If you mean by "screen capture" the ability to record the current screen with command-shift-3 or -4 then that won't work. That form of screen capture just records the image before it is sent to the display (it's more of a reflection of the output from the video card rather than what you see on the display). You can't read the physical display pixels from the display and save that as a new image.
If you don't believe this just take a screen capture at a reduced brightness or even a reduced (minimum) resolution. You'll find that the screen capture looks nothing like the image seen on the display (it's the image being sent to the display, not an image of the actual pixels on the display). I should add that these tests aren't particularly good examples of what a screen capture is (for example, brightness is controlled by the backlight -- not by the display pixels), but they do begin to illustrate that what you see on the display is not necessarily what you will get in a screen capture. Here is perhaps a better example, take a screen capture on a "bad" display and move it to a "good" display. What do you expect to see? As long as the video cards are working correctly (and using the same color and resolution settings) the image from the "bad" display will look good on the "good" display.
Probably the only way to record the actual image quality produced by a display is with a photograph. However, that's pretty difficult to do since the camera taking the photograph will have its own limitations in color and uniformity (let alone the difficulty in taking a good close-up).