What is Colour?
Visible light (VIBGYOR) or visible spectrum is that part of light (flow of photons) which is visible to naked eye. Or our eye can detect. Since light travels in wave, each wavelength defines a particular Colour and is unique.
Description of a Colour is highly subjective as each observer interprets same Colour of a flower or for that matter any coloured object on personal references and also verbally defines a Colour differently.
Red rose may be viewed and defined by some one as dark red or light red or pinkish red etc etc which may be ok for a drawing room discussion but in professional print environment we need to communicate objectively so as to eliminate ambiguity hence CIE (International Commission of illumination) proposed CIE LAB model and has been accepted since 1976 a s standard model to define Colour . has defined 3 dimensional model of Colour space (A 3D map of visible Colour) where and each and every Colour in the Colour space can be measured and communicated by CIE LAB numbers accurately across different platforms where “L” defines LIGHTNESS ( Black to White), “A” denotes red/ green Values and “ b” denotes “yellow/Blue” values.
THIS L,a, b can define most colors mathematically and thus can be communicated through L,a,b values to define a particular Colour.
A s a Colour specification and communication format, spectral data offers TRUE objective values and has distinct advantages over conventional TRISTIMULUS formats such as RGB or CMYK values because latter method is dependent upon Viewing conditions, type of device that is rendering the Colour, type of lighting under which the Colour is viewed.
What is Colour management?
When we refer to Colour management actually we refer to “ ICC Colour management”. The framework of “profile connection space and format of profiles has been described by ICC (International Colour Consortium) – A regulatory body that supervises Colour management protocols between software vendors, equipment manufacturers and users.
In digital environment, Colour management is essential because there are as many RGB spaces as there are computer monitors and as many colors spaces in CMYK as there are printers. Scanned colors also may not loo the same as they are displayed on monitors and different monitors will display colors differently. To create even bigger confusion in Colour rendition, source images (camera) may be in a different Colour space all together.
Under such an environment of disunity amongst various devices one must remember “ RGB (Monitors) and CMYK (printers) ARE ONLY INSTRUCTIONS FOR A DEVICE AND DO NOT PROVIDE A RELIABLE OR PRECICE SPECIFICATION OF A COLOUR”.
Therefore the final exact Colour will be dependent upon the device and its configuration. A RGB or CMYK file can be created but if printed on different devices it WIL NOT produce same result with respect to Colour. Or in other words it is NOT feasible to communicate Colour information across different devices in an open Colour managed system.
So, Colour specification falls into 2 categories:
- Device dependent Colour (RGB and CMYK)
- Device INDEPENDENT Colour information that can be communicated across devices with same consistent result ie L,a,b spectral data model.
Colour management in a simple language, creates a language that different devices can read and follow across RGB and CMYK Colour spaces commonly called PROFILES or CHARACTERIZATION of devices by converting RGB or CMYK values in L,a,b values .
Colour CONVERSION Process
Colour management works by converting RGB or CMYK values into L,a,b values as ICC protocol . For example a Photograph of a Red Colour taken by a camera (Say Nikon D3200) The digital camera operates in Adobe RGB Colour space and the image is therefore in RGB Colour space. When opened in Photoshop (say) the information palette indicates RGB value of 173,4, 37(Red)
Now, each camera will have it’s own characteristics response for the same coloured object thus RGB values may be different for different Nikon or for that matter any other camera of any other make.
So camera RGB values are converted to L,a,b values using camera profile . For example in this example if we presume that profile is Adobe RGB (1998) the corresponding L,a,b values are L=43, a=68 and b=46 . By converting these RGB data into L,a,b values ( device independent) Colour management ensured that Colour information from camera to printer is NOT lost
We the same image is printed process is reversed.
In previous case, camera profile was the source profile and L,ab profile was destination profile .( RGB to L,a,b)
For printing , L,a,b profile becomes the source profile and CMYK profile becomes the destination profile ( as all printers print in CMYK) BUT CMYK may be different to produce the same l,a,b values depending upon printer.
For example L,a,b = 43,68,46 when printed on EPSON SC P 5070 it could be CMYK=56,76,44,5 for same RED out put ( let us say Coca-Cola red)
L,a,b = 43,68,46 when printed on ROLAND XR 640 it may have CMYK values of 59,70,34 ,15 for same RED. And Same L,a,b = 43,68,46 when printed on XEROX 700 Digital Colour press it may have CMYK values of 48,71,46,8 for same RED.
Colour management can be defined in terms in order of 3 C’s:
- Calibrations It involves establishing a fixed, repeatable condition for a device. For a Monitor this may mean adjusting the brightness and contrast settings. For a printer it may mean agreeing to a paper type (media) and printer resolution setting. Anything that CAN alter the Colour of an image must be locked down.
- Characterizations Also commonly known as profiling which refers to sending a known standard Colour charts (test charts) to the printer device and measuring its response / During characterization Colour characterics and gamut of device is ascertained and is stored in in an ICC device profile.
- Conversion is a process in which images are converted from one Colour space to another using device profiles . typically from a camera to printer scenario it would mean using a camera profile and a printer profile. The conversion process relies on external or internal software.
3 C’s are hierarchical which mans each process is dependent upon preceding step. A stable calibration is foundation of Colour management ie device must be consistent and not drift from original calibrations .If calibration changes then it can shake the whole pyramid as characterizations becomes INVALID which is detrimentally affect results of conversion.
A printing press is a complex machine that operates at a high speed. A printing press operates so as to produce density/L,a,b values that are in accordance to with accepted standards for offset printing known as REFERENCE PRINTING CONDITIONS.
Instead of profiling each individual press,it is more convenient to run a press according to specific reference printing conditions and then for the designer, pre press and proofing to use a standard profile that represents this condition.
One of the major disappointment in colour imaging is that “printed image does not match with the monitor screen .
For example from above image: The gamut of an RGB (bigger volume) is much larger than that of a CMYK print process ( Smaller volume) which results in elimination of many colors that may have been captured by camera ( let us RAW or PROPHOTO) or which could be viewed on a screen during print process in CMYK.
Major issue lies with difference Colour gamut. The gamut of a device is measured during characterization process