Re: Monitor Calibration/Color Correction Questions

trouble, 10/7/2008 07:29 hrs:

The purpose of calibration is to map the colors your monitor displays into the color space of your printer in an objective manner.
What you see is subjective: your eye/brain automatically white balances; additionally the way the brain works you tend to see what you think should be in the image rather than what is really there.
Monitor calibration for Photoshop has nothing/nada to do with what you see on the monitor screen, it is strictly to translate what the monitor displays into what the printer can replicate.

With respect, that is not right. The purpose of monitor calibration is to remove any color casts or errors in the monitor display, so the displayed image matches the original image color as closely as possible.

This why the calibration software displays a series of varying colors on the screen which the colorimeter measures, and the difference between the known RGB values of the displayed color and the screen color is then corrected by the calibrating software producing a 'monitor profile'.

Subsequently, images on the monitor are displayed via the profile which corrects the screen colors *for visual purposes only. The original image values are not altered by this process.*

Likewise, a printer profile is generated by printing a known series of RGB values - some hundreds of different shades of color - on a given paper with given inks. The print is then analyzed in the same manner as was the screen to produce the printer profile. Subsequently, any image to be printed is corrected by the profile on its way to the printer. It should be obvious that different inks and different papers will need another profile; any one profile is only for the ink/paper combination it was generated with.

Profiles can be generated for cameras, scanners, monitors, and printers. The purpose in every case is to correct RGB errors in the equipment, and is always done with accurately calibrated standard images or targets

If you can be bothered to follow this post a bit further, consider the case of an uncalibrated system handling an image from your camera.

Let's say the task is to take and print a photograph of a Kodak grey card. We set up the card outside in sunlight, with the camera white balance set to auto. For simplicity, we shoot a jpeg shot with the card filling the entire frame.

Next, we load the image into Photoshop, and find it looks a bit greenish on the screen. Immediate question: is it really greenish, or is the monitor just showing us that color because of monitor errors?

So we open Window/Info (F8) and use the eyedropper to look at the RGB values in the image (which should be a uniform grey - if it is not, you have other problems!). Since the image should be an 18% gray, we can expect RGB values around 128 if the exposure was correct.

The eyedropper gives us values R 128, G 120, B 121. As the eyedropper is looking at the image and not the screen, we deduce that the camera image is a bit reddish, so right at the start the camera auto white balance is a bit out.

We have no way at this point of measuring the screen color except by eye, but as it looks greenish even with a reddish image, it is clear that the monitor is not reproducing colors accurately.

However, we press on, and adjust the screen color with Image/Adjust/Variations and choose the nearest to grey by eye that we can. Now, the screen looks to be a fairly good grey, so we decide to make a print.

But, what has happened to the image in the meantime? We reduced the green cast by shifting the image balance - by eye - toward the magenta, correcting the displayed image on screen, *but this move has added magenta to the already reddish original image*.

So, we come to doing a print. The printer, with its canned profile from the manufacturer, will print a reasonably acceptable print, so, fed with a reddish-magenta image, it proceeds to print a reddish-magenta print, nowhere near the grey we have on the screen. Worse, the print comes out very dark, much darker than the screen image.

Why did we get a dark red/magenta print? Well, as we saw, we deliberately pushed the balance towards magenta in an attempt to make the screen look right, but we failed to notice that the screen was too light because the gamma was wrong and the brightness was too high.

So, in summary, we have a camera that is a bit red, and a monitor that is too green and too light. Only the printer is about right - and it gets the blame for the bad print.

Translate that scenario into taking regular images and printing them with that setup. Not surprisingly, we are dismayed and confounded by the results.

Practically, the camera and printer can be taken as reasonable, but the monitor will be the main cause of poor printing. Monitor calibration is essential for good prints, with printer and camera calibration next in that order. If you scan color material, you should profile your scanner as well.

If you have persevered this far, congratulations. I hope it clarifies the situation.

Colin D.
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