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3ShapeSupportGuy
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Hi guys
I have been following the discussions about technology vs. performance for different kinds of dental scanners. I really find everybody’s input very interesting.
I would like to contribute to the discussion by throwing some additional light on the technical differences between “White Light scanners” and “Laser scanners”.
The plain answer first: It really doesn’t make a difference – other factors are decisive for scanner performance.
Now for the detailed story. In general, there is a lot of confusion about supposed technical advantages of various kinds of light and light sources. Let’s have a look at this.
White light images are sharper:
The opposite is true. Optical lenses refract light of different colors differently. As white light is a combination of all colors, not all these colors can be in perfect focus. This phenomenon is called chromatic aberration [1]. Its effect can be minimized by careful design and assembly of a scanner’s lens system, but it will never disappear completely, and thus it will limit the accuracy of a white light scanner. Lasers, in contrast, are single color light sources and hence don’t suffer from chromatic aberration.
White light penetrates objects less and is reflected better:
An object that appears white reflects all colors equally well. Gypsum casts are at least close to white. As one can easily see, gypsum is very matte which indicates that there is essentially no light penetration through gypsum in general. So all colors work equally well with gypsum. The debate may be muddled by confusion of white light dental scanners with white light interferometers [2] or white light confocal microscopes [3]. These latter instruments are indeed very accurate (and expensive),but they exploit other physical phenomena than dental white light scanners, which are based on the simple principle of fringe projection.
White light sensors are preferred in industry:
Not correct. The majority of scan heads for coordinate measurement machines, which are the reference standard in industrial metrology with accuracies down to 3-5 micron, use lasers. Note that coordinate measurement machines are the most widely used high-accuracy measurement devices. In addition, some industries use white light and laser 3D scanners for less demanding measurement tasks.
White light sources have a longer life time:
The opposite is true. Halogen lights have a mean lifetime of thousands of hours and usually have to be replaced, whereas lasers and LED’s generally outlast other electronics components [4].
White light gives better resolution than laser light:
Camera resolution is what really matters, i.e. how many megapixels the camera sensor has. High end dental scanners have 5 megapixels, typical dental scanners have 1.3 megapixels.
Lasers are imprecise due to speckle:
Again, true in theory, but the effects are too small to matter in practice. Speckle is an effectively random variation in laser light intensity across a dot or line of laser light, resulting in some uncertainty in detecting its center in a scanner’s camera. However, lasers in dental scanners can be driven in such a way that the magnitude of this uncertainty is only approximately 1 micron [5]. Furthermore, advanced image processing algorithms can correct for speckle artifacts, such that the overall effect is negligible.
White light scanners save one axis in the motion system:
Somewhat true - if you are willing to live with a serious disadvantage. White light scanners project a multi-line pattern from a central light source, while a laser only gives a single line and thus has to be moved along a linear axis. However, the additional degree of freedom gives better coverage. From the extreme position of the linear axis, the laser can illuminate regions that are shadows when illuminated from a central position only. As linear axes can be built to be very precise (1-2 micron),the advantage comes at no noticeable accuracy cost.
White light sensors capture data faster:
There is no reason why this should be true. A sensor’s frame rate has nothing to do with the color of light that it senses. In fact, several white light scanners in the dental market are much slower than some laser scanners.
In conclusion, there is no fundamental law in physics that determines that one type of dental scanners is better than others. What really matters for obtaining optimal accuracy are advanced image processing software algorithms, high-resolution cameras, a well-designed mechanical motion system, and sound craftsmanship. To ascertain a consistent level of quality in every-day use, it is furthermore imperative that scanners are supplied with a reference / calibration object and procedure
References
[1] Chromatic aberration - Wikipedia, the free encyclopedia
[2] Encyclopedia of Laser Physics and Technology - white light interferometers, dispersion measurement, low-coherence interferometry
[3]Frank JH, Elder AD, Swartling J, Venkitaraman AR, Jeyasekharan AD, Kaminski CF: A white light confocal microscope for spectrally resolved multidimensional imaging, Journal of Microscopy, vol 227, pt 3 September 2007, p. 203–215.
[4] Sony Global - Laser Diode
[5] Dorsch, RG, Häusler G, Herrmann JM: Laser triangulation: fundamental uncertainty in distance measurement, Applied Optics, vol 33, no 7, March 1994, p. 1306 – 1314. (See Figure 7. In dental scanners, lasers can be driven to behave like an LED.)
