Thursday 5 June 2008

Colour management part II - the digital workflow

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As mentioned in an earlier post, digital and electronic devices struggle to handle colours accurately. However, this can be measured and corrected by colour profiles - a process known as colour management.

Let's have a look at the digital photography workflow, which for most people will consist of a digital camera, monitor and printer. You could potentially colour manage all three devices and create individual colour profiles for them, but in practice, most people will use the generic profiles provided by the manufacturers. People with more than just a casual interest in photography, however, will tend to profile monitors as their colour output changes over time.

You can callibrate all three devices though. To do this you need a colour standard (something that defines what red is, etc) such as the colour chart shown above - these are expensive and have to be replaced regularly. These charts are supplied with software containing digital descriptions of the coloured squares on them - in the RGB colour space as red, green and blue channels (there's also saturation and luminosity information) which have values between 0 and 255 ie 8-bit. There are various RGB colour spaces with different colour ranges or gamuts - sRGB, Adobe, ProPhoto - for home digital work stick to sRGB. So to create a colour profile of your digital camera you'd take a lot of photos of the colour chart under varying lighting conditions and then use the software to compare to the standard and create a colour profile (essentially a database of colour corrections). I imagine that digital sensors don't produce significant colour shifts over time and production methods give good consistency between cameras as most people use the generic colour profiles that come with digital processing software - as in Adobe Camera Raw used by Lightroom and Photoshop - and don't have any problems colour managing their digitial workflow.

Monitors, however, are the weak link. As mentioned in an earlier post, think of the range of colours produced by TVs in a showroom all tuned to the same channel. All models have unique colour biases, there are big differences between individual examples of the same model (hence generic profiles are not as useful) and the colour bias shifts over time! It's for this reason that if you're serious about colour management (you're regularly sending your shots off to be printed elsewhere and the results are important) you need to profile your monitor - I use the Gretag Macbeth Eye-One Display 2. Let's say your monitor has a colour cast. You make corrections to your shot so that it looks good on your monitor but by doing so you've shifted all the colour data in the file you send off for printing! The profiling software displays known colours on your monitor and a spectrophotometer (see the picture above) measures the actual output. Comparison of the two allows a correcting colour profile to be constructed.

Finally you can do the same for your printer. Print off a colour standard and use a spectrophotometer to measure the output or send it off to someone who can. For home use I'm not so fussy (for weddings I use professional printers) so I stick to the generic colour profiles for printer, paper and ink combinations that came with my Epson printer - that's right these profiles are only accurate for the branded papers and inks. You should have a different profile for every paper type that you can use with your printer.

I hope that makes sense. Questions and comments are always welcome!

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Anonymous said...

Hi David and thanks for all the advice. There is so much debate on the net about colour space, I wondered what you do and what your thoughts were in relation to wedding photography? Is it a better idea to work on the photos within photoshop as sRGB and supply the customer with a jpeg in the sGRB colour space? Knowing that the web is sRGB and so will be the vast majority of labs etc the customer may use? Is there any point in supplying a different colour space unless specifically asked to do so? Many Thanks.

David said...

A pleasure!

I used to fret about things like this but I ended up plumping for sRGB.

Nobody's ever complained and I've never had a request for an alternative colour space!