色温
维基百科，自由的百科全书
色温是可见光在摄影、录象、出版等领域具有重要应用的特征.。光源的色温是通过对比它的色彩和理论的热黑体辐射体来确定的。热黑体辐射体与光源的色彩相匹配时的开尔文温度就是那个光源的色温, 它直接和普朗克黑体辐射定律相联系。
目录 |
[编辑] 不同光照的分类
Because it is the standard against which other light sources are compared, 黑体辐射体的色温等于它表面的开尔文温度, 使用了以19世纪英国物理学家威廉 汤姆逊, 1st Baron 开尔文的名字命名的温标。(标注：it should not be construed that the color temperature refers to the thermal temperature of anything other than the black-body radiator.) 白炽灯就非常接近于一个黑体辐射体。 However, 许多其他光源，诸如荧光灯，do not emit radiation in the form of a black-body curve, and are assigned what is known as a 校正色温 (CCT), which is the 色温 of a 黑体 which most closely matches the lamp's perceived color. Because such an approximation is not required for 白炽灯, the CCT for an 白炽灯 is simply its unadjusted 开尔文 value, derived from the comparison to a heated black-body radiator.
As the sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing colors of the sun over the course of the day is mainly a result of refraction and, to a lesser extent, scattering of light, and is unrelated to 黑体 radiation.
Even when the Sun is low over the horizon, we can estimate its apparent 色温 and correct it to compute its effective temperature. So, even if the sun looks red, and showing an apparent 色温 of 2500 K, a simple calculation can demonstrate that its effective temperature is in reality close to 5770 K.
天空的蓝色不是因为黑体辐射，而是由于阳光雷利散射 from the atmosphere which tends to scatter blue light more than red. 这一现象黑体的性质无关。
[Note this diagram is only a symbolic-representation; the colors shown have not been calculated with any colorimetric accuracy. A colorimetrically-accurate diagram is available.
一些常见的例子：
- 1700 K: 火柴光
- 1850 K: 蜡烛
- 2800 K: 钨灯 (白炽灯)
- 3350 K: Studio "CP" light
- 3400 K: Studio lamps, photofloods, 等...
- 4100 K: 月光
- 5000 K: 日光
- 5500 K: 平均日光, 电子闪光 (因厂商而异)
- 5770 K: 有效太阳温度
- 6420 K: 氙弧灯
- 6500 K: 日光°
- 9300 K: 电视屏幕 (模拟电视)
5000K和6500K are also called respectively D50 (美国标准) and D65 (欧洲标准) in all professions working with light (photographers, publishers, etc.). It is a temperature similar to the 黑体 temperature but not strictly identical.
灯泡的功率 (20或100瓦) seems to change its color but in reality it only affects its luminosity (luminance) to which our eyes are very sensitive.
From these observations, it becomes clear for all colors 基于黑体 that 蓝色是"更热"的颜色，同时事实上红色是"较冷"的颜色。This is the opposite of the cultural associations both colors have taken on, with "red" as "hot", and "blue" as "cold". The 传统的associations come from a variety of sources, such as water and ice appearing blue, while heated metal and fire are of a reddish hue. However, the redness of these heat sources comes precisely from the fact that red is the coolest of the 可见 colors: the first color emitted as heat increases. To see this, observe that while incandescent bulbs glow a reddish to yellowish color throughout their lifetimes, when one blows out, the flash of light is noticeably bluish - the filament is hotter when it burns out, as evidenced by the scorch mark often left on the glass.
"色温" is sometimes used loosely to mean "白平衡" or "white point". Notice that 色温 has only one degree of freedom, whereas white balance has two (R-Y and B-Y).
In photography, an alternative numerical measure used is the mired. 色温s and mireds are convertible to each other via a simple formula (see the mired page for details of the computations, and the reasons for the use of the alternative unit).
[编辑] 色温的应用
[编辑] 胶片摄影术
Film sometimes exaggerates the color of the light. An object that appears to the naked eye to be under white light may turn out looking very blue or orange in a photograph. The color balance may need to be corrected while shooting to achieve a neutral color print.
Film is made for specific 光源 (most commonly daylight film and 钨film), and used properly, will create a neutral color print. Matching the color sensitivity of the film to the 色温 of the light source is one way to balance color. If tungsten film is used indoors with 白炽灯, the 橘黄 light of the 钨丝[白炽灯] will appear as white (3200 K) in the photograph.
Filters on a camera lens, or color gels over the 光源) may also be used to correct color balance. When shooting with a bluish light (高温) source such as on an overcast day, in the shade, in window light or if using tungsten film with 白或者蓝光, a yellowish-orange filter will correct this. For shooting with daylight film (calibrated to 5600 K) under warmer (low temperature) light sources such as sunsets, candle light or tungsten lighting, a bluish (e.g. #80A) filter may be used.
荧光 light varies in color and may be harder to correct for. Because it is often greenish, a magenta filter may correct it, though this could take some trial and error.
If there is more than one light source with varied 色温s, gels (placed over each light source) in conjunction with daylight film is the best way to balance the color.
[编辑] 桌面出版
In the desktop publishing industry, it is important to know your monitor’s 色温. Color matching software, such as ColorSync will measure a monitor's 色温 and then adjust its settings accordingly. This enables on-screen color to more closely match printed color. Common monitor 色温s are as follows:
5000 K (D50), 5500 K (D55), 6500 K (D65), 7500 K (D75) and 9300 K.
Designations such as D50 are used to classify 色温 of light tables and viewing booths. When viewing a color slide at a light table, it is important that the light be balanced properly so that the colors are not shifted towards the red or blue.
