Liquid Crystal Display (LCD) Inspection System

Author(s):
Doug Wilson - Engineering Technology Center

Industry:
Telecommunications

Products:
LabVIEW, Vision, Motion Control

The Challenge:
Automated testing of the function of each pixel of a dot matrix LCD to determine if the proper characters are being displayed.

The Solution:
Building a PC-based systems 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 high-resolution LCDs can be tedious and error prone. With the increased speed of PC 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.

Testing Requirements
Dot matrix LCDs, as well as seven-segment 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 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, 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 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 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 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 high-speed, 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.

For more information, 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

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