Thursday, January 13, 2011

A prediction for machine vision lenses

It seems like yesterday that a 2Mp sensor was “high resolution” but today 5Mp is the new 2Mp and I’m pretty sure that in another year or so 12Mp will be on its way to commonplace. Much of this is driven by the consumer camera market where megapixels sell, but it’s fair to say that in machine vision we want higher resolution too.

Now if you study sensor specs you’ll know that as pixel numbers go up, each pixel gets smaller. If it didn’t we’d have image sensors the size of tennis courts by now! But I think there’s a limit to how small pixels can get.

The first issue is noise. As pixels get smaller so does their “full well capacity,” yet because noise is proportional to the square root of numbers of photons captured, smaller pixels tend to exhibit more noise.

Second, there’s that pesky business of diffraction. If you go back to my post of February 8th, 2009 (Wavelength matters,) you’ll see reference to something called the Airy disc. What this means is that light can’t be focused down smaller than its wavelength. Now as red is around 0.660 µm, that pretty much sets a limit on how small a pixel can be.

Right now it seems that most sensors used in machine vision – like the 5Mp Sony ICX625 - have pixels of 3.5 µm or greater, but consumer applications are already edging into the sub micron area. For the reasons listed above, that’s not going to work for us, so what’s the conclusion?

Well that brings me to my prediction: as resolutions increase we’re going to reach the point where the trusty C-mount lens will no longer do the job. Quite simply, we will have to use larger format lenses (I’m thinking F-mount,) to project our images onto the larger CCD and CMOS sensors that we’ll want to use.

In other words, large format lenses could be a real growth area a couple of years from now.


Jon Vickers said...

But there are problems with F-mount - that bayonet fitting allows for some tilt movement. I'm currently testing a Canon EF-mount lens that shows the same problem. In a typical industrial environment some vibration to the lens manifests itself as blurring even if the camera is securely mounted. Screw-threads (M42 etc) are a lot better suited.

Spencer Luster said...

Pixels: You can't live with them, you can't live without them!

Especially for very large format image planes, the good old enlarger lens might make a comeback. The typical 6 or more element designs offered by Nikon & Rodenstock are excellent. Very flat field, distortion under 0.1%, great color correction. Furthermore, in light of Jon's comment, they are threaded mounts.

When I worked for Automatic Inspection Devices I specified enlarger-type lenses a lot. The down sides are: 1) Usually no faster than F/2.8 and more typically F/4 or F/5.6 for longer focal lengths. 2) No inherent focus adjustment 3) Unlike c-mount lenses, the back flange focal length varies with effective focal length, and usually about the same size. Thus, a 105mm fl. lens, for example, will have about 90mm to 95mm flange focal length. 4) I'm not aware of any enlarger lenses that have focal lengths shorter than 28mm. Maybe not shorter than 40mm these days.

A good thing for certain apps is that focal lengths up to 360mm are available!

There are also M39 to c-mount adapters -- the former being the thread diameter of most typical enlargers.