Ideal when paired with SONY IMX990 or SONY IMX991 sensors, Edmund Optics’ C-Series fixed focal length SWIR lenses support a 2.8µm pixel pitch far smaller than classic SWIR pixel sizes in the 5 – 15µm range.
Fixed focal lengths help the lens designers achieve great performance while minimizing production costs due to fewer parts.
Certain sensors marketed as Vis-SWIR (Visible plus SWIR spectrum coverage) are far less expensive than those traditionally designed for SWIR alone – and perform really well in the SWIR range (900 – 1700nm). The SONY IMX990 and SONY IMX991 are two such sensors, the former available in AVT Goldeye 130, and the latter in AVT Alvium 1800. So are SONY IMX992 and SONY IMX993, as featured in AVT Alvium cameras with diverse interface options.
So while certain users buy those sensors for applications that generate an image in both the visible and SWIR portions of the spectrum – MOST buyers are purchasing these sensors “just” do do SWIR applications in a cost-effective way.
It’s a bit like buying a dual-function toaster oven and never using one of the functions – but if it creates a valuable solution for you, who cares about the feature not used?
Edmund Optics saw the opportunity to create a lens series for the customers using the sensors referenced above to do dedicated SWIR applications. So they created their C-Series fixed focal length SWIR family, with 7 members, and focal lengths from 6 – 50mm.
Did we mention performance?
Recall that lens performance is typically expressed by the Modular Transfer Function (MTF). Below is the MTF chart for the 6mm FL at 1.3µm wavelength, from the Edmund Optics C-Series fixed focal length lenses. All 8 members of the family show comparable performance – see spec sheets for details.
MTF graph for the 6mm FL at 1.3µm wavelength” – Courtesy Edmund Optics
Shorter focal lengths not always easy to find
With fixed focal lengths at 6mm, 8.5mm, 12mm, 16mm, 25mm, 35mm, and 50mm, knowledgeable customers may note that especially the shorter focal length offerings are not that common in the machine vision optical market.
Compact and cost-effective
As fixed focal length lenses, each member of this lens series only need a focus adjustment – fine tuning – which is lockable against vibration slippage. They do NOT need the complexity of a varifocal lens. That means fewer glass elements and less metal, yielding a smaller form factor, handy if space is an issue.
It also means the lenses are less expensive to manufacture, a savings the user can enjoy in achieving a cost-effective way to get good performance in the SWIR spectrum.
Built as a variation on another lens series
It’s worth noting this SWIR-optimized lens series piggybacks on Edmund Optics visible spectrum C-Series fixed focal lenses. The key difference is the new lens series are optically coated for the SWIR spectrum. The benefit to the user is that Edmund Optics could do a spin on an existing lens series, which is cost-effective for the customer as well.
Optimized for factory automation applications
Both the visible and SWIR versions of the C-Series lenses have been optimized with factory automation in mind, particularly with respect to WD, size, and cost.
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Metrology, when done optically, requires that an object’s representation be invariant to the distance and position in the field of view. Telecentric lenses deliver precisely that capability. Telecentric lenses only “pass” incoming light rays that are parallel to the optical axis of the lens. That’s helpful because we measure the distance between those parallel rays to measure objects without touching them.
Telecentric lens eliminates the parallax effect – Courtesy Edmund Optics
Parallax effect
Human vision and conventional lenses have angular fields of view. That can be very useful, especially for depth perception. Our ability to safely drive a car in traffic derives in no small part from not just identifying the presence of other vehicles and hazards, but also from gauging their relative nearness to our position. In that context parallax delivers perspective, and is an asset!
But with angular fields of view we can only guess at the size of objects. Sure, if we see a car and a railroad engine side by side, we might guess that the car is about 5 feet high and the railroad engine perhaps 15 or 16 feet. In metrology we want more precision than to the nearest foot! In detailed metrology such as precision manufacturing we want to differentiate to sub-millimeter accuracy. Telecentric lenses to the rescue!
