Line Scan Cameras ideal for High-Performance Quality Inspection
November 23, 2012
A line scan camera uses a single row of photosensors, instead of a matrix of them, to capture images of a constant stream of
moving material, often called a web. The data stream is then processed within a machine vision system to create two-dimensional
image data for industrial quality inspection purposes.
Many machine vision applications lend themselves to scanning a scene across a line of photosensors (with line scan cameras)
instead of attempting to capture the whole scene at once via a two-dimensional array of photosensors (with area scan cameras).
Examples of applications ideal for line scan include inspections of components moving along a conveyor line, or materials produced
on a web, such as fabrics or paper. High-speed manufacturing processes for printed circuit boards (PCBs), solar panel cells,
flat-panel displays, woven and non-woven fabrics, and high-speed printed materials are all manufactured by such processes.
Line scan cameras use a linear array or arrays of photosensors to map illuminance of a scene that passes across them in a direction
at right angles to the pixel array. They are thus ideal for situations where the subject of a scene moves at a predictable rate.
(Note: in many situations, web speed is not constant, it varies, but this can be handled by using an encoder.)
For many applications, the object is moving continuously. When the product being inspected consists of discrete items,
such as photovoltaic solar cells, imaging software can “snip” the steadily moving data stream from the camera into individual
images in post processing. These individual images can then be processed just as if they were separately captured by an area scan
camera after the subject was halted for static imaging.
The choice of line scan technology is even more obvious for web inspection. Window film, for example, is produced in arbitrary
length rolls by a continuous web process. The process operates at a continuous speed for hours upon hours – ideally forever.
Line scanning takes advantage of this continuous motion to produce a continuous stream of data on film quality. Any defects can
be precisely located on the web according to the time they passed the imaging line.
For line scan, one of the key issues is how to increase responsivity. Throughput is critical for a production line and many
inline inspection systems have to match the throughput of the production line, which requires very high speed imaging.
Only line scan can meet the requirement. In many situations, the speed is so high that the application is often light starved.
Teledyne Dalsa’s Piranha4 high performance line scan camera, can illustrate several such situations. To improve responsivity,
the sensor used in the Piranha4 camera consists of two parallel lines of CMOS photosensors with on-chip circuitry to rapidly
read out the signal and summing using time-delay integration (TDI). TDI is a technology used to allow multiple exposures to
increase responsivity in light starved situation. The Piranha4’s sensor also includes on-chip Analog to Digital converter (ADC)
and correlated double sampling (CDS) techniques to reduce noise and improve sensitivity. The sensor consists of two lines of 8,192
CMOS pixels, each 7 microns across, capable of being read out at a line rate of 70 kHz in the summing mode.
The CDS technique used to read out each pixel reduces noise and readout errors by sampling the readout voltage twice: once with
the signal present, and once during the reset cycle. The reset-cycle voltage is subtracted from the signal-bearing voltage to
produce a pixel output dependent only on the illumination signal.
The two-line chip doubles the effective integration time – improving the image’s signal to noise ratio (SNR) by a factor
of √2 – by combining signals from equivalent pixels in the two rows with a suitable time delay to compensate for image motion.
Thus, the effective integration time is doubled, without introducing motion blurring.
In addition, the camera’s electronics allow 2x2 binning, effectively increasing pixel sensitivity by a factor of two.
Additional features include anti-blooming, exposure control, and others.
SUBHEAD: Ideal Applications
PCBs, for example, are typically produced by silk screening photoresist patterns onto copper-clad blank panels.
A typical panel is 24” across and likely includes circuits for several boards to be manufactured together.
Most PCB manufacturers inspect patterned boards after the photoresist has been applied, but before etching away the unwanted copper.
Thus, any panel containing pattern defects can be saved and recycled. In addition, catching defective patterns at this stage prevents
adding value to already ruined units.
High-speed automated processing systems typically send patterned boards via a conveyor line directly from the photoresist
developing and fixing process to the etching bath. This provides an ideal opportunity to set up a line scan camera over the
conveyor to image panels as they exit the patterning process. Panels containing smears, spots, or other defects can be shunted
away from the etching bath before the defective pattern is made permanent. The defective photoresist pattern can be cleaned off,
and the blank panel sent back for re-use.
Line scan technology is even more critical for forest products manufacturers. Plywood veneers begin as thin sections shaved from
rotating logs on a lathe. Before these veneers can be sandwiched together to form finished panels, serious defects, such as
knotholes, must be punched out and patched.
Automated inspection of veneers as they roll off the lathe can spot these defects, and signal their locations to downstream
machines, which can then punch them out and install patches automatically without interrupting the product flow. Again, line scan
imaging equipment is the clear choice.
If engineers use area scan cameras for the inspection, they would have to use high shutter speeds to stop the motion, and then
digitally stitch the images together to provide a continuous record of the long stream of veneer exiting the lathe. All of which
happens automatically using line scan technology.
In short, line scan techniques using line scan cameras like Teledyne DALSA’s Piranha4 provide an ideal solution for a large class
of inspection applications in high-speed production processes across a broad range of industries. Whenever product is being
transported from station-to-station, choosing this technology for automated inspection is a no-brainer.
About the Author
Xing-Fei He, Senior Product Manager
Xing-Fei He manages Teledyne DALSA’s line scan and TDI product lines. In this role, he is responsible for all phases of
product planning, development, marketing and sales, as well as execution throughout the product lifecycle. He has many years
of experience in sensors and machine vision. Prior to this position, Xing-Fei served as Product Manager at JDS Uniphase
(Milpitas, California) and Business Development Manager at LUXELL Technologies (Mississauga, Ontario). He holds a B.Sc.
and M.Sc. Degree in Semiconductor Devices from Zhongshan University (Guangzhou, China) and a Ph.D. in Photonics from the
Australian National University (Canberra, Australia). His areas of expertise include: imaging, displays and photonics.
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