AUTOMATED TESTING
Wireless
Liquid Crystal Display (LCD) Inspection System
by Doug Wilson,
Engineering Technology Center
The Challenge: Automating the
inspection of high-resolution
displays commonly found on pagers,
cellular phones, medical instruments,
and other electronic devices to
determine if the proper characters
are displayed.
The Solution: Building a PCbased system using an IMAQ™ image
acquisition board, IMAQ Vision
software, and LabVIEW.
Introduction
Automated testing and inspection of the
liquid crystal displays (LCDs) and light
emitting diodes (LEDs) commonly found
on pagers, cellular phones, and medical
instruments needs to be accurate, detailed,
and fast. Even subjecting an LCD display
to traditional electronic functional testing
using automated test techniques does not
guarantee that the proper pixels and
characters are displayed, because voltage
and current readings do not correlate
perfectly with pixel display states. The best
method for verifying display performance
is inspection. Human inspection of highresolution LCDs can be tedious and error
prone. With the increased speed of PC
The positions are determined to
subpixel accuracy using IMAQ Vision
edge-detection tools with quadratic
interpolation.
hardware, a PC-based optical inspection
system can acquire and process images
with sufficient speed and accuracy to be a
far superior approach to other inspection
methods. We have designed a PC-based
automated optical inspection system for
conducting functional testing of LCD and
LED panels.
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The ETC LCD Inspection System software uses IMAQ Vision edge-detection tools to create an
advanced positioning algorithm to locate character positions to subpixel accuracy.
Testing Requirements
Dot matrix LCDs, as well as sevensegment LCDs and LEDs have certain
characteristics that we must test.
• Pixel and character spacing – to ensure
that each pixel is situated properly
• Pixel functionality – to check that each
pixel can be switched on and off
• Character recognition – to test the
electronics controlling the pixel
switches because faulty circuits can
produce incorrect pixel combinations
for a particular character
• Contrast – to ensure that the display has
the proper brightness
Dot matrix LCDs are generally more
difficult to test than seven-segment
displays because failures are more difficult
to detect. We require precise image
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • www.ni.com/telecom
sampling because pixels are more
numerous and much smaller than the
segments. The placement and orientation
of the LCDs can vary from part to part,
creating a problem of locating the pixels
within an acquired image.
Therefore, we needed imaging analysis
algorithms capable of locating the
characters within a range of orientations.
In addition, the display is often covered by
a protective film that can create a glare
and/or become dirty during the assembly
process. This condition creates distorting
artifacts in the image that need to be
distinguished from the actual display.
The ETC LCD Inspection System can
perform all the tests required for
maintaining high standards of quality
control. The system is also versatile,
AUTOMATED TESTING
stepper/servo motor can move the LCD
units through the camera’s field of view
for testing. In this configuration, the
entire test system is integrated into a
single PC.
Another useful feature of the system is
the image database. We can store the
images from failed tests in the database so
we can view them at a later time to help
determine the source of the production
errors. We can also use the image
database to verify that the system is
calibrated and operating properly.
Hardware
The intuitive user interface compares sampled LCD pixels to a known test string.
accommodating any dot matrix
arrangement as well as seven-segment
displays and LEDs. The system can test
the display and automatically accept or
reject it based on tolerances for the
The National Instruments IMAQ
PCI-1408 image acquisition board
speeds up image processing by
performing several functions in
hardware, such as image scaling,
decimation, binary image
thresholding, and gamma correction.
characteristics under test.
Software
Before the system can test a display, it
must find the LCD characters within the
image. This is accomplished with a robust
positioning algorithm that compensates
for angles of rotation varying from one
unit to another as well as XY translation
and scale. The positions are determined
to subpixel accuracy using IMAQ Vision
edge-detection tools with quadratic
interpolation. This algorithm ensures an
accurate definition of sampling
coordinates regardless of image
magnification or variations in the
placement of the unit under the camera.
The ETC software employs IMAQ
Vision filtering functions and background
normalization to eliminate glare and other
distorting artifacts in the images acquired
so we can test through the protective film
covering LCDs. The software filtering also
reduces misdetections and improves
overall accuracy.
Once the system filters the image and
locates the pixel positions, the image is
sampled to determine each pixel value.
Several tests can be performed with this
data. The system can perform a pixel
functionality test to determine if all the
pixels are working, and at the same time,
check for proper pixel spacing. An optical
character recognition determines the
specific characters displayed from the
image for comparison with a known test
string. The contrast of the display can also
be measured from the image to determine
if it is within the specified tolerance.
In addition to image acquisition and
processing, the software can incorporate
motion control into the test sequence. A
dual-axis motion control stage or a single
The National Instruments IMAQ
PCI-1408 image acquisition board speeds
up image processing by performing
several functions in hardware, such as
image scaling, decimation, binary image
thresholding, and gamma correction.
This capability greatly reduces the
computational load on the central
processing unit (CPU) to provide quicker
image processing.
Results
The ETC LCD Testing System is a highspeed, accurate, and versatile solution to
the many challenges of display testing. The
LCD Testing System can perform several
different tests on dot-matrix LCDs and
seven-segment LCDs and LEDs. It also has
integrated motion control capabilities, so
that you can perform all display tests on a
single system. The result not only saves
production space and time, but also
standardizes test reports and unifies all the
testing under one user interface.1
For more information, please contact Doug
Wilson, Engineering Technology Center,
240 Oral School Road, Mystic, CT 06355-1208,
tel (860) 572-9600, fax (860) 572-7328,
e-mail dwilson@etc.atinc.com,
www.etc.aati.com/noframe/testmeas.htm
National Instruments
Tel: (512) 794-0100 • Fax: (512) 683-9300 • info@ni.com • www.ni.com/telecom
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