CloudCompare forum

Hello:

I know this question is not directly related with CloudCompare, but could be interesting for people interesting in TLS measurements. I'm working with PTS files, exported by Faro Scene software (also observed with a Faro TLS), and I'm confused about the format. I know the first line contains the total number of points and all remainder lines stores X Y Z intensity R G B. My problem is related to the 'intensity' column. I can't find any document where the PTS format is explained, so I confused with the intensity values limits. I've read in some forums that the intensity values take a range between -2048 to 2047, but in other places I've read that the limits are -4096 to 4095. Also, here (viewtopic.php?f=9&t=1281&hilit=pts) a user puts an example where intensities are up to 7500. Or maybe each software (Scene, Cyclone, etc.) has its own criteria?

First of all, which is the meaning of such values? I think the upper limit means 100% of relative emitted intensity signal, and lower limit means 0% intensity, i. e. there is not received signal. Am I right? Another question is about the true limits. Anybody knows them? Anybody knows where is (if exist) the official PTS file format specifications document?

Thanks

In fact I don't think there are any specifications on the intensity values (to be more correct there may have been some internal specifications written by the first one who came up with this format - I believe it was Leica but I might be wrong - but now that everyone uses it as an almost 'standard' exchange format, this has been long forgotten and gladly ignored ;). If it was indeed Leica, then the 'true' range should be [-2048 ; 2048] (see "Cyclone pointcloud export format - Description of ASCII .pts format").

Today what is stored in the 'intensity' field highly depends on the scanner manufacturer (or the software - CloudCompare will simply save the active scalar field there ;). And what the manufacturers put in this field is generally either the reflectivity/reflectance or the signal amplitude. Some expresses it in dB (or a multiple of it), but others simply put what their sensor stores (and I guess that it may even be the raw ADC component output, in which case only them know how to convert this to a 'physical' value!).

By the way here is what one of them (kindly) explained to me recently:

• Typically intensity values don’t have any physical unit. When explained, I often hear it has something to do with how much energy is reflected from the object. We tried to put a meaning in intensity by measuring the signal strength of the echo and setting up the context with hardware parameters. This is why we decided for dB as a unit as it expresses the ratio of two figures.
• For amplitude the reference value is the detection threshold. This is the lowest signal we can detect on an instrument. All signals below that threshold are drowned in the environment noise. Everything above is measured and expressed as Amplitude.
• Reflectance is slightly different. We know a white diffuse plate is the perfect target for a laser scanner. What we do is measure the echo amplitudes of such reference targets at the various distances. This is stored in the instrument. For an actual measurement we compare the measured amplitude to the amplitude of a white target at the same distance. The ratio is the reflectance. The beauty is, that this ratio is range independent. So the same ‘material’ shows up in similar intensity at 10 meters or at 500 meters. This gives you a much more homogeneous visual representation of your scene.

Daniel, thank you for your explanation. This is the problem with pseudo-standards...

The reason because I asked about the PTS and intensity column is I'm working with a Faro Focus X 330 and I would like to understand some data about the range uncertainty. The TLS brochure says that range error has a fix part of +/- 2mm, but also shows a variable part called "Ranging noise", which is defined as: where 90% refl. means 90% reflective surface. So, assuming the PTS 'intensity' positively correlated with reflectivity, I think the ranging noise could be estimated using the 'intensity' PTS column and the data in the brochure. But also should be stated if the upper 'intensity' bound is 100% relf. and the lower bound is 0% refl., and also which are the bounds used by the Scene software.

Ok I understand.

I have no info about Faro intensity ;)

A good place to ask about this would be the laser scanning forum (http://www.laserscanningforum.com).