File Preparation for Screen Printing with Adobe Photoshop®.
Properly prepared source files always improve a color separation and result in a better print. File preparation should be done prior to color separation in the RGB mode. The checklist below outlines some basic elements of an image file prepared for separation.
• Use the RGB Color Mode
• Image Resolution between 150 and 300 ppi
• Text merged into layers with AntiAlias Off
• Antialias off on imported Vector Graphics
• Transparent Background
• Color Profile Attached
• Save as a Photoshop file
File Preparation in the RGB Color Mode
Images should be in the RGB Mode prior to starting a color separation. The RGB mode provides the widest color range and the tools needed to prepare and adjust an image. Avoid converting to CMYK or other color modes before preparing a file for separation as the image quality may be reduced.
Using the RGB color mode
An image’s resolution needs to be adequate for the print size. Resolutions between 150 and 300 pixels per inch are recommended for images at 100% size. For photographs, scanned art, or images to be printed with a halftone, use a resolution of 150 ppi or higher. For text and graphics with or without an image use a resolution of 300ppi or higher.
In most cases increasing an images resolution by resampling will not improve image quality. However, If you plan on adding graphics or text to an image, masking the background, or making other changes, increase the images resolution prior to making these adjustments.
To change an images size use the Image Size dialog:
Image / Image Size
Checking the image size and resolution
Checking the maximum print size.
Use the image size dialog to determine an images maximum print size.
• With the image size dialog open, Uncheck the Resample Image box. Set the resolution to 120ppi and note the images new Width and Height, this is the maximum print size for reproducing the image with Halftones. Image resolution should be 2.5 times the halftone line screen or higher.
Increasing an images resolution to add text and graphics
• With the Resample Image box Unchecked, Set the image Width and Height to the desired output size in the image size dialog. The resolution can be any amount above 120ppi.
• With the Resample Image box Checked, choose Bicubic as the resampling method and increase the resolution to 300 ppi. Click OK and save as a new file.
Increasing an images resolution to add text.
Resampling changes the actual pixels in an image, resulting in a loss of detail. Resampling should be used only once, and sparingly, on an image. For most images, a small amount of bicubic resampling may not be noticeable or may be mitigated. For text and graphic that are merged into layers, bicubic resampling will soften the edges
Antialias is used to smooth the appearance of high resolution images on screen. This smoothing effect creates a slight blurring of edges when viewed up close, but actually creates a clearer image when viewed at various sizes. When printed, Anti-alias is represented with the line count used to reproduce the image. With Screen printing, an anti-alias edge would be represented with a 50 lpi dot. This 50 lpi anti-alias creates a visibly rougher edge then can be reproduced by screen printing with a higher resolution output.
The Anti Aliased edge looks smoother up close , but will actually print
at a lower resolution.
• Edges with Anti Alias turned off print at the resolution of the file.
• Edges with Anti Alias On print at the resolution of the halftone line screen.
To keep edges sharp, graphics can be imported or bitmapped at a high resolution with the anti-alias off. Text set at 300 ppi can have the antialias set to None before merging into a normal layer. Be sure to check on small text and graphics as some details may be lost when antialias is removed.
Set the type’s Antialias (aa) to None near the bottom right of the Character window.
Text in documents will require fonts that may not be available on another computer. To transfer files containing text, text layers should be merged into normal, transparent layers with document resolution at 300 ppi or higher, and Anti Alias set to Off.
Exporting / Importing Vector files
A common approach to creating text and graphics is to use a Vector program such as ®Adobe Illustrator. Vector graphics are resolution independent, meaning they do not use pixels and bits to form an image. To use vector graphics in a color separation images need to be converted to a Raster format, ie; one that uses pixels and bits. When converting to a raster format, or rasterizing, image parameters such as resolution, antialias, color mode and color profile are set.
Importing a vector file into Adobe Photoshop requires the same settings as exporting from a vector application;
Resolution, Size, Anti-alias, and Color Mode.
Most vector applications provide a method to save files in a rasterized format. These options are usually under the Export menu. Export files as Tiff with Zip compression to rasterize all the data. Some vector formats can be opened and rasterized by Adobe Photoshop. Whether importing or exporting vector files use the following settings when available.
• Set resolution to 300 ppi.
• Export with Layers
• Export with AntiAlias OFF.
• Include the Color Profile.
• Use 8bit RGB mode
To isolate the white channels in a color separation an image needs to be masked onto a transparent background. Using a transparent background provides control over the transition from the garment color to the printed colors, and can be used to manage the ink deposit as well. Image areas that fade to shirt color when screen printed should fade to a transparent background in the image file.
The checkered pattern behind the image indicates the background is transparent.
Some file types do not support transparent layers, images that do not contain layers are refered to as Flattened. Jpeg, PDF and older tiff formats do not display layered files and only support flattened images. Flattened image files will need to have the white background removed.
In Adobe Photoshop, a Background layer can be turned into an editable layer by double clicking on it in the layers palette.
Double click on a flattened layer in Adobe Photoshop to create an editable layer
With an editable layer the background can be removed using the erasure tool, or by creating a layer mask. Active selections created using any of Adobe Photoshop’s selection tools can be converted to a transparency mask by clicking the layer mask icon at the bottom of the layers palette.
A layer mask is visible as an additional, linked thumbnail in the layers palette, and as a mask channel in the channels palette. With a layer mask active, a layers transparency can be edited by painting or applying adjustments to the mask channel.
A layer mask can be viewed and edited in both the layers palette and the channels palette.
• All layers and layer effects over transparency should be in the “Normal” mode.
• Layered images should be previewed on black, gray and white backgrounds to ensure stray light and dark pixels are removed.
• Vector files can be exported with a transparent background when using a file format that supports layers.
The Color Profile.
A color profile is a bit of information that identifies the exact color space of the file, or device, it is associated with. Files should have a color profile attached so the correct colors can be displayed and printed. The color profile is used when opening a file and Adobe Photoshop will either use the profile to display the correct colors, or to convert the image to the working color profile, depending on the settings.
If a document has a color profile attached, the option to include the color profile will be available when saving. To see what profile is being used for an open file, and to convert an image to another profile, open the Convert to Profile window under the Edit menu.
Edit / Convert to Profile
If a color profile is not attached when a file is opened, Adobe Photoshops working profile will be used. If the working profile does not match the documents actual color the images colors will shift. The correct profile can be assigned if available with the Assign Profile option in Adobe Photoshop’s Edit menu:
Edit / Assign Profile
Assigning a color profile when one is not embedded in the document
• Only assign a profile if a document does not have one attached already. If your not sure what color profile to assign to an image file start with Srgb, click the preview box and see how the image changes. If the image does not change when the preview is turned off then it is the correct profile. Try different profiles to find one that fits the imageClick OK
Save a .PSD file with layers and an attached color profile.
Save files as Adobe Photoshop® files with layers, and an attached color profile. Avoid compression formats, such as Jpeg and Gif, when preparing files for screen printing.
Save as a Photoshop file with layers and the color profile embedded.
The Color Separation.
Files prepared in this way are ready to be separated. Start by merging all the visible layers, and discarding any hidden layers. The YRGBK script can be launched from the script menu to begin the color separation process.