Advanced Imaging – April 1998

Integral Vision Introduces LCD Inspection System
by Sheldon Liebman

LCD and LED panels are everywhere. Mobile telephones, electronic organizers and test equipment are just three examples of products that utilize this technology. In almost every case, the specifications of the display are custom tailored to the needs of the application. How many characters are on each line? How many lines must be displayed? These are just a couple of the questions that need to be answered.

Recently, one of the world’s largest electronics manufacturers needed to update their current LCD inspection system. Their existing system was proprietary and monochrome only. This made it very expensive to maintain. In addition, the company’s cellular phones are manufactured with both LCD and LED displays, so a color system was needed to inspect both display types on one manufacturing line.

Usually, companies that need to test this type of equipment develop their own systems using tools and equipment from a variety of manufacturers. In this case, though, they contracted with Integral Vision, developers of the VisionBlox machine vision software, to create a system for them. VisionBlox is a collection of 32-bit OCX vision algorithms running under Windows NT. For this application, Integral Vision created a turnkey application on top of VisionBlox. The result is called LCI-Professional and was delivered to the customer last year. Since it was developed by Integral Vision, though, it is also available as a standard product for sale to any company requiring similar capabilities.

LCI-Pro is a non-contact display inspection system based on VisionBlox. For this customer, the software is combined with Imaging Technology’s IC-RGB frame grabber, although the open nature of the system allows virtually any frame grabber to be used. Images can be captured from normal area-scan video cameras, digital cameras or linescan cameras. Since the system runs on a PCI-bus equipped PC, it is also affordable and easy to upgrade over time.

When the customer asked Integral Vision to develop this system, they wanted a vision system that would work with a variety of displays. As a result, LCI-Pro can be used with LCD displays and both dot matrix and seven segment LED displays.

Seven segment LED displays are the easiest to inspect because failures are obvious – one segment does not illuminate or illuminates at the wrong time. Segments can also be too bright or too dark. Dot matrix LED displays are more difficult to inspect because they are small and the placement of the individual LED chips can vary from part to part. These displays switch on in groups to form characters or patterns and failures occur when they do not work or when a short circuit causes two or more LEDs to illuminate at once. LCD displays can suffer from missing or additional pixels just like LEDs. In addition, if the manufacturing process contaminates the display, it can contain pinholes or distortions in the shape of individual pixels. The illumination can also be too low resulting in a display that can’t be read properly in low-light conditions. Partial or total failure of illumination must also be checked. These illumination tests can also reveal defects in the display assembly that are invisible under normal conditions.

One special requirement for this type is inspection is that the system must work correctly through the protective film that is used to cover these displays. Initially, this film is clean and clear. As an OEM integrates the components into an end product, however, the film can become damaged. It’s important that the system not reject a part simply because the film is damaged and distorts the image.

The LCI-Pro system uses direct measurement to inspect the parts, which allows it to detect even a single pixel defect. LCI-Pro also checks for a number of other conditions. It can detect short circuits that cause incorrect pixels to be activated and open circuits that cause missing lines or columns. It verifies characters and icons are operating correctly. It also checks for illumination failure or poor contrast over all or part of the display. Taken together, these checks ensure that the display is operating correctly or it is rejected.

The key to using the LCI-Professional system is proper training of the system with a good model. An operator sets parameters that determine whether a part is rejected or accepted for every test that will be performed. The parameters allow for some deviation in color or appearance while still being acceptable. All of the information is stored on the system disk so that it can be recalled at any time or changed as needed.

Once the "ideal" images have been captured and the parameters set, inspections can begin. The system uses a template tool to analyze an image from the assembly line and compare it to the stored image. The system is set up to analyze an image even when a part has been rotated and/or moved within an image frame. The region of interest used by the system is large enough to distinguish one or more identifying marks on a component. Alignment tools then bring the captured image to the same orientation as the stored image so that a proper inspection can occur.

As specified by the customer, one of the requirements of the system is that it works with a variety of displays. However, all of these displays are combined onto a single assembly line, so the system must be able to distinguish between products and perform the appropriate tests on each. This is accomplished through a configurable task sequencer in the system.

The task sequencer configures each inspection sequence for a specific application. It can easily be connected to other test equipment via a serial or digital I/O interface. This can be used with a barcode reader, for example, to identify each phone on the assembly line and determine which type of display it has and what tests need to be performed.

LCI-Pro is not limited to being used for LCD and LED inspection. Since it is built on a general-purpose image-processing library, other operations can be performed. For example, once the display is determined to be working correctly, other features can be checked. Is the company’s logo in place? Are the keys on the keypad labeled correctly? Are there cracks or chips in the phone casing? LCI-Pro can be set up to detect all of these conditions and more.

In addition to performing the actual inspections, LCI-Pro stores a database of failed part images. QA engineers can use this database to diagnose production faults. It can also be used to verify that the system is configured correctly.

When all the features of the LCI-Professional system are added up, the result is a system that improves quality and reduces costs. If defects can be detected earlier, this prevents complex products from being built and discarded. In the case of the original customer, the goal was to achieve a failure rate of less than one part per hundred thousand. With LCI-Pro, they have been able to achieve this.

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