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infinity_microscope_basics [2019/02/26 12:48]
jon [Infinity Microscope Basics]
infinity_microscope_basics [2019/02/26 12:52] (current)
jon
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 The "​infinity"​ signifies that the distance between the two lenses is not important. ​ Optically this is because because both the objective and tube lens have one side focused at infinity, unlike older objectives and tube lenses which needed to be mounted a certain distance from each other to yield the stated magnification. The "​infinity"​ signifies that the distance between the two lenses is not important. ​ Optically this is because because both the objective and tube lens have one side focused at infinity, unlike older objectives and tube lenses which needed to be mounted a certain distance from each other to yield the stated magnification.
  
-The region between objective and tube lens is often called "​infinity space,"​ but we prefer the term "​collimated space" to signify that rays originating from a single point in the sample plane are collimated in this region. ​ The distance occupied by collimated space doesn'​t affect the magnification. ​ The exact length of collimated space usually does not matter, as long as it is short enough to avoid [[#​vignetting | vignetting]]. ​ Filters, polarizers, and other elements are usually placed in collimated space.+The region between objective and tube lens is often called "​infinity space,"​ but we prefer the term "​collimated space" to signify that rays originating from a single point in the sample plane are collimated ​or parallel ​in this region. ​ The distance occupied by collimated space doesn'​t affect the magnification. ​ The exact length of collimated space usually does not matter, as long as it is short enough to avoid [[#​vignetting | vignetting]]. ​ Filters, polarizers, and other elements are usually placed in collimated space.
  
-Internally, objective lenses have many individual elements (often more than 10) in order to sufficiently correct aberrations with a relatively short focal length (i.e. the light rays need to bend quite a lot and in specific ways). ​ Usually ​microscopes ​are built to accommodate different objective lenses. ​ The tube lens is usually mounted in the microscope, and optically are comparatively simple lens (usually just a few elements).+Internally, objective lenses have many individual elements (often more than 10) in order to sufficiently correct aberrations with a relatively short focal length (i.e. the light rays need to bend quite a lot and in specific ways). ​ Usually ​microscope bodies ​are built to accommodate different objective lenses. ​ The tube lens is usually mounted in the microscope, and optically are comparatively simple lens (usually just a few elements).
  
  
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 ===== Vignetting ===== ===== Vignetting =====
  
-Recall that light coming from a point in the sample will be turned into a bundle of rays coming out of the objective lens, and the further the point is from the optical axis the more tilted that bundle will be.  That bundle of rays is collected and refocused to a point by the tube lens, **if** the whole bundle makes it to the tube lens (the tube lens is only a certain size, plus there may be spots in the microscope'​s optical path where rays may hit a side wall or miss a mirror en route to the tube lens). ​ The further from center of the sample the point is, the more pronounced the angle and the more likely some of those rays won't reach the tube lens.  This leads to vignetting or darkening around the edge of the image. ​ Also the longer the space between the objective and the tube lens ("​collimated space"​) ​the more likely ​to get vignetting. ​ The larger the back aperture of the objective the more this is a concern.  ​+Recall that light coming from a point in the sample will be turned into a bundle of parallel ​rays coming out of the objective lens, and the further the point is from the optical axis the more tilted that bundle will be.  That bundle of rays is collected and refocused to a point by the tube lens, **if** the whole bundle makes it to the tube lens (the tube lens is only a certain size, plus there may be spots in the microscope'​s optical path where rays may hit a side wall or miss a mirror en route to the tube lens). ​ The further from center of the sample the point is, the more pronounced the angle and the more likely some of those rays won't reach the tube lens.  This leads to vignetting or darkening around the edge of the image. ​ Also increased distance ​between the objective and the tube lens (more "​collimated space"​) ​leads to increased ​vignetting. ​ The larger the back aperture of the objective the more this is a concern.  ​
    
-The formula for vignetting-free ​space, assuming everything is perfectly aligned, is:+The formula for vignetting-free ​distance, assuming everything is perfectly aligned, is:
  
 \begin{equation} \begin{equation}
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 $F_{TL}$ is the tube lens effective focal length $F_{TL}$ is the tube lens effective focal length
  
-$⌀_{sensor}$ is the diameter of the image sensor (18.8 mm for standard sCMOS)+$⌀_{sensor}$ is the diameter of the image sensor (18.8 mm diagonal ​for standard sCMOS full-frame)
  
 Rearranging this equation you can come to the following two equations in terms of the diameter of the vignette-free field of view at the sensor ($⌀_{sensor,​max}$) and sample ($⌀_{sample,​max}$):​ Rearranging this equation you can come to the following two equations in terms of the diameter of the vignette-free field of view at the sensor ($⌀_{sensor,​max}$) and sample ($⌀_{sample,​max}$):​
infinity_microscope_basics.txt · Last modified: 2019/02/26 12:52 by jon