Maintaining colour accuracy is possibly the most important aspect of digitisation within the cultural heritage sector.
This process requires multiple steps to achieve, including consistent preparation before each shoot, as well as ongoing maintenance. This blog post will outline the importance of colour management and provide a walk through of the colour management process within the photographic studio at the Chester Beatty.
What is colour? And how does colour work?
To begin with lets go over what colour is and how colour works. Colour is the perception of light emitted at a particular wavelength. These wavelengths are a part of what is known as the electromagnetic spectrum, which consists of all types of electromagnetic radiation that vary in frequencies and vary in their respective wavelength. Each component of the electromagnetic spectrum emits a colour, however, the human eye is only able to see a small part of the electromagnetic spectrum, which is referred to as the visual spectrum. (1)
The visual spectrum is the foundation of how digital colour and colour management tools are built. In this case the visual spectrum can also be referred to as a map or key that indicates all of the humanly perceivable colours, which a digital colour model can be created from
A colour model is a mathematical approximation of the visual spectrum used to describe different colours using calculations. The two main colour models are RGB and CMYK. The RGB colour model is used for computers, mobile and T.V displays as well as digital cameras, whilst CMYK is commonly used for printing. A colour model does not define what is considered to be the true colours of red, green and blue and the result of mixing these colours, instead they are only relative. When the exact specification of the quality of a colour is defined (chromaticity) the colour model then becomes an absolute colour space such as sRGB or Adobe RGB or ProPhoto RGB. (1), (2)
Finally! A colour profile represents the specific numerical model of a given colour space for a specific device and is needed to maintain consistency across different monitors and devices. A colour profile helps to describe the way a particular device represents colour and ensures a colour is represented as accurately as possible. (3) In other words a colour space can be seen as a kind of map or key which helps a computer know what colour to use and where to put it in an image. Whilst a colour profile helps to refine this map in accordance to a specific device, phew!
Moving on to colour management
Colour management is the practice of ensuring all of the hardware and software used for digitisation maintain consistent colour accuracy throughout your entire workflow. At the Chester Beatty, our colour management process begins with the colour calibration and colour profiling of each monitor that will be used for digitisation. Profiling means measuring and fixing up any remaining inaccuracies in the device’s colour output. Calibration sets the device into a known and repeatable state. (3) These processes are done reasonably infrequently. The Eizo monitors that we use, contain self-calibration units, and are scheduled to do this once every two weeks.The software we use for image capture and postproduction is Capture One, regular software updates of Capture One are performed simultaneously on all workstations when required. When performing these updates, it is important to ensure all files and previous configurations from previous updates are removed.
Setting up the studio
The profiling and calibration of our equipment happens relatively infrequently. However, the next set of steps need to happen whenever we start a day’s work. Before establishing correct exposure and white balance the studio lighting needs to be fully set up specifically according to the object to be digitised’s requirements. Once the setup is complete, the object is carefully placed to one side and replaced by an X-Rite Colourchecker SG. This tool is used to set the correct white balance for each shoot and is a core part of ensuring colour accuracy in the image capture phase.
What do we mean by setting the white balance?
To understand what we mean by setting the white balance one needs to first understand what colour temperature is. Each light source has its own colour temperature which can be measured in units of kelvin. An example of varying colour temperature can be seen when comparing the difference between midday sunlight and evening sunlight, the sunlight has a different colour temperature each time and every artificial light source has its own colour temperature also.
These variations in colour temperature can cause the whites in an image to have what is called a colour cast if it is not adjusted in camera or by image capture software. Setting the white balance corrects the colour temperature to show the whites as white and renders each colour within the image more accurately.
White balance setting in practice
Within our photographic studio at the Chester Beatty, we generally use two flash heads equipped with soft boxes placed about one and a half metres equidistant on opposite sides of the collection object. This consistency reduces the amount of exposure variation, however in order to ensure better colour accuracy a white balance reading needs to be taken at the beginning of each shoot and when the lights are repositioned. The white balance reading is taken by photographing the colour checker chart and then by clicking on patch F5, which is neutral grey, using the white balance picker tool. The Curve in Capture One is also set to Linear Response.
Maintaining exposure consistency using a Golden Thread
To maintain correct exposure consistency across all of our images, an object level target known as a Golden Thread is placed beside the object. To keep track of our exposure we monitor the colour value of patch 13 on the Golden Thread. When working with RGB values this patch should give a reading of 201 for red, green and blue. It is important that the Golden Thread is in the same focus plane as the part of the object being photographed otherwise the Golden Thread may appear out of focus in the image which can lead to exposure inaccuracies.
Throughout each shoot the colour value of the Golden Thread is checked every few images to make sure all of the readings are consistent. The Golden Thread is kept in the archival images created for each image, raw and high-resolution tiffs. However, the colour bar is cropped out for the Jpeg images made to go on the Chester Beatty collections online site.
Colour management within a photographic studio is a complex system to set up and maintain. However, when a protocol is set up, it allows the studio to manage their digitisation processes more easily and, most importantly, it becomes easier to spot inconsistencies. We faced challenges within our colour management system at the Chester Beatty, which have since been overcome. The most significant challenge we have faced so far was trying to figure out why one of the three computers that are used for digitisation was showing different colour values compared to the other computers. After multiple tests were conducted this was confirmed to be caused by old configuration files that were linking up to old colour settings. This problem was overcome by simply deleting the old files. However there was, of course, plenty of hair pulling and head scratching that went on prior to the discovery of these files. To avoid this problem in future, when updating digitisation software, one needs to ensure all old files and configurations from the previous update are removed.
I greatly hope this blog on our colour management practices at the Chester Beatty was useful and interesting. Furthermore, I greatly hope you will visit our Chester Beatty collections online, we would also be delighted to have you visit us once the Chester Beatty building is open once again.
Caroline Harding, Digital Photographer at the Chester Beatty