Laser Scanning: Back to basics part 1
31 May 2011
3D Scanning can be daunting but our man Stuart Brown gives us the low-down in the first of a series of articles
As a newcomer to the world of scanning, the whole process can appear confusing. White light, lasers, point clouds, foot spray (yes really), post processing and polygon meshes can all appear daunting at first and possibly coupled with the well intentioned prejudices of your peers, it can be easy to become disillusioned with data overload. Scanning can appear akin to discovering the secrets of alchemy, but at its heart, the principals are not that difficult to grasp.
Three steps cover it:
- Post-process to get the scan data into the format you require.
- If necessary finish off using your chosen CAD tool to refine and get the file into a format your CAM system requires.
The ease or otherwise of the above steps is determined by:
- The suitability and quality of your equipment and programs.
- Your knowledge, skill and ability with your chosen tools.
Before I continue, I ought to tell you what this article will not cover and that is opinions on different scanning types and debates on technical issues. What it will cover is a basic overview of scanning at a practical level from someone who has made more than his fair share of mistakes on the way to being scanning proficient.
Firstly, your enthusiasm for learning is key to becoming a scanning expert. Remember Luke Skywalker in Star Wars being presented with a large bill to exit Mos Eisley? He comes up with the line something like he could almost buy his own spacecraft for that price, to which Han Solo retorts “But whose going to fly it kid?!” Same applies with scanning. If you won the lottery tomorrow and could buy whatever equipment you wanted, unless you take time to master it, you will always have problems.
Essentially scanning involves either firing a series of light patterns - structured light systems - or laser beams…well that will be a laser system then, at a subject so that the equipment you are using can collect the data and turn it into a point cloud or on some systems directly into a polygon mesh.
A point cloud is quite simply thousands of points defined by an x, y and z with a common origin. A polygon mesh is the point cloud; you guessed it, converted into polygons.
These two styles - notice not type as there are many file types - of file are the basic building blocks of creating the end result which is typically either an accurate record of the subject being scanned or a solid model that can be used as an aid to manufacture. An accurate record could be for an historical artifact or inspection of the accuracy of an “as built” item, whereas manufacturing is as the name suggests is to enable the accurate production of the scanned subject.
This leads me on to the error the vast majority of people make when deciding on their choice of scanning system. Like blinkered horses, they focus on two factors being price and resolution. The emphasis naturally on the former. Little, if any thought is given to the accuracy ACTUALLY required for the given task. Is +/- 100 microns necessary when scanning a sculpture when the person who will make it by hand has zero hope of achieving this level of accuracy?
Additionally, can your system of choice achieve the task you require of it? For example if you need to scan in the open air, a system that requires little or no ambient light to operate effectively is not going to be of much use.
An example, that embarrisingly eminates from my firm. Hey, nobodies perfect! When buying our first scanner, we thought we would think smart and do a table style analysis of the scanning systems within our budget ask a few key questions, then make our choice. So moving onto our preferred and ultimately purchased system.
Q: Will it scan something the size of a car?
A: Yes, we have a client in the USA that uses the system for this kind of work. Very successful he is to.
Q: Anything else we need to know on the way to success?
A: You will need some space around the car and the subject will need to be covered in foot spray to matt the surface, as with most systems.
Happy that we had covered all bases - although worried about footspray on car paint, we bought the system and frankly, it has proved completely unusable for our chosen application.
- After difficulties, we discovered the client in the USA had a unit specially prepared for scanning and the client’s brought their cars to him. In our business, we had to go and see the client and therefore accept whatever environment we were presented with.
- The system required a completely white matt surface in order to operate effectively, considerable space around the subject and virtual blackout conditions. At no point did we ever gain a contract where the system in question had a full tick box of affirmative answers to its environmental requirements.
- Accuracy was assessed by the user on set up and the equipment was susceptable to being knocked or key parts moved. In other words, you were always worried about calibration. Was it still accurate? After all, the whole point of you being at the client is to produce an accurate record - to his requirements on accuracy - and if you are not confident about this, then everyone is in trouble. The spectre of consequential loss should quite rightly send a shiver down your spine….even if you believe the wording of your Terms & Conditions will come to your rescue. You are quite rightly expected to be an expert in your field.
I must stress that the above predicament is entirely of our own making and the system chosen could have useful application. Tragically, not for us though.
As a newcomer to scanning you will probably hear about foot spray or similar to matt down a surface. The reason for this being required is that transparent, dark or reflective surfaces are typically hated by most systems as they cannot “get a grasp” for want of a better phrase on the surface they are trying to record. A spray is used to turn the surface white and matt in appearance.
This leads me onto foot spray. To my mind foot-spray is designed to be put on feet. At no point did the designer consider putting it on paintwork and a myriad of other possible surfaces. Even worse are home made concoctions of meths and chalk or talc. At 3D Engineers, we only use Ambersil Flaw Detector Developer 3 as it comes with a data sheet, telephone back up and is an engineering product. Put it this way. Would you like to stand up in court and defend an action against the paint falling off say an aircraft part you scanned and hear the prosecution say along the lines of “Can you explain your use of foot spray on this delicate item of equipment”? I would prefer to be able to wave data sheet and telephone record details to prove my competence. At considerably less than £10 a can, it can hardly be called expensive either.
So subject prepared, we can move onto scanning. However, wait. You have a system that demands you put positioning targets on the subject. What is all that about?
Using as examples the Creaform and ZCorp range of scanners, positioning targets are used to enable the unit to locate itself in 3D space. Essentially all - and this is simplifying greatly an incredible set of algorithms - the dots do is provide the system with a text file containing an X, Y and Z. In other words, we are back again to a point cloud being made using a common origin from which the XYZ is created.
Moving onto scanning, the key points to be considered are:
- Light - Many systems like low light or in some cases virtually blackout conditions.
- Temperature - A consistent comfortable temperature is ideal.
- Space around the subject - Varies according to system. One metre is probably the minimum.
- Health and Safety - Lead management. Other people operating in YOUR low light environment.
- Not damaging the subject. For example after a day of waiving say a Creaform scanner over a surface - especially if it is elevated - the scanner gets to feel like it weighs a lot. In fact, it feels like your arm is going to fall off at times, so make sure whatever system you use that you do not push yourself and end up with the scanner clattering on the subject surface. If potential client issues do not worry you, sending your scanning equipment back to the manufacturer for recalibration should…along with your attendent losses.
So you now have a scan or series of scans. Time to go home? No. Back up your data preferably on an encrypted device kept separate from your main equipment and ensure you leave the work area as you found it or better.
So there you go. Scanning is not as difficult as trying to find the Higgs Boson particle, but rather a set of logical steps with knowledge and appropriate equipment the foundations for success.
Stuart Brown is the main man at 3D Engineers (as featured in DEVELOP3D) and is now the UK distributor for the Noomeo Scanner. His favourite quote from a scanning job was having his ear bent by a spotter (slang: generally a time waster interested in a laser based device). Upon trying to change the subject by inquiring, “What do you think of your boss?”, said spotter replied “He is like a James Bond villain but instead of having an underground lair, he has the crap office in the corner.” then pointed.