1、Day 8 VIS201 Class NotesMonitor CalibrationA typical imaging system will consist of input and output devices, for example: scanners, digital cameras, monitors, and printers. Unfortunately, they will each reproduce the same color differently (i.e. color is device dependent). Therefore, some means of
2、ensuring that color data is reproduced in a predictable way throughout the entire imaging system is essential. This is the purpose of a Color Management System (CMS).Color Numbers, their Meaning and ICC ProfilesA digital image will usually comprise many millions pixels, each of which is represented
3、by a numeric value. The value assigned to each pixel will describe many attributes but for right now lets focus on the color value or mix (e.g. RGB value). When a color is device-dependent, the appearance of pixels with identical values will very often differ because each device has its own unique w
4、ay of translating the color “value“ or “number“ into visual color. ICC profiles ensure that the discrepancies of each device are known to the color management system. If we were discussing spoken language rather than color, then the ICC profile would be synonymous with a translator.Device profiles c
5、ome in two basic forms, i.e. Input and Output. Input profiles are for scanners and digital cameras, whereas Output profiles are for monitors, printers and film recorders. Input profiles are often referred to as one-way since they represent the source device. For example, we can never convert a docum
6、ent into the color space of our scanner or digital camera. Output profiles on the other hand are two-way meaning we can convert From or To them. The following flow diagram demonstrates a typical imaging workflow, with the document being passed between devices: - from scanner/digital camera - to - co
7、mputer - to - monitor - and printer. In order to make sure our color values can be translated throughout the diagram, we have to calibrate the system and create an ICC profile. Imaging System Diagram OnlineCalibration is a process whereby a device is brought to a standard state (e.g. a color tempera
8、ture of 6500K and gamma of 2.2), whereas characterizing the monitor is the process of determining how the monitor represents or reproduces color . We characterize the monitor by measuring how it displays known color values, then creating an ICC profile. The ICC profile is simply a data file that inc
9、ludes a description of the monitors color handling characteristics (i.e. its gamut). The calibration data will also be written into the ICC profile. Photoshop then uses the monitor profile to automatically optimize the display of documents. It does so by carrying out an on-the-fly conversion between
10、 your document profile and your monitor profile. This conversion does not alter the actual document in any way; just its appearance on the monitor.Monitor Calibration:CS3 is the first ICC aware version of Photoshop not to include its own monitor calibration utility because there is a good apple util
11、ity available and Vista doesnt do well with the previously shipped Adobe utility (Adobe Gamma). So Windows users will have to find their own program to help calibrate their monitors. This should be easy enough to do online. Obviously, software only monitor calibration applications use the human eye
12、to determine tone and color differences between a series of white/grey/black/color patches. However, it should go without saying that the eye is likely to be the most accurate method of measuring these differences.Lets calibrate our monitors together:* From Mac OS X System Preferences panel open the
13、 Displays applet* Ensure you display is set to the Native resolution (LCDs should always be set to the native resolution otherwise images will appear soft)*Ensure Colors is set to Millions*Set the Brightness slider to your preferred value (the Apple default is approximately 60%, although a slightly
14、brighter level might be necessary on aging displays). With Apple PowerBooks its actually better that you adjust the Brightness control to maximum. *Choose the Color tab*Search the list of Display Profiles for one that matches your display and select it. In truth this step shouldnt be required since
15、the calibration utility automatically picks up the necessary data from the display via the DVI/ADC connector.*Press the Calibrate button. Note: if after calibrating your display you find that it appears to revert to the uncalibrated state then repeat the step outlined above but this time select the
16、new calibration profile. Very often this problem arises after the computer wakes from sleep mode and is especially prevalent with Apple PowerBooks. Its actually a bug in the Operating System that for one reason or another Apple seems unable or unwilling to fix. *Expert Mode should be selected so tha
17、t all the available settings can be adjusted.Determine the Native Gamma of the DisplayThe checklist down the left side of the Display Calibrator Assistant dialog is only a guide and not all steps will be applicable to all display types. The following screenshot shows the first of five steps used to
18、determine the native gamma of the display. In each step you will be adjusting the Brightness and Tint thus obtaining much finer control of the gamma curves.*Begin by adjusting the Brightness slider (1) vertically so that the apple symbol blends with the background.*Next using the Tint slider (2) tri
19、m out any colour imbalance that may be present. You may need to readjust the left hand brightness slider a little more after this step.*Its helpful to squint when trying to blend the inner and outer shapes.The remaining four steps are basically a repeat of the first but effect other parts of the gam
20、ma curve. You should follow the instructions given by Apple in the dialogs.Setting the Target GammaOnce the native gamma has been determined it only remains for you to decide on your preferred target Gamma. For many years the standard gamma setting for Macs was 1.8. Compared to gamma 2.2 a setting o
21、f 1.8 will appear quite bright. In other words gamma 2.2 will appear darker and more contrasty than what many long time Mac users are used to. If you dont have a need (or love affair) with gamma 1.8 I recommend that you choose gamma 2.2. The majority of displays (CRT and LCD) will have a Native gamm
22、a somewhere between 2.0 and 2.2 so choosing a lesser value requires a fair amount of adjustment on the part of the graphics card LUT and will tend to cause banding or posterisation.Choosing the White PointHigh-end LCDs such as those from Apple tend to be factory set to a value of around 6500oK (i.e.
23、 their Native White Point is 6500oK) and so it only remains for the user to check the box labelled Use native white point. However, it is worth mentioning that as the display ages the backlight will slowly begin to discolour. At time of writing this tutorial my display is measuring around 6350oK whi
24、ch is well within the margin of error when using the eye-ball as a measuring device (see footnote).Naming and Saving the Display ProfileThe final step requires that you give the display profile a name. Pressing the Create button will crate the new display profile and saves it to the appropriate fold
25、er for use by the system. Photoshop will automatically read and use this display profile until it is superseded or deleted.Done.PrintingThe print dialog in CS3 must be reconfigured for each and every image rather than once for any given print session. Remember, he Print menu option is only available
26、 for selection when an image is open on your desktop. Open The Marina Insert.psd image.Go to File Print.Document: denotes the ICC profile embedded within or assigned to the source document. The example shown in figure 1 shows ProPhoto RGB, but it could be any user specified profile (e.g. sRGB, Adobe
27、 RGB (1998), ColorMatch). If the document has already been converted to a printer/media profile using the Convert to Profile command, then it will be the printer profile name shown rather than a Photoshop working or color space profile.Close the Print Dialog and go to Edit Convert to Profile. Choose the Profile we created earlier. Go back to the Print dialog. Notice the difference. Proof: this field will normally display as (Profile: N/A). Once its activated via the radio-button it tells Photoshop to convert the document on-the-fly from the source