Assorted telecentric lenses – Courtesy Edmund Optics
Telecentric Tutorial
Telecentric lenses only pass incoming light rays that are parallel to the optical axis of the lens. It’s not that the oblique rays don’t reach the outer edge of the telecentric lens. Rather, it’s about the optical design of the lens in terms of what it passes on through the other lens elements and onto the sensor focal plane.
Let’s get to an example. In the image immediately below, labeled “Setup”, we see a pair of cubes positioned with one forward of the other. This image was made with a conventional (entocentric) lens, whereby all three dimensions appear much the same as for human vision. It looks natural to us because that’s what we’re used to. And if we just wanted to count how many orange cubes are present, the lens used to make the setup image is probably good enough.
Courtesy Edmund Optics.
But suppose we want to measure the X and Y dimensions of the cubes, to see if they are within rigorous tolerance limits?
An object-space telecentric lens focuses the light without the perspective of distance. Below, the image on the left is the “straight on” view of the same cubes positioned as in “Setup” above, taken with a conventional lens. The forward cube appears larger, when in fact we know it to be exactly the same size.
The rightmost image below was made with a telecentric lens, which effectively collapses the Z dimension, while preserving X and Y. If measuring X and Y is your goal, without regard to Z, a telecentric lens may be what you need.
Courtesy Edmund Optics.
How to select a telecentric lens?
As with any engineering challenge, start by gathering your requirements. Let’s use an example to make it real.
Object of interest is the circled chip – Image courtesy Edmund Optics
Object size
What is your object size? What is the size of the surrounding area in which successive instances of the target object will appear? This will determine the Field of View (FOV). In the example above, the chip is 6mm long and 4mm wide, and the boards always present within 4mm. So we’ll assert 12mm FOV to add a little margin.
Pixels per feature
In theory, one might get away with just two pixels per feature. In practice it’s best to allow 4 pixels per feature. This helps to identify separate features by permitting space between features to appear in contrast.
Minimum feature size
The smallest feature we need to identify is the remaining critical variable to set up the geometry of the optical parameters and imaging array. For the current example, we want to detect features as small as 25µm. That 25µm feature might appear anywhere in our 12mm FOV.
Example production image
Before getting into the calculations, let’s take a look at an ideal production image we created after doing the math, and pairing a camera sensor with a suitable telecentric lens.
Production image of the logic chip – Courtesy Edmund Optics
The logic chip image above was obtained with an Edmund Optics SilverTL telecentric lens – in this case the 0.5X model. More on how we got to that lens choice below. The key point for now is “wow – what a sharp image!”. One can not only count the contacts, but knowing our geometry and optical design, we can also inspect them for length, width, and feature presence/absence using the contrast between the silver metallic components against the black-appearing board.
Resuming “how to choose a telecentric lens?”
So you’ve got an application in mind for which telecentric lens metrology looks promising. How to take the requirements figures we determine above, and map those to camera sensor selection and a corresponding telecentric lens?
Method 1: Ask us to figure it out for you.
It’s what we do. As North America’s largest stocking distributor, we represent multiple camera and lens manufacturers – and we know all the products. But we work for you, the customer, to get the best fit to your specific application requirements.
Give us some brief idea of your application and we will contact you to discuss camera options.
Method 2: Take out your own appendix
Let’s define a few more terms, do a little math, and describe a “fitting” process. Please take a moment to review the terms defined in the following graphic, as we’ll refer to those terms and a couple of the formulas shortly.
Telecentric lens terms and formulas – Courtesy Edmund Optics
For the chip inspection application we’re discussing, we’ve established the three required variables:
H = FOV = 12mm
p = # pixels per feature = 4
µ = minimum feature size = 25µm
Let’s crank up the formulas indicated and get to the finish line!
Determine required array size = image sensor
Array size formula for the chip inspection example – Courtesy Edmund Optics
So we need about 1900 pixels horizontally, plus or minus – with lens selection, unless one designs a custom lens, choosing an off-the-shelf lens that’s close enough is usually a reasonable thing to do.
