Hello, welcome to this second course on the design of optical systems with geometrical principles. My name is Robert McLeod. I'm a professor of optics at University of Colorado at Boulder. With my colleague, Dr Amy Sullivan, we're going to teach you the next level in optical design in this course. In our first course, we learned that rays, straight lines, could be used to describe how lenses work. And because it's very simple geometry, we derived the principal properties of optical systems with these rays, and used that to design real optical systems. We took those optical designs, turned them into real glass, put them in Zemax OpticStudio, optimized their performance, and began the process of real optical design. Now we left some stuff out. So in this course we're going to take it up a level. And in particular, the thing we're going to address is that the ray picture that we worked on before said that if you have a perfect lens, at the focus spot here, you'd confine the light to an infinitely small spot. And, of course, you'd have infinite energy and density, and you'd blow up your laboratory. Given that that doesn't happen too often, we must have left something out. So we're going to learn about diffraction. We're going to learn that we have to patch up, we have to add something to our ray description that will tell us light has a finite size at this focus, for example. And that's important. If you're trying to look at cells in a microscope, you need to know how big the cell is in relation to that finite size. If you're trying to look at a binary star system and try to figure out is it a single star or is it two? Well, your stars had better be separated more than your blur caused by diffraction there. So it's very important in all optical systems, what's your resolution? And this is what's going to set it. To understand that, we will start with Gaussian beams. These are an ubiquitous shape of light that goes through optical systems, particularly common in laser based systems, because most lasers put out beams that can be approximated by a Gaussian. So this kind of field that we'll learn to manipulate is important when you're building lasers, perhaps a laser resonator or a fiber to fiber coupler that you might use in telecom. Or the laser going out of a lidar, that, again, would distance range for your self-driving car. Conveniently, it turns out that the ray techniques that we learned in the first course can be used. So we'll learn, there's a couple of special rays we shoot through an optical system that, when we combine them in the right way, tell us about how this calcium beam diffracts and diverges as it moves through a system. And these very simple techniques would be the ones you would use to design a fiber to fiber coupler, for example, in the telecommunications industry. Fundamental the course is about the fact that optics are a finite size. And so we'll learn, really, there's only two stops, two finite sized elements that we'll need to use and understand when we design an optical system. And we'll give them names. They're called the aperture and field stop. We'll shoot and learn there's two critical rays which describe how those stops work. And from that, we'll learn what are the critical properties of finite optics like the field of view and the resolution, because these are the things that set those quantities. Once we do that, we'll be able to solve problems like this. And this is the, besides resolution, the other major technique we want to learn from this class. Which is how to take light from, let's say, a light bulb, design an optical system here in the black box, and get it into an optical system. This could be perhaps the projector of a digital cinema system. This is the light that's going to illuminate the screen eventually. And you're going to have a real significant requirement in getting a lot of light out here to the eventual screen. How much light can you get off this light bulb? The answer turns out to be a shockingly low amount in many cases. But conveniently, it's described by those rays that I just pointed to. And so we'll learn all of these techniques so that you can now take the simple optical designs we worked with in course one and understand their resolution and their efficiency.