Problems like extremely poor display of shadow areas, blown-out highlights, or images prepared on Macs appearing too dark on Windows computers are often due to gamma characteristics. In this session, we'll discuss gamma, which has a significant impact on colour reproduction on LCD monitors. Understanding gamma is useful in both colour management and product selection. Users who value picture quality are advised to check this information.

Below is the translation from the Japanese of the ITmedia article "Is the Beauty of a Curve Decisive for Colour Reproduction? Learning About LCD Monitor Gamma" published July 13, 2009. Copyright 2011 ITmedia Inc. All Rights Reserved.


Just what is monitor gamma?

The term gamma comes from the third letter of the Greek alphabet, written Γ in upper case and γ in lower case. The word gamma occurs often in everyday life, in terms like gamma rays, the star called Gamma Velorum, and gamma-GTP. In computer image processing, the term generally refers to the brightness of intermediate tones (grey).

Let's discuss gamma in a little more detail. In a PC environment, the hardware used when working with colour includes monitors, printers, and scanners. When using these devices connected to a PC, we input and output colour information to and from each device. Since each device has its own unique colour handling characteristics (or tendencies), colour information cannot be output exactly as input. The colour handling characteristics that arise in input and output are known as gamma characteristics.

The colour reproduced on a PC monitor is based on a combination of three primary colours: red (R), green (G), and blue (B). Each RGB colour has eight bits (28 = 256 tones) of data. Approximately 16.77 million colours (known as "full colour") result from 256 x 3 (256 R tones * 256 G tones * 256 B tones).

While certain monitors are also compatible with colour handling at 10 bits per RGB colour (210 = 1024 tones), or 1024 x 3 (approximately 1,064,330,000 colours), operating system and application support for such monitors has lagged. Currently, some 16.77 million colours, with eight bits per RGB colour, is the standard colour environment for PC monitors. 

When a PC and a monitor exchange colour information, the ideal is a relationship in which the eight-bit colour information per RGB colour input from the PC to the monitor can be output accurately—that is, a 1:1 relationship for input:output. However, since gamma characteristics differ between PCs and monitors, colour information is not transmitted according to a 1:1 input:output relationship.

How colours ultimately look depends on the relationship resulting from the gamma values (γ) that numerically represent the gamma characteristics of each hardware device. If the colour information input is represented as x and output as y, the relationship applying the gamma value can be represented by the equation y = xγ.

Gamma characteristics are represented by the equation y = xγ. At the ideal gamma value of 1.0, y = x; but since each monitor has its own unique gamma characteristics (gamma values), y generally doesn't equal x. The above graph depicts a curve adjusted to the standard Windows gamma value of 2.2. The standard gamma value for the Mac OS is 1.8.

idealer Gamma-Wert vs. Windows-Standard vs. MacOS-Standard

Ordinarily, the nature of monitor gamma is such that intermediate tones tend to appear dark. Efforts seek to promote accurate exchange of colour information by inputting data signals in which the intermediate tones have already been brightened to approach an input:output balance of 1:1. Balancing colour information to match device gamma characteristics in this way is called gamma correction.

A simple gamma correction system. If we account for monitor gamma characteristics and input colour information with gamma values adjusted accordingly (i.e., colour information with intermediate tones brightened), colour handling approaches the y = x ideal. Since gamma correction generally occurs automatically, users usually obtain correct colour handling on a PC monitor without much effort. However, the precision of gamma correction varies from manufacturer to manufacturer and from product to product (see below for details).

Farbverwaltung zur Annäherung an den Idealwert „y = x“

The gamma relationship between the operating system and the LCD monitor

In most cases, if a computer runs the Windows operating system, we can achieve close to ideal colours by using a monitor with a gamma value of 2.2. This is because Windows assumes a monitor with a gamma value of 2.2, the standard gamma value for Windows. Most LCD monitors are designed based on a gamma value of 2.2.

The standard monitor gamma value for the Mac OS is 1.8. The same concept applies as in Windows. We can obtain colour reproduction approaching the ideal by connecting a Mac to a monitor configured with a gamma value of 1.8.

Bild bei Gammawert von 2,2 und 1,8

An example of the same image displayed at gamma values of 2.2 (photo at left) and 1.8 (photo at right). At a gamma value of 1.8, the overall image appears brighter. The LCD monitor used is EIZO's 20-inch wide-screen EV2023W FlexScan model (ITmedia site).

EIZO's LCD monitors allow users to configure the gamma value from the OSD menu, making this procedure easy. In addition to the initially configured gamma value of 2.2., one can choose from multiple settings, including the Mac OS standard of 1.8.

