Learning about LCD monitor gamma and colour reproduction.read more
In this article, we'll focus on colour temperature, a fundamental parameter in picture quality adjustments. While colour temperature dramatically affects the picture quality of an LCD monitor, more often than not, people simply use the default settings. A good understanding of the meaning of colour temperature will enable better adjustments of LCD monitor picture quality.
Note: Below is the translation from the Japanese of the ITmedia article "Altering a colour dramatically with a single setting: Examining colour temperature on an LCD monitor" published March 30, 2009. Copyright ITmedia Inc. All Rights Reserved.
Why is "temperature" used to describe colour?
Most of today's LCD monitors feature colour-temperature adjustment options in their OSD menus. Since colour temperature settings affect colour reproduction significantly on an LCD monitor, if a user wants to display an image with the appropriate colour cast, he or she must choose the correct colour temperature.
We'll start with a brief explanation of the meaning of colour temperature. Colour temperature refers to the colour of light, serving as the standard index for colour balance for a range of products, including monitors, cameras, and lighting equipment. Colour temperature is specified in units of Kelvin (K) of absolute temperature, not the degrees Celsius (C) used to express the temperature of air and other materials. While Kelvin is less familiar that Celsius, it should present no problems if we keep the following two basic points in mind: the lower the Kelvin value for colour temperature, the redder a white object appears; the higher the colour temperature, the bluer it appears.
The tables below indicate rough colour temperatures for various lighting sources, including sunlight. As you can probably guess, lower colour temperatures mean redder light, while higher temperatures mean bluer light. Most photographers shooting pictures with digital SLR cameras might set their white balance to 5000-5500 K. Since daylight has a colour temperature of 5000-5500 K, setting the white balance to this figure makes it possible to capture photos with colour reproduction close to that perceived by the eye.
A diagram of colour temperature. As colour temperature decreases, white becomes yellow, then red. As colour temperature increases, white gradually turns to blue. Note that this diagram is merely a rough representation of how to think about colour temperatures, not a precise indication of colour temperatures under specific conditions.
|Clear sky||12.000 K|
|Shade on clear sky||8.000 K|
|Cloudy sky||6.500 K|
|Average noon sunlight||5.300 K|
|Two hours after sunrise||4.500 K|
|One hour after sunrise||3.500 K|
|Sunrise, sunset||2.000 K||Referenz-Farbtemperatur 1 (Sonnenlicht)|
|Light source||Colour temperature|
|Fluorescent light: daylight||6.500 K|
|Fluorescent light: day white||5.000 K|
|Fluorescent light: white||4.200 K|
|Fluorescent light: warm white||3.500 K|
|Fluorescent light: soft white||3.000 K|
|Incandescent light||3.000 K|
|Candlelight||2.000 K||Referenz-Farbtemperatur 2 (z. B. Kunstlicht)|
Colour is expressed as a temperature due to the relationship between the colour of light and temperatures when objects are heated to high temperatures. Here we'll touch briefly on the technical definition of colour temperature. First, assume a subject that can completely absorb heat and light, then radiate this energy back out. This object (an idealized object, not one encountered in reality) is a black body, or perfect radiator. Second, assume that this black body radiates light when heated and that the wavelength and spectrum of this light varies with the temperature of the black body. Third, assume that the temperature of the black body when it radiates a certain colour of light is also understood to describe that colour. This is how colour temperature is defined.
While any object will radiate various light frequencies when heated to high temperature, the temperature at which the light becomes a certain colour differs from object to object. For this reason, a black body is an idealized object, used to generate standard values by matching specific colours of radiated light to specific temperatures. While this is a complex topic with detailed explanations grounded in physics and mathematics, we do not need to understand this in depth to adjust the colour temperature of an LCD monitor. Anyone with a deeper interest is encouraged to consult reference works.
Colour temperature for LCD monitors
As mentioned in passing at the start of this session, most current LCD monitors allow users to adjust colour temperatures using the OSD menu. As we would expect, reducing the colour temperature on an LCD monitor gives the entire screen an increasingly reddish cast, while increasing the colour temperature makes the colour cast increasingly blue. The menu items for adjusting colour temperature vary from product to product. Some ask users to choose from terms like "blue" and "red" or "cool" and "warm"; others ask users to set numerical values like 6500 K or 9300 K.
If the options for selecting colour temperature are "blue" and "red" or "cool" and "warm," choose "red" or "warm" to lower the colour temperature and "blue" or "cool" to raise the colour temperature. While these options make it easier to understand how the eye will sense the colour white, since the user is not given specific Kelvin values, they can be inconvenient when trying to adjust the monitor to a specific colour temperature.
