2018 International Forest Industries IFI April May 2018 Digital | Page 31

SCANNING & OPTIMISATION – LMI TECHNOLOGIES
LUMBER PROCESSING
Figure 5: Area cameras read a “patch” of pixels rather than a line Figure 6: W
 ith an area camera, the light is only ON during the time it takes
to expose one row
Figure 7: Low duty cycles and strobed LED lighting allow light placement
close to the camera Figure 8: 2D data is white balanced and scaled based on 3D
height variation
Area cameras and longer
lighting lifetime physically out of the way of board
movement (Figure 7).
Once we have generated a 2D
colour image, the data is further
white balanced for accurate colour
representation, and scaled based
on the height variation of the board
using profile data taken from a 3D
scan of the same region (Figure 8).
A colour pixel has a different
physical size on the board surface
at one height than at another
height. If colour pixels are not
corrected for height, then the
dimension of defects (e.g. knots)
will be incorrect.
The alternative to a linear camera
is an area camera. An area camera
is composed of a 2D array of
pixels that are mapped by a lens
onto an area of the board surface.
Area cameras use a colour mask
to encode pixels into R, G, and B
elements in a pattern known as
a Bayer filter. This Bayer pattern
is decoded later by software to
produce colour for every pixel on
the 2D array.
For the purposes of this
discussion, assume we use a 2D
array with 10 rows (note: a 2D array
is just a linear with more rows). If
we wanted 0.5 mm resolution for
each row, like we did in the linear
example, then the encoder will
trigger the capture of an area when
the board moves every 5 mm (0.5
mm/row x 10 rows = 5 mm) (Figure
5).
Now we are reading a small
‘patch’ of pixels – not just a row.
Each patch is then stitched to
build a 2D colour image based on
encoder stamps that identify the
exact start location of each pixel
patch.
Now, let’s consider what
happens with the lighting in this
type of system design. We still
need white illumination to produce
colour images, but the duty cycle is
very different. The ON time of the
light spans the duration it takes
to expose one row (since we want
to ‘stop’ motion for 0.5 mm – just
like in a linear camera). The rest of
the time (9 rows), the light is OFF
while we wait for the 5mm of board
motion to complete (Figure 6).
During the time the light is ON,
all 10 rows are exposing. The duty
cycle is therefore very low (1/10 or
10%), not the 100% cycle of a linear
system. This means the light can
be strobed for a very short period
– ON for one row, and OFF for nine
rows. Strobing an LED light can
lead to very intense light output as
long as the duty cycle is very low,
so the LED never heats up, which
results in a much longer lifetime
(e.g. 10 years versus one year).
With strobed LED lighting, LEDs
can be overdriven at a much higher
current to produce five times more
intensity. This allows lighting to be
conveniently mounted and wired
close to the cameras, and kept
Modular, area camera system
design with easy bolt-on
lighting
At LMI, the Gocator 200 series
of modular scanner systems
is designed around the area
camera principle. An LED light bar
generates white light illumination
and is strobed for a short ON time
but at high intensity to scan even
the darkest board surfaces.
The timing of LED lighting is
synchronized to the area camera
exposures precisely. The resulting
colour patches are stitched into
a single seamless image, white
balanced, and then scaled based
on board height variation.
The 3D profiling scan data from
a Gocator 210, 230, or 250 scanner
is aligned to the colour scan plane
of a bolt-on Gocator 205, so 3D
data can be used to scale the
colour-image data.
All of the software needed
to capture 2D colour with 3D
profile and tracheid is provided to
customers in an open source SDK.
The SDK shows how to manage the
many sensors in an optimizer in
order to build high definition data
models.
These models are processed
by machine vision algorithms
(supplied by OEM) in order to
extract wane and defects, and
compute optimal cutting patterns.
Gocator makes it easy to mix 3D
with 2D colour in order to build
custom solutions for a variety of
machine centres in saw and planer
mills.
International Forest Industries | APRIL / MAY 2018 29