Konfiguration des Gammawerts im OSD-Menü bei EIZO Monitoren

To digress slightly, standard gamma values differ between Windows and Mac OS for reasons related to the design concepts and histories of the two operating systems. Windows adopted a gamma value corresponding to television (2.2), while the Mac OS adopted a gamma value corresponding to commercial printers (1.8). The Mac OS has a long history of association with commercial printing and desktop publishing applications, for which 1.8 remains the basic gamma value, even now. On the other hand, a gamma value of 2.2 is standard in the sRGB colour space, the standard for the Internet and for digital content generally, and for Adobe RGB, the use of which has expanded for wide-gamut printing.

Given the proliferating use of colour spaces like sRGB and Adobe RGB, plans call for the latest Mac OS scheduled for release by Apple Computer in September 2009, Mac OS X 10.6 Snow Leopard, to switch from a default gamma value of 1.8 to 2.2. A gamma value of 2.2 is expected to become the future mainstream for Macs.


Internal gamma correction to improve LCD monitor tones

On the preceding page, we mentioned that the standard gamma value in a Windows environment is 2.2 and that many LCD monitors can be adjusted to a gamma value of 2.2. However, due to the individual tendencies of LCD monitors (or the LCD panels installed in them), it's hard to graph a smooth gamma curve of 2.2. 

Traditionally, LCD panels have featured S-shaped gamma curves, with ups and downs here and there and curves that diverge by RGB colour. This phenomenon is particularly marked for dark and light tones, often appearing to the eye of the user as tone jumps, colour deviations, and colour breakdown.

The internal gamma correction feature incorporated into LCD monitors that emphasize picture quality allows such irregularity in the gamma curve to be corrected to approach the ideal of y = x γ. Device specs provide one especially useful figure to help us determine whether a monitor has an internal gamma correction feature: A monitor can be considered compatible with internal gamma correction if the figure for maximum number of colours is approximately 1,064,330,000 or 68 billion or if the specs indicate the look-up table (LUT) is 10- or 12-bit.

An internal gamma correction feature applies multi-gradation to colours and reallocates them. While the input from a PC to an LCD monitor is in the form of colour information at eight bits per RGB colour, within the LCD monitor, multi-gradation is applied to increase this to 10 bits (approximately 1,064,330,000 colours) or 12 bits (approximately 68 billion colours). The optimal colour at eight bits per RGB colour (approximately 16.77 million colours) is identified by referring to the LUT and displayed on screen. This corrects irregularity in the gamma curve and deviations in each RGB colour, causing the output on screen to approach the ideal of y = x γ.

Let's look at a little more information on the LUT. The LUT is a table containing the results of certain calculations performed in advance. The results for certain calculations can be obtained simply by referring to the LUT, without actually performing the calculations. This accelerates processing and reduces the load on a system. The LUT in an LCD monitor identifies the optimal eight-bit RGB colours from multi-gradation colour data of 10 or more bits.

grayscale
Left: colour seepage (1) and tonality breakup (2). Right : smooth greyscale display
ideale Gamma-Kurve vs. unkorrigierte Kurve
red = ideal gamma curve; blue = uncorrected gamma curve
Ideale Gammakurve
red = ideal gamma curve; blue = corrected gamma curve

An overview of an internal gamma correction feature. Eight-bit RGB colour information input from the PC is subjected to multi-gradation to 10 or more bits. This is then remapped to the optimal eight-bit RGB tone by referring to the LUT. Following internal gamma correction, the results approach the ideal gamma curve, dramatically improving on screen gradation and colour reproduction.

EIZO's LCD monitors proactively employ internal gamma correction features. In models designed especially for high picture quality and in some models in the ColorEdge series designed for colour management, eight-bit RGB input signals from the PC are subjected to multi-gradation, and calculations are performed at 14 or 16 bits. A key reason for performing calculations at bit counts higher than the LUT bit count is to improve gradation still further, particularly the reproduction of darker tones. Users seeking high-quality colour reproduction should probably choose a monitor model like this one.


Checking the gamma value of an LCD monitor

In conclusion, we've prepared image patterns that make it easy to check the gamma values of an LCD monitor, based on this session's discussion. Looking directly at your LCD monitor, move back slightly from the screen and gaze at the following images with your eyes half-closed. Visually compare the square outlines and the stripes around them, looking for patterns that appear to have the same tone of grey (brightness). The pattern for which the square frame and the striped pattern around it appear closest in brightness represents the rough gamma value to which the monitor is currently configured.

Based on a gamma value of 2.2, if the square frame appears dark, the LCD monitor's gamma value is low. If the square frame appears bright, the gamma value is high. You can adjust the gamma value by changing the LCD monitor's brightness settings or by adjusting brightness in the driver menu for the graphics card.

Naturally, it's even easier to adjust the gamma if you use a model designed for gamma value adjustments, like an EIZO LCD monitor. For even better colour reproduction, you can set the gamma value and optimise colour reproduction by calibrating your monitor.

Gamma Test