Technical Link 技術鏈接

COLOR REPRODUCTION

Products may be printed in two or more colors by almost all of the processes described earlier. The simplest method of color reproduction is flat-color printing. A separate plate is prepared for each color desired. Plates are then printed in sequence on a single sheet of paper. Each plate is covered with the desired color ink and then printed in register with all of the other color images.

The process-color printing method is used to reproduce full-color pictures such as the colored continuous tone photographs. Process-color activities include: color separation, color correction and color reproduction.

A beam of white light passed through a glass prism results in a rainbow of color. This is because white light is really a mixture of all the colors of light. Each color refracts or bends differently as it enters and leaves the prism, Figure 5-1.

Three colors of light - red, blue, and green - can be used to reproduce white light, Figure 5-2. Where all three colors of light overlap, white is produced. Red, blue, and green are called additive primary colors because added together they form white light.

Where any two additive primary colors of light overlap, a third color is formed. Red and blue combine to form magenta. Blue and green form cyan. Red and green combine to produce yellow. Magenta, cyan, and yellow are called subtractive primary colors because they subtract colors from white light to form black. The subtractive process is shown in Figure 5-3.

Subtractive primary color inks are transferred to the paper when the plates made from the separation negatives are printed. Magenta (blue+red) ink is printed by the plate made from the green separation negative. Cyan (blue+green) ink is printed by the plate made from the red separation negative. Yellow (red+green) ink is printed by a plate made from the blue separation negative.

Process-color prints are made by printing yellow, magenta, and cyan plates in perfect register on a single sheet of paper. Figure 5-3 also shows how additive primary colors are created by overprinting pairs of subtractive primary color inks. By controlling the amounts of yellow, magenta, and cyan inks, you can produce all other colors of the visible spectrum.

In theory, only the three subtractive primary colors of ink are necessary to reproduce all colors of the spectrum. This is not true in actual practice, however. A fourth ink, black is needed to improve the grays and shadows in the print.

色彩復制

用前述幾乎任何一種印刷工藝都可印刷雙色或多色產品。一種最簡單的彩色復制方法是實地套色印刷。所印的每一種顏色都要制作一塊單獨的印版。隨后印版按照順序在一張單張紙上印刷。每一種印版都涂上要印的顏色的油墨，并與其他顏色的圖文套準印刷在一起。

三原色彩色印刷方法用來制作全色彩圖畫，比如連續調彩色照片。三原色復制過程包括：分色、色彩校正及色彩復制。

一束白光通過玻璃棱鏡會產生彩虹的效果。這是因為白光實際上是由所有顏色的光合成的。每一種色光在進入和離開棱鏡時折射和彎曲的程度都不同。如圖5-1所示。

紅、藍、綠三種色光可用來形成白光，如圖5-2所示。只要把這三種色光疊加起來，白光即可產生。因為紅、綠、藍三種顏色疊加在一起形成白光，所以人們把它們叫做加色三原色。

任何兩種加色光疊加都能得到第三種顏色。紅色和藍色合成品紅色；藍色和綠色合成青色；紅色和綠色形成黃色。品紅、青、黃叫做減色三原色，因為它們從白光減掉色光后形成了黑色。如圖5-3所示的是減色過程。

使用由分色陰圖片制得的印版印刷時，減色三原色油墨被轉移到紙上。品紅（藍＋紅）油墨用在由綠色分色陰圖片制成的印版上印刷。青色（藍＋綠）油墨用在由紅分色陰圖片制成的印版上印刷。黃色（紅＋綠）油墨用由藍分色陰圖片制得的印版印刷。

三原色彩色印刷是由黃、品紅、青三色印版在同一張紙上準確套印而成的。圖5-3也顯示了加色三原色是怎樣由減色三原色油墨兩兩疊印而成的。通過控制黃、品紅、青三色的墨量，可以得到可見光譜內所有其他顏色。

從理論上講。只要用減色三原色油墨就可復制光譜內的所有顏色。然而，實際上并非如此。還有第四種油墨——黑色，被用來增進印品的灰色調和陰影部分。