Precision Inspection of OLED & MicroLED Displays Requires High-Resolution, Low-Noise Imaging Solution

Posted:  
Tue, June 23, 2020
Author: 
Anne Corning  | 

As the demand for higher and higher resolution displays continues to expand, emerging technologies such as OLED, quantum dot, and microLED are rapidly gaining ground in the market. For example, active matrix OLED displays (AMOLED) are forecast to account for 50% of all smart phones sold in 2023.1

At the same time, the worldwide microLED market is projected to grow at 78.8% CAGR by 2025, driven primarily by the microLED displays segment, which is predicted to grow an astonishing 81.1% CAGR to reach more than $16.2 billion in revenues by 2025.2 MicroLEDs are appealing for devices such as smart watches, head-up displays (HUD), and augmented and virtual reality (AR/VR) devices where high luminance and resolution are particularly important. Meanwhile, the quantum dot market is expected to grow from $3.5 billion this year to $10.6 billion by 2025, a CAGR of 24.6%3, with quantum dot displays responsible for driving a significant amount of that growth.
 

An OLED microdisplay designed for use in virtual reality devices. (Image: Road to VR)
 

The Need for High-Resolution Inspection at the Pixel & Subpixel Level

What these emerging display technologies have in common is their reliance on increasingly small light-emitting elements (e.g., LEDs) to produce precise visual detail, display brightness, vivid colors, and sharp contrast. As devices from televisions to smartphones to AR/VR displays increase their resolution and pixel density dramatically to meet these performance demands, so too measurement systems for display qualification must increase their measurement precision to accurately inspect these new displays.
 

The pixel pattern of an OLED smart watch.
 

Visual quality inspection must be able to measure at the pixel and subpixel level to detect differences in luminance and color and identify and quantify defects. It might seem an obvious conclusion that using imaging systems with higher resolution sensors (e.g., 80 MP or more) for display measurement are the answer. But high resolution alone doesn’t influence measurement performance. Other factors such as the mechanics, electronics, calibrations, and settings that combine to influence overall imaging system performance are key—a system's response, noise control, and high signal-to-noise ratio (SNR) ultimately determine the quality of images captured my measurement systems and thus the accuracy of any visual inspection regimen. Radiant designs and manufactures systems to meet these exacting performance standards. In fact, our ProMetric® Y43 Imaging Photometer has been recognized by Vision Systems Design 2020 Innovators Awards program, receiving an award in the  “Cameras – Specialty (High-speed, scientific)” category.
 

Used for precise measurement of qualities of light across a display, the award-winning ProMetric Y43 Imaging Photometer objectively quantifies spatial luminance, contrast, and uniformity values in addition to defects like mura, light leakage, dead pixels, lines, and particles.
 

The ProMetric Y43 system identifies and quantifies display qualities that human visual inspectors and competitive measurement systems miss—but which affect can negatively affect the end-customer experience if they are mistakenly shipped to market. The Y43 system completes measurements containing over 43 million data points in less than 1.5 seconds, providing the fastest photometric imaging solution for production-level applications such as in-line quality control and correction.
 

With sensor resolution options including 29 and 43 megapixels, the ProMetric Y is ideal for capturing measurements at the pixel and subpixel for light output characterization.
 

In emissive displays like microLED, mini LED, and OLED, independently emitting pixels can vary from one to the next, causing non-uniformity in brightness or color that can worsen at different display bright states. Within a single measurement, the ProMetric Y measures an entire display in exact detail across thousands of emissive pixels, acquiring values for each pixel in the display.
 

A Complete Subpixel Measurement Solution: TrueTest® Software, ProMetric Imaging System, and Microscope Lens

Defects in images can be identified and quantified using Radiant’s TrueTest™ Automated Visual Inspection Software (available with tests for comprehensive display defect detection, mura identification & assessment, or application-specific testing of automotive displays, head-up displays, and augmented or virtual reality displays). TrueTest applies objective analysis and repeatable pass/fail criteria to Y-series images for fully automated quality control. With quality, highly detailed images provided by the Y43 imaging system, TrueTest’s ability to qualify displays becomes even more precise.

Radiant’s ProMetric Y Imagers (in 29-megapixel or 43-megapixel options) can also be combined with our Microscope Lens for objective measurement with 5X or 10X zoom, to perform detailed inspection of individual emissive elements of any shape. Microscope lens measurements have been proven for measuring subpixel luminance and color output, registering each pixel (for example, each individual microLED) using ROI that are hundreds of image sensor pixels in area for ultimate accuracy. This is especially useful when capturing exacting measurements at the wafer level during design and production of new mini- and microLED displays.

Using any high-resolution, low-image-noise ProMetric system (with or without the Microscope Lens add-on) means that every display pixel is captured over several sensor pixels for optimal measurement precision. The system is effective for evaluation of display pixel structures and subpixels of individual microLEDs.
 

Radiant’s ProMetric imaging system and Microscope Lens (left) can be used to characterize individual emitters, subpixel layouts, shapes, and color patterns (right) with TrueTest™ Software, achieving a significant amount of data per element for accurate luminance and chromaticity measurements.

 


CITATIONS

  1. According to the Smartphone Display Intelligence Services, as reported in Chin, S., “Report: Half of smartphone displays to be OLED by 2023”, Fierce Electronics, November 7, 2019.
  2. Global Micro-LED Market Analysis, Trends, and Forecasts, 2019-2025”, report published by ResearchAndMarkets.com, February 28, 2020.
  3. Quantum Dot Market by Product (Display, Medical Devices, Solar Cells, Photodetector/Sensors, Lasers, Lighting (LED), Batteries and Energy Storage, and Transistors), Material (Cadmium-Based, Cadmium-Free), Vertical, and Region – Global Forecast to 2025”, report published by Markets and Markets, March 2020.
     

 

 
 
 
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