ICDM Decides There’s More to Resolution than Pixel Count
The International Committee for Display Metrology convened for its general meeting in San Francisco on May 24. As a result, display quality measurement methods are expected to change. With different display manufacturers now offering various types of technology for producing images, the committee concluded that new methods for measuring display resolution quality are necessary. For our readers, Samsung Newsroom has summarized the details of the meeting in 6 sets of questions and answers:
Q1. What is the ICDM and what proposals were submitted at this year’s general meeting?
A. At the most recent ICDM general meeting, international standards for measuring display performance were discussed, given the limits of conventional measurement methods.
The ICDM is an international organization that develops display performance measurement standards. It is a part of the SID (Society for Information Display), which is an industry organization for electronic displays. Display experts from around the world, certifying authorities, and display manufacturers—including Samsung Electronics, LG Electronics, and Panasonic—are members of the ICDM.
At the general meeting, revising display measurement standards was discussed in a way that would complement current display quality measurement methods, because current methods don’t clearly show performance differences among displays when measuring resolution.
Conventional displays employ RGB (red, green, and blue) technology, but new display layout technologies use RGBW (RGB plus white), triggering a discussion on the need for a change in how display quality is measured. The current measurement methods were developed in the analog era, so are not appropriate for more recently developed displays. Therefore, it is necessary to develop new measurement methods. (Further details on RGB and RGBW are described below.)
Q2. What decisions were made at the general meeting?
A. “Contrast modulation” was determined to be the optimal criterion for display resolution measurement. Reporting contrast modulation has become mandatory.
At this year’s ICDM general meeting, information display measurement standards were revised according to three major points.
First, it’s not appropriate to employ current resolution measurement methods to measure newly developed display layout technologies like the RGBW. This is because a) the new technologies do not fit in the previous definition of sub-pixels, b) there are differences in resolution between black-and-white and color scene, and c) there are differences in clarity when tested with grille pattern by an arrangement order of black-and-white lines. In this regard, the ICDM clarified that new measurement methods should be developed.
New display technologies (pixel layouts) make the current measurement and calculation of display resolution, according to 7.1, 7.2 and 7.8, incomplete. Care should be taken when interpreting the results. New resolution measurement methods should be added.
IDMS (ICDM display measurement standard) version 1.03, Section 7.8, page 131
Second, contrast modulation is the best way to describe display resolution. This statement indicates that contrast modulation is the best single-metric for describing quality differences, so it should be adopted as the criterion for measuring effective resolution.
Contrast modulation (Michelson contrast) Cm is considered by some to be the best and most complete single-metric description of the ability of a display to exhibit information.
IDMS (ICDM display measurement standard) version 1.03, Section 7.8, page 131
Third, manufacturers must not only count the number of lines, but also report contrast modulation when measuring resolution until new methods are established. In the past, manufacturers didn’t have to describe contrast modulation if it was above 50 percent. However, from now on manufacturers must report resolution and contrast modulation together: for example, “1920 lines at Cm=76%” An example is as follows.
(3) Reporting Michelson Contrasts Using Thresholds: When thresholds are used for a pass-fail test, it is mandatory that the Michelson contrast obtained for the 1×1 grille also be reported. For example, suppose a near-eye display advertises a resolution of 1920×1080 and the horizontal 1×1 grille using § 7.8 is found to have a Michelson contrast of 76%, then the manufacturer must report that the resolution as “1920 lines at Cm = 76 %” or something similar to report the Michelson contrast of the 1×1 grille.
IDMS (ICDM display measurement standard) version 1.03, Section 7.2, page 113+5
Q3. What is the difference between RGB and RGBW displays?
A. RGBW displays use a white LED, so picture quality and contrast modulation are reduced.
Displays use red, green, and blue lamps as the primary color lights. Red, green, and blue subpixels are arranged in order on an RGB display so that the display can exhibit colors and straight lines more accurately. However, a RGBW display uses a white subpixel as well as the three primary color subpixels. This allows RGBW displays to have a relatively higher aperture ratio, but at the cost of color reproduction and contrast modulation.
Q4. What is contrast modulation?
A. The higher the contrast modulation is, the clearer a line is. But it is impossible to measure contrast modulation using current resolution measurement methods.
At this year’s general meeting, the ICDM stipulated that contrast modulation should be used to redefine what resolution is. The picture below shows one of the methods the ICDM employs to measure the resolution of a display. Contrast modulation shows how clear the letters “mmme” look on a single line of a display with the resolution of 3840×2160. You can see how the higher contrast modulation is, the clearer the letters are. With a lower contrast modulation, the letters become more blurred because it is difficult to distinguish between the black and white lines.
Q5. How much contrast modulation is necessary?
A. For analog TVs, a contrast modulation of 50 percent was considered sufficient. But now, a contrast modulation of 95 percent is the average for RGB TVs.
Display signals were less clear for analog TVs, therefore, a contrast modulation of 50 percent was the standard for whether a certain display could exhibit a given resolution input signal. However, applying that standard to today’s TVs would make a high-definition video look like an analog TV. The average contrast modulation of RGBW TV displays on the market is known to be 60 percent, whereas the average contrast modulation of RGB TV displays is 95 percent. The pictures below show the difference between a RGB display and a RGBW display exhibiting 12-point texts. The pictures were taken with a magnification ratio of 1:1. You can easily see the difference in contrast modulation between the two pictures.
Q6. What changes will these revised standards bring?
A. The conventional pass-fail method of evaluation is about to completely change. Contrast modulation will be the only method to describe display resolution.
The ICDM said that the current display resolution measurement methods were incomplete because they use a pass-fail test that applies random criteria, determined by each manufacturer, to black-and-white measurement values. Therefore, the ICDM said it would look to change the current pass-fail method for a system that combines black-and-white measurements and color measurements, and makes those results open to the public. Until that new measurement system is established, the ICDM announced that all manufacturers should report both the number of lines (or in the form of #### x #### pixels) as well as contrast modulation (Cm).
Once the new measurement methods are established, contrast modulation will be the only method to describe display resolution, giving users a better understanding of display performance.
(3) Reporting Michelson Contrasts Using Thresholds: … (We realize that making a mandatory requirement of reporting the contrast can be viewed as a departure from the general philosophy of this document as noted in the abstract on the title page and in the Introduction. However, we also wish to avoid the lack of a measurement result that the pass/fail thresholds can introduce; thus, we want to be sure that the measurement result is also available to all.)…
(6) Use of Thresholds: We also anticipate that the use of thresholds as guidelines will discontinue in any new metric and will be replaced by measured levels.
IDMS (ICDM display measurement standard) version 1.03, Section 7.2, page 113+5
For full list of ICDM updates, please visit: http://www.icdm-sid.org/updates/index.html (documents available for download at bottom of page).
