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Experiment setup

I have the above experiment setup where a triangulation-based laser scanner projects a laser (line) towards a transparent glass material. I then took a picture using my phone (indicated in the above image).

The image from the phone is provided below. I was trying to understand where does each laser line reflection from the phone image come from, but I couldn't figure out how does reflection (1) come about.

Here is my attempt at explaining reflection (2) - (4):

(2): As the laser hits the glass surface, some of the light is reflected diffusely towards the phone camera (laser scanner -> glass -> phone camera)

(3) As the laser hits the glass surface, some of the light is reflected specularly towards the phone rear surface, which then gets reflected back to the glass before reflecting back to the phone camera (laser scanner -> glass -> phone rear surface -> glass -> phone camera)

(4) As the laser hits the glass surface, some of the light is transmitted and gets reflected diffusely by the ground to reach the phone camera (laser scanner -> ground -> phone camera)

Image taken from phone camera

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  • $\begingroup$ Have you considered a reflection toward the camera from the back surface of the glass plate? $\endgroup$ – S. McGrew Dec 5 '18 at 6:56
  • $\begingroup$ Yes. From my observation, the specular reflection from the back surface of the glass plate is almost contained in reflection (3) as the glass plate is very thin (about 2 mm) $\endgroup$ – nasw_264 Dec 5 '18 at 8:02
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The glass sends back light in two different ways.

(2) is scattering of the laser light. This is correctly depicted on the drawing: light is sent back in all directions. This is a new source of light located on the surface of the glass.

(1) is reflection of the laser's aperture. Light is sent back in a single direction, forming a mirror image behind the surface of the glass (at the same distance the laser is).

Note that (1) coincides with the mirror image of the laser scanner itself. On the other hand, (2) extends laterally far beyond it, and so cannot be a reflection of the aperture.

Since the scattering does not happen at the same place as the mirror image, they appear separate in the picture.

enter image description here

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A small amount of the laser light source is scattered by its aperture and optics and it is like a flashlight (not laser like anymore). This is reflected off the glass to form an image in your camera.

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