Article by Johnny Shell, SGIA (Mon, Jun 12, 06)

A great deal of color management confusion revolves around the Adobe® Photoshop® commands Assign Profile and Convert to Profile. Have you ever wondered what the proper use of each command is in a real world environment? What about the advantages and disadvantages of using them? If you're like many, you've probably poured over the Photoshop® documentation which, buried deep in the Help section, ultimately states this: When using the Assign Profile command, "the color numbers in the document remain the same, but the new profile may dramatically change the appearance of the colors as displayed on your monitor." Convert to Profile, however, converts "the colors in the document to the color space of a different profile. The color numbers are shifted in an effort to preserve the original color appearances." Helpful, right? Let's see if we can clear up some of the confusion.

Photoshop® Thinking

Adobe® Photoshop® is unable to accurately display, print or convert a file unless the application knows the source space of the file. There are only three ways this is accomplished:
1. The file is tagged with an embedded profile.
2. The correct profile is assigned.
3. The source space of the file equals the working space set in Photoshop®.

If the file is tagged, we want to choose to open it using the embedded profile. However, numbers two and three above are a bit more challenging. If I choose to assign the correct profile, it means that I must first know the correct profile to assign. But, what if I don't? Well, I have to ask or guess. Not a place I'd like to be. Number three above occurs when the customer supplies the file in the same working space you use or when all images are created on your machine, in your working space. So, profiles are important but how they are used is just as important.

When you choose Edit>Assign Profile, Photoshop® uses the profile you select to describe how the RGB numbers in your image should be interpreted, but does not convert them based on this new description. So, Assign Profile changes the meaning of the numbers (from the perspective of Photoshop®) but never actually changes those numbers. Assigning a profile is most commonly used, for example, for input profiles where the raw numbers captured by a scanner do not produce the same color on the monitor. So, by assigning a profile, the monitor can then know how to display the scanner values. Think of assigning a profile as placing a colored filter over an object. It will modify the appearance of an object, but it will not modify the object itself. If the file is saved and closed with the profile embedded, it can easily be removed at a later date with no effect on the data whatsoever.

Converting is a very different story. When we select Convert to Profile, we convert a file from one profile to another. When we assigned a profile we requested the image's raw numbers remain unchanged, while its appearance changed to reflect the RGB values of the new profile. When we convert, on the other hand, we are asking the color management system to change the numbers describing the file in order to preserve its appearance in the destination space. Converting is most common for output when we want to preserve the image's appearance as much as possible, but we want to describe it with numbers the printer will understand.

The two key questions you should ask yourself when deciding whether you want to assign or convert are:

1. Do I wish to alter the appearance of the image? If so, assign.
2. Do I wish to preserve the appearance of the image in the new space? If so, convert.

Basically there is little use for Assign Profile unless you are receiving images that do not have a profile attached and even then you need to ask or guess which profile to use. Convert to Profile should be used if you are sending a file to a printer or you are converting into your working space from the native colors in the file.

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