To scan a picture, view it on a screen and 'proof' it on an inkjet before sending to press, and to maintain colour throughout the process you need colour profiles. Here's how they work.
I've got a job to design, I need to scan the photo (or take off a camera), I'd like to make sure everything is OK before committing to press. It would help if I could show the client an accurate representation and get them to sign it off. Well it's possible but there are differences between RGB & CMYK devices.
My eyes are can differentiate billions of colours, a photo has a good colour range but somewhat less than an eye, a scanner has a reduced colour gamut. So looking at an object, photographing it and scanning it into a computer gradually loses colours. A monitor is able to match the scanner, . Inkjets and presses are different, they have a relatively limited colour gamut compared to photos and eyes. Digital cameras,scanners and monitors are RGB devices, a screen is composed of red, green and blue pixels, their colour is dictated by the voltage placed across the pixel. RGB devices start with zero voltage (i.e. a black screen) and reach their greatest output in the white areas. We refer to this as additive. Printing on paper, ink jets and presses are CMYK devices, CMYK devices are subtractive, they start with a white background filters (i.e. inks) reduce the light reflecting from the substrate. Their signal is greatest in the black. Just to be difficult my inkjet's magenta is different to your magenta and they both differ from the press.
We need a way of defining colour without reference to light or ink, a colour space independent of device. It's known as Lab. Lightness, the saturation of the colour is the vertical axis and on two axes horizontally the colours are defined. These axes are labelled a & b. This provides a reference to which all RGB and CMYK colours can be converted.
An ICC colour profile is a just a database that lets the computer look up what Lab values are equivalent to a CMYK or RGB colour in your job. Once that colour is converted to Lab it can be converted to any other CMYK or RGB device.
There are limitations which we will deal with later.
