In this module, we're going to formally address the second of the two major concepts of the course. In general, the course is about the shape of light. The first concept we spent most of our time on is understanding how apertures in the system limit resolution, the minimum spot size you can make or cutoff spacial frequency, what patterns you can get through an optical system. But we refer to several times the idea that this is somehow also fundamentally related to how much light you can get through a system. It doesn't really seem obvious that those two should be so fundamentally related, but of course they are. And we're going to develop all of that now. The field of study worrying about the amount of radiation you can get through an optical system is radiometry. So that's formally what we're going to be talking about. I've shown here a cartoon of the prototypical problem that one might face as an optical engineer. You're given the task of taking a certain amount of electrical power, that through some form of light bulb, turns into a certain amount of optical power. And finally, you're asked to get that power through an optical system and get a certain amount of power out or onto a target. And so you have sort of two questions of what amount of power can you get in watts? And what should the optical system be that takes the light from the light bulb and gets it into your optical system? I've shown kind of a typical or maybe a generically nice optical system here, which is a doubly telecentric system. And the reason it's nice, is notice that every single point in the field of a particular size h, is able to collect a certain aperture angle, alpha, and that's because of the nature of a doubly telecentric system. So hopefully, since we've dealt with this before, the idea of a Lagrange invariant given by these two quantities at this initial plane comes to mind. And the question is how does that relate to the optical system that we have going in? So this module, we will understand that problem and be able to design systems like this.