As sensor technology has improv ed in leaps and bounds, the old rule of thumb familiar to many line scan camera users has lost much of its punch. The idea that CCD equals high image quality while CMOS equals speed is only true under very limited circumstances.
While charge-coupled device (CCD) and complementary metal-oxidesemiconductor (CMOS) sensors perform similar functions — collecting light (photons) and converting those into charges (electrons) — there are a few fundamental differences between their methods. The biggest difference is in the sensor architecture. While CCD sensors generally employ a majority of the pixel surface for capturing light, CMOS sensors use a portion of that surface for conversion electronics.
Because up to a few years ago the conversion electronics required a significant amount of space, CCD sensors were generally considered to be more light-sensitive. Modern CMOS sensors are now outfitted with space-saving microelectronics. The numerous, comprehensive improvements have brought microlenses directly on the sensors, to bundle the incoming light and direct it straight to the light-sensitive parts of the sensors.
The results are clear: CCD and CMOS sensors are almost identically effective at capturing photons.
CMOS Area and Line Scan Cameras
If you look at two different applications of CMOS technology — area scan and line scan sensors — you’ll find that there’s another difference of major importance: unlike area scan sensors, modules with line scan sensors have only one row of pixels available. This generally provides extra space for the necessary conversion electronics to be located outside the pixel. In practical terms, the fill factor for line scan CMOS sensors totals almost 100%.
High line frequencies are the major strength of CMOS technology. Its structure inherently keeps delays between transfer and processing of image data to a minimum, which in turn enables the very high line frequencies that are essential to industrial applications such as print quality inspections, as well as for medical technology and research. For many of these applications, it is so important to keep the highest possible localized resolution (including sampling rate) that area scan cameras are no longer sufficient to implement the image processing. Only line scan cameras are up to the task.
The current standard for line scan camera applications calls for line rates of 10-20 kHz (fps). Constantly rising demands on throughput, such as where image processing is needed for quality assurance applications, has fed a clear trend toward higher speeds, even in line scan cameras. Higher line frequencies in the range above 40-140 kHz remain rare even today, but in the future, they will become commonplace.
Because the speed limit for line scan cameras based around CCD technology top out around 30 kHz for a 2k line, the trend towards higher speeds in line scan applications means that CMOS technology will eventually be required for line scan cameras as well.
Selection of Interface Technology
The latest generation of CMOS sensors allows for line frequencies of up to 51 kHz via a GigE interface, as well as up to 80 kHz via Camera Link, with resolutions of 2-12k as needed. This makes clear the tremendous potential for this combination of CMOS sensor and highperformance camera.
The use of quicker CMOS technology has also had an impact on various interface technologies. While speed limitations inherent to CCD sensors just a few years ago meant that it was not possible to achieve high line frequencies of 50 kHz or greater at 2k resolution when using Camera Link, the classic line scan standard, the CMOS sensors at work in today’s cameras are just starting to test the tremendous potential and bandwidth available through the GigE interface. More than ever, CMOS line scan cameras with GigE interface are a real alternative to cameras outfitted with a Camera Link interface.
AOI settings that limit the resolution of the camera to 1024 pixels mean that the interface is no longer the limiting factor, with the maximum attainable camera speed corresponding more or less to the maximum read-out speed on the sensor. In many modern line scan applications, the speed potential, the flexibility, and attractive price for the overall concept of a GigE-based line scan solution have proved so convincing that the interface has enjoyed lasting tremendous success, including in the line scan segment.