How We Test Mobile Displays

Dr. Raymond M. Soneira

President, DisplayMate Technologies Corp.

This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated

Into any other work without the prior written permission of DisplayMate Technologies Corporation

Article Links: Overview MP3 and Video Players Smart Phones Game Consoles

Printing: If your browser is improperly printing some pages with text cutoff

on the right edge then either print in Landscape mode or reduce the font

size (View Menu - Text Size) and margins (File Menu - Page Setup).

Introduction

This sidebar provides additional technical details and background information on the lab measurements for the Mobile Devices LCD Comparison and Picture Quality Shoot-Out article series. The devices were tested using the same technology and methodology as in the Display Technology Shoot-Out article series for CRT, LCD, Plasma, DLP, and LCoS HDTVs, which includes additional details, in-depth explanations, interpretations and analysis for all of the tests discussed here.

DisplayMate Test Patterns

All of the testing was done using a very large set of test patterns produced by a special version of the DisplayMate Multimedia Edition, our most advanced Windows based video diagnostic product. This is the same product used by PC Magazine, ExtremeTech, and most major publications for testing and evaluating monitors, projectors and HDTVs. The Multimedia Edition is able to produce accurate test patterns for all of the device’s unique resolutions because it generates the patterns real-time from scale free mathematical equations. Producing the test patterns at the native resolution of each device was essential for all of the testing.

NIST Flat Panel Display Measurements Standard

We also used the NIST (National Institute of Standards and Technology) FPDM, Flat Panel Display Measurements Standard, which is produced in association with VESA (Video Electronics Standards Association). See the Shoot-Out Part I for details. The Screen Reflectance measurements in Table 2 were done in accordance with FPDM 308-1, Reflectance with Diffuse Illumination, using an integrating hemispherical dome and a calibrated diffuse white reflectance standard. All of the devices were tested with their Backlight set for maximum brightness and connected to their AC Adapter, so their battery state was not an issue.

Photometry and Colorimetry

All of the Photometry and colorimetry for the article was done with a Konica Minolta CS-200 Chroma Meter spectroradiometer, which was provided by Konica Minolta Sensing Instruments. A spectroradiometer was necessary because the more common and inexpensive colorimeters would have delivered inaccurate results (see the Shoot-Out Part I for an explanation). This spectroradiometer also has a narrow one degree acceptance angle, which is very important when measuring LCD panels. The NIST-VESA FPDM calls for a maximum acceptance angle of two degrees.

Konica Minolta CS-200

Gray Scale and Gamma

Figure 1 in each article shows the LCD Screen Brightness (technically referred to as the luminance) in cd/m² (candelas per square meter) for each of the displays as a function of the signal/image intensity level (0-255) expressed as a percentage of maximum (0-100 percent). The open symbols plotted on the graph are the measured data points. They are plotted on dual logarithmic scales because the desired relationship is a mathematical power-law, which appears as a straight line on a log-log graph. The graphs are all staggered vertically for clarity.

Many of the devices depart significantly from the desired gray scale. See Figure 2 in the Shoot-Out Part D for examples of displays with ideal gray scales. Many of the devices show significant compression (rounding and flattening) at both the bright and dim ends of the gray scale, which is typical for under compensated LCD panels. The Gamma (Table 3) was calculated from the logarithmic slope in the central 40 to 80 percent signal range. The optimum Gamma values are in the range of 2.2 (the official standard) to 2.5 (high contrast). Some of the devices greatly exceed this, which produces too much picture contrast. The Bright Compression (Table 3) is calculated by extrapolating the Gamma fit to 100 percent intensity and comparing that to the data value.

Chromaticity

Colors are measured in special chromaticity coordinates. We measured the primary colors for each device with the spectroradiometer and plot them on a 1976 CIE Uniform Chromaticity Scale diagram with u’,v’ coordinates in Figure 2 of each article, together with the ITU Rec.709 standard primary colors for HDTVs and sRGB computer monitors, which are marked by the black triangle. The outer white line defines the pure spectral colors (upper horseshoe) and the purples connecting the extreme ends of the visible spectrum (lower diagonal line). The White Points for the devices are plotted together with the D6500 standard (in black). See the Shoot-Out Part II for an in-depth discussion of this topic.

Article Links

Series Overview

MP3 and Portable Video Players

Smart Phones

Handheld Game Consoles

How We Test Mobile Displays

About the Author

Dr. Raymond Soneira is President of DisplayMate Technologies Corporation of Amherst, New Hampshire, which produces video calibration, evaluation, and diagnostic products for consumers, technicians, and manufacturers. See www.displaymate.com. He is a research scientist with a career that spans physics, computer science, and television system design. Dr. Soneira obtained his Ph.D. in Theoretical Physics from Princeton University, spent 5 years as a Long-Term Member of the world famous Institute for Advanced Study in Princeton, another 5 years as a Principal Investigator in the Computer Systems Research Laboratory at AT&T Bell Laboratories, and has also designed, tested, and installed color television broadcast equipment for the CBS Television Network Engineering and Development Department. He has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American. If you have any comments or questions about the article, you can contact him at dtso@displaymate.com.

Copyright © 1990-2006 by DisplayMate Technologies Corporation. All Rights Reserved.
This article, or any part thereof, may not be copied, reproduced, mirrored, distributed or incorporated
into any other work without the prior written permission of DisplayMate Technologies Corporation