彩色印刷品常采用CMYK四種原色油墨，以不同的網點比例印刷在紙上，就得到了各種印刷色彩。為了檢驗印刷品的質量，就可以使用密度儀對印刷油墨密度、網點進行測量。

  印刷色彩與網點是什么？

  印刷色譜是印刷行業顏色系統。彩色印刷通常采用青（C）、品紅（M）、黃（Y）和黑（K）四種原色油墨，這四種原色油墨以不同的網點比例印刷在紙上，就得到了各種印刷顏色，各種印刷網點形成顏色的總和就構成了印刷顏色系統，通常有稱為CMYK顏色空間，因此印刷的顏色系統也是一種混色系統。在印刷制作過程中和設計印刷版面時，經常需要知道印刷特定顏色時所需要的原色網點比例，也經常需要知道所設計的原色網點比例印刷后是什么效果，在這種情況下都需要對照印刷色譜來確定顏色。

  油墨在紙張上以網點的形式存在，各網點之間有兩種個相互的存在關系：網點疊合和網點并列。疊合部分的網點以色料減色法原理形成顏色，并列部分的網點以色料減色法原理和色光加色法原理呈現顏色。網點疊合形成的顏色又與某一網點并列呈現新的顏色。網點疊合發生在中間調以上的暗調區域，而網點并列主要發生在中間調以下的亮調區域。

  當黃、品紅、青三種原色油墨互相疊印時，形成二次色紅、綠、藍和三次色黑。因此網點疊合形成二次色和三次色，網點并列時，反射回的色光即該網點原色油墨的顏色。并列后所形成的新顏色與其他網點顏色之間仍然是并列的關系。當網點非常小，觀察的距離又足夠大，眼睛不能觀察到單獨的小網點存在，就發生了顏色相加的混色效果，各種顏色小網點根據比例形成各種各樣的混合色。

  如何檢測印刷品的質量？

  在印刷中，GATF彩色六角形中，互不相鄰的三個角上是理想的三原色（YMC），另外三個角上是疊印的復合色（RGB），中心的為中性色。彩色六角形的中心密度為零，自中心向頂點密度逐級增加，通過C、M、Y、R、G、B的位置關系可以直觀地判斷色相偏移等現象。此外，采用了25%、50%、75%和實地四個階梯組成的4個環進行研究，通過它們之間的相對位置關系，可以直觀地判斷印刷反差等屬性。

  基于密度的印刷品質量評測技術是目前用于印刷品質量檢測與控制的主要手段之一，通過使用反射密度計測量不同網點覆蓋率下的網點密度和實地密度，采用顏色六角形對印刷品質量進行評測。

  密度計對四色印刷品的密度測量：

  對于單色測量來說，單色密度儀的設計，只需使用一個符合明視覺光譜光效率函數V(λ)濾即可，所測密度值為視覺密度。彩色印刷測量的是彩色密度，需要獲得圖像上某一色相的濃淡層次的定量表示，即顏色的飽和度。由于彩色印刷采用紅、綠、藍的補色青、品紅、黃來控制進入眼睛的紅、綠、藍光的數量，用光學密度表示彩色，就是要記錄下被測物體對紅/綠/藍三種光線的透過或反射的光量，換算成物體的反射或透射率，取其倒數的負對數，來表示顏色的深淺，反射或透過的光量不同，相應的紅/綠/藍密度數據不同。能夠實現這種測量的儀器是光學密度儀，稱為彩色密度儀。用它可以測量印刷品的分色密度。彩色密度計分為透射密度儀和反射密度儀兩類，透射密度儀用于測量膠片上的密度;反射密度儀可以測量的是樣品對紅、綠、藍光的吸收量，因此在光電探測器前要分別放置紅、綠、藍和視覺密度濾，用來分別透過紅、綠、藍光，或者將濾置于光源后面，用來產生紅、綠、藍照明光。由于青、品紅、黃油墨分別吸收紅、綠、藍光，并且墨層厚度越大，對紅、綠、藍光的吸收就越多。所以，測量經過油墨吸收后剩余的紅、綠、藍光量，就可以得到油墨的密度值，間接得到墨層的厚度。

