EO Imaging Lab Module 3.1: Introduction to Illumination Concepts

Hi, I am Greg Hollows. Welcome to the Imaging Lab. In this segment, we are going to discuss how lighting interacts with different objects and putting together the illumination for an imaging system. Much like playing pool, if you shoot a pool ball into the bumper on a pool table, the angle of incidence is equal to the angle of reflection of that ball going off the bumper. Lighting works the same way going off of any given object. Let's start with the simplest example. If we take light, a laser let's say, something of a collimated sort of variety, and shine it onto a mirror, we are going to get the angle of incidence equal the angle of reflection. Basically, if I was to shoot a laser into that object and it hit down and came back off, we would be able to see this angle coming off of the object itself and we would be able to calculate those angles and they would be equal, the in and the out. Now when we start thinking about light in general, when I think about even the lights that are around me here, they are very diffuse, and they scatter across the entire surface that I have here. So, there are lots of different angles that are coming into this object, but anywhere they strike it, they are still going to come off with an equal angle of incidence and angle of reflection from that object itself. Now what we want to think about now as we start looking at different objects and different materials is how that light truly interacts with those objects and what creates the different effects that we see. If we were to take a light source and take it into a mirrored surface, we would see that absolute angle come off and we would see hot spots actually coming off of that mirror when we looked at it at that angle of reflection coming towards our eyes. We would actually be able to see those lights themselves. The other thing that can happen is when you get a diffuse or scattered sort of reflection. That has more to do with the material itself, and can also have to do with the lighting, but when those angles of the rays of light are coming into the object, we are hitting an object like this table cloth here, it has a very complex geometry to it. This table cloth is going to have angles going at all sorts of different direction. That gives us a very scattered effect. That is why we get a nice diffuse reflection if it was a white table cloth or one of these boxes or something like that. The next thing that can happen with light as it goes into an object is that it can actually be transmitted through. We see this in every day of our lives as we look at things like window panes or through the lens of a camera system. If we were to look through it, we would see light transmitted through to our eyes and you may see some amount of subtle reflections there but depending on that material, you can actually see light passing through the object itself. The other thing that can happen, it can be absorbed, as we take a look at again this table cloth that we have here. Some of the lens housings, they're blackened. The light is actually being absorbed by them and that is why we see a black coloring here because all the lighting is basically being absorbed and we see an absence of color reflect the light to our eyes. The other thing that can happen is we could get emission in the system. Fluorescence is an example of this where light of one wavelength goes in and is absorbed and then the actual object itself becomes a light source of sorts where the light itself is actually retransmitted. It could be at the same wavelength, usually at a different one, and it's coming off and being scattered out to the system relating to the geometry of the object itself. So, there are lots of different ways that lighting can interact with the object and it really depends on the geometry of the object itself. Now, why is all that important? The critical part of that is that you go through all these different types of materials and the ways that light can interact with the object. It is going to define what sort of light source we are going to have to use for the system, the sort of geometry it is going to have to allow us to see whether it is a scratch or a defect or bring out some edges or other information that we want to see in our imaging. We're going to have to choose a light source that matches the geometry of that very very well to see that information. What you are going to find is there is a variety of geometries that can be used and those really constitute or work their way over to the different types of light source that are available. And there is a wide range of them because you could have some objects that are like this table here - very flat, kind of diffuse material. That is going to need one type of lighting. And then you might have something that is a curved reflective surface or something that is translucent or transparent that is also curved and trying to pick up details, and that is going to require a different lighting system to be able to take into account these angles of incidence and reflection and be able to highlight which reaction we want to see. The next segment we are going to look at is called W and how that is going to tell us how the geometries of light are affecting the actual object that we are looking at. And that allows us to transition over to what type of light sources that we need to see in the following segments. That is the end of this segment. You can click on any of the links shown on the screen to go on to an other segment or you can continue on to the W in Illumination.