Data capture
The methods used in the cultural heritage typically involve no physical contact between the 3D sensor and the heritage asset being digitised to avoid damaging fragile artefacts and structures. Several approaches and systems have been developed to address different circumstances and characteristics.
Active systems emit energy (in most cases light) projected on to the object being measured which is reflected back to a sensor. These systems are affected by:
How shiny the object is (reflectance)
How transparent the object is (transmittance)
How much the surface absorbs light (absorbance)
Devices used include:
Laser scanning
Structured light
Range sensing
Time of Flight (TOF) devices
Phase Shift (PS) laser scanners
Confocal microscopy
X-Ray devices
Passive methods used ambient energy to detect the surface of the object being measured. Digital photogrammetry is the most widely used passive method.
Devices used include:
Cameras
Triangulation devices
The capabilities of the different technologies vary and several different factors need to be balanced when planning data capture. These include:
Cost - of the equipment and software to buy or hire
Skills - is extensive training required to use the equipment
Working environment - is the object indoors or outside, does the technique require a dark environment or specific lighting conditions
Range - how close or far away can the device be from an object?
Accuracy required - what is the maximum level of recorded accuracy?
Resolution required - the minimum distance between two consecutive measurements
Sampling rate - the minimum time between two consecutive measurements
Use of the 3D data - is it required for scientific analysis or visualisation
There are significant variations between approaches. For example, triangulation techniques can produce greater accuracy than time-of-flight, but can only be used at relatively short range. Where great accuracy is a requirement, this can normally only be achieved with close access to the heritage object to be digitised (< 1m). If physical access to the artefact is difficult or requires the construction of special scaffolding, alternative non-invasive techniques may be considered. If physical access is impractical then sensing from a greater distance may be required using direct distance measurement techniques (TOF, Phase Deviation) leading to less accurate results.
Consideration must also be given to the length of time available to carry out the data collection process and the relative speed of data capture of each technology.
Module 1.3 of the video training course illustrates a range of active and passive 3D digitisation techniques, the equipment, camera setups and the results.
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