Testing Equipment And Parameters For LED Displays

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Rapid developments in technology are powering the indoor and outdoor LED display segments. However, display manufacturing is a complicated business that requires many high-level measuring instruments, analysis software, and production equipment.

By Deepshikha Shukla

Advanced LED technology has enabled big breakthroughs in the colour, resolution and  appearance of the display. Quality, accuracy and the standard display production operations have also been enhanced. Displays, single components and panels must be objectively tested at all stages of the manufacturing process. The measuring instrument employed simulates the human eye or visual system. It ensures that the displays meet several standards through the objective evaluation of display characteristics like:

• Standard evaluation of ergonomic and safety-relevant characteristics.
• Determining product specifications.
• Product and technology optimisation.
• Screen colour measurement.
• Display production quality control.
• Monitoring of brightness requirements.

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Key parameters to characterise full colour LED displays To ensure that the display
image does not distort, an LED display’s surface roughness should be within +1mm. The flatness of the LED display is determined by the production technology. Partial convex or  concave shapes can lead to a blind visual angle of the display screen. The display visual angle is mainly determined by the LED chip encapsulation method. The visual angle of a display is determined by the size of the audience, so the better the visual angle, the better the audience experience.

The display’s luminous intensity can be measured in units of nit (cd/m²). The indoor LED  displays’ brightness should be low (between 800cd and 1000cd/m²), while outdoor displays’ brightness should be a little high—between 1500cd and 10,000cd/m². This enables people to see clear and vivid images. To ensure seamless operation, the indoor display requires a high grayscale of 16 bits. The brightness is mainly decided by the LED chip, but too high a  brightness can shorten the lifetime of LEDs. One of the most important indicators of a LED display is the white balance effect, which is mainly determined by the control system of the display screen.

To ensure the authenticity of the image, the screen displaying colours need to stay highly  congruent. The amount of dead light represents the quality of the LED chip. Dead light refers to constant bright or black points in the display screen. It also determines the display  connector quality.

Display measurement systems must be configured to perform different types of display tests in such a way that they do not introduce error into the measurement process. Parameters such as chromaticity, gamma, white balance, contrast, flicker and uniformity are important when characterising LED displays. A few LED display testing equipment available in the market today are described briefly below.

Display Test and Characterisation Solutions (Source:https://www.gamma-sci.com/ products/ned-lmd-near-eye-display-measurement-systems)

Near eye display (NED) measurement system
The Gamma Scientific-1290 NED system  provides test capabilities for a wide variety of parameters including luminance and colour uniformity, pixel/line defects, contrast, image virtual distance, image distortion, the field of view (FOV), resolution and response time. It can cover over 160-degree FOV with high- resolution measurements in the virtual image space. It provides precision angular alignment to the device under test.

Display test system
Instrument Systems has been providing all-in-one systems for display measurement since many years. The DTS 140D display test system is an all-in-one system optimised for high measurement accuracy, maximum stability and high variability. It includes the following components:

  • The spectrometer of the CAS 140D series with a spectral range of 360nm – 830nm, or  380nm – 1040nm.
  • A 200 telescopic optical probe with an alignment camera for focusing on the measurement point and recording the data.

    Spectroradiometric display test system with the CAS 140D (Source:https://www.
    instrumentsystems.com/en/systems/dts-display-measurement/)
  • SpecWin Pro lab software for controlling the spectrometer and telescopic optical probe, together with complete measurement analysis.
  • A manual or a fully automatic five-axis positioning system.

 

 

Spectroradiometer
The MK550T handheld spectroradiometer`developed by UPRtek determines the colours, resolution and appearance of the display. It is different from the traditional filter-type architecture, and is equipped with advanced spectral optical know-how to assist  manufacturers increase accuracy on display colours and brightness. The production  programme calibration software ensures that manufacturing conditions meet the standard operating procedures.

Imaging colorimeter
The ProMetric I is a high-resolution imaging colorimeter designed to address the demands of high-volume display manufacturing. Whether you need to expand test coverage or increase throughput, it delivers the required performance for accurate colour and luminance  measurements in an automated manufacturing environment. It is built around scientific-grade CCD sensors, ranging in resolution from 2 megapixels in the I2, to 29 megapixels in the I29. These sensors enable pixellevel measurements of displays.

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