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Part II ? Gray-Scale and Color Accuracy
Introduction
This is Part II of an article series describing an in-depth comparison between CRT, LCD, Plasma and DLP display technologies in order to analyze the relative strengths and weaknesses of each. In Part I we measured, analyzed and compared primary specs like Black Level, Color Temperature, Peak Brightness, Dynamic Range, and Contrast for each display. Click here to read Part I. In this installment we examine the Gray Scale and Gamma in order to explore color and gray scale accuracy. In Part III we'll study the complex world of display artifacts -- just a fancy name for the image peculiarities -- of each technology for both computer and television applications.
We chose the top performer for each display technology using our own 2004 DisplayMate Best Video Hardware Guide. The candidates included a 40" direct-view LCD (NEC LCD4000), a 61" Plasma (NEC 61XM2), a 50" DLP Rear Projection (Optoma RD-50), and a much smaller CRT 19" professional High Definition studio monitor (Sony PVM-20L5), which was used as the reference standard for color and gray scale accuracy. It's important to emphasize that this article is designed as a comparison of four different display technologies and not as an editorial review of the above models. By comparing the top performing model in each technology we are in effect examining the state-of-the-art for that technology. We will be looking at fundamental image and picture quality performance issues and not the implementation idiosyncrasies of any particular model.
Instrumentation for Parts I and II:
All of the test patterns were generated with DisplayMate for Windows Multimedia Edition on both DVI and component video. For HD signals we used an ATI Radeon 9800 Pro with an ATI HDTV Component Video Adapter, which provides high quality computer-generated 720p and 1080i component video outputs YPBPR. We also used a pre-release version of the DisplayMate Professional DVD that has DisplayMate's proprietary test patterns on DVD (available later this year). All of the photometry and colorimetry measurements were made with a Konica Minolta CS-1000, which is a high-end laboratory Spectroradiometer with a narrow 1º acceptance angle for light emitted by the display. Most of the photometers and color analyzers that are used for display measurements are actually accurate only for CRTs because they rely on filters calibrated to the light spectrum of a CRT. They also have broad acceptance angles that are not accurate for many flat panel technologies because of their variation in light distribution with viewing angle. The Spectroradiometer measures the light spectrum directly and was crucial for making precise comparisons between the different display technologies. The Spectroradiometer and all of the displays (except for the Sony) were generously provided on a long-term loan basis by their manufacturers. We offer special thanks to all of them for agreeing to participate. It was especially challenging to get all of this high-end hardware together at the same time.
Gray Scale
In Part I we measured the extremes of display brightness: the black-level and peak intensity white. Here we're going to carefully examine all of the intensities in between, which is referred to as the display's Gray Scale. This is the signature of a display; it's what gives the display its own unique look and performance characteristics. While each display technology has its own native gray scale, known as a Transfer Characteristic, signal processing electronics within the display modifies this to produce the gray scale that we actually see (and measure). There are two reasons why this is necessary. First, the native gray scale for most display technologies is either unsuitable or sub-optimal for accurate image reproduction. Second, we need to have a standard gray scale so that images will be accurately reproduced on any display or display technology. The accepted standard is the CRT's own native gray scale. There are two reasons why the CRT is the standard: first, it was until recently the only prevalent display technology so new technologies had to mimic its behavior if they were to be accepted; second, it turns out that the CRT's native gray scale is actually very close to the ideal. We are incredibly lucky that the CRT came first and has served us well as an imaging device for over 75 years.
It turns out that the term Gray Scale is an unfortunate word choice because it actually describes the intensity scale for all colors, not just the grays, which are shades of white. As we'll see shortly, the shape of the gray scale has a major impact on not only image brightness and contrast, but also on hue and color saturation.
Before going any further we first need to define exactly what is meant by the gray scale. It is the brightness or amount of visible light that a display produces for a given level of input signal. (This applies for every pixel in the image.) For example, a maximum signal produces peak white and a zero signal produces the closest approximation to black that the display can produce. We measured the gray scale using a set of DisplayMate test patterns with a Minolta CS-1000 Spectroradiometer (we recommend the Kayye and Multimedia Editions of DisplayMate for the ProAV market). As we increase the signal from zero to maximum the display brightness also increases in a particular way that we can measure and then plot on a graph. This graph of brightness versus signal intensity level is called the Gray Scale. The input signal can be specified in many different and equivalent ways. For computers it's generally on a scale of 0 to 255, with 0 for black and 255 for peak white. For most digital video it's 16 for black to 235 for peak white and for analog video it's generally specified in IRE units, from either 0 or 7.5 for black to 100 for peak white. To simplify matters we'll describe the input signal intensity level as a scale going from 0 percent for black to 100 percent for peak white. The brightness scale will be luminance in cd/m2, the same as in Part I.
Gamma
Gamma is a popular, yet widely misunderstood number that describes the steepness of a display's gray scale as it increases from black to peak white. The gray scale is not linear as most people presume, but instead logarithmic (mathematically it's actually called a power-law, which behaves linearly on logarithmic scales) because that's how standard CRTs behave, and also because that corresponds well with the eye's own logarithmic response (which is also a power-law).
While you normally see the gray scale plotted as a linear graph, that's really not the proper graph to use. Here's why: what matters to the eye are ratios of brightness not differences in brightness. When comparing two intensities or mixing two colors it is their brightness ratio that determines what your eye sees. (Ratios are just divisions and differences are just subtractions between any two values.) A linear graph shows differences uniformly. Since the eye responds to brightness ratios we need a graph that displays ratios uniformly. That's just what a logarithmic graph does. You don't need any advanced math to understand logarithmic graphs, just pay attention to the scale values. They're arranged so that any given ratio corresponds to a fixed distance anywhere along the scale. For example, the distances between 80, 40, 20 and 10 (on the horizontal axis) are all the same because each is a 2:1 ratio.
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