4. All About Profiles 5. Measurement, Calibration, and Process Control 6. Building Display Profiles GED119 Colour Science and Digital Applications
What are Device Profiles A profile is just a “table” correlating device colour values (e.g. RGB numbers) with device-independent colour values (LAB numbers). Conceptually: RGB numbers: 255, 255, 255 255, 255, 254 255, 255, 253 … 0, 0, 1 0, 0, 0 LAB numbers: 92,0,0 92,0,1 91,0,2 3,1,2 3,1,0 GED119 Colour Science and Digital Applications
Profiles
Profiles Classes One-way input profile Two-way display/output profile
One-way Input Profiles An input profile matches an input device colours to LAB (one-way) RGB numbers: 255, 255, 255 255, 255, 254 255, 255, 253 … 0, 0, 1 0, 0, 0 LAB numbers: 92,0,0 92,0,1 91,0,2 3,1,2 3,1,0 RGB numbers: 255, 255, 255 255, 255, 254 255, 255, 253 … 0, 0, 1 0, 0, 0 LAB numbers: 93,0,1 93,1,1 90,1,2 4,1,3 3,2,0
Two-way Display Profiles A display profile is two-way: LAB colours must be matched to device’s RGB numbers for display (output) Colours according to RGB numbers must be matched to LAB (input) RGB numbers: 255, 255, 255 255, 255, 254 255, 255, 253 … 0, 0, 1 0, 0, 0 LAB numbers: 99,99,99 99,99,98 92,0,0 92,0,1 92,0,2 91,0,2 3,1,2 3,1,0 3,0,1 Out-of-gamut
Two-way Output Profiles An output profile is also two-way: LAB colours must be matched to device’s RGB/CMYK numbers for output Colours printed according to RGB/CMYK numbers must be matched to LAB for display/proofing on other devices CMYK numbers: 0,0,0,0 1,0,0,0 1,0,0,1 … 89,69,69,69 90,70,70,70 LAB numbers: 99,99,99 99,99,98 92,0,0 92,0,1 92,0,2 91,0,2 3,1,2 3,1,0 3,0,1 Out-of-gamut LAB numbers: 92,0,0 92,0,1 91,0,2 … 3,1,2 3,1,0
Making Profiles For a display device such as a monitor, send RGB values (signals) to the screen and measure exactly what colours in LAB come out on the display (screen). For an output device such as a printer, send RGB /CMYK values (signals) to the device and measure exactly what colours in LAB come out on the output medium (e.g. paper).
Making Profiles For an input device such as a scanner or a digital camera, a scanning target with pre-measured LAB values are used. The input device scans the target and produces an RGB file which has to be correlated with the pre-measured LAB values of the target.
Profile Limitations The profile maker cannot measure all possible device (colour) signals. A profile can address only those colours within the gamut of a device. A profile is only as accurate as the measurements from which it is made.
Calibration vs. Profiling Calibration is the act of changing a device’s behaviour to achieve some desired state. Profiling is the process by which we record a device’s behaviour into a profile. It does not change the device’s behaviour. Some devices do not support calibration because they cannot be adjusted, e.g. desktop scanners. GED119 Colour Science and Digital Applications
Controlling Variables Devices have their internal software (firmware) which control their colour behaviour, e.g. auto colour correction in printers. Printer profiles are dependent on inks and papers. Be careful of manufacturing variations/changes in these consumables. For devices with hardware settings, be careful of keeping them constant.
Calibrating Devices Aim: adjusting a device so that it always produces the same colour in response to a given set of (RGB/CMYK) numbers. Other goals: Stability – when devices change their behaviour over time. Optimization – exploit the full dynamic range and colour gamut Simulation – adjust a device to behave like another device, e.g. soft proofing
Building Display Profiles What to adjust: Monitor control settings Video Lookup Table (videoLUT) Calibration and Profiling Tools Packages bundled with a monitor Standalone instrument-and-software bundles Standalone software packages that support different instruments Visual calibrators
Before Calibration Allow sufficient warm up time Check monitor settings Clean the screen
Demonstration Calibrate a notebook PC LCD screen X-rite Colormunki Photo: a software-and-instrument package