      Color prints often use CMYK four primary color inks and print them on paper with different dot ratios to obtain various printing colors. In order to check the quality of printed matter, a densitometer can be used to measure the density and dots of the printing ink.
  What are the printing colors and dots?
  Printing color spectrum is the color system of the printing industry. Color printing usually uses four primary color inks of cyan (C), magenta (M), yellow (Y) and black (K). These four primary color inks are printed on paper with different dot ratios to obtain a variety of printing Color, the sum of the colors formed by various printing dots constitutes the printing color system, which is usually called the CMYK color space, so the printed color system is also a color mixing system. In the printing production process and when designing the printing layout, it is often necessary to know the primary color dot ratio required for printing a specific color, and it is often necessary to know the effect of the designed primary color dot ratio after printing. In this case, you need to control printing. Color spectrum to determine color.
  Ink exists in the form of dots on the paper, and there are two mutual relations between the dots: dot overlap and dot juxtaposition. The dots of the superimposed part form colors based on the principle of pigment subtractive method, and the dots of the parallel part show colors based on the principle of pigment subtractive color method and color light additive method. The color formed by the overlapping of the dots and a certain dot side by side presents a new color. The overlap of dots occurs in the dark areas above the midtones, while the juxtaposition of dots mainly occurs in the bright areas below the midtones.
  When the three primary colors of yellow, magenta, and cyan ink are superimposed on each other, the secondary colors red, green, blue and the tertiary black are formed. Therefore, the dots are superimposed to form the secondary color and the tertiary color. When the dots are aligned, the reflected color light is the color of the primary ink of the dot. The new color formed after juxtaposition is still in juxtaposition with the colors of other dots. When the dots are very small and the observation distance is large enough, the eyes cannot observe the existence of individual small dots, and the color mixing effect of color addition occurs. The small dots of various colors form a variety of mixed colors according to the proportion.
  How to check the quality of printed matter?
  In printing, in the GATF color hexagon, the three non-adjacent corners are ideal three primary colors (YMC), the other three corners are overprinted composite colors (RGB), and the center is a neutral color. The center density of the colored hexagon is zero, and the density increases step by step from the center to the vertex. The hue shift and other phenomena can be intuitively judged through the positional relationship of C, M, Y, R, G, and B. In addition, 4 rings composed of 25%, 50%, 75% and four steps on the ground are used for research. Through the relative position relationship between them, the printing contrast and other attributes can be judged intuitively.
  Density-based printing quality evaluation technology is currently one of the main methods used for printing quality inspection and control. It uses a reflection densitometer to measure dot density and solid density under different dot coverage, and uses color hexagons to evaluate printing quality.
  Density measurement of four-color printed matter by density meter:
  For monochromatic measurement, the design of the monochromatic densitometer only needs to use a filter that conforms to the optical efficiency function V(λ) of the photopic vision spectrum, and the measured density value is the visual density. Color printing measures color density, and it is necessary to obtain a quantitative expression of the shades of a certain hue on the image, that is, the saturation of the color. Since color printing uses the complementary colors of red, green, and blue, cyan, magenta, and yellow to control the amount of red, green, and blue light entering the eyes, the optical density is used to express the color, which is to record the red/green/blue three The amount of transmitted or reflected light of a kind of light is converted into the reflection or transmittance of the object, and the negative logarithm of the reciprocal is taken to indicate the depth of the color. The amount of light reflected or transmitted is different, and the corresponding red/green/blue density data different. The instrument that can achieve this measurement is an optical densitometer, called a color densitometer. It can be used to measure the color separation density of printed matter. Color densitometers are divided into transmission densitometers and reflection densitometers. The transmission densitometer is used to measure the density on the film; the reflection densitometer can measure the absorption of red, green and blue light by the sample, so it is in front of the photodetector Place the red, green, blue, and visual density filters separately to transmit red, green, and blue light separately, or place the filters behind the light source to produce red, green, and blue illuminating light. Since the cyan, magenta, and yellow inks absorb red, green, and blue light respectively, and the greater the thickness of the ink layer, the more red, green, and blue light are absorbed. Therefore, by measuring the remaining amount of red, green, and blue light after the ink is absorbed, the density value of the ink can be obtained, and the thickness of the ink layer can be obtained indirectly.