Organic light emitting diode (OLED) technology is promising for flat-panel displays, but image quality issues must be resolved for it to be commercially viable.
Organic light emitting diodes (OLEDs) are emerging as the next wave of technology in the flat-panel display market. This is exciting because OLED displays promise improved display appearance at lower cost and power than other display technologies. They have superior contrast ratios and sharper images with deeper blacks and more vibrant colours.
High costs due to manufacturing yield issues have hindered widespread OLED technology adoption, most dramatically in large-format implementations, as they drive up end-user prices. The smartphone market has been the most successful segment for OLED technology to date and will likely be the catalyst that drives long-term adoption of OLEDs for other applications.
Manufacturers have found it difficult to achieve consistent picture quality on large-format OLED displays. This impacts the timing of viable market entry and drives up retail price for OLED large-screens.
Manufacturing challenges
OLED technology faces several unique challenges to the manufacturing process, regardless of the size of display.
Line mura
In the manufacturing process, material is deposited that forms the individual sub-pixels. If this process is not completely uniform, the result may be line mura which will have well-defined horizontal and/or vertical orientation in the OLED display (see Fig. 1).
 Fig. 1: OLED display with |
 Fig. 2: Sub-pixels combine to create pixels with various colours and brightness. |
Sub-pixel luminance performance
OLEDs use emissive organic semiconductor material which lights up when electric current is applied. Because of this, OLED displays require no backlight. Their display pixels are composed of red, green, and blue sub-pixels, the output of which is controlled individually. Brightness (luminance) and colour are determined at the pixel level by the combination of the sub-pixel outputs.
Due to production variations, there may be variations in luminance for the same electrical signal input throughout the population of same-coloured sub-pixels on the display. This results in variations in brightness from pixel to pixel (see Fig. 2).
This sub-pixel level variability in OLED displays results in different performance issues than those found in liquid-crystal displays (LCDs). In LCD panels, adjacent pixels generally have the same luminance because LCDs use a common backlight to ensure that the brightness of adjacent pixels is fairly uniform.
Display colour non-uniformity
A second impact of inconsistent brightness levels is reduced colour accuracy and colour non-uniformity across the display. The brightness of each individual sub-pixel must be within tight bounds to achieve accurate and uniform colours. The reality is that, even with a well-controlled manufacturing process, sub-pixels in OLED displays will have significant variations in brightness levels. When these variations are not compensated for, a lack of colour uniformity occurs across the display, reducing picture quality to potentially unacceptable levels and so reducing production yields (see Fig. 3).
Imaging colorimeter applications
Imaging colorimetry-based testing systems have demonstrated success in improving quality and reducing production costs for LCD and LED display screens. These techniques can be adapted to OLED display production testing.
The two key components of these systems are:
- Imaging colorimeters, which provide accurate measurement of display visual performance matching human perception of brightness, colour and spatial (or angular) relationships. High-performance imaging colorimeters can measure the luminance and overall display uniformity of individual sub-pixels in OLED displays.
- Test execution and analysis software – production-line software for image analysis to identify defects and quality issues, quantify their magnitude and assess the measurements to make pass or fail determinations. This software can also include display performance correction methods that can be adapted to identify and correct variations unique to OLED displays.
Improving quality
In a typical manufacturing process, display visual performance is tested by human inspectors, resulting in high variability in the quality of delivered product. With the improved image quality of OLED displays, this is becoming an even more significant issue. Human inspectors are not able to evaluate display quality on high-contrast, high-resolution displays consistently and repeatedly.
Fig. 3: Incorrect brightness levels crate non-uniformity across OLED displays. On the left is an ideal “white” pixel. To the right is an uncorrected “white” pixel. The green brightness is 10% too low.
Automated visual inspection (AVI) using imaging colorimeters has multiple quality benefits:
- Improved consistency in test application, from line to line and location to location, as all systems share the same calibration and test definitions.
- Quantitative assessment of defects, with precise filtering of good from bad.
- Increased testing speed, which allows more tests to be run in the same time interval, ensuring a more careful assessment.
- Simultaneous assessment of full display quality, e.g. uniformity, colour accuracy and fine scale, e.g. pixel and sub-pixel level defects.
When applied to OLED display testing, imaging colorimeter-based AVI simplifies testing while improving delivered product quality.
Correcting OLED displays
As the OLED display size scales, yields decline drastically and the cost of each component is much higher. At this point, it becomes viable for manufacturers to perform correction or electronic compensation to improve display image quality. The concept is simple: by modifying the inputs to individual sub-pixels, known dim sub-pixels can be brightened, resulting in improved luminance uniformity and correct colour across the display.
Performing electronic compensation for OLED displays requires, firstly, in-display electronics that can control brightness of the individual sub-pixels accurately and adjust this based on a set of pixel-specific correction factors.
Secondly, a system is needed to measure individual sub-pixel brightness and colour accurately, and to compute specific correction factors for each sub-pixel. This method has been used widely for LED display screens made up of individual LEDs, and Radiant Vision Systems has adapted this technique to OLED flat-panel displays.
When the OLED display is completely assembled, test images are displayed to enable measurements and calibration values to be computed. For example, a “green screen” with all green sub-pixels turned on can be used as a sample image and the imaging colorimeter can measure and record the brightness of each individual green sub-pixel. This is repeated for all the primary colours and, usually, for white. This data can be gathered in the course of ordinary quality testing of the OLED display.
Once these values are known, unique correction factors can be computed and applied to the electrical input of each individual sub-pixel so that brightness will be accurate and uniform across the entire display. When this correction map is applied to the finished OLED display, there is a significant improvement in colour and brightness accuracy and uniformity. The net effect is that OLED displays that would have failed quality inspection without electronic compensation will now pass, and so increase production yield.
OLED technology is very promising as the next generation for flat-panel displays, but technical issues related to image quality and production yields must be resolved before it can be considered commercially viable. The issues facing OLED manufacturers, although unique, are similar to technical issues that have been solved by Radiant Vision Systems and by LED screen manufacturers.
ProMetric imaging colorimeters are charge-coupled device (CCD)-based imaging systems calibrated to match human visual perception of spatial and angular distributions of brightness and colour. The appropriate system for specific OLED display testing scenarios will depend on measurement accuracy and resolution requirements.
Accurate measurement of display performance is important, as is the analysis of the measurement data. Automated visual inspection software such TrueTest implements test sequences against user-defined pass/fail criteria. It offers a test suite and sequencer with built-in tests available for OLED uniformity testing; line and pixel defect detection and contrast measurement.
TrueTest incorporates software alignment and Moiré pattern removal to simplify test setup. On the production line, it runs in operator mode where access to test parameters is locked, preventing changes. It also stores configuration information, test parameters, and pass/fail criteria for multiple models of display.
Contact Shaina Warner, Radiant Vision Systems, info@radiantvs.com
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