Edmund Optics Imaging Lab 3.2: The W of Illumination Geometry
What is the W of illumination geometry? How can it be used to choose the correct light source for an imaging application? Join Gregory Hollows, Director of Machine Vision Solutions, as he reviews the similarities and differences between brightfield and darkfield illumination, and explains how these two illumination techniques allow one to see different pieces of information from the object under inspection.
Hi, I am Greg Hollows. Welcome to the Imaging Lab. Today, we are going to talk about the W and how it relates to Illumination. You might be thinking what in the world is the W in Illumination. What it's going to show you as we go through some of the geometry of the way lighting interacts with surfaces is we are going to be able to understand how to define whether we are looking at a Bright Field Illumination technique or a Dark Field Illumination technique to pull out the desired features on an object that we are going to be looking at. So where does this W come from? Let's move back a little bit and understand the way light interacts with the surface again quickly. Let's take for an example that we have a mirrored surface, something that is completely specular, complete reflection off of it. If we take a point source of light, that can be like a laser pointer or even think of it like a maglight, something that has a defined light source of a small size that is going to be shining down on the object, if we were to take that and run that at an angle across a mirrored surface so it is reflecting off the mirror and back off, first off we know that the angle of incidence is going to equal to the angle of reflection that is there. Basically whatever angle that light is coming in at is going to reflect at the same angle off. If we have a camera system and a lens that are looking down at this there are going to be positions where we're going to be able to shine that light down and it's going to reflect back into the lens and we're going to be able to see the reflection of that light source. As we slide this light source down to the side, we are going to hit some sort of angle where it is going to reflect off of this surface and it's going to be going in a direction that is no longer be able to be captured by the illumination or the lighting system, the camera system, through the lens and we will no longer will be able to see the lighting itself. You can imagine at some point here, we are going to get to that spot. It's going to go from being seen to not being seen. If we did it on the other side, we would have that same sort of angle going on and you would have both of these directions coming in here and it creates what looks like a W. Anything that is inside the W itself is what we consider Bright Field Illumination. Bright Field because as we put the lighting through there, we see a bright object in the field. As we slide past that angle, the light is going away from the lens, is no longer seen, and instead of seeing that bright specular object now, the bright light coming off of that object, we are actually seeing nothing. It is dark, and this is where we move into Dark Field Illumination. Now this is going to be variable depending on the focal length of the lens or the angular field of view of the lens that you are using, the sensor size, the combination of those things together. So, it is not an absolute for every single system that lighting is either going to be bright field or dark field for using the same light. Now, in many cases, we will call out certain lights as a Bright Field light or a Dark Field light because some of them are designed to be up close to the lens and have angles that come straight down to fill the Field of View. Some are designed to be very very low angle and do the same purpose and come in at very very low angles to fill the Field of View. So, they could be considered Bright Field or Dark Field but in actuality you can use any light source in a Bright Field or Dark Field configuration in your system depending on the purpose that you are trying to achieve with it. Now the reason why this is important as we look at the different light sources that are available and the way they are designed, is it's going to allow us to see different pieces of information off of the surface. As an example, if we go back to that mirror, think of like a CD, and you are going to be looking at this compact disc that has a high amount of reflectivity on it. If you have any sort of scratches or etching that is on that surface, when we have it in the Bright Field, all the light shining down, and reflecting right back up into the camera system, we are not going to be able to see that information, it is going to be basically blown away by the rest of the lighting that is there. As we bring this lighting down to the edges here, and it comes in sideways, the entire background is going to go away and any of those defects, because the angle is changing there, that angle of reflection off of that portion of the object is now changed, we are getting lighting up into the system and can see those defects. We'll see a lot more about that as we look at the different light sources that are available. That is the W. You can click on any of the links that are shown to take you to another segment of interest or you can continue on to different illumination techniques by product.
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