How to create an extra/alpha channel in Adobe Photoshop?

In digital printing, achieving high-quality color reproduction is essential. One effective technique is extra-channel rasterization, which extracts an additional alpha channel from a raster image, such as a TIFF file, for white or other special ink separation, enhancing detail, and color accuracy.

Preparation in an image processing application like Adobe Photoshop is crucial to successfully implementing this. Read on to learn how to create an extra/alpha channel step by step.

Part of the information below is a summary of the official Adobe documentation. If you want to know more details, please refer to their site directly.

About channels in Photoshop

Channels in image editing are grayscale images that store various types of information:

• Color information channels are automatically created based on the image's color mode, such as RGB, which includes separate channels for red, green, and blue, along with a composite channel.
• Alpha channels (seen in this article) are used to store selections and masks for manipulating or protecting parts of an image
• Spot color channels specify additional plates for printing with spot color inks.

An image can contain up to 56 channels, all matching the original image's dimensions, and the file size for a channel varies based on its pixel information, with certain formats like TIFF and Photoshop compressing this data to save space.

Step 1: Create the Alpha Channel

1. Open Adobe Photoshop.
2. To create the alpha channel from scratch, begin by opening the Channels panel. Click on the Window menu and select Channels to display the panel.
3. Next, create a new channel by clicking the New Channel button located at the bottom right corner of the panel. This action will generate a new channel named Alpha 1.
4. Ensure that the CMYK (or RGB) channels are visible so you can see your image in the background.
   • To adjust the properties of the newly created channel, double-click the Alpha 1 icon. In the channel properties window, confirm that the type is set to "Selected Areas" or "Spot Channel," rather than "Masked Areas".
   • You have the option to change the channel color (the default is red), adjust the opacity, or rename it (for example, to "White"). Keep in mind that these changes will only affect the Photoshop preview and will not alter the TIFF file.
     
     
     
5. Understanding the channel values is crucial. A value of 0% indicates no white, while 100% represents full white. In the spot channel view, it is normal for 0% to appear white and 100% to appear black; only the densities matter in this context. You can refer to the sample images for clarity:
   
   "White" channel disabled "White" channel only All channels

    

   

   (With CMYK disabled)

   

   (Where the “White” is set to red for better visibility)

   

6. (Optional) If you prefer to create the white channel from a selection, start by selecting the areas you want to fill with white using the Magic Wand tool or Quick Mask.
   • After making your selection, save it as a channel by clicking Save Selection as Channel (in the Channels panel).
   • This will create a new channel named Alpha 1.
   • To check and adjust the properties of this channel, double-click it. If the type is set to "Masked Areas," change it to "Selected Areas" to avoid inverting the selection. If necessary, you can convert it to a "Spot Color" after ensuring it is set to "Selected Areas":
     

       Info

     After conversion, you can still edit the channel. For instance, you can add text to it.

Step 2: Export the file

At this step, you have two possibilities:

• Save as TIFF: Once you have filled the channel with the appropriate data, save the file in TIFF format. Ensure that all options for Spot Colors and/or Alpha channels are enabled.

• Save in Photoshop PDF Format: Set the channel type as "Spot channel" for recognition in Caldera software. You can also pre-assign the correct Spot color ("White", with all components at 0%) to save time.

Step 3: Import the file into CalderaRIP

1. Open CalderaRIP.
2. Open the FileManager module and select the file (no need to open it yet). The FileManager will detect the extra channel and display it in the top right corner of the window:
   

     Info

   If your image contains an alpha channel instead of an "unspecified" extra channel, the alpha channel will also appear in the list in FileManager. You must explicitly convert it to an extra channel (either undefined or colored) before loading it. If you do not perform this conversion, you will need to convert it later in the Print Module, just before printing.

     Good to know

   In Fileman > Setup > Misc tab, you can force the assignment of alpha channels in files to a spot channel (such as white or other special inks). This improves your experience by ensuring that the Rasterize Spot Colors > Auto option functions correctly.

3. If everything appears to be in order, proceed to Open (import) the file.
4. (Optional) You can also check the extra channel of your file by dragging and dropping the file into the Info module (once it is already imported into the software).

Step 4: Print the file

1. Open the Print Module.
2. Drag and drop the file into it:
   
3. Click on Page Setup (first icon at the bottom).
4. Open the White tab (or Varnish, Gloss, etc.) and click on Enable Separation:
   • If you have correctly named the channel “White”, you do not have to do anything else.
   • Otherwise, select the extra channel in the list after checking “Show all possibilities”.
   • You can also click the red patch in the Preview options (just below the image) to enforce the appearance of the White:
5. Click Print when ready (or close Page Setup and continue configuring the Print module).

Considerations

If your image contains a true alpha (transparency) channel, you must convert it before printing. CalderaRIP interprets the alpha channel in a specific way that cannot be overridden during the ripping process.

You can convert the file when loading it in the FileManager or the Print Module. The alpha channel will appear in the Available Sources list.