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CMOS Takes Line-Scan Speed to New Levels

POSTED 03/17/2015

 | By: Winn Hardin, Contributing Editor

hine vision customers in any industry want the same three things when it comes to their cameras. They want the cameras to run faster. They want higher-resolution cameras. And they want this speed and resolution for less money. For many applications, line-scan cameras are the perfect answer.

Unlike area cameras, which capture an entire image in a single exposure, line-scan cameras build an image of theoretically infinite length in a single dimension by acquiring one line of image data (pixels), moving the object (or camera) and acquiring the next line of image data. While movement is usually a problem for cameras, line-scan arrays embrace it, which is why these cameras are regularly used in web inspection applications, such as printing and textiles. They are also used in remote sensing and scanning applications, where the camera is mounted to a moving platform. Acquiring hundreds of thousands of image lines per second makes integrating the camera and processor timing critical or the final image will suffer.

It’s easy to see the cost benefit of a line-scan camera that has less than a handful of rows of pixels versus an area array with thousands of rows of pixels. An area-array camera that measures roughly 2500 x 2000 pixels, for a total of 5 million pixels, running at 100 frames per second generates 500 megapixels (MP) of data. Compare that to a 4,000-pixel line scan running at 200 kHz that delivers 800 MP for less than the cost of the high-speed area-array camera.

“During the last one-and-a-half years, the average price for a line-scan camera has dropped by 30%,” says Henning Tiarks, head of product marketing at Basler AG (Ahrensburg, Germany). “When you design in GigE Vision instead of Camera Link, the cost for the system drops another $300, and that’s opening up a lot of applications that previously wouldn’t have considered line-scan cameras.”

But it’s about more than just speed and cost. Advances in complementary metal oxide semiconductor (CMOS) fabrication processes are expanding the market for line-scan cameras by improving sensitivity, speed, and cost compared to area array cameras.

CMOS vs. TDI: Simplicity Wins
“Customers have always wanted higher responsivity, and dual line [line scan] is the most efficient way to do that without having to get into TDI [time delay integration] products,” explains Mark Butler, manager of product management for Teledyne DALSA (Waterloo, Ontario, Canada).

TDI cameras typically use multiple CCD linear-array sensors stacked one parallel to the next. These cameras can run at higher speeds (and shorter exposure times) than a single line-scan CCD camera because the signal from each sensor is combined with signals from the other sensors in the stack, increasing the total amount of light gathered and resulting in an image with sufficient intensity and contrast for automated inspection systems. 

CMOS fabrication techniques allow the individual rows to be located on a single die and much closer together due to CMOS chip architecture, yielding several benefits. The closer the sensor rows, the less time needed to pass between exposures in a multi-line CMOS line-scan camera. This means vibrations common to industrial environments, especially high-speed machinery, essentially do not affect the image-acquisition process. This is because the subsequent exposures happen so closely and so quickly that vibration doesn’t have time to impact the camera’s field of view.

CMOS and CCD imagers also differ in the way that signals are converted from signal charge to an analog signal and finally to a digital signal. In CMOS area and line-scan imagers, each CMOS pixel has its own amplifier, resulting in relatively low bandwidth for each amplifier. While high-speed CCDs have a large number of parallel fast output channels, each pixel doesn't have its own amplifier, resulting in higher relative bandwidth for a CCD amplifier compared to a CMOS amplifier, resulting in higher noise within the image. Consequently, high-speed CMOS imagers can be designed to have much lower noise than high-speed CCDs.

Finally, windowing CMOS imagers, or using a subset of pixels from the sensor, is much easier than with CCD sensors, allowing users to run high-speed imaging applications even faster.

The ability to put multiple CMOS sensors on a single die also simplifies color line-scan camera architectures. In the past, multi-line CCD cameras could use Bayer filters on a single line scan, reducing resolution, or three or more line sensors with a prism, or a TDI approach. As mentioned earlier, vibration can pose a problem for TDI, but it can also cause internal prisms to shift out of alignment, effectively rendering a prism-based color line-scan camera useless.

“The use of color line scan is growing as the costs come down,” says Basler’s Tiarks. “While you’ve needed color imaging for 100% inspection of printed materials, it also makes a lot of applications easier for the operator when you present a true color image using a line-scan imager. With careful sensor and filter design, we’ve been able to develop dual-line-sensor color cameras with color response accurate enough to make colorimetric measurements, not just generate a RGB image.”

Teledyne DALSA recently released the Piranha4 multispectral camera that adds an NIR channel to the standard red, green, and blue CMOS trilinear color platform on a single die. The quadlinear multispectral camera runs at a max line rate of 70 kHz.

Running line-scan cameras at such high frame rates can create a bottleneck between the camera and processing unit. While most line-scan cameras today run on Camera Link due to higher available bandwidth than standard GigE Vision, many companies are looking at even higher bandwidth protocols for the largest line-scan cameras. When it comes to Teledyne DALSA’s new CLHS Piranha XL 16k CMOS camera with TDI capability and e2v’s (Saint Egreve, France) 16k multiline ELiiXA+ camera, both products had to go to higher speeds and use Camera Link HS and CoaXpress interfaces, respectively.

“Our new Piranha XL 16k outputs over 2 Gpix/sec data throughput that exceeds standard Camera Link,” adds Teledyne DALSA’s Butler.

Speed, resolution, relative cost and flexibility, for these reasons and more, when it comes to next-generation line-scan cameras, it’s easy to see why CMOS imagers outperform CCDs on every relevant metric.