Machine Vision

 

Basler launched the new sprint camera series at the Vision show in Stuttgart (November 7-9, 2006). After seeing a live demonstration, visitors to our booth were impressed both by the camera’s speed and by its performance. The demonstration camera was a model with 4096 pixel resolution running at a line rate of 70 kHz.

The CCD technology used in most industrial digital cameras was invented in the late 1960s and has been continuously improved over the last 30 years. Today’s CCD sensors have high quantum efficiency, a small pixel size, low dark current, and low noise. But due to the internal design needed to transport charges out of the sensor, the main drawback of CCD sensors has always been limited readout speeds.

CMOS technology was applied to imaging sensors much later than CCD technology. The first CMOS sensors provided poor image quality with lots of noise. Until recently, the performance of CMOS sensors has been restricted by the limits of the CMOS process technology used to produce the sensors. But one innovation in particular, the use of active pixel sensor (APS) design, improved quality tremendously. And in the last few years, CMOS process technology has dramatically improved in several ways, including:

	Improvements in the basic CMOS fabrication technology
	Smaller CMOS structure sizes resulting in an increased number of pixels in a smaller area and a much higher pixel fill factor
	Improved technology for processing CMOS wafers into finished sensors

These improvements, and may others, addressed the disadvantages that CMOS sensors had in the past, such as low fill factor, high dark noise, and poor signal-to-noise ratios.

With process improvements and APS design, CMOS technology started its journey along the road that lead to the high quality sensors we have now. Today, CMOS sensors built with advanced CMOS technology operate at performance levels directly comparable to CCD based sensors. Two significant advantages of CMOS versus CCD sensors are that CMOS sensors are free of blooming and smearing and they have a lower power consumption. CMOS sensors also have the ability to directly access an area of interest (AOI) resulting in a nearly linear speed increase when the AOI size is decreased. In addition, CMOS sensors have fewer data rate limitations, so they are open to even more speed increases as their development continues. Now and in the future, the high speed sensor of choice will be a CMOS sensor.

After summarizing all the advantages of CMOS sensors, it is obvious that they are becoming more attractive and that the CMOS sensor market share is growing rapidly. Basler’s recently introduced sprint line scan camera series is based on a new dual line sensor that uses the most advanced CMOS technology including APS. When focusing on the sprint sensor, in addition to the advantages mentioned above, the unique architectural concepts used in the sprint sensor offer even more benefits:

	A maximum line rate of 140 kHz at either 2k or 4k resolution
	A fill factor of 100%, which leads to high quantum efficiency (comparable to CCD sensors) and outstanding sensitivity
	An increased signal-to-noise-ratio of 3 dB when using sprint’s line sum feature

When the camera’s line sum feature is enabled, both of the sensor’s lines are used. Each area on the object being imaged is scanned sequentially by the sensor’s two lines, and the information collected by the two lines is combined into a single output. This doubles the available image information for each area on the object and means that total noise can be reduced by the square root of 2 while the signal-to-noise ratio is improved by 3 dB. In addition, the camera’s responsivity is doubled.

As users of line scan cameras know, there is often a trade off between line rate, light level, and cost. As line rates increase, integration times are shorter and noise becomes problematic. This means that more elaborate lighting systems are required to achieve good image quality at high line rates. Of course, more elaborate lighting systems cost more. With its very high sensitivity and extremely low noise level, the sprint series virtually eliminates these tradeoffs and gives you superior image quality, even at high line rates.

In color models of the sprint, the camera’s dual line sensor will have a Bayer pattern filter. Use of a Bayer filter is a proven, widely accepted method for producing color images in many applications. Since a standard image data output protocol will be used, most existing frame grabbers will be compatible with the new Basler sprint series.

Because there is no gap between the two lines in the sprint sensor, you will see three major advantages when comparing a color sprint camera to a color camera that uses a three CCD sensor:

	No elaborate spatial correction is necessary
	There are larger alignment tolerances
	Synchronization to the speed of the object is much easier

From a financial standpoint, the sprint’s color concept makes system integration much less costly than a three CCD camera. The choice of an appropriate lens system is also much easier and less expensive compared to three CCD cameras. Several reasonably priced lens systems are available that can be used with either 2k or 4k sprint models. And the smaller dimensions of sprint cameras as compared to three CCD cameras can be another major advantage during the system design process

Production of sprint series cameras will start in the second quarter of 2007. The first available models will be 4k mono cameras, followed by 2k versions. Color models will be available in the third quarter of 2007.