Basic Guidelines for Image Capture and Optimisation
This document provides a guide to good practice in still image capture to help the user to design a workflow for capture and optimisation. The document highlights the main points that should be considered to ensure the highest quality output from your equipment.
No matter how high the specification of the equipment that you use to digitise your original images, the image-files can easily be degraded by poor capture technique or inattentive processing within the workflow.
These guidelines and procedures offer examples of good practice that will help you design a workflow that creates and maintains the highest quality digital images possible from your originals. The advice on these procedures is laid out in the approximate order that they will be encountered within the workflow.
The basic skills of image capture and manipulation are not hard to master, however it should not be presumed that they are so easy that operators can learn how to excel in them without training and practice. In theory it might be possible to create a workflow that is so automated that operator skill is kept to a minimum. In reality it will be found that the varied demands of creating high quality images will always make it a task for highly skilled and well-trained operators.
The image capture process can be undertaken either by digitising direct from the original work or by digitising an analogue surrogate image, such as a transparency or print of that original. The capture device can either be in the form of a scanner designed to capture 2D works or a camera which can be set up to work with either 2D or 3D originals.
Pre- image capture
|Equipment, software & environment||All equipment should be tested and calibrated. The capture studio should be prepared with consistent controlled lighting and the digitisation team trained in the use of all capture equipment and software within an established and standardised workflow.|
|Testing and establishing operational specifications for image capture||Before any image capture can be undertaken it is imperative that the operators know the required File sizes and File Types. Once these have been established they will become part of the Operational Specifications. These should be established and agreed with all interested parties before the onset of project.|
|Establish capture workflow||All tasks within the capture workflow should be considered and a manual of 'Good Practice' established to guide all operators.|
|Consider the use of an 'Objective' or 'Subjective' colour management system within the capture workflow||
In an objective capture workflow the colour of the digital images can be objectively measured and compared against the original work. This is normal within a 'Direct Digital Capture' workflow where there has not been any intermediary image. This is a typical capture workflow when working with a digital camera or when scanning direct from the original.
An objective capture workflow should be calibrated and characterised using the ICC profiling system and then operator colour adjustment prevented.
In a subjective capture workflow the colour of the digital image can only be compared against another 'analogue intermediary' image (often a copy transparency of the original work). As this analogue intermediary has already introduced a subjective element to the digital image, it will be necessary to allow the operator to make colour adjustments according to his skill and understanding of the original work to correct any possible fault within the 'analogue intermediary'.
This 'subjective' workflow is typical of any system that is working with some form of copy of the original.
|Collect and prepare original images||Agree with curators / conservators that all originals are in a stable state and can withstand the necessary handling. If this is not the case, now is the time to have any required conservation work done. It is very important that originals can be provided for capture at a sufficient rate to prevent any bottleneck in workflow.|
|Clean originals||If the original is a work of art then the method of cleaning will be dependent upon the requirements of the curator, conservator or owner. However any cleaning at this stage will provide higher quality and speed than having to digitally 'clean' them after capture.|
|Clean capture device||Scanners must be kept scrupulously clean, however cameras need to be cleaner still. All capture equipment should be externally cleaned every day and internally cleaned as required, which is normally at least each week. Dust is a particular problem with digital SLR cameras frequent swapping of lenses exposes the sensor to airborne particles. All cleaning should be done in strict accordance with the manufacturer's recommendations. This may require sending the camera to an authorised service centre. Try to reduce the camera's exposure to dust by keeping it in a clean bag or case with all lens and body caps fitted.|
|Capture at the highest quality colour settings offered by the capture device||
Capture at the full colour depth offered by the capture device. This should certainly be at least 24bit (8bit per channel), however many scanners now offer the ability to capture at higher colour depths (30-48bit).
This will be slower, uses more memory and will increase storage requirements. However the archive image will then have been stored at the best possible quality from the device allowing re-use with confidence.
These 'high-bit' files will provide higher quality images (increased maximum density and shadow detail) which can then be used to down-sample to 24 bit colour images.
