In China printing services, we understand color printing is the process of copy the color from artwork. Or can understand as that it is the process of transfer the color on the print matter from artwork. The artwork quality is one of the factor to affect book printing quality.

The artwork quality is depend on color balance and contrast two parts. We often say a good artwork without color cast, moderate contrast and rich in layers.

The color balance of book printing artwork

The artworks have chromatic aberration is the common issue. We observed the color cast often from two parts: Neutral gray and memory colors.

1. Neutral gray

The human eye is very sensitive to the color cast caused by the gray in the image. Because the neutral gray only changes in brightness, and it is easy to detect a slight color cast. Since the color around gray also affects the human eye's perception and judgment of it. Thus, at the same time of visual evaluation. It is necessary to check the gray balance data with the help of a colorimeter or densitometer. 

When detecting gray in an image (such as a white shirt). When using the L·A·B color mode, the lightness value L is not equal to zero. And the chromaticity values a and b should be equal or close to zero. If use the RGB color mode, the measured values of the three components of RGB in the gray area should be equal. If it is expressed in CMYK color mode, the three component values of CMY (dot area ratio) are not equal. And the proportional relationship is related to the ink and paper selected for book printing. As well as printing conditions, such as printing pressure. 

Gray balance data sheet to determine if gray balance is achieved. If the neutral gray casts a color cast, the entire printing artwork has a consistent color cast. If there is no gray tone in the image, look for the most familiar memory color in the image to judge the color cast or not.

2. Memory color

People have a deep memory in some color. Such as different races, different age, different gender skin color. Sky's blue, grassland's green, desert's yellow. And various of food's color, like fruits, vegetables, meat, etc. Once these colors happen color cast. The human eye can identify soon.

For the color value of memory color, we can trough three ways. One is through practice accumulation. Second, we can measure with a colorimetric tool. Third is, take the value of the color blocks on the color spectrum as a reference. And remember the numerical relationship of some memory colors. Such as the CMYK proportion of sky blue about C=60%, M=23%, Y=0, K=0. If increase the color value of sky blue, the sky is not bright blue, is overcast. For convenient to observed the color cast. We often placing the gray scale ruler under the artwork. When printing the books, we can observe the restore situation from neutral gray of gray scale ruler. 

How to correct the color cast?

We correct the color cast often with below two ways.

1. Color balancing way

Color balancing way is use curve to adjust. Such as in Photoshop, we can adjust the rate of RGB or CMYK. 

2. Color correcting way 

We can divide color correcting way into major color correct and Pantone color correct. Although use digital way can fix the artwork message. Even add the content that artwork without. But to the whole copy effect, we don’t recommend you use the artwork with a serious color cast.

Sometimes, we feeling color cast, perhaps people hope to see that. The color cast with personalized and create an atmosphere to strong the visual effect, don't need to correct it. Such as, to highlight the magnificent effect of the palace. When shooting will add a yellow color filter, and the image tone is yellowish. Shooting the scenes before sunrise, there will happen the yellow red color of cast warmth. Thus, when we printing the books. Such as art book printing. Don't blindly remove every color cast that we feel. We need to consider the usage and theme of each artwork. If some kind of color cast can help you transfer some kind of message. Such as sunrise or sunshine message. Please keep it. 

The contrast of book printing artwork

The contrast refers to the difference between the density values of the darkest and brightest points in the image. That is the largest range of density variation in an image. The density range of color transmission originals is between 0.1-3.0D. The contrast can sometimes reach over 3.0D. Such artwork with bright color and rich in layers, the contrast is large than the printing matter. The contrast that the print can reproduce is about 2.0D. So, when doing color separation, we need to compress the density of artwork. It will lose the dark of subtle layers. Besides, if the density range that the scanner can recognize is smaller than the density range of the original. Then the details and color saturation of the scanned image will be lost. Reflective artwork usually has a moderate density. And the density range is generally between 0.2-2.0D. Reflective artwork often same dynamic range and density range of prints as most scanners.

According to the density range and tone distribution of the artwork. We can divide artwork into the following three types:

1. Thick artwork

Like some kind of coffee table book printing. Such artwork are mostly caused by dim light and underexposure during photography. The picture is dominated by dark and medium tones, occupying 60%-80% of the area of the picture. Lacking highlights, high density, and partial congruence in the dark tones. The relationship between tone distribution and occupied area is shown in Figure 5-13. The least density is about 0.4-0.6D, and the density contrast is in the range of 2.5-2.8D. When printing such artwork, adjust the shadow layer curve to open the shadow layer. Brighten the highlights, increase the bright parts of the picture. And increase the contrast of the image. Appropriately reduce the color density, emphasize the main color. And reduce the complementary color.

2. Thin artwork

This type of artwork photography is for pictures of snow-capped mountains, deserts, lakes, oceans, etc. There is excessive sunlight or the environment is full of sunlight. The thin original picture has less dark tone and low density. The least density is 0.1~0.2D, and the contrast is small. In the range of 1.4~1.8D, the light color pixels are mostly concentrated in the high and middle tone. And the high light tone has poor levels and tone distribution. curve. When book printing factory print such originals. The method of adjusting the gradation curve can be used to appropriately increase the contrast between the high and middle tone. And improve the contrast between light and dark.

To achieve this, the copy curve should be bent downward or an "S" shaped copy curve should be used. The highlight calibration point is selected at the point with lower density value in the original. And the density value of the dark calibration point. Should be appropriately reduced to enhance the color saturation of the mid-tone. Make the mid-tone level fuller, and strengthen the perspective effect.

3. Normal contrast artwork

Such originals are exposed normally. The low density is 02~0.3D, the high density is 2.1-2.9D, and the contrast is in the range of 1.9-2.4D. The picture is rich in layers, including high, medium and low-key, with bright colors and good clarity. Such originals should be mainly faithful originals when copying. And the copy curve should be brightened by 5%-8% in the middle tone, as shown in Figure 5-17. Which can make the layers richer and enhance the contrast between light and dark. Select normal shading point scaling for highlights and shadows.

To sum up, no matter how the artwork quality is. The book printing company need to know the artwork features. To make sure the book printing with the effect of bright color, rich in layers and correct memory color, etc.