High-Resolution, Pixel-Dense Displays: Why Megapixels are Important for Measurement

For years, the trend in displays has driven relentlessly towards higher-resolution screens that feature an increasing amount of smaller pixels that are packed together more densely.

In the 1990s and early 2000s, digital imagery went mainstream, and LCD screens with LED backlights grew in popularity. It wasn’t until 2007 that the sales of LCD screens overtook older cathode ray tube (CRT) televisions.

The emergence of new technologies like microLED and OLED has since helped accelerate the trend. High-resolution viewing experiences have been brought to the mass market with HD (high definition), 4K and 8K display devices.

Projected sales of high-resolution 4K television screens (which, along with 8K screens, are called ultra high definition or UHD) continue to grow around the globe

Projected sales of high-resolution 4K television screens (which, along with 8K screens, are called ultra high definition or UHD) continue to grow around the globe. Image Credit: IHS Markit

However, these high-resolution screens are not cheap, and in return for a high price, consumers expect high performance. Consumer electronics device manufacturers are tasked with ensuring their display devices provide a flawless visual experience with crisp, bright and defect-free screens.

Manufacturers have long relied on automated visual inspection (AVI, also called automated optical inspection, AOI) solutions when it comes to ensuring display quality—typically applying imaging systems and cameras.

These tools can be used to detect spot defects, such as dead pixels and uniformity issues (or mura, from the Japanese word for “blemish”) of each display panel.

There has been a parallel trend towards higher resolution specifications for inspection systems among display metrology providers like Radiant. Simply put, to measure a high-resolution display, a high-resolution system is needed.

High resolution is important not only for big screens like televisions, but also for small displays such as smartphones and smart watches that are viewed close up. Users expect a crystal-clear image. Shown here: the Apple Watch 5 and iPhone 11 Pro.

High resolution is important not only for big screens like televisions but also for small displays such as smartphones and smartwatches that are viewed close up. Users expect a crystal-clear image. Shown here: the Apple Watch 5 and iPhone 11 Pro. Image Credit: Radiant Vision Systems

Are We Talking About Pixels? or Pixels?

The term “pixels” is frequently utilized, but it can mean different things. The word pixel is derived from the “picture element” and is employed to describe the individual ink dots of a printed image or the individual illuminated elements of a display screen.

In photography, a pixel is the smallest element of a camera’s image sensor (a photosite) that can record the light (photons) that enter the camera when the shutter opens. By converting photons to electrons, the sensor pixels store an image in digital form.

Display inspection applications are concerned with two specific types of pixels: image sensor pixels and display pixels. A common specification when the consumer electronics industry discusses displays (like laptop, smartphone or television screens) is the number of display pixels

For instance, a display device listed at 1520 x 720 means the screen is a grid that has 1520 pixels horizontally and 720 pixels vertically. Multiplying 1520 x 760 = 1,094,400 pixels in the entire display. Usually, more pixels equal higher resolution and better display quality and image clarity.

Display makers have been able to shrink the size of display pixels and squeeze them closer together over the years. This is known as decreasing the pixel “pitch” (the spacing between pixels), which generates a denser array of pixels and a higher-resolution visual image on screen.

When observing different display panels in the image supplied, notice how the “dots” (display pixels) in the lower screen (A) are larger, and the black space between them is more obvious.

This display has lower pixel density, fewer pixels per inch (“PPI”) or a larger pixel pitch, and these descriptions are the same in meaning. The pixels are quite far apart, so not as many will fit into a given area.

Examples of display panels with different pixel sizes, pixel pitches, and pixels per inch (PPI)—all of which impact display resolution

Examples of display panels with different pixel sizes, pixel pitches, and pixels per inch (PPI)—all of which impact display resolution. Image Credit: Radiant Vision Systems 

The other displays in the image show the pixel size and the space between the pixels shrinking as they progress backward. The top/back-most panel (B) has the tiniest pixels, spaced the closest together (smallest pixel pitch), so this display supplies the highest resolution and the highest PPI.

Sensor Pixels

During production, imaging systems such as Radiant’s ProMetric® cameras are employed to inspect and measure displays for quality control. Each camera has a certain MP (megapixel) specification, and the higher the number of pixels, the higher the resolution of the camera.

For example, Radiant’s latest product launch includes 45 MP and 61 MP models (with 45 million and 61 million sensor pixels, respectively).

However, when referring to the MP of a ProMetric imager, it is not to be confused with display pixels, but it is the number of sensor pixels on the camera’s image sensor. Sensor pixels capture light and translate it into digital information that is utilized to measure light or color values while display pixels emit light.

To supply the most accurate assessment of today’s pixel-dense, high-resolution displays, it is crucial to be able to capture high-resolution images for measurement and analysis. More sensor pixels in your camera can make a big difference in achieving this.

A high-resolution camera can dedicate a larger number of sensor pixels to measure finer details in a display. The camera’s images can be employed to distinguish and measure individual display pixels if a camera has an equal or greater number of pixels than the display.

A low-resolution camera possessing fewer pixels than the display will have to image multiple display pixels with each sensor pixel, and it will lose the ability to distinguish individual display pixels in the image and decrease the precision of the measurement.

A high pixel count translates into high spatial resolution, which determines a system’s ability to distinguish fine detail within an image. For example, the image at far left  has a low pixel count (low resolution), so there is little discernable detail. The image at far right has a high pixel count (high resolution), providing excellent detail and clarity.