I have been following the discussions about technology vs. performance for different kinds of dental scanners. I really find everybody’s input very interesting.
I would like to contribute to the discussion by throwing some additional light on the technical differences between “White Light scanners” and “Laser scanners”.
The plain answer first: It really doesn’t make a difference – other factors are decisive for scanner performance.
Now for the detailed story. In general, there is a lot of confusion about supposed technical advantages of various kinds of light and light sources. Let’s have a look at this.
White light images are sharper:
The opposite is true. Optical lenses refract light of different colors differently. As white light is a combination of all colors, not all these colors can be in perfect focus. This phenomenon is called chromatic aberration [1]. Its effect can be minimized by careful design and assembly of a scanner’s lens system, but it will never disappear completely, and thus it will limit the accuracy of a white light scanner. Lasers, in contrast, are single color light sources and hence don’t suffer from chromatic aberration.
White light penetrates objects less and is reflected better:
An object that appears white reflects all colors equally well. Gypsum casts are at least close to white. As one can easily see, gypsum is very matte which indicates that there is essentially no light penetration through gypsum in general. So all colors work equally well with gypsum. The debate may be muddled by confusion of white light dental scanners with white light interferometers [2] or white light confocal microscopes [3]. These latter instruments are indeed very accurate (and expensive),but they exploit other physical phenomena than dental white light scanners, which are based on the simple principle of fringe projection.
White light sensors are preferred in industry:
Not correct. The majority of scan heads for coordinate measurement machines, which are the reference standard in industrial metrology with accuracies down to 3-5 micron, use lasers. Note that coordinate measurement machines are the most widely used high-accuracy measurement devices. In addition, some industries use white light and laser 3D scanners for less demanding measurement tasks.
White light sources have a longer life time:
The opposite is true. Halogen lights have a mean lifetime of thousands of hours and usually have to be replaced, whereas lasers and LED’s generally outlast other electronics components [4].
White light gives better resolution than laser light:
Camera resolution is what really matters, i.e. how many megapixels the camera sensor has. High end dental scanners have 5 megapixels, typical dental scanners have 1.3 megapixels.
Lasers are imprecise due to speckle:
Again, true in theory, but the effects are too small to matter in practice. Speckle is an effectively random variation in laser light intensity across a dot or line of laser light, resulting in some uncertainty in detecting its center in a scanner’s camera. However, lasers in dental scanners can be driven in such a way that the magnitude of this uncertainty is only approximately 1 micron [5]. Furthermore, advanced image processing algorithms can correct for speckle artifacts, such that the overall effect is negligible.
White light scanners save one axis in the motion system:
Somewhat true - if you are willing to live with a serious disadvantage. White light scanners project a multi-line pattern from a central light source, while a laser only gives a single line and thus has to be moved along a linear axis. However, the additional degree of freedom gives better coverage. From the extreme position of the linear axis, the laser can illuminate regions that are shadows when illuminated from a central position only. As linear axes can be built to be very precise (1-2 micron),the advantage comes at no noticeable accuracy cost.
White light sensors capture data faster:
There is no reason why this should be true. A sensor’s frame rate has nothing to do with the color of light that it senses. In fact, several white light scanners in the dental market are much slower than some laser scanners.
In conclusion, there is no fundamental law in physics that determines that one type of dental scanners is better than others. What really matters for obtaining optimal accuracy are advanced image processing software algorithms, high-resolution cameras, a well-designed mechanical motion system, and sound craftsmanship. To ascertain a consistent level of quality in every-day use, it is furthermore imperative that scanners are supplied with a reference / calibration object and procedure
References
[1] Chromatic aberration - Wikipedia, the free encyclopedia
[2] Encyclopedia of Laser Physics and Technology - white light interferometers, dispersion measurement, low-coherence interferometry
[3]Frank JH, Elder AD, Swartling J, Venkitaraman AR, Jeyasekharan AD, Kaminski CF: A white light confocal microscope for spectrally resolved multidimensional imaging, Journal of Microscopy, vol 227, pt 3 September 2007, p. 203–215.
[4] Sony Global - Laser Diode
[5] Dorsch, RG, Häusler G, Herrmann JM: Laser triangulation: fundamental uncertainty in distance measurement, Applied Optics, vol 33, no 7, March 1994, p. 1306 – 1314. (See Figure 7. In dental scanners, lasers can be driven to behave like an LED.)