It helps to be able to specify precise Kelvin values when we adjust the picture quality of an LCD monitor. For example, on most EIZO LCD monitors, users can choose from about 14 levels (in 500-K intervals from 4000 to 10,000 K, plus 9300 K). This is industry-leading precision. Some other LCD monitors allow users to designate colour temperature by Kelvin value. Most offer significantly fewer options in the OSD menu: 5000, 6500, and 9300 K, for example.
On most EIZO LCD monitors, users can adjust colour temperature precisely from the OSD menu in 500-K intervals (photo at left). Using the bundled ScreenManager Pro software for LCD monitors to configure various display settings from the PC, users can easily adjust colour temperatures simply by moving the position of a slider at the top of the screen (photo at right).
Ideally, due to the need to choose the optimal colour temperature corresponding to individual applications and circumstances, we should be able to adjust colour temperature using Kelvin values. Some major real-world examples are given below.
A colour temperature of 6500 K is standard for ordinary PC use and for the sRGB standard. Most LCD monitors offer a setting of 6500 K among their colour temperature options. If a monitor offers an sRGB mode, setting it to this mode should present no problems. In most cases, even products whose colour-temperature settings use terms like "blue" and "red" will be adjusted to close to 6500 K for standard mode, although accuracy may be lacking. The LCD monitors on some laptop PCs are set to higher colour temperatures.
In the field of video imaging—television, for example—the current standard under Japanese broadcasting standards (NTSC-J) is 9300 K. This is significantly above the 6500 K standard for PC environments. Television pictures actually have a pronounced blue cast. However, most people appear to be accustomed to television and often perceive PC screens as having a reddish cast. Some products offer a picture mode with a colour temperature around 9300 K as a "movie" or similar mode. When viewing the picture from a television tuner in a PC environment, one can generally choose a colour temperature of 9300 K for colour reproduction similar to a home television display.
On the other hand, the U.S. broadcasting standard (NTSC) calls for a colour temperature standard of 6500 K. The international standard for digital high-definition television (ITU-R BT.709) also specifies a colour temperature of 6500 K. When watching video on a PC, users should set the LCD monitor's colour temperature between 6500 K and 9300 K, checking for differences in colour reproduction.
As a rule of thumb, most Japanese film titles assume a 9300 K environment, while non-Japanese films assume a 6500 K environment. This means one is highly likely to achieve colour reproduction close to that intended by filmmakers by setting the colour temperature of an LCD monitor to 9300 K when viewing a Japanese film and 6500 K when viewing a non-Japanese film. (Naturally, this doesn't apply universally.) When using a model with a wide range of choices in Kelvin values—an Eizo Nanao LCD monitor, for example—users can adjust the colour temperature to whatever looks best.
A colour temperature of 5000 K (D50) is standard in the field of desktop publishing (DTP) for printing or publishing. This is the colour temperature recommended for lighting by the Japanese Society of Printing Science and Technology when evaluating colours for print applications. While this standard might give a distinct reddish cast to whites in pictures prepared to the standards of television video or similar images, it is intended to reproduce the look printed colours have when viewed under conditions close to direct sunlight.
Sample display of white under the colour temperatures 5000, 6500, and 9300 K (from left). Since the photo was shot with the colour temperature of the digital camera set to 6500 K, white in the 6500 K image in the centre appears pure white. It appears red in the 5000 K image and blue in the 9300 K image. Naturally, when changing the colour temperature setting for the camera, the look of whites in those images will be shifted accordingly: the image with a colour temperature lower than the set value will appear reddish and the one with a higher colour temperature will look bluish.
Sample colour bars displayed at colour temperatures 5000, 6500, and 9300 K (from left). The photo was shot under the same conditions as the photo above. As colour temperatures change, the apparent colour of the white, or the overall colour balance, is affected. Colours at lower colour temperatures tend to appear warm; at higher colour temperatures, they tend to appear cool.
Special tools are needed to adjust colour temperature precisely
The preceding page explained the basics needed to set the correct colour temperature based on the intended application. However, for applications like retouching digital photographs or colour adjustments for printing or video editing, where users are professionals or high-end amateurs for whom colour reproduction significantly affects the final quality of the work, managing LCD colour temperatures with greater accuracy is critical. If colours differ between the output of photo retouching and the colour reproduction in printing, or colours appear unnatural when a video is viewed on another computer, it could not only impair the work itself, but also significantly reduce the efficiency of image processing.
Addressing these demands adequately requires an LCD monitor that supports colour management based on hardware calibration. A hardware calibration system uses a colour sensor to measure colours on screen and controls the look-up table (LUT) in the LCD monitor directly. This makes it possible to correct for differences in colour temperature attributable to differences between individual LCD monitor units or to an aging display and to generate accurate colours, an important feature when handling colour.
Here we'll use an EIZO LCD monitor with a good reputation for enabling high-precision colour management to briefly explain the knowledge and specialised tools required to work with colour temperatures at a deeper level. We also recommend reading the articles below for more information on hardware calibration, colour gamut, and look-up tables.
EIZO offers the ColorEdge series of colour management-capable LCD monitors. All models in the ColorEdge series support hardware calibration, allowing users to manage in detail all aspects of colour reproduction, including screen colour temperature and colour gamut.
Designed for advanced colour management, the ColorNavigator software is bundled with all models in the ColorEdge series. ColorNavigator offers a wide range of functions, including a function for matching the colour temperature of the LCD monitor with the white of a particular paper. Using a colour sensor (sold separately), users can measure a white point on the paper and set this to white when performing a hardware calibration of the LCD monitor. This makes it possible to precisely match the on-screen white and the paper white, ensuring that colours on screen are very close to those on the printed paper.
ColorNavigator also offers an advanced function for emulating any colour gamut. This lets users reproduce on screen, with high precision, the Adobe RGB, sRGB, or NTSC colour gamut, using a wide colour-gamut panel. ColorNavigator can also be set to emulate colour gamuts by reading existing ICC profiles, rather than relying on preset software gamuts. For example, for commercial applications, emulating the client's LCD monitors using their ICC profiles lets users streamline the colour-proofing workflow by reproducing the colour reproduction of the client's monitors on a ColorEdge monitor.
ColorNavigator also features functions that encourage users to perform periodic hardware calibration of their LCD monitors and to maintain accurate colour reproduction through precise manual adjustments. Since screen brightness and colour reproduction change as a monitor is used over many years, colour temperatures will also change. In applications for which accurate colour reproduction is paramount, merely selecting preset colour-temperature settings is not enough. It's a good idea to perform hardware calibration once a month or so.
ColorNavigator software is designed for use with the ColorEdge series.
Using lighting and LCD hoods to improve the colour work environment
In addition to adjusting LCD monitors with special tools like ColorNavigator or EasyPIX, one should closely examine worksite (environmental) lighting and LCD hoods.
Most worksites use fluorescent lighting. Some fluorescent lighting is suitable for working with colour; others are not. The majority of fluorescent lights sold to the general public are not suitable for colour work. Ordinary fluorescent lights have a highly biased light spectra, and colour divergence is readily apparent if we compare the LCD monitor screen to paper. Accurately printed colours, for example, may appear greenish under fluorescent light.
Fluorescent lights suitable for working with colour are known as high colour-rendering fluorescent lamps or fluorescent lights for colour evaluation. These lamps feature light spectra similar to the sun and generate very little colour divergence between the LCD monitor screen, printed paper, and human colour recognition. Colour rendering describes the colour an object appears to have under a certain light. Colour-rendering performance is expressed in terms of the average colour-rendering index (Ra). An Ra value of 100 means the lighting is identical to natural light. The closer the value to Ra, the higher the colour-rendering performance. The International Commission on Illumination (CIE) recommends fluorescent lighting with an Ra of 90 or above at locations where art is viewed or colours evaluated.
Most high colour-rendering fluorescent lamps are tubes, making them difficult to use in most homes without modification. In these cases, we recommend three-wavelength fluorescent lamps, which offer relatively high colour-rendering performance for fluorescent lamps and are readily available to the general public. To determine if a fluorescent lamp is a three-wavelength model, simply look at the lamp's specifications. With respect to the colour temperature of the fluorescent lamp itself for evaluating printed materials, a daylight lamp (4600-5400 K) is ideal.
An LCD hood is attached to the top and sides of an LCD monitor to reduce the effects of environmental lighting on the screen display and to make it possible to view the true screen colours while working.
We've examined some basic aspects of colour temperature and of using and adjusting colour temperatures on an LCD monitor. The colour cast of an LCD monitor varies dramatically with colour temperature settings—the difference is hard to miss. If you've used nothing but your monitor's default settings up to this point, we encourage you to explore the OSD menu and see how colours change at different colour temperature settings. While 6500 K, sRGB mode, or "standard" mode is recommended for general PC use, you might find that you prefer a different colour temperature for watching films, playing computer games, or other uses.