Many scanners also offer a choice of quality levels to the user. The 'High Quality' scan provides a higher quality by multiple sampling where the scanner effectively scans twice (or more) and then averages the results. This improves the quality but extends the scanning time. It is certainly recommended as long as the project can justify the extra time taken to capture the work
|Capture at the highest resolution necessary for your uses (or larger than your current needs)||Operational specifications should be set that quantify the required file sizes and formats for your project. Images must be captured at a resolution that provides files to this size or larger. Remember that capturing at a higher resolution will provide some extra future-proofing at the price of extra archive and storage costs.|
|Record the metadata||It is important to collect as much administrative metadata (technical file details such as device settings, operator, bit depth) as possible and that is entered as close to the time of creation as possible. This should either be done automatically within software or manually by the operator at the time of capture. It is bad practice to go back and do it at a later date as people are likely to forget and data will then be lost forever.|
|Visually check each and every image||Every image should be given a visual check for any obvious faults, re-capturing at time of creation is easier and less time consuming than having to return to the original at a later date.|
|Once image is captured, save it||Save the image within a 'standard' open uncompressed file-format. This is normally TIFF. The image file should be saved with a unique name following a standardised name structure that provides enough information to locate the original image. See our document on Choosing a File Name.|
Once the image has been captured by the input device (scanner or camera) and saved as a master archive file, it is necessary to optimise the image-file to prepare it for its proposed use. This optimisation can be considered in two parts; an initial generic optimisation is made to all files and then a further specific optimisation that creates the surrogate image for a targeted use such as Web delivery or print use.
Initial generic optimisation
|The exact method of undertaking these tasks will depend upon the Image manipulation program that you choose for the job, however all stages here are quite generic and will be available in all common image manipulation programs.|
|Create a working copy||It is best practice to first create a working copy of the image from the master archive file, then if for any reason things go wrong within the manipulation you know that the master archive image is safe. This file should be named in such a way as to both connect it to the original and also show that it is a new working copy of the original image.|
|Crop if necessary||Check the size, shape and orientation of image and adjust if needed. There is no point in saving image area that is surplus to needs.|
|Optimise density range||Use levels tool to adjust shadow and highlight points to best use the whole range of available tones within original image.|
|Check and correct any fault in the colour of image file||Use curves tool to adjust and modify the colour balance within each separate channel.|
|Check image for any faults or artefacts||Each file should be visually checked for any marks or dust. If it is easy to fix these within software, then it should be done. If not then the image should be considered to have failed the QA and should be marked for subsequent re-capture.|
|Apply sharpening if necessary||It is best practise to add no sharpening at this point, however digital cameras and some scanners have an inherent softening effect on the images and it can be necessary to apply some small element of Unsharp Masking. This should certainly be limited to only very slight work that repairs image rather than augments image.|
|Save your work often||Some image editors have a limited undo capability (often just the last operation). Others keep a ‘History’ of operations that can be selectively deleted or changed, however remember that the number of ‘History’ levels will quickly expand the file size and therefore memory requirements.|
Further specific optimisation
|After the image has been generically optimised, it will be necessary to undertake further optimisation specific to the required use of the image.|
|Save again||It is best practice to again save the image at this juncture so you have both the original 'Master archive' image and an 'Adjusted master' image for later use. Both file names should be unique and reflect the differing use.|
|Resize||Any surrogate images should be made by re-sizing direct from the 'Adjusted master' archive image. It is important that images are only made smaller rather than larger.|
|Sharpen image||If it is necessary to re-apply any sharpening, now is the time to do it, between the re-sizing and any compression made to image. Sharpening should be undertaken with great caution! It is imperative that image is not over sharpened (only to remedy rather than augment image).|
|Save image (within appropriate file-format)||
Choose the appropriate file-type for the proposed use. This is likely to be either TIFF (for print or archive) or JPEG (for delivery on monitor).
If an image is being saved as a JPEG, it will be necessary to choose a level of compression for the file; this choice is a compromise between quality and file-size and will need to be deduced by testing. As a rule of thumb a JPEG compressed to 10% of its original size is normally thought to be visually acceptable in quality terms, however a larger size file can normally be compressed more than a smaller file.
|Naming File||Again it is important that the file is saved using a consistent naming structure that enables you to identify both the original file and the proposed usage for the new file.|
The complete workflow from capture through optimisation to delivery includes many decisions that will affect the quality of the image file. It is very important that all of these decisions (and who made them) are recorded as metadata for use by the project management team.
As many of these variables as possible should be decided on and standardised as part of the Operational Specifications. However if at any time there is a variable choice to be made by the operator then it should be recorded as part of the administrative Metadata for that image.
Once a standard 'image capture and optimisation' workflow has been established and shown to support all elements of 'best practice' it should be documented as the 'Image Capture and Optimisation Manual' so that it may support the quality assurance system for the project. It is imperative that once established all operators work exactly to the manual so that all image capture and optimisation is standardised.
Taking the time to fully document all the capture and optimisation can seem, at the time, less important than getting on with the project, however experience has shown that it is a great help in extending the usefulness of the images well into the future. There can never be too much documentation, but it is all too easy to be thwarted in the future by not knowing enough about the images and how they were made.
Published in: Digitisation