a specific wavelength range, $λ \to λ + d λ$
- at an angle, $θ$ , with the $z$ -axis
$I_{λ}$ depends on the direction, the location, and the wavelength of light
the flux, $F$ , which is the energy rate in a direction per unit area, in the z-direction:

F_{λ} = \int_{Ω} I_{λ} \cos θ d Ω

d E_{λ} = I_{λ} \cos θ A d t d λ d Ω