Display Metrology: What Is It (Good for)?
Michael E. Becker
Display Metrology, the measurement of the optical properties of electronic displays, is intended to support the supply of customers with display devices that satisfy visual performance requirements resulting from specific applications.
Display metrology is practiced in order to provide physical data as an objective basis for rating of the visual performance of electronic-display devices; e.g., luminance is measured in order to estimate the brightness perceived by a human observer. Display metrology thus contributes to bridging the gap between physical measurements and the human visual perception of electronic displays and thus their ergonomic performance. It is subject to a variety of boundary conditions (canonical rules) and the way it is exercised or the results are featured may severely affect commercial competition. For example, in a court case from 2003 (NEC Mitsubishi Display of America vs. ViewSonic Corp., Illinois Federal Court, Case No.: 02 C 08304), NEC Mitsubishi had charged that ViewSonic could have been misleading or confusing customers with the way they had specified the contrast of ViewSonic LCD monitors. While this was a case between competitors, similar cases involving consumer advocacy groups holding manufacturers to task for issues of exaggerated technical claims have been initiated many times over the years as well. Display metrology and the resulting data is of significant commercial interest and thus often subject to manipulations. Hence, the stakes for understanding and properly exercising the practice of display metrology are very high.
In the first step, physical measurements that are significant have to be carried out, providing characteristics that are meaningful with respect to human visual perception, such as measuring light instead of infrared or ultraviolet electromagnetic radiation. The object of measurement has to be in a state that corresponds to a realistic application situation (e.g., not in a completely dark room), and its controls have to be set accordingly (e.g., luminance and contrast settings).
It is taken for granted here that display metrology is reproducible, i.e., providing the same results when the specifications of the measurement procedures are followed, simple, and robust (i.e., insensitive to small variations of the instrumentation and its geometry).
The measuring methods, usually specified by international standards (no qualified competent monographs are available on this complex subject; the FPDM-2 provides the required solid basis), should be clearly described to be easily understood, and all details that are important for implementation and accomplishment of the method must be disclosed (see ISO/IEC Directives, Part 3: Drafting and Presentation of International Standards). A comprehensive compilation of compulsive terms and definitions is a prerequisite for unambiguous communication and understanding between any two parties.
In addition, the methods should be applicable to a wide range of different display effects and technologies. They should be honest (i.e., not devised to hide deficiencies), allowing for a broad range of instruments (not restricted to unusual, highly specialized, or hardly accessible instrumentation). International metrology standards should not be misused as marketing instruments for metrology instrumentation manufacturers.
The above requirements for measurement methods and procedures imply that there should be as many of them available as required for carrying out the daily work in the metrology laboratory, but these days, unfortunately, there are too many applicable but unsynchronized standards providing confusion of terms and definitions and measurement approaches rather than the required clarity and assistance. This confusion is often exploited by marketeers by selecting such measurements that are yielding the "best numbers" for the product data sheet. In line with that mentality of specsmanship (i.e., abuse of technical data to establish putative superiority of one device over another) is the measurement of contrast in a dark room, yielding (very) high numbers, especially for emissive displays, but under conditions that do not represent the actual application situation (about 99% of all display-application cases are under ambient illumination). The extension of image formation times according to ISO 13406-2 to transitions between different levels of gray was eagerly adopted by certain LCD and monitor manufacturers because an "improvement" by a factor of 2 was granted by the fact that ISO was using the sum of both transition periods between ON and OFF while the alternative gray-to-gray response times(not standardized before the introduction of ISO 9241-300) only specified one of the transition periods (usually the faster one).
The international standard ISO 13406-2 proves that a standard can actually help to advance technology and the quality of products to which most of us are exposed to for many hours every day. ISO 13406-2 for the first time introduced methods for the measurement and rating of directional variations and reflections from LCDs, and with the enclosed ergonomic rating of the physical data into performance classes it has been pushing the improvement of these features, thus making better displays available on the market.
We can now simply trace back the justification for solid display metrology also from an economical starting point: Every customer (private and corporate) deserves an unbiased, honest set of data for specification of the visual performance of an electronic-display device for a specific application (office work, video and television, home-theater, medical diagnostics, etc.) as a basis for purchasing decisions. The visual performance of the display is being evaluated from a set of physical data that must be obtained by significant, well-defined measurements and specified by standardized technical terms and performance characteristics.
So, if you are interested in purchasing electronic-display devices that are fulfilling your performance expectations without regrets, disappointment, or hangover, you should be interested in display metrology and the international standards that define them. You then should also be interested in knowing what the specifications mean and how relevant they are for your intended application.
In response to the question posed in the title, display metrology can be helpful in acquiring electronic displays with the visual performance adequate for a specific task at an affordable price. The articles in this issue were written with this in mind.
In his contribution "Diffuse Clarification," Edward F. Kelley, the spiritus rector of the FPDM2, provides a clarification of display-related terms that are often confused. This appetizer is intended to illustrate that unambiguous terminology is the basis for factual understanding and thus for the communication of technical specifications.
Carsten Dolar from the Technische Universität Dortmund (Germany) describes his numerical model for simulation of the perception of moving images displayed on LCD screens. This model is based on generalized measurement results (rules and laws expressed in mathematical formalism), and it offers the advantage of accurate control of all involved parameter values. The presented model comprises the electro-optical display together with human visual perception, and it has been developed for systematic optimization of the chain of signal transmission and processing.
Joe Miseli from Sun Microsystems introduces the International Committee for Display Metrology (ICDM)), which is currently producing a Display Measurement Standard (DMS), the updated successor to the most comprehensive reference on the "art of measuring electronic displays," the FPDM2, issued by VESA in 2001. In the Spring of 2007, the FPDM working group migrated to become the ICDM, under the auspices of SID.
The field of display metrology and its evolution during the past 30 years is described by Jurgen Laur from autronic-Melchers GmbH in his article, "Measuring and Rating Electro-Optical Display Performance," from the perspective of a long-term manufacturer of display-metrology instruments.
As guest editor of this Special Issue on Display Metrology, I sincerely hope that the contributions on display metrology stimulate your interest in the subject and provide you with useful information and some helpful clues. •