packaging inspections. Early versions of this
technology were not widely adopted though,
because they were subject to vibration, which
discouraged users from choosing them.
Further advances soon followed to deliver
more robust technology. The next generation
of cameras were trilinear CCD color cameras that featured a much faster line rate at
about a third of the price, making it an attractive option for applications such as printing, packaging, food, pharmaceutical, electrical, cotton, or web. These advances were
achieved by using a different sensor architecture. There were three pixel lines fabricated
on the same die, each line covering red,
green, and blue colors respectively (Figure 3).
One challenge with trilinear camera technology is the three colors’ separation, as the
color lines on the trilinear sensor do not share a
common optical axis. This results in three separate object points for the three different color
lines. Because of this, the color image is mis-aligned along the scanning direction. Instead,
trilinear cameras deal with color separation
via a technology known as spatial correction.
Spatial correction allows the user to input an
appropriate parameter to re-align color images.
In addition to spatial correction, this generation of trilinear cameras included more auxiliary functions to accommodate users and drive
adoption of color imaging for some machine
vision applications. Some features allowed
users to fine tune each pixel’s reaction to the
incident light, and to upload one user set to
multiple cameras to make set up easier. Some
cameras included a color correction feature
that enabled nearly true-color tone images.
This function is especially useful for applications that require extremely high quality color
images such as banknote inspection or high
quality packaging inspection.
CMOS-based cameras bring better speed
and lower costs
The next big leap in the development of cameras for color inspection were CMOS-based
tri-linear cameras. These CMOS-based cameras built on the advances of previous generations, and added a significant increase in performance at a lower price.
Speeds on these cameras went as high as
70 k Hz, a vast improvement over previous generations, which averaged around 17 kHz. This
generation of camera also included improved
spatial correction technology, allowing precise correction down to the subpixel level, and
also featured horizontal dispersion corrections
(also sometimes called
The parallax correction feature allowed
users to mount the
camera at angles (the
sensor surface is not
perpendicular to the
object surface), but still
get non-dispersion color
images. Some users
take advantage of this feature to operate one
camera in the place of two cameras.
For example, machine vison inspection of
cigarette packages needs to check both color
printing and concave-convex shape of the
package. Previously, this required two different cameras, but technical advances enabled
some users to angle-mount a single camera to
allow it to check both items.
Current color line scan camera
High-speed inspection, in many cases, is
light starved due to a short exposure time.
These kinds of applications require extremely
responsive cameras, and single-line-per-color
trilinear cameras may not be capable of performing this kind of task.
To meet the needs of this kind of application, the machine vision industry has developed CMOS-based cameras with TDI ( Time
Delayed Integration) technology. Teledyne
DALSA recently introduced the Piranha XL,
a color line scan camera with a TDI color
sensor (Figure 4).
Where a trilinear camera is a single line
per color, the Piranha XL is four lines per
color, making the responsivity four times
higher with very low noise. Because cameras
Figure 2: The prism-based sensor architecture.
Figure 3: The trilinear sensor architecture.
Figure 4: The Piranha XL TDI color line scan
sensor architecture and working principle.
Figure 5: The bilinear sensor architecture, the third colors are interpolated.