Reviewing a catalog of candidate area scan cameras with horizontal pixel counts around 1900, we find Allied Vision Technology’s (AVT) Manta G-131B, where G indicates a GigEVision interface and B means black-and-white as in monochrome (vs. the C model that would be color). This camera uses a sensor with 2064 pixels in the horizontal dimension, so that’s a pretty close fit to our 1920 calculation.
Determine horizontal size of the sensor
H’ is the horizontal dimension of the sensor – Courtesy Edmund Optics
Per Manta G-319 specs, each pixel is 3.45µm wide, so 20643.(45) = 7.1mm sensor width.
Determine magnification requirements
The last formula tells us the magnification factor to fit the values for the other variables:
Magnification = sensor width / FOV Courtesy Edmund Optics
Choose a best-fit telecentric lens
Back to the catalog. Consider the Edmund Optics SilverTL Series. These C-mount lenses work with sensor sizes 1/2″, 2/3″, and 1/1.8″ sensors, and pixels as small as 2.8µm, so that’s a promising fit for the 1/1.8″ sensor at 3.45µm pixel size found in the Manta G-131B. Scrolling down the SilverTL Series specs, we land on the 0.50X Silver TL entry:
Some members of the SilverTL telecentric lens series – Courtesy Edmund Optics
The 0.5x magnification is not a perfect fit to the 0.59x calculated value. Likewise the 14.4mm FOV is slightly larger than the 12mm calculated FOV. But for high-performance ready-made lenses, this is a very close fit – and should perform well for this application.
Optics fitting is part science and part experience – and of course one can “send in samples” or “test drive” a lens to validate the fit. Take advantage of our experience in helping customers match application requirements to lens and camera selection, as well as lighting, cabling, software, and other components.
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Need a close-up image your preferred sensor and lens can’t quite deliver? A glass-free extension tube or close up ring can change the optics to your advantage.
C-mount extension tube kit – Courtesy Edmund Optics
What’s an extension tube?
An extension tube is a metal tube one positions between the lens and the camera mount. It comes with the appropriate threads for both the lens and camera mount, so mechanically it’s an easy drop-in procedure.
By moving the lens away from the optical plane, the magnification is increased. Sounds like magic! Well almost. A little optical calculation is required – or use of formulas or tables prepared by others. It’s not the case than any tube of any length will surely yield success – one needs to understand the optics or bring in an expert who does.
S-mount extension tube kit – Courtesy Edmund Optics
Note: One can also just purchase a specific length extension tube. We’ve shown images of kits to make it clear there are lots of possibilities. And some may want to own a kit in order to experiment.
Example
Sometimes an off-the-shelf lens matched to the sensor and camera you prefer suits your optical needs as well as your available space requirements. By available space we mean clearance from moving parts, or ability to embed inside an attractively sized housing. Lucky you.
But you might need more magnification than one lens offers, yet not as much as the next lens in the series. Or you want to move the camera and lens assembly closer to the target. Or both. Read on to see how extension rings at varying step sizes can achieve this.
Navigating the specifications
Once clear on the concept, it’s often possible to read the datasheets and accompanying documentation, to determine what size extension tube will deliver what results. Consider, for example, Moritex machine vision lenses. Drilling in on an arbitrary lens family, look at Moritex ML-U-SR Series 1.1″ Format Lenses, then, randomly, the ML-U1217SR-18C.
ML-U1217SR-18C 12mm lens optimized for 3.45um pixels and 12MP sensors – Courtesy Moritex
If you’ve clicked onto the page last linked above, you should see a PDF icon labeled “Close up ring list“. It’s a rather large table showing which extension tube lengths may be used with which members of the ML-U-SR lens series, to achieve what optical changes in the Field-Of-View (FOV). Here’s a small segment cropped from that table:
Field-Of-View changes with extension tubes of differing lengths – Courtesy Moritex
Compelling figures from the chart above:
Consider the f12mm lens in the rightmost column, and we’ll call out some highlights.
Extension tube length (mm)
WD (far)
Magnification
0
100
0.111x
2
58.2
0.164
5
13.5
0.414
5mm tube yields 86% closer WD and 4x magnification!
Drum roll here…
Let’s expand on that table caption above for emphasis. For this particular 12mm lens, by using a 5mm extension tube, we can move the camera 86% closer to the target than by using just the unaugmented lens. And we quadruple the magnification from 0.111x to 0.414x. If you are constrained to a tight space, whether for a one-off system, or while building systems you’ll resell at scale, those can be game-changing factors.
Any downside?
As is often the case with engineering and physics, there are tradeoffs one should be aware of. In particular:
The light reaching the focal plane is reduced, per the inverse square law – if you have sufficient light this may not have any negative consequences for you at all. But if pushed to the limit resolution can be impacted by diffraction.
Reduced depth of field – does the Z dimension have a lot of variance for your application? Is your application working with the center segment of the image or does it also look at the edge regions where field curvature and spherical aberrations may appear?
We do this
Our team are machine vision veterans, with backgrounds in optics, hardware, lighting, software, and systems integration. We take pride in helping our customers find the right solution – and they come back to us for project after project. You don’t have to get a graduate degree in optics – we’ve done that for you.
Give a brief idea of your application and we’ll provide options.
Related resources
You might also be interested in one or more of the following:
About you: We want to hear from you! We’ve built our brand on our know-how and like to educate the marketplace on imaging technology topics… What would you like to hear about?… Drop a line to info@1stvision.com with what topics you’d like to know more about.
With 9 members in the Kowa FC24M lens series, focal lengths range from 6.5mm through 100mm. Ideal for sensors like the 1.1″ Sony IMX183, 530/540, 253 and IMX304, these C-mount lenses cover any sensor up to 14.1mm x 10.6mm, with no vignetting. Their design is optimized for sensors with pixel sizes as small as 2.5µm – but of course work great on pixels larger than that as well.
Kowa FC24M C-mount lenses – Courtesy Kowa
Lens selection
Machine vision veterans know that lens selection ranks right up there with camera/sensor choice, and lighting, as determinants in application success. For an introduction or refresher, see our knowledge base Guide to Key Considerations in Machine Vision Lens Selection.
Give us a brief idea of your application and we will contact you with options.
Noteworthy features
Particularly compelling across the Kowa FC24M lens series is the floating mechanism system. Kowa’s longer name for this is the “close distance aberration compensation mechanism.” It creates stable optical performance at various working distances. Internal lens groups move independently of each other, which optimizes alignment compared to traditional lens design.
F is for fixed. With focal lengths at 9 step sizes from 6 – 100, lens design is kept simple and pricing is correspondingly competitive.
C is for C-mount. It’s one of the most popular camera/lens mounts in machine vision, with a lot of camera manufacturers offering diverse sensors designed in to C-mount housings.
24M is for 24 Megapixels. Not so long ago it was cost prohibitive to consider sensors larger than 20M. But as with most things in the field of electronics, the price : performance ratio keeps moving in the user’s favor. Many applications benefit from sensors in this size.
And the model names?
Model names include LM6FC24M, LM8FC24M, …, LM100FC24M. So the focal length is specified by the digit(s) just before the family name. i.e. the LM8FC24M has a focal length of 8mm. In fact that particular model is technically 8.5mm but per industry conventions one rounds or truncates to common de facto sizes.
1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera and components selection. With a large portfolio of cameras, lenses, cables, NIC cards and industrial computers, we can provide a full vision solution! We’re big enough to carry the best cameras, and small enough to care about every image.
About you: We want to hear from you! We’ve built our brand on our know-how and like to educate the marketplace on imaging technology topics… What would you like to hear about?… Drop a line to info@1stvision.com with what topics you’d like to know more about.