A high pixel count translates into high spatial resolution, which determines a system’s ability to distinguish fine detail within an image. For example, the image on the far left has a low pixel count (low resolution), so there is little discernable detail. The image on the far right has a high pixel count (high resolution), providing excellent detail and clarity. Image Credit: Radiant Vision Systems

Why Sensor Megapixels Matter

Included in this article is a conceptual illustration of display pixels vs. camera sensor pixels, and each square represents a single pixel in this simplified schematic.

The data from each display pixel can be captured fully by a single pixel on the camera’s sensor if the display panel (A) and the camera sensor (B) have the same number of pixels (a 1:1 ratio).

Conceptual illustration of display pixels and camera sensor pixels.

Conceptual illustration of display pixels and camera sensor pixels. Image Credit: Radiant Vision Systems

Yet, if the display (C) has a lot more pixels than the camera sensor (B), it means each sensor pixel has to store the data for numerous display pixels (a ratio of 4:1), limiting the amount of detail captured.

To capture more detail, a low-resolution camera could capture multiple images of the display, but this slows the inspection speed (increases cycle or measurement time), which is not viable for high-speed manufacturing lines.

A high-resolution display (C) would ideally be imaged by a high-resolution sensor (D), with several sensor pixels dedicated to gathering the data for each display pixel, a 1:4 display-to-sensor-pixel ratio in this theoretical example.

High-resolution imaging is also vital for measuring each display pixel’s constituent parts. In today’s microLED and OLED displays, each pixel is made up of three sub-pixel elements (red, green and blue diodes), which generate their own light.

It is vital that a display measurement system is able to capture detail down to the subpixel level, as light emissions can vary per subpixel (causing visible quality issues in the display).

Ideally, the measurement system should be able to capture this level of detail in a single image (a single camera shot) to keep up with the pace of manufacturing production lines.

Magnified view of OLED display pixels, each containing one red, one blue, and two green subpixel elements.

Magnified view of OLED display pixels, each containing one red, one blue, and two green subpixel elements. Image Credit: Radiant Vision Systems

New High-Resolution Inspection Solutions: ProMetric 45MP and 61MP

Radiant’s latest high-resolution measurement systems include the ProMetric Y45 and Y61 Imaging Photometers and the ProMetric I61 Imaging Colorimeter. The “45” and “61” in the product names refer to the number of megapixels in each camera’s sensor.

Increasing camera resolution enables users to apply a greater number of sensor pixels per display pixel, increasing the amount of detail that their cameras can capture and, for instance, enabling them to measure individual subpixels.

Furthermore, Radiant has developed software algorithms and other methods to maximize the performance of their cameras for measuring and distinguishing display pixels and subpixels with repeatable accuracy.

Radiant’s product development team is always challenged to engineer their systems to strike the right balance of imaging speed and imaging detail and to ensure efficiency and accuracy for display inspection.

This means creating more advanced image processing techniques and higher-resolution imaging systems to keep up with the increasing resolution of emerging displays.

Radiant’s fractional pixel measurement method (US Patent 10,971,044) and spaced-pixel measurement method (US Patent 9,135,851) were created to address the industry’s increasing display resolutions utilizing existing hardware.

These new 45MP and 61MP imaging solutions enhance a user’s ability to image the highest-resolution displays with even better accuracy and detail while capturing an entire display at the speed of production in a single image for measurement.

Radiant’s new ProMetric I61 Imaging Colorimeter (left) and ProMetric Y45 Imaging Photometer (right).

Radiant’s new ProMetric I61 Imaging Colorimeter (left) and ProMetric Y45 Imaging Photometer (right). Image Credit: Radiant Vision Systems

All Pixels are NOT Created Equal

However, how much data (how many electrons) the pixel can hold is determined by pixel size and capacity (“well depth”), limiting the precision of each sensor pixel’s measurement.

Dynamic range and signal-to-noise ratio (SNR), a measure of how much good signal (electrons from the display image) versus noise (electrons from other sources) each sensor pixel captures, are other qualitative differences in sensor pixels. A lot of image noise and poor dynamic range limits measurement accuracy.

Radiant carefully tests sensor components for these and other criteria before selecting the sensors for their ProMetric systems.

Next, to ensure they are performing to the high standards that customers have come to expect, they rigorously test each sensor before, during and after the assembly of their cameras.

Of course, besides the sensor, there are many other factors that contribute to the superior quality of Radiant’s imaging systems; some of these include:

  • Optical design and lens selection to identify and eliminate issues like stray light
  • Design and engineering of the electronics to control temperature and limit thermal noise
  • TrueTest™, the industry’s leading image analysis and automation software
  • Lens calibration to eliminate distortion or “fisheye” effects from wide-angle optics
  • Integration of photopic and tristimulus color filters with advanced calibrations for accuracy to light-measurement standards (CIE color-matching functions)

Radiant’s new ProMetric imaging systems can carry out the inspection of pixel-dense, high-resolution displays at high speeds to keep pace with manufacturing production lines by combining careful engineering with high-resolution sensors.

One of the highest-resolution displays available today: Samsung offers 33 million pixels across its 65” NEO QLED screen

One of the highest-resolution displays available today: Samsung offers 33 million pixels across its 65” NEO QLED screen. Image Credit: © Samsung Display

Radiant is positioned to solve measurement challenges for display manufacturers now and into the future with the new 45MP and 61MP ProMetric models in their product portfolio, as microLEDs, quantum dots (QD) and other nano-scale display pixel technologies become more common.

Acknowledgments

Produced from materials originally authored by Anne Corning from Radiant Vision Systems.

This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.

For more information on this source, please visit Radiant Vision Systems.

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