General 电脑用户 should set their PC monitor color-temperature to "sRGB"o或者"6500K", as this is what 数码相机, 网页图像, 以及DVD等通常的设计。Indeed the sRGB standard stipulates (among other things) a 6500 K display whitepoint.
[编辑] 电视，视频和数码静止照相机
The NTSC and PAL TV norms call for a compliant TV screen to display an electrically "black-and-white" signal (minimal color saturation) at a 色温 of 6500K. On many actual sets however, especially older and/or cheaper ones, there is a very noticeable deviation from this requirement.
Most video and digital still cameras can adjust for 色温 by zooming into a white or neutral colored object and setting the manual "white balance" (telling the camera that "this object is white"); the camera then shows true white as white and adjusts all the other colors accordingly. White-balancing is necessary especially when indoors under 荧光 lighting and when moving the camera from one lighting situation to another. The setting called "Auto white balance" is not recommended for optimum quality video or stills.
[编辑] Artistic application via control of 色温
Experimentation with 色温 is obvious in many Stanley Kubrick films; for instance in Eyes Wide Shut the light coming in from a window was almost always conspicuously blue, whereas the light from lamps on end tables was fairly orange. Indoor lights typically give off a yellow hue; 荧光 and natural lighting tends to be more blue.
Video camera operators can also white-balance objects which aren't white, downplaying the color of the object used for white-balancing. For instance, they can bring more warmth into a picture by white-balancing off something light blue, such as faded blue denim; in this way white-balancing can serve in place of a filter or lighting gel when those aren't available.
Cinematographers do not "white balance" in the same way as video camera operators: they can use techniques such as filters, choice of film stock, pre-flashing, and after shooting, color grading (both by exposure at the labs, and also digitally, where digital film processes are used). Cinematographers also work closely with set designers and lighting crews to achieve their desired effects.
[编辑] 校正色温
The 开尔文 system for lamp description works well for an incandescent lamp. Since these are very nearly 黑体 radiators, their chromaticity coordinates land directly on the Planckian locus in the CIExy color space. 荧光 lighting is not incandescent and presents a new challenge. 荧光灯 are made using myriad combinations of phosphors and gases. The illumination that they produce is almost never described by a point in color space that lies on the Planckian locus.
The question then becomes how to describe the quality of light from a 荧光灯. The method used is called the "correlated 色温", which is a method for assigning a 色温 to a color near, but not on, the Planckian locus. The above plot shows lines crossing the Planckian locus for which the correlated 色温 is the same. Nevertheless, the colors are not the same, and the method gives only an approximate specification of a particular color. Due to this shortcoming, the rated CCT of any 荧光 tube does not completely specify its color.
To be more precise: A number of color spaces have been developed in which the distance between them on a chromaticity diagram may estimate the difference between two colors. These include the 1960 CIELuv (which is now outdated) and the 1976 CIELu'v' and CIELab spaces. On a chromaticity diagram for which distances specify color distances, the best estimate of the 色温 of any point will be the 色温 of the point on the Planckian locus closest to that point. Although it is outdated, the CIE specifies distances in the 1960 CIELuv chromaticity space to define correlated 色温.
Photographers often use 色温表。色温表 are designed to read only two regions along the 可见 spectrum (red & blue), more expensive ones read three regions (red, green & blue). They are almost useless under 荧光 light. There are general guidelines and some specific filters recommended to obtain optimum quality under such frustrating circumstances.
[编辑] 显色指数
The CIE developed a newer model for describing and rating light sources, called the color rendering index (CRI), which is a mathematical formula describing how well a light source's illumination of eight sample patches compares to the illumination provided by a reference source. The index provides a number up to 100 for ideal light. This index is useful in determining the suitability of illuminating spaces occupied by humans, since there are adverse health effects of over-illumination by artificial lights or by mismatch of natural light sources.
[编辑] Spectral power distribution plot
The spectral power distributions provided by many manufacturers may have been produced using 10 nanometre increments or more on their spectroradiometer. The result is what would seem to be a smoother (fuller spectrum) power distribution than the lamp actually has. 2nm increments are mandatory for taking measurements of 荧光 lights. Here is an example of just how different an incandescent lamp's SPD graphs compared to a 荧光灯:
[编辑] Recommendations for those without expensive equipment
Only those with expensive spectrophotometers and spectroradiometers can obtain accurate data. Those without these tools should take the time to review the lamp's specifications, and if they seem good, then the eyes are one's best tool.
New mathematical indices are being proposed. There are at least 3 new ways to categorize lamps more accurately. They are not in use as of yet. One is referred to as the color rendering capacity (CRC).
[编辑] 参见
- Luminous efficacy
- Over-illumination
[编辑] 参考书目
- Berns，Roy S. (2000). Billmeyer and Saltzman's Principles of Color Technology，3rd edition，New York: Wiley. ISBN 0-471-19459-X.
- Stroebel, Leslie; John Compton; Ira Current; Richard Zakia (2000). Basic Photographic Materials and Processes，2nd edition，Boston: Focal Press. ISBN 0-240-80405-8.
- Wyszecki, Günther; W. S. Stiles (1982). Color Science Concept and Methods, Quantitative Data and Formulae，New York: Wiley. ISBN 0-471-02106-7.
[编辑] 外部连接
- Charles Poynton's Color FAQ for the basics.
- Frequently asked questions about Color Physics - also includes history of the CIE color specification
- What color is a blackbody? - Colorimetrically-calculated blackbody RGB color values and comment
- Color-temperature: visualising blackbody radiation - Blackbody curves for typical lightsources, and colorimetrically-calculated blackbody color strip
- Specifying Color GE's "Learn About Light" pages
[编辑] Film- and video-related
- White Balance - Intro at nikondigital.org
- Understanding White Balance - Tutorial
- White Balance - Understanding its use in digital photography
