Why is the Sun visible with 3D Laser Scanning?

I LOVE finding out about different ways to appreciate the Sun and light in general. Use this forum to post your info or questions about various outside the mainstream ways to appreciate our life giving star!
Post Reply
AnthonyM
The Sun?
The Sun?
Posts: 6
Joined: Tue Jun 15, 2021 4:57 am
Has thanked: 1 time
Been thanked: 3 times

Why is the Sun visible with 3D Laser Scanning?

Post by AnthonyM »

Hi,
I just wanted to know why the Sun appears in 3D laser scans I have taken when viewing the preview images from the scanner (i.e. using a FARO Focus 3D laser scanner). The images are in gray scale as they represent the intensity levels of the reflections from the laser. I notice that the sun itself is not captured but what appears to me to be the corona is somehow still recorded. Attached some of the laser scan preview images. Just really asking out of curiosity, any advice regarding what I'm looking at appreciated, thank you!
Image -  01.jpg
Image - 01.jpg (329.43 KiB) Viewed 882 times
Image -  02.jpg
Image - 02.jpg (350.73 KiB) Viewed 882 times
Kind Regards,
Anthony


User avatar
GreatAttractor
Almost There...
Almost There...
Posts: 964
Joined: Sat Jun 01, 2013 1:04 pm
Location: Switzerland
Has thanked: 747 times
Been thanked: 753 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by GreatAttractor »

Hi Anthony,
The Sun is the small black disc, correct? I suspect the rings are due to some internal reflection in your optics. If they were e.g. due to an actual halo in the atmosphere (matching your sensor's expected wavelength), I'd expect them to be circular without this truncation.


My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scopes (90 mm, 200 mm), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)
User avatar
Montana
Librarian
Librarian
Posts: 34527
Joined: Mon Oct 17, 2011 5:25 pm
Location: Cheshire, UK
Has thanked: 17523 times
Been thanked: 8763 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by Montana »

A very warm and sunny welcome Anthony :) these are quite incredible pictures.

Alexandra


AnthonyM
The Sun?
The Sun?
Posts: 6
Joined: Tue Jun 15, 2021 4:57 am
Has thanked: 1 time
Been thanked: 3 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by AnthonyM »

"The Sun is the small black disc, correct?" Yes that is right, it is actually the absence of the sun (i.e. black= no recorded data or no reflection of the laser light bouncing back to the scanner from the surrounding surfaces and grey scale for the reflection value of everything in between). Sounds plausible regarding internal reflections within the scanner as you mentioned the visible rings are truncated/cut off on either end.

Its still odd to me that the sun itself is not recorded at all yet some artifacts in terms of the rings etc are still there in the 3D scan. So the scanner is picking up some solar wavelengths but not the sun itself. Also strange that the scanner is recording anything at all in terms of solar radiation since even if the solar wavelengths recorded match those of the laser, the 3D laser scanner is meant to record the time of flight of the laser light generated internally and bouncing back to give an exact distance to surfaces and hence their geometry, repeated millions of times during the scanning process to generate a 3D pointcloud of data - there is no time of flight/distance data associated with the sun yet there it is :)
Close up image with brightness/contrast enhanced
Close up image with brightness/contrast enhanced
Image - 03.jpg (559.93 KiB) Viewed 838 times


User avatar
Montana
Librarian
Librarian
Posts: 34527
Joined: Mon Oct 17, 2011 5:25 pm
Location: Cheshire, UK
Has thanked: 17523 times
Been thanked: 8763 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by Montana »

Do you know what wavelength of light the laser is (nm)?

Alexandra


AnthonyM
The Sun?
The Sun?
Posts: 6
Joined: Tue Jun 15, 2021 4:57 am
Has thanked: 1 time
Been thanked: 3 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by AnthonyM »

Hi Alexandra,
The tech sheet for the scanner mentions a wavelength of 1550nm, thanks.


User avatar
Montana
Librarian
Librarian
Posts: 34527
Joined: Mon Oct 17, 2011 5:25 pm
Location: Cheshire, UK
Has thanked: 17523 times
Been thanked: 8763 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by Montana »

Thanks Anthony,

I was wondering whether it was picking up any known absorption lines and thus faint emission in that region. The Sun is black hinting at an absorption line, otherwise I would have thought it would have emitted plenty of light at 1550nm. Here is a link showing the main lines which solar observers look at
https://solarnutcase.livejournal.com/9556.html

Your scanner is looking for light at 1550nm, the question is, is that what it emits or what it is looking for when it is rebounded? would that be slightly red shifted when rebounded? I have no idea.

But there is one line at 1565nm Fe I, however that is used for the photosphere and magnetic lines. Nothing in the corona. You are about 15nm further into the blue which equates to 150 angstroms which is quite wide. Usually for these spectral lines you need a width of 2-4 A to see anything (within the width of the band).

It is quite a remote thought but possible and a nice idea, but I am absolutely no expert. Very interesting though. The most probable cause is as said before some reflections.

Alexandra
PS I will copy this topic and add it to the spectroscopy part of the forum. There are some geeks in there who may know more and may spot this post.


User avatar
p_zetner
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1644
Joined: Sun Mar 18, 2012 4:59 pm
Location: Winnipeg, Manitoba, Canada
Has thanked: 166 times
Been thanked: 719 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by p_zetner »

Hi.
If this is a 3D time-of-flight scanner, then it operates by measuring the time interval between a start laser pulse (launched by the device) and the reflected version of this pulse received at the scanner. The pulse is launched, the scanner's internal counting electronics starts whirring away and the count stops when the reflected pulse (a somewhat dimmer pulse of 1550nm light) is received. The Sun is intensely bright (even at 1550nm) and is likely swamping the detector. There is a "reflected" pulse measured at the scanner for all time intervals because of the continuous arrival of a huge flux of solar photons. My guess is that the detection electronics are then saturated and heavily biased toward counts of extremely short timing intervals, hence, the Sun appears black. The concentric rings around the Sun are, as Filip mentioned, likely due to internal reflections in the glass cover of the scanner which will also introduce continuous arrival times of photons but with far less intensity...not saturating the electronics.
Cheers.
Peter


User avatar
Carbon60
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 14175
Joined: Wed Mar 07, 2012 12:33 pm
Location: Lancashire, UK
Has thanked: 8355 times
Been thanked: 8103 times

Re: Why is the Sun visible with 3D Laser Scanning?

Post by Carbon60 »

I agree with Peter. The Sun looks black probably because of over saturation of the sensor. I had the same effect with a webcam based solar targeting device. The Sun appeared as a black disk for this reason.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
Post Reply