Images:Images are another complicated element for Preparing a Print-Ready File. An image file is a rasterized digital file that a grid of pixels defines its content. By standard, images are to be scanned or prepared actual print size (100%) at 300 dpi. The most important thing to remember is increasing the resolution after scanning or creating the file will not improve the quality of the print, but it only increases the file size.
To understand this, let’s imagine a 1”x1” box containing 300 dots. If we distribute these dots evenly in the box, the space between dots is much tighter than if we had half as many dots in that area. Probably there is twice as much space available for 150 versus 300 dots. Now let’s increase the size of the box to 2”x2” where the new box can contain four boxes of 1”x1”. In this case, we have increased the space between each dot by a factor of 4. Or another word, we can fit ¼ of 300 (75) dots on each of 1”x1” boxes. The significance of this calculation reflects on the clarity and detail of that image at the printing stage. At the print production, the space between each dot translates to distortion; the less dot in a defined space creates a more distorted image and more dot in that space creates a sharper and clearer image. After a certain point, the increased dpi does not have an impact on the quality that is visually noticeable. In most cases, defining an optimum quality for print production is when creating or scanning a file at 300 dpi for Grayscale, RGB and CMYK color spaces, and 1200 dpi for Bitmap (black and white) images.
The relationship between the size and dots are inversely proportional. As we increase the size, we are reducing the number of dots per inch (dpi), and as we reduce the size, we are increasing dpi. The programs like Photoshop® provide you with an option to make this transition.
The condition, quality, age, paper type dictates the resolution and size of scanning images. The scan resolution defines how much detail the scanner can detect and register. The higher the resolution, the more pixel details are detected by a scanner. For example, scanning of a textured photograph like old matt finish photo at high resolution is a mistake due to the amount of detail the scanner detects. The light of the scanner and the texture of the photo creates shadows. The result is many stains on the scan. The solution is to scan at a low resolution like 100dpi or even lower like 72 dpi to detect less detail, but at higher magnification like 700% to 1000%. Later we can convert the size to more pixel.
Color Space:In printing, there are four color spaces to consider and understand the differences between them; Bitmap (black and white), Grayscale, RGB, and CMYK.
Bitmap or (black and white) is an image that has only black or white pixels. Due to lack of gradation or additional colors, the best way to scan this type of images is to prepare them at 1200dpi. This is the only way to compensate for the lack of additional colors or shades. If it is not possible to scan at that resolution, scan it at a lower resolution but increase the size. For example, if you are scanning at 600dpi, you want to increase the size to 200% and so on. Later you can reduce the size and increase the resolution as mentioned above.
Grayscale is an 8 bit (256 shades of gray) file. That means for every pixel there are 256 possible shades of gray. As the result, the file has more detail than Bitmap. Also, this file prints in one channel (same as a bitmap) which results in the entire file outputted into one plate (hence one color). Since the image has more details available for each pixel you can scan and prepare the file at 300dpi.
RGB file has a combination of three colors Red, Green, and Blue. RGB is a 24-bit file (16,777,216 shades of color) and is not suitable for traditional printing. The RGB has a larger color gamut than CMYK, but it is not suitable for offset printing. On the other hand, for digital printing, it is a different story. Usually, the digital printers and copiers can print RGB files better than CMYK with correct settings; even, Spot colors are closer to their intent colors in RGB than CMYK color space.
CMYK color space has four basic colors (Cyan, Magenta, Yellow and Black) to create all possible colors. In comparison to RGB, it has a smaller color gamut, but it will have successful separation for full-color offset printing. One thing worth mentioning is, not all Pantone colors have a successful translation to CMYK, and even some have a drastic color shift.
Printing process:The printing process defines how a file gets printed. Full-color printing uses CMYK color space; Spot Color printing uses Pantone color system, and digital printing uses CMYK or RGB color space to print.
Collectively, full color and spot color printing refer to offset printing where a printing press is used to print. The offset printing uses plates that are mounted on the inked up printing press. In Four color printing, each pixel of the image is broken down to ratios of basic colors (CMYK) and transferred to a corresponding plate. For example, if a pixel is analyzed to have 10% Cyan, 50% Magenta, 100% Yellow and 2% Black, on each respective plate a dot is created with its relative density value. During printing, the impression of each plate for that exact pixel is transferred to paper and consequently creating the color for it.
On the other hand, spot color uses one color per plate. The color is not broken down to basic colors, but the color is created based on Panton formula guide first and then placed on the press for printing. For example, for two colors, red and blue; each of two towers of the press are filled with the respective color, and the content of the artwork separated to their respective plates. The accuracy of color in this process is much higher than CMYK process.
Digital printing is an advanced form of duplication. Another word, digital printer is nothing more than a copier that can receive and process files digitally via a computer. An internal or external computer called RIP connected to the copier; where RIP can receive a file and translate the colors for the copier to print. Obviously, the digital printing is more complicated; this oversimplification of the digital printing process is for comparison with the traditional printing process.
Paper:The paper is another element affecting the quality of printing. The fiber content, the sheen, color and density of paper can impact the final product. The amount of cotton fiber in the paper change the absorbency rate of the paper. The more cotton fiber, the more it absorbs which can change the density of ink on the paper. The impact of ink absorption for Digital printing is less unless it uses liquid ink.
The sheen or glossiness of the paper can impact the brightness of the color. More glossy the paper is the brighter the color appear on that paper; due to the coating on the surface which minimize the ink absorption to the paper and as the result, the entire density of the ink sits on the surface of the paper.
Ink absorption rate is also affected by the density and rigidity of the paper. We consider more rigid and dense paper as cardstock or cover and less dense and rigid as paper or book; so 100lb book is less rigid than 100lb cover and so on. The density of paper also affects the ink absorption for a different reason. The density of paper prevents the ink sip through the paper.
The color of paper can also impact the quality of print. For example, if you are using a yellow paper and printing blue ink, your colors can shift toward green. When using a color paper, make sure to understand its impact on the artwork.