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LCD Pinhole Confocal Microscope

Somewhat geeky.
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Apologies for a somewhat geeky post.

First to explain confocal microscopy. In a conventional microscope, only a narrow plane is in focus, but light from out-of-focus areas (closer to and further from the objective lens) also contributes to the image. The blurred foreground and background images contribute noise to the image, especially if (as is usually the case in biology) you're looking at a semi-transparent object like a cell.

The confocal microscope is a brilliant solution to this problem. The light coming from the objective is focussed through a point (imagine the light rays forming an hour- glass shape, or rather the shape of a diabolo - two cones point-to-point). At this point in space, you put an opaque screen with a pinhole in it, like putting a tight collar around the waist of the hourglass. The result (easier to understand if you sketch out the light rays on paper) is that light coming from behind or in front of the focal plane is blocked (because those rays do not focus to the same point, and hence hit the screen rather than going through the pinhole).

The result is that you can image a perfect "optical section" through the cell or whatever. You can also take multiple optical sections, to reconstruct a true 3-D image.

The downside, though, is that the pinhole blocks all of the image except for a focal point (ie, light rays coming from points on either side are also blocked by the screen, as well as those from in front or behind). Therefore, confocal microscopes use a raster-scan technique, collecting the image point-by-point.

The raster scanning can be done in various ways. Typically, a pair of mirrors (X and Y) is used to shift the focal point in a raster pattern. On good systems, the scanning can be done at better-than-TV rates, so you can collect maybe a hundred full images per second (important for looking at dynamic processes). Another solution is to use a "Nipkow disc", which is a spinning disc with multiple pinholes; each pinhole moves across the image-plane in turn, to give the same effect as a single pinhole scanning the image.

(Note - to avoid confusion - this isn't a "pinhole lens"; the microscope objective is a normal one, and collects much more light than a pinhole would; that light is then funnelled through the pinhole to block the out-of-focus parts of it.)

Anyway, as far as I know, the raster-scanning of the pinhole is always done mechanically.

Now, it is possible to make LCD screens with very small pixel sizes, and I'm guessing that it would be possible to make a small LCD screen with pixel sizes similar to the pinhole size in a confocal microscope.

So, why not replace the pinhole (and the mechanics needed to scan the point-image) with a small, dense LCD array. The array would be set to "black", except for a single clear pixel, and the position of this clear pixel could be scanned to produce the confocal image, without any mechanics.

(Note: the pixel-size of the LCD array isn't really very important; you can funnel the light in such a way that a 100µm "pinhole" correponds to a diffraction-limited spot on the image, for instance. But small pixels would be convenient.)

I know this would play havoc with polarised light but, in most biological applications, polarization doesn't enter into things (it does in a few case, though, for which this method would not work.)

Incidentally, Googling did turn up a "virtual pinhole" confocal using LCDs. However, as far as I can tell, it's a light-field camera which collects data allowing a 3D image (or slice thereof) to be reconstucted in silico, rather than using the LCD as a moving pinhole.

MaxwellBuchanan, Nov 07 2011

patent WO 9940471 http://worldwide.es...&NR=9940471A1&KC=A1
uses LCD "spatial light modulator" [xaviergisz, Nov 07 2011]

[link]






       This sounds like it could work [+].   

       Coincidentally I was thinking about something similar to this the other day. I was wondering if a scanning pinhole could be used to improve sharpness in a conventional camera, unaware this was in use in microscopes.
mitxela, Nov 07 2011
  

       Look into digital mirror devices - they might be better than LCD for this application.
spidermother, Nov 11 2011
  

       From Wikipedia, "Confocal Microscope":   

       Programmable Array Microscopes (PAM) use an electronically controlled spatial light modulator (SLM) that produces a set of moving pinholes. The SLM is a device containing an array of pixels with some property (opacity, reflectivity or optical rotation) of the individual pixels that can be adjusted electronically. The SLM contains microelectromechanical mirrors or liquid crystal components. The image is usually acquired by a CCD camera.   

       So, baked!   

       [bigsleep] AFAIK, the sample is moved relative to the lens assembly to alter the position of the focal plane in the sample, as in most microscopes. The pinhole(s) must be scanned because only a point on the sample is illuminated and visible at a time.
spidermother, Nov 11 2011
  

       If an LCD can effectively block enough light, why haven't conventional cameras started using them as aperture diaphragms? Would be much quicker to respond than the mechanical iris. Only thing I can think of is the effect of polarized light on the autofocus system.
mitxela, Nov 11 2011
  

       Also, LCD elements block at least half the light; and they would create artifacts by interacting with the polarisation of the incoming light; and only relatively high-end cameras have irises now, so compromising picture quality would not go down well. Otherwise they would be fine.
spidermother, Nov 11 2011
  

       //sn't it just an up/down movement of the pinhole through the vertical focal field, like, done manually like ? Lenses being 2D and all that.//   

       No, if it's truly confocal, then the image itself only covers a few microns (or much less) of the image in the plane of focus (ie, the imaged volume is very small and roughly barrel-shaped), so it's scanned in 2D (as well as in the 3rd D if you want multiple optical sections).   

       //So, baked! // I did mention the "light field" camera, which I think is what you're referring to. But my understanding is that it collects light in a different way to a confocal; it collects depth information, but doesn't reject out-of-focus light at any particular depth. Or maybe it does - I couldn't understand it well enough.   

       As for the question of whether LCDs are opaque enough - yeah, I'm not sure. When I was a kid I thought I had a brilliant idea for an SLR camera that used an LCD shutter, and I worried about speed and opacity. So, I don't know. Speed should be OK (LCD screens run at a hundred hertz maybe). As for opacity, I think probably the transmission of an "open" LCD is pretty high, so if necessary you could stack them to get the opacity when "closed". (in a confocal microscope, there are two or three planes where you can put the pinhole, so you could put an LCD pinhole at each so the opacities added).
MaxwellBuchanan, Nov 16 2011
  

       //sn't it just an up/down movement of the pinhole through the vertical focal field, like, done manually like ? Lenses being 2D and all that.//   

       No, if it's truly confocal, then the image itself only covers a few microns (or much less) of the image in the plane of focus (ie, the imaged volume is very small and roughly barrel-shaped), so it's scanned in 2D (as well as in the 3rd D if you want multiple optical sections).   

       //So, baked! // I did mention the "light field" camera, which I think is what you're referring to. But my understanding is that it collects light in a different way to a confocal; it collects depth information, but doesn't reject out-of-focus light at any particular depth. Or maybe it does - I couldn't understand it well enough.   

       As for the question of whether LCDs are opaque enough - yeah, I'm not sure. When I was a kid I thought I had a brilliant idea for an SLR camera that used an LCD shutter, and I worried about speed and opacity. So, I don't know. Speed should be OK (LCD screens run at a hundred hertz maybe). As for opacity, I think probably the transmission of an "open" LCD is pretty high, so if necessary you could stack them to get the opacity when "closed". (in a confocal microscope, there are two or three planes where you can put the pinhole, so you could put an LCD pinhole at each so the opacities added).
MaxwellBuchanan, Nov 16 2011
  

       As an undergraduate, I my third-year project was to evaluate the properties of an SLM (as mentioned by [spidermother] - essentially it was a very small 48x48 LCD matrix). The problem with it was that the contrast between on and off was pretty poor.
hippo, Nov 17 2011
  

       Ah well. Perhaps it's a non-starter then. You'd probably want >95% transmission (of at least one polarization) when "open", and maybe 90% occlusion when "off".
MaxwellBuchanan, Nov 17 2011
  
      
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