Metrology is “the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology,” as defined by the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, BIPM).
The term ‘display metrology’ essentially means using a scientific approach to quantify, or measure, the visual output of a display. Light is the primary output of today’s digital displays, which can be measured in a number of different ways – in terms of a display’s color, contrast, brightness, or another visual quality.
Display screens illuminate modern life. Image Credit: Radiant Vision Systems
Spectral Sensitivity vs. Spectral Power Distribution
Of course, it must be noted that there is more than one approach to quantifying and characterizing a light source. The light’s electromagnetic output is called its Spectral Power Distribution (SPD). Typically, a graph is used to display the SPD of a light source that represents the relative intensity of the source at each wavelength.
Normalized spectral power distributions of (a) halogen light, (b) fluorescent light, (c) daylight, and (d) another type of fluorescent light. (Image Source)
However, the human eye neither perceives every single wavelength nor the raw power of light emitted at every wavelength in the electromagnetic spectrum. Human beings therefore do not perceive the entire SPD of many light sources. There are three types of cones in human eyes (S, M, and L), each sensitive to a range of wavelengths. Below is a rough illustration of each of the cones within our eyes and the normalized response of each — in other words, how powerful we perceive each wavelength to be.
The spectral sensitivity of each cone in the human eye. Image Credit: Radiant Vision Systems
Individual wavelengths cannot be detected by the cones in our eyes. Instead, our eyes see the sum of the total light that all three cones are sensitive to, defined by the area under each of the graph’s curves. Plotting how we perceive various SPDs is helped significantly by understanding this human visual response — or our spectral sensitivity.
Human Perception of Measuring Displays
Measurement and characterization of display performance (in terms of light and color output) uses human spectral sensitivity as the yardstick, as display devices are of course designed to be used by humans. It is therefore important to match the human visual response.
Our experience of light is ultimately determined by human visual perception, including the accuracy and appearance of colors in devices like displays or light sources.
- It therefore makes sense that the objective in testing displays is to measure color and light in the same way it is perceived by a human being, in order to ensure that the visual quality of a display matches the human experience.
- We can only perform automated display metrology with meaningful results by obtaining values standardized against human perception. In order to evaluate displays, we need to understand the two fundamental aspects of light: the light output produced by the display (the SPD), and the light input we as human beings receive via sight.
Photometry is the name given to the branch of metrology that measures the intensity of emitted light (luminance) as perceived by humans. Similarly, there is a field called colorimetry which measures the color (chromaticity) of light as seen by the human eye. There are, therefore, certain types of display measurement which depend on colorimetric and photometric instruments.
Key Performance and Quality Characteristics of Displays
Today’s display manufacturers perform a wide range of measurements, tests, and inspections of display devices to ensure that quality and performance meet brand standards and customer expectations. With complex global supply chains, the industry relies on the consistent, objective evaluation of displays and components at all steps of the manufacturing process.
Some of the key characteristics for display evaluation include:
- Color Gamut Area
- Luminance and Chromaticity
- Gamma
- Color Volume
- Mura & Defects
- Uniformity
- Reflection
- Directional View
- Flicker and other time-domain characteristics
Display Metrology Instruments
Both in the R&D lab and on the production line, display makers use a range of different metrology instruments to measure all these characteristics, such as spectroradiometers, spot meters, photometers, and colorimeters:
Source: Radiant Vision Systems
| Instrument Type |
Typical display characteristic to be measured |
| Spot Meter |
Color, luminance, contrast, gamut, gamma, flicker... |
| Imaging System (Photometric or Colorimetric) |
Color, luminance, contrast, uniformity, mura, defects, pixel-level analysis, distortion… |
| Spectroradiometer |
Spectral data... |
| Others |
Response time, view-angle-dependent performance... |
The “imaging system” category is where Radiant’s ProMetric® Y-Series Imaging Photometers and ProMetric I-Series Imaging Colorimeters fit in. Such imaging systems truly lead by example when measuring display devices: in particular, measuring display appearance as human viewers perceive it. Simultaneously, an image-based measurement system uses a 2D detector in order to measure multiple points of light. This, therefore, captures the entire display at once just as our eyes take in an entire scene in one glance, simulating the human visual experience.
An imaging system can evaluate and compare millions of data points simultaneously by capturing a complete display image at once. This capability permits:
- Location, severity, and identification of defects (pixels, lines, blobs)
- Determining distortion, dimensions, and focus quality (projection displays)
- Simultaneous measurement of multiple regions of interest (LED arrays, pixels, subpixels)
- Contextual evaluation: gradient, uniformity, mura, contrast, distortion
- Performing multiple advanced analyses per image captured
Fundamentals of Display Metrology: Deeper Learning
This article has outlined some of the simple facts about display quality measurement. If you’d like to learn more about display quality measurement, why not listen to some of the experts in the field in the Short Course “Fundamentals of Display Metrology” which was originally presented as part of the 2021 Display Week virtual event program (and is now available as an on-demand webinar).
“Fundamentals of Display Metrology” was presented by Jens Jensen, Vice President of Product Development at Radiant Vision Systems; Dr. Recto Häring, Vice President of Customer Solutions at Instrument Systems GmbH; and Yutaka Maeda, Business Development Lead at Konica Minolta. The course covers the following:
- Science of light and color and measuring human visual response (e.g., SPD, CIE standard functions)
- Units of measurement (e.g., luminance, color gamut), measurement standards (e.g., IEC, ISO), and metrology systems from imaging equipment to spot meters
- Test methods used to adress display performance parameters such as mura, pixel uniformity, and flicker
- The newest metrology solutions for the testing of emerging displays such as microLED and AR/VR devices, and many more.
Acknowledgments
Produced from materials originally authored by Shaina Warner and 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.