The nature of colour.
You are sitting in a windowless room at midnight, there is no light source in the room, so the room is pitch dark. What colour is the wall. You might think this is a stupid question but it goes to the central core of what colour is. We tend to think that the absence of a light source in this situation as academic, but the colour of an object that we see is dependant not just on the properties of the object but also on the constituent wavelengths of the incident lightsource.
The colours that we see everyday in nature are usually subtractive. That means an object reflects some of the light that is incident on it and absorbs others. Say you are in a room that is light by a red light source that has very little blue light. If someone comes in wearing blue clothing their clothes might appear black because there is no blue light to reflect off it, while the red wavelengths get absorbed by the clothing material.
The colours that we see on the printed page are very similar in nature to the colours that we see in everyday life in that they are subtractive in nature. Cyan,Magenta, Yellow and Black pigments or dyes are deposited on paper, these absorb some of the incident light and reflect others
The colours that we see on computer monitors are very different. Computer monitor generated colours are additive, that means they are generated by adding the colours red, green and blue together in varying amount, this also means they are not dependent on the incident light. Because of the way onscreen colours and printed colours are generated the colours we see onscreen may not always match the colours on paper. Some other possible problems are:
- The gamut of colours that your printer can print may not match up to the gamut of the monitor.
- The interaction of the dye or pigments with the paper can vary from paper to paper.
- The white point, brightness/contrast and saturation settings of the monitor are adjusted on the monitor not on the computer which means that unless the monitor has been calibrated the computer will not know the exact colour displayed on the screen.
- Our eyes can get used to the colour balance of incident light sources affecting the way we perceive colours.
Colour management.
When we edit a photo on a computer we see the effects on a monitor. The computer has no way of knowing the exact colour that the display is showing as the user may have changed the brightness, contrast or colour temperature of the display via the on screen buttons. Also a monitor has a range of possible colours that it can display (this is called its gamut) and the computer can be asking for the monitor to display a colour that is outside of its gamut. The way we cope with this is to calibrate the monitor. This involves placing a colorimeter ( a device about the size of a mouse which measures the colour that the display is showing) and running the software that accompanies it. The software asks the monitor to display a range of colours and the colorimeter measures the colour that the monitor is actually showing. This is then used to generate a monitor profile which photoshop and other colour managed applications can use to know what colour the monitor is showing. Once a profile is generated you should not adjust your monitor settings as it will invalidate the profile. Monitors should be profiled frequently as they can drift over time.
A similar process is used for photo papers. You print an array of colours and use a device to measure what the colour/paper combination looks like. Many paper manufacturers offer a profiling service for free you just download a file from their site, print it on each of the papers that you use, send the printed pages to them and they email you printer profiles for each printer/paper combination which you then install on the computer, Alternatively when you install a printer, a set of profiles covering the printer manufacturers own papers will be installed.
Photoshop can use these printer and monitor profiles to understand how the colours of an image are being displayed on the screen and printed on paper. They are an important part of our attempt to make the image we see on screen and the image we print as close as possible.
Printing in photoshop.
When you go to print an image make sure that you set the following four settings in the print dialog box:
- Set colour handling in the print dialog box to “photoshop manages colors”
- Set the printer profile to the profile of the paper you are using.
- Set the rendering intent to “perceptual” or “relative colorimetric”
- Check that “Black point compensation.” is ticked.
Rendering intent is concerned with how a printer manages to print out of gamut colours.
Perceptual means that photoshop will try to retain the visual relationship between all the colours in an image so that the colours look natural. This will be done at the expense of colour accuracy as some in gamut colours will be printed differently than they appear in the screen image. Effectively the image colour space will be squashed to fit into the printers colour gamut. This is best suited to images with a lot of out of gamut colours .
Relative colorimetric means that colours that are in gamut are printed as normal but colours that are out of gamut are substituted with colours that are in gamut. Using relative colorimetric results in a print that more accurately reflects colours in the original image but the visual relationship between out of gamut colours and in gamut colours may be impinged. Relative colorimetric is the default intent in the US and Europe and is better suited when an image has fewer out of gamut colours.
Black point compensation should be ticked because there is a maximum amount of black ink that a paper can take. Put more ink than that on the paper and it will not change how black it is. This can result in shadow and dark areas where there is a lot of ink to be deposited loosing detail and looking like a splodge. Ticking black point compensation causes photoshop to compress the gamut in order to allow details in these dark and shadow areas to be expressed.
The image you see on the left hand side of the dialog box is Photoshops best attempt to show you how your print will look like.