SLS on a budget - camera and lens choice

Discuss about cameras, projectors, calibration panels, turntables etc.
Tzk
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Joined: 17 Mar 2020, 13:28

SLS on a budget - camera and lens choice

Post by Tzk » 11 Apr 2020, 20:15

Greetings all,

i'm a long time lurker on this (and the "old") forums, but never managed to register. I'm currently in the process to build myself an extremely low-buget SLS scanner (~300 bucks). I've read a lot of posts on here and noted that OBNracer uses a low-budget 5mp C-mount camera and lens. That said, i'd like to choose the following components for my setup:

projector: Viewsonic PLED-W500, 800p DLP LED, 1.4x throw ratio
camera: YW500U3, 5mp, YUY2: https://aliexpress.ru/item/32864044502.html
lens: ZLKC HM1214MP5: 12mm 5mp F1.4 2/3" https://de.aliexpress.com/item/32713339739.html

Now, my questions regarding this setup:
1. How did you choose the correct focal length? I assume the camera should be able to see the whole scanning pattern plus a small border around it.
2. Are these components worth buying at all? Are there other (better?) cameras or lenses available on a similar budget?

You guys can probably shed some light on this... Thanks in advance,
Oliver

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OBNRacerMan
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Location: Obninsk, Russia

Re: SLS on a budget - camera and lens choice

Post by OBNRacerMan » 12 Apr 2020, 15:47

To select a lens with the required parameters, I used an optical calculator, having previously measured what projection size the projector gives out from a certain distance (for example, from a distance of 0.5m). I also know the size of the camera matrix.
Using a calculator, I selected the focal length of the lens so that the camera’s field of view was slightly smaller or equal to the projector’s lighting area.
camera_objective_01.jpg
https://www.beward.ru/calculators/#2
However, it should be noted that good lenses are designed for larger matrices (on smaller matrices, the lens will actually have a slightly longer focal length). Therefore, initially, in my case, 8 or 12mm went well with 1 / 2.5 "arrays of my old cheap cameras. After buying expensive cameras with 1 / 1.8" arrays, I think I will buy 16mm lenses

Tzk
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Joined: 17 Mar 2020, 13:28

Re: SLS on a budget - camera and lens choice

Post by Tzk » 12 Apr 2020, 18:30

Thanks for your response. I'm aware that usually the focal length is stated for a certain sensor size, i'm also familiar with the crop-factor on regular photocameras. Luckily ZLKC states the FOV of their lenses for different sensor sizes which makes it a lot easier to calculate everything. Here's what i did so far:

First i had a look at the datasheet of the projector which gave me the following results:

distance projector to wall: 900mm
width of projected image: 642mm
height of projected image: 402mm

I assumed that my camera must ideally view the full projected image, but not much less or more. The datasheet of the cheap YW500U3 camera stated that it has a 1/2.5" CCD sensor (5.702mm wide, 4.277mm high). Then i assumed that the angle between the camera and projector is 20° to 25° (i used 22.5°). That gave me a camera-to-object distance of ~975mm at projector distance 900mm, assuming that the projector is mounted perpendicular to the aluminium support bar on the tripod and that the camera is rotated by 22.5° to the support bar.

I then fed both values into a lens/focal length calculator for the ZLKC HM0814MP5 8mm "5mp" F1.4 2/3" lens:
https://de.aliexpress.com/item/32710545879.html
Capture2.PNG
https://www.scantips.com/lights/fieldofview.html#top

I noticed that the horizontal view field of the camera is a bit bigger than the projectors image width, but not by much.
camera horizontal view: 693mm
camera vertical view: 520mm

I also did this for the ZLKC HM1214MP5 lens and got:
camera horizontal view: 462mm
camera vertical view: 347mm

Is my assumption correct that a 8mm lens will be the better choice in my case?

The 8mm lens will almost perfectly match the width of the projectors image, but will also have a bit too much view field height. However when i go for the 12mm lens, then i'll miss some parts of the projected image on the sides and the viewable height will be a bit too narrow.

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OBNRacerMan
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Location: Obninsk, Russia

Re: SLS on a budget - camera and lens choice

Post by OBNRacerMan » 12 Apr 2020, 19:04

When making my decision at that time what to buy - an 8mm or 12mm lens - I was guided by the thought that it would be better if the camera did not capture the entire projection. Therefore, I chose 12mm. This is purely my personal opinion.
Now, even with a 12mm lens, my 1/1.8 "camera captures more area than the projector shines. And I can fix this only by moving the camera about 100mm forward relative to the installation location of the old cameras.
But here a new problem arises - from a closer distance to the object, the current camera with a resolution of 6.3 megapixels starts to “see” the pixels of a projector of low (1280 * 720) resolution quite well.

Tzk
Posts: 23
Joined: 17 Mar 2020, 13:28

Re: SLS on a budget - camera and lens choice

Post by Tzk » 13 Apr 2020, 11:05

I went further and modelled all parts and viewing angles in CAD as i'll need that anyways for my mounting brackets. Here's what it looks like with 1/2.5" sensor (YW500U3) and 12mm lens (ZLKC HM1214MP5). The blue rectangle is the FOV of the camera (~1150mm distance to object) and the yellow one is the projector (900mm to object). The angle between camera and projector is 22.5°. I'd say this fits well and thus i'll choose the 12mm lens. I can still move the camera further forward or back to compensate for a bit of missing FOV if needed. I'll order the parts and post again when i'm starting to build the scanning rig.

So your suggestion to use a 12mm lens on this setup was spot on.Thank you!
Capture4.PNG

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OBNRacerMan
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Re: SLS on a budget - camera and lens choice

Post by OBNRacerMan » 13 Apr 2020, 20:36

Один из моих старых вариантов с применением "чёрных" камер YW500U3:
20190515081554s.jpg
The cameras originally had different angles from the projector
и "красных" YW500U3M:
20190915143337s.jpg
There is already an opportunity to expose them at the same distance from the center of the projection.

mading
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Joined: 31 Jan 2017, 13:09

Re: SLS on a budget - camera and lens choice

Post by mading » 14 Apr 2020, 20:35

using a combined vertical and horizontal angle has proven to be useful in increasing the precision.
At least in my experience.
HP 3D scan, Acer K132, LG PF50, 2x DMK23UX236, 2xComputar M1214, David Calibration panels, Melamine panels, DIY rotary table

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Micr0
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Re: SLS on a budget - camera and lens choice

Post by Micr0 » 15 Apr 2020, 15:09

Are you talking about positioning one camera above but to the left of the projector, and the other below and to the right? Is the reduction in the scanable area or field of view worth the increase in accuracy? Can you approximate the benefit?
µ

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OBNRacerMan
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Re: SLS on a budget - camera and lens choice

Post by OBNRacerMan » 15 Apr 2020, 15:34

I think that he meant my forced setup, when the cameras turned out above the lens of the projector and to the sides of it. In my case (at that time, and in combination with an existing projector), this was the only solution that allowed me to install cameras with the same horizontal angle of 22.5 ° relative to the optical axis of the projector. Also, because of this, a certain vertical angle is obtained.

mading
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Re: SLS on a budget - camera and lens choice

Post by mading » 15 Apr 2020, 17:07

Micr0 wrote:
15 Apr 2020, 15:09
Are you talking about positioning one camera above but to the left of the projector, and the other below and to the right? Is the reduction in the scanable area or field of view worth the increase in accuracy? Can you approximate the benefit?
Yes. Because of the use of offset lens projector (and of the combination of camera+lens and projector FOV) I'm forced to do this way. In the caos of my tests I remember an increase of quality in alignment and a reduction of waves.
My projector is above, with the two cameras below, approximately at the same angle with projector (aroun 10°).
But the latter could be a combination of other things (new laptop, etc..)
HP 3D scan, Acer K132, LG PF50, 2x DMK23UX236, 2xComputar M1214, David Calibration panels, Melamine panels, DIY rotary table

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