There is a wide range of contactless metrology systems available these days all with their own advantages and disadvantages. In the blog post below we’ll look and compare some of the most commonly used systems on the market and the principals on which they are based.
Laser Scanning
The term laser scanning very often refers to laser triangulation; it is based on the principle of emitting many thousands of laser points in the form of a line on to a surface of the object to be scanned then recording the refection via a receiver (usually high power optics). Surface shape and texture can then be determined by the deformation in this laser line, as the distance and angle from laser source to receiver is known the system can then establish the depth of each point on the surface via Pythagoras theorem. As this process happens with many thousands of points per second the system is able to determine the shape of the object being scanned.
The main advantages of the laser triangulation method are speed, versatility and accuracy. With the modern forms of metrology laser triangulation systems capable of measuring data with an accuracy of 0.02 mm and able to capture up to 800,000 points per second and based in a unit not much bigger than your hand. All of this means you can work your way round an object building up a very accurate digitized model of point cloud data.
The main disadvantages of laser triangulation are historically the systems can be affected by surface finish, a darker surface requires a more intense laser setting in order to get the surface to reflect enough of the laser points. However if the surface has a gloss or polished finish it will cause this more intense laser to disperse and form glare causing the system to read false data. The latest scanning systems used by professional companies such as Qualis Engineering have developed ways around this, such as instead of altering the laser intensity changing the exposure levels on the receiver to avoid this glare and software features to recognise and remove any incorrect data captured. Or the introduction of blue light lasers, which are further on the light spectrum than red in relation to natural light meaning the system picks up less glare. The other big challenge faced by the triangulation method is putting the data in to context, as the system only picks up data as the laser line passes over the surface the system requires another form of reference to determine the units location relative to the surface. Many of the high accuracy systems are mounted on metrology arms that rely on rotary encoders to track movement and orientation in the joints of the arm, others such as hand held units rely on optical targets and reference to patterns to track location, others rely on infer-red light and a stationary tracking unit in order to reference the movement in the unit.
Lidar (Laser pulsed 3D scanning)
Lidar relies on much of the same principals as Laser Triangulation, with the exception that it is applied over a much greater distance and therefore relies on the time taken for a single laser point to be returned to the sensor. Modern systems can increase the accuracy and amount of data captured by pulsing or phasing the laser source so multiple points are returned to the sensor every second.
The Advantages of Lidar are that it can capture data over a large area (100’s of meters) within a couple of mm accuracy, largely used in the surveying and construction industry a Lidar scanner is capable of capturing its entire environment from one stationary point eliminating the need to put the scan data into context.
Due to the nature of Lidar relying on a single laser point the system must emit the laser a full 360 degrees around the target object to capture shape. This is often performed by a rotating mirror built into the system, but very often an object will need to be scanned from multiple angles if a full scan of a single object is required which can be very time consuming and require some post processing in order to stitch the scan data together. Another disadvantage is the resolution and accuracy whilst a lidar system is very accurate over large distances it cannot compete with the resolution and accuracy of laser triangulation or structured light over smaller distances.
Structured light
Structured light scanners work on the principal on shining different light patterns over a target object and recording the difference in shadow between the patterns. A pattern is projected then an image captured and analysed, the system then uses these differences in how the patterns and shadows fall on the faces of the target object to determine surfaces and edges.
The Advantages of Structured light is that early fringe projection systems were one of the most accurate methods of contactless metrology on smaller objects (although modern laser triangulation systems are now just as accurate) modern systems can capture surface colour and texture, which is very useful for model rendering and widely used in the animation industry. Each projection of light can also captures some environment this alongside the ability to capture colour and texture makes it easier to put scanned data into context than laser triangulation (although not as accurate without the use of additional optical targets).
The disadvantages of structured light is it’s susceptibility to variations in environmental lighting, if there is a dramatic change in the environments light whilst the measurement operation is occurring it can have a dramatic effect on the results. Another disadvantage is the more accurate metrology systems require laboratory conditions and must remain stationary during operation this limits there versatility as to change from large to small target objects requires a big change in set up.
Photogrammetry
Photogrammetry is the process to constructing 3D measurements from 2D photographs. The process starts by taking multiple pictures of the target object from multiple angles. All of these images must include a reference scale. The system then uses this reference scale and the image pixels much like the point cloud data of a laser scan to stitch all of the 2D information together and produce a 3D model.
The main advantages of photogrammetry are versatility; as this method can be applied to any set of photographic images it can be applied to virtually any object in any environment, with the correct equipment it is possible to capture very high resolution and accuracy on either smaller target objects or large objects which would otherwise rely on Lidar.
The main disadvantage of Photogrammetry is cost; specialist photographic equipment is required alongside the requirement for a lot of post processing from high spec computers. It can also be very time consuming given that multiple images of each target object are required and the outcome can be heavily influenced by the human operation of the photographic equipment.
In summery the type of contactless metrology equipment required very much depends on the target object, budget and environment. If high accuracy is required over an area 0.1 to 20 meters then laser triangulation will provide the highest accuracy and versatility. If the area to be covered is around 20 meters to 600 meters then Lidar is would be the preferred option. If the environment is more specialists then photogrammetry may be the most appropriate method.
Qualis Engineering provides high accuracy mobile 3D Scanning Services across the UK, for more information take a look at our website https://www.qualisengineering.co.uk/or drop us an email info@qualisengineering.co.uk
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