Quick Guide to Precision Measuring Instruments

48/76

PG46Depth of Focus (DOF)unit: mmAlso known as ‘depth of field’, this is the distance (measured in the direction of the optical axis) between the two planes which dene the limits of acceptable image sharpness when the microscope is focused on an object. As the numerical aperture (NA) increases, the depