How to Measure Magnification of a Lens System with Liquid Surface

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Discussion Overview

The discussion revolves around the challenges of imaging the bottom of a small plastic container filled with liquid using an optical system. Participants explore issues related to illumination, optical distortion caused by the liquid surface, and potential solutions for achieving even illumination across the image.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes a setup involving a camera and a lens with 1x magnification, noting that the illumination works well for an empty container but becomes uneven when liquid is added.
  • Another participant suggests that adding liquid alters the path length and may require re-adjustment of the optical setup.
  • Concerns are raised about the meniscus effect of the liquid surface, which appears to create optical distortion, particularly at the edges of the container.
  • Some participants propose filling the container to the top to flatten the meniscus as a potential temporary solution for testing.
  • There are discussions about the possibility of using a flat coverslip or a corrective lens to mitigate the lensing effect of the liquid surface.
  • One participant mentions the optical density differences between air and water, suggesting that flooding the tube's surroundings with water could help reduce distortion.
  • Another participant raises the idea of using post-processing techniques, such as Fourier transform options, to correct image issues.
  • Several participants express uncertainty about how to measure or calculate the optical effects caused by the liquid surface.

Areas of Agreement / Disagreement

Participants generally agree that the meniscus is causing optical issues, but there is no consensus on the best method to mitigate these effects or how to measure the resulting distortion. Multiple competing views on potential solutions remain present.

Contextual Notes

Participants note limitations in their ability to control the liquid level and the need for the system to function within specific operational parameters. There are unresolved questions regarding the optical properties of the liquid surface and the best corrective measures.

  • #31
HomeTinker said:
Surely if I did that the image of the elements on the bottom of the container that I am trying to take will be distorted, right?
Which comment are you replying to? And I'm pretty sure that neither of them like to be called Shirley. :wink:
 
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  • #32
Gleb1964 said:
What if you place a plano-covex lens at the bottom of container, flat side towards bottom and observe the cuvette through the convex side of the lens? The aim of such lens is to form sort of Galilean telescopic system in combination with the negative water meniscus. In that case the illumination system woud function as it should.
The problem is that by the time the light passes through the bottom of the sample much of it has already been lost. Plus that will require modification of the camera end of the system since you have introduced a positive lens into it.
 
  • #33
berkeman said:
Which comment are you replying to? And I'm pretty sure that neither of them like to be called Shirley. :wink:
Apologies, that was a reply to the idea of adding plano-convex lens on the bottom of the container.
 
  • #34
Drakkith said:
The problem is that by the time the light passes through the bottom of the sample much of it has already been lost. Plus that will require modification of the camera end of the system since you have introduced a positive lens into it.
Modification of the camera lens is possible, there is a complete freedom on that side. What I cannot do is to interfere with the container itself or it's content.
 
  • #35
HomeTinker said:
Modification of the camera lens is possible, there is a complete freedom on that side. What I cannot do is to interfere with the container itself or it's content.
Like I said, I think you will have already lost too much light by the time it reaches the new lens, but you could always try it. I'd recommend reaching out to an optical engineer and getting their professional opinion on the matter.
 
  • #36
Drakkith said:
The problem is that by the time the light passes through the bottom of the sample much of it has already been lost. Plus that will require modification of the camera end of the system since you have introduced a positive lens into it.
I haven't seen specification for the system, but I guess it not on a limit of signal/noise. The aim is to get evenly illuminated background field, not the best efficiency of using every photon. You may mention, there is a caple of apertures truncating a lot of light.
 
  • #37
Gleb1964 said:
I haven't seen specification for the system, but I guess it not on a limit of signal/noise.
Indeed. That wasn't what I was trying to get at. My apologies for the confusion.
 
  • #38
Gleb1964 said:
I haven't seen specification for the system, but I guess it not on a limit of signal/noise. The aim is to get evenly illuminated background field, not the best efficiency of using every photon. You may mention, there is a caple of apertures truncating a lot of light.
There are two diaphragms in the system as per koehler design (field and aperture diaphragms) however to be honest they may very little difference here. Having them completely opened or closed do not improve on this situation.
 
  • #39
I have raytraced two options, with plano-convex lens above cuvette and below. Both options are working somehow, but not the same.
Variant with lens above cuvette does not give even illumination over entire field. Illumination is even at the central field but does not goes to the edge. Vignetting is increasing close to edge and at the very edge there is a dark rim because light undergo a total internal reflection and does not pass through.
Another option with the same lens under cuvette so the imaging optics is looking at cuvette bottom through the planoconvex lens. That does destroy telecentricity condition of imaging optics but makes possible to achive even illumination over entire cuvette bottom.
What trade of is preferable is at the topic starter choice.
picture-3.png
 
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  • #40
Gleb1964 said:
Both options are working somehow, but not the same.
That doesn't surprise me too much. There is a right end and a wrong end for binoculars too.
 
  • #41
Many thanks for that, I will try to get hold of LA1859 and see what results that makes for both designs.
 

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