CMOS cameras rival their CCD cousins
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 10/1/2008 2:00:00 AM
 AL SABEH VP of sales and applications, Toshiba Teli |
The resolution of cameras based on cost-effective CMOS image sensors has continued to improve. Newer sensor technology has led to better light-sensitivity levels and has helped reduce noise levels in the CMOS-based cameras that are penetrating high-speed machine-vision applications. Al Sabeh, VP of sales and applications for Toshiba Teli, explained some of the improvements in CMOS cameras and the sensors on which they are based.
Q: Where are CCD cameras still used in machine vision?
A: Today, the major difference in usage between CMOS and CCD cameras for machine vision occurs in situations where light levels are very low. In those cases, you still need CCD sensor technology because it is more sensitive to light: The minimum sensitivity for CCD cameras is 1 lux, but for CMOS it’s 5 to 10 lux.
But if there’s sufficient illumination during inspection, CMOS camera performance is excellent. And that’s the case with the majority of machine-vision applications, since they are usually well lighted. Also, some applications that need a higher near-infrared response, like about 900 to 1000 nm, will require CCD cameras, because those cameras are more sensitive at a higher spectrum.
Traditionally, CCD technology has also delivered lower noise levels, or a higher SNR (signal-to-noise ratio).
Q: What are the benefits of CMOS cameras?
A: For one thing, CMOS sensors are less expensive than their CCD counterparts. So, for some customers, if CMOS technology meets their sensitivity needs, that’s what they will use.
But at least as important is the ability of CMOS cameras to increase frame rates by letting users choose a smaller, lower-resolution area within the image—a window of interest—and sending out only those pixel addresses. Let’s say you have a 1020x768-Mpixel camera, but you’re interested in an area that’s 700x500 Mpixels. On a PC, you can set the camera to that resolution and select the rectangle anywhere on the monitor. So, you can view that area of interest while also increasing camera speed.
For example, our 12-Mpixel CleverDragon camera’s window-of-interest feature can be programmed to display a maximum of 28 square windows within a single image. The camera can perform at 4096x3072-pixel resolution, at 25-fps full-frame speed, with an SNR greater than 54 db. No 12-Mpixel CCD camera will do that. They can only give you 5 fps at that resolution.
Q: Why did Toshiba Teli design its own proprietary image sensor instead of using a commercially available chip?
A: A CMOS image sensor lets you set the camera to any resolution you want, including lowering the resolution to increase speed. We designed our 12-Mpixel, 1.9-in., CMOS monochrome progressive-scan sensor to be extremely flexible. For instance, you could set our 12-Mpixel camera to 2000x2000 pixels and get 30 fps.
We also designed it for higher performance because there are emerging applications in machine vision and inspection that will demand that performance. A major example is the industry move from 6-in. to 12-in. wafers. Now those bigger wafers can be inspected at much faster speeds.
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This is really a whole lot of camera for a very small price. Toshiba Teli is a great company to work with and Al Sabeh gives as much support as each application dictates. If you are interested in this camera it can be purchased through Audio Video Supply (800) 284-2288, ask for Krista at ext.431
Krista Jewell - 2008-2-10 13:35:00 EDT
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