Solutions to your machine vision challenges.
Frequently Asked Questions (and some answers!):
1. Telecentric Lenses
Q: I’ll get a better image with a telecentric lens, won’t I?
A: It depends on what you mean by a better image. If you mean a sharper or better focused image than as seen with a conventional lens, the answer is no. Given the same basic high quality, the same magnification and the same f-number, the two lens types will produce the same image sharpness.
Also, the image sharpness depth of field, or MTF Depth of Field (MTFDOF), is not necessarily any greater with a telecentric lens than with a conventional lens.
Q: Then why use a telecentric lens?
A: There are two primary advantages of a telecentric lens. The first is that it provides a constant or nearly constant perspective across the field of view. Another way to say this is that the view angle is constant across the field, as if you were looking at the object from infinitely far away. This property is very useful when inspecting the insides of tubes, or making sure that tall objects don’t "shadow" adjacent shorter objects.
The other advantage of a telecentric lens is that the magnification remains constant or nearly constant even if the object distance is changed. This makes telecentric lenses ideal for many gauging applications.
Q: I’ve tried other telecentric lenses, but didn’t get the result I wanted. Why should I use your lenses?
A: Not all telecentric lenses are created equal. There are two types of telecentric lenses that are typically commercially available. The first is called Object Space Telecentric; this is the most common type. The second is called Double or Bi-Telecentric. Double Telecentric lenses inherently have a larger range over which the object can move and still maintain the same magnification. We call this Magnification Depth of Field, or MAGDOF. In fact, Double Telecentric lenses can theoretically have infinite Magnification Depth of Field. Manufacturing tolerances prevent this from being actually realized. Light Works’ Super-Eye™ and Mini-Super-Eye™ lenses are true Double Telecentric designs. For more on the subject, click here [Or however you want to link to the VSD article]
Q: Why don’t you state the focal lengths of your lenses?
A: Unlike other telecentric lenses that have real focal lengths, such as 55mm for one popular model, Double Telecentric lenses have infinite focal lengths. Optical engineers call this being afocal. It’s why the Double Telecentric design can have much larger Magnification Depth of Field.
Q: If the focal length is infinite, how do I make lens calculations?
A: The magnification is fixed, so there’s no need to calculate this property. What’s left to calculate is the working distance (WD) that results for a given lens extension or spacer between the lens and the camera.
For any of our Double Telecentric lenses, the change in WD is inversely proportional to -(the square of the magnification) times the (spacer length), or:
Change in WD = -[1/(magnification2)] x (spacer length)
Let’s look at a specific example:
Our Mini-Super-Eye™ model SES-400 has a magnification of 0.400, and a WD (with no spacers) when used with a c-mount camera of 332mm. What happens when you add a 10mm extension between the lens and the camera?
1) Change in WD = -[1/(0.4002)] x (10mm)
2) Change in WD = -6.25 x 10mm
3) Change in WD = -62.5mm
That is, the working distance gets shorter by 62.5mm, or reduces to 269.5mm.
NOTE: If you are using a cs-mount camera without a 5mm c-mount adapter, this is the same as using a negative 5mm spacer. Using the formula above, you would find that WD for the SES-400 increases by about 31mm.
Q: I need a different working distance or different magnification or different lens size than what you offer. What do I do?
A: Light Works has always been happy to custom design lenses. Very often, these designs don’t cost too much more, nor do they require excessive lead time. More than half our current customers have purchased our custom lenses, and they come back for more. Let us know what you need; we think you’ll be pleasantly surprised.
Q: How do I know your custom lenses will work?
A: By speaking with you and learning in detail about your application, we’re able to guide our design to best fit your needs. Sometimes we’ll conclude that we can’t provide a matching product, and we’ll happily recommend alternate solutions. (Some of our best customers are people we regularly steer toward our competitors. No kidding!)
When we do have a good solution, however, we’ll provide the calculated data and interpretation to back up our recommendation. What’s more, we are often able to suggest simple experiments you can do with conventional lenses that will allow you to see comparable image quality and light collection results before you buy a telecentric lens.
Q: I need high magnification and large MTF depth of field. Can you help?
A: This question comes up about once a month. The short answer is, maybe. It depends on how high a magnification and how large a depth of field you need. These two factors compete with each other. A third factor is lens f-number.
Generally speaking, image sharpness or MTF Depth of Field (MTFDOF) is linearly proportional to lens f-number, and inversely proportional to the square of the magnification.
So, all other things being equal, if you double the f-number (make the aperture half the diameter), you will double the MTFDOF. You will also, however, collect 1/4th as much light because your aperture is now 1/4th the area. Furthermore, even if you had as much light as you wanted (which almost never happens!), eventually the aperture would be so small that diffraction effects at the aperture edge would ruin the image quality. This is referred to as the diffraction limit.
But things get worse! (Or better, depending on your point of view.) Keep in mind that MTFDOF is inversely proportional to the square of the magnification. So, again all other things being equal, if you double your magnification you will reduce your MTFDOF to 1/4th of what it was before. Furthermore, you are taking the same amount of collected light, and now spreading it over four times as much area at the camera detector. What do you do to make up for that reduction in light level? You decrease the f-number of the lens which reduces your MTFDOF further.
Arggh! It’s a losing game, so why bother? Well, most of the time we can help you find a happy medium and an excellent solution. Give us a try and remember …
2. Optical Field Splitting
Q: What will I see on the monitor when I use the Adjustable Field Splitter?
A: You’ll see two (or more) images next to each other, with an overlap or “dead” zone between them. The overlap zone is a small region where light from both object fields land on the detector. Usually this small region is not useful for most machine vision purposes because the two images interfere with each other.
The size of this overlap zone depends on two factors. The first is what f-number the lens is set to. The smaller the f-number (larger the lens aperture), the more overlap there will be. The second factor is how far away from the entrance pupil the actual optical split occurs. The farther this distance, the smaller is the overlap zone.
This is the reason that the very best type of lens for use with an optical splitter is often a telecentric lens. Almost all commercially available telecentric lenses have their entrance pupils infinitely far behind the lens. Nevertheless, a majority of our customers quite successfully use the AFS with conventional lenses. Contact us to receive a copy of our AFS User’s Guide for more detail on optical splitting theory and practice. We always try to help insure that an optical field splitting application goes right.
Q: Does the field splitter attach to the camera?
A: This question comes up a lot. In almost every case, the lens first mounts to the camera, and then the field splitter mounts to the front filter thread of the lens. You not only save the cost of multiple cameras, but also the cost of multiple lenses. Even so, we have provided splitters that screw into the camera, with a separate lens at the end of each arm of the splitter. In this way, different magnifications may be applied to each sub-field of view.
Such applications usually do require other auxiliary lens elements to be mounted inside the optical splitter extension tubes, and care must be taken when laying out the design. You cannot simply just screw a lens to the ends of the splitter arms; the back focal lengths of most lenses are too short to be used this way.
Q: The field splitter is more expensive than a second camera. Why should I use it?
A: If you are using low-end cameras with no on-board processing power, maybe you shouldn’t. But camera cost alone isn’t the only factor. Even if your camera board can handle two inputs, there is the cost of an extra lens, cable set, power supply, and mechanical mounting. It adds up. Each case must be reviewed individually.
On the other hand, if you are using more expensive smart cameras, or high-end, high resolution cameras, the cost savings often makes the decision a no-brainer. Furthermore, the AFS or other optical splitters from Light Works provide other benefits. With two or more images on the same camera, there is no need for two separate cameras to coordinate processing tasks, timing, etc. There is also very often a space savings when using an AFS or other optical splitter as compared to using two or more cameras.
Q: Can the Adjustable Field Splitter be used for large fields of view?
A: In most cases, no. The AFS can certainly be used to view fields that are larger than the approximately 1” clear aperture of its components, but the object being viewed must be some distance away. As the object gets bigger, it must get farther and farther away for the limited view angle through the AFS to capture the required area. Picture yourself looking through say a four to twelve inch long pipe with an ID of about an inch. If you’re standing next to a car, you can see the keyhole in the door, or a couple of letters of the cars name logo. To see the whole car, however, you have to be a few dozen feet away.
We have and do make optical splitters for larger fields of view. In one such case, the two sub-fields of view were three feet wide! For a more reasonable, but still large field splitter, take a look at this example.
We make Light Work for you.
