Resolution and Electronic Imaging

Resolvable Detail
The smallest resolvable detail imaged by an objective lens can be estimated by 0.550m/2xNumerical Aperture. That detail appears in the virtual image produced by the objective lens, enlarged by the magnification of the lens. The detail in the virtual image is projected via a tube lens onto the surface of a CCD chip in a color camera. The smallest resolvable detail in the virtual image observable with the camera is determined by the pixel periodicity of the CCD chip. The output of the camera goes through a video controller board for pixel interpolation and is transferred to the monitor for display. The smallest resolvable detail observable at the monitor is determined by the monitor pixel periodicity which is usually based on the display mode. In very high resolution systems observable resolution is limited by the Red-Green-Blue triad periodicity of the cathode ray tube.
 
The diagram below shows an example of the smallest resolvable detail at each level in an imaging path from an objective lens to a monitor. The resolution-limiting component is the one that has the largest detail size shown for that objective lens.

Field-of-View

The field-of-view for a microscope with oculars is equal to the Ocular Field # / Objective Lens Magnification (if there are no magnifying intermediate lenses). For Field # 20 oculars, the field-of-view is shown below, along with the area displayed on a typical electronic imaging system.

Obj Field-of-View with Oculars Video
Lens Total Diameter Perceived Area Displayed Area
5x 4.0 mm 2.40 x 2.00 mm 1.20 x 0.90 mm
10x 2.0 mm 1.20 x 1.00 mm 0.60 x 0.45 mm
20x 1.0 mm 0.60 x 0.50 mm 0.30 x 0.22 mm
50x 0.4 mm 0.24 x 0.20 mm 0.12 x 0.09 mm
100x 0.2 mm 0.12 x 0.10 mm 0.06 x 0.045mm

 

When performing rapid screening through oculars, the area that information is gleaned from is not the total area illuminated1,2. The perceived area shown in the table is an average (some see more, while others see less). But the reality is that the best one can expect to glean information from is 75% of the diameter on the horizontal axis and 66% on the vertical axis. The lower limit is 50%/33%. The Perceived Area data reflects the average. Since a video image is 100% perceived, to achieve the same screening area, a 2x lower objective is required.

Total Diameter/Perceived Area
 
  1. "How Did I Miss Those Cells?" Gary W. Gill, CT (ASCP), ADVANCE for Medical Laboratory Professionals, Nov. 20, 1995
  2. "Visual Conspicuity, visual search and fixation tendencies of the eye" Engel F.L., Vision Res. 17:95-108, 1977

 


The Next-Generation of Optical Microscopy
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