Today, we're going to continue in the module on realistic lighting & materials, and go deep in radiometry & photometry: measurement systems for lighting.
Radiometry is a measurement system & a set of units for illumination. The goal is to measure the spatial properties of light. In order to do this, we introduce a handful of new terms - radiant flux, intensity, irradiance, and radiance.
The underlying assumption for the model that we'll be working with is geometric optics (as opposed to wave optics). This assumption assumes that photons travel in straight lines, represented by rays. For graphics (and creating photorealistic appearances), a particle model (geometric optics model) of light is an excellent model.
When we talk about light, we center our discussions around the electromagnetic spectrum.
Later, we'll have more in-depth discussions about color. Today, we'll be talking about the net energy of this light.
How do lights work? Let's think about a LED lightbulb.
With a lightbulb, we have some physical process converts energy into photos. Over some amount of time, light consumes some amount of energy (Joules). Some energy is turned into heat, and some is turned into photons. The energy of photons hitting an object can cause exposure (e.g. film, sensors, sunburn, solar panels, etc.).
In graphics, we generally assume a "steady state flow" where the rate of energy consumption is constant, so flux (power) and energy are often interchangeable.
The flux is the flow rate: how fast do photons flow through a sensor? Each photon carries a small amount of energy.
Radiant (luminous) energy* is the energy of electromagnetic radiation. It is measured in units of joules. It's denoted by the symbol $Q$.
Radiant (luminous) flux* is the energy emitted, reflected, transmitted, or received, per unit time.
*We can also define these terms in photometric quantities & units.