How does PS use a monitor's profile?

I edit and print a lot of images with Photoshop. I understand why I need to calibrate my monitor regularly to have confidence in my editing and printing. What I don't understand is why I need to identify the monitor profile to Photoshop in the Color Settings window. I've been told that the monitor profile contains "target values," not the actual "physical values" of the monitor. If this is true, it must mean that when the physical values of the monitor drift over time, Photoshop cannot dynamically adjust what it displays by referencing the monitor profile. True? If so, then linking Photoshop with the monitor's profile must have a purpose that I don't understand.

Hi kenigma,

Okay, you said that you don't understand why you need to identify the monitor profile to Photoshop in the Color Settings window. Well, you shouldn't need to do that. Where, in Photoshop's color setting window are you putting that information?

You can select a profile from the RGB drop-down menu in that window. I first create a profile by using a colorimeter to calibrate my monitor, and then I see the profile's name in the drop-down. You can alternatively select a default profile, but if you calibrate your monitor, you won't want to choose second best.

Okay, first of all, regarding what you've been told about monitor profiles containing "target values," and not the actual "physical values" of the monitor".... Well, I'm not sure we could say it contains either one Although, it was certainly created using target values (from your colorimeter) and physical values (from you monitor).

Here's the deal... As you know and have done, you calibrate your monitor using a colorimeter and the software that comes with it. Basically, what happens is the software outputs a series of color patches, of known color values, on the monitor and the colorimeter then reads the values that your monitor displays. The software then compares those measured monitor values to the actual values sent to the monitor. Knowing the difference between the actual values sent to the monitor and the values output from the monitor, the software can create a profile of the monitor. That profile can then be used to create an output correction curve (lookup table) that the video card uses to adjust your monitor (hopefully correcting the monitor's output value so it matches the known color values output by the software).

As you mentioned, over time, the physical values of the monitor drift. So, it makes sense to calibrate often in order to allow the calibration software to create a new correction curve.

Secondly, you should not select your monitor profile in that RGB drop-down menu. That's for your working space (sRGB or Adobe RGB 1998) not your monitor profile. Photoshop will automatically use the system's default display profile for the screen display. If you use your monitor profile for your working space, your Photoshop images will look great on your monitor but not on other people's monitors. Their monitor profile is different than yours! You can easily check to see if Photoshop is using the right monitor profile. Just open that RGB drop-down menu you mentioned and look for "Monitor RGB"; the profile created with your colorimeter should be listed there.

kenigma,
You should be able to find numerous places on the web telling you not to use your monitor profile as your working space. Here's one quick example (see link below)... this webpage shows two windows, one on top of the other. Notice that, in the lower window, the RGB working space is set to sRGB. The top window is showing how to check to make sure Photoshop is properly using your monitor profile. The main thing to notice here is that the author says "Be careful not to change your working space in the process. To be safe, click "cancel" when done looking." In other words, he wants to make sure you don't end up setting your working space to your monitor profile

http://www.lashier.com/home.cfm?dir_cat=38451

OK, point taken about not using the monitor profile for the working space. But how does Photoshop know that I've created a custom profile for the monitor if I don't enter a thing about it on the PS GUI?

Well, your colorimeter software knows where your monitor profile should be and puts it in the right place for you. Where it goes depends on your operation system. Then, Photoshop automatically uses the monitor profile defined by your operating system (which of course is the one the colorimeter software put in the right place for you). As mentioned earlier, the profile that Photoshop will use is the one listed in that RGB drop-down menu (the one you've been using for your color space). You'll see that it is the proper profile and it is the profile that Photoshop will use.

Here's an interesting webpage that may be helpful....

http://www.earthboundlight.com/phototip ... le-go.html

Thanks to all, with special thanks to artd. I have been enlightened.

You're very welcome, glad we could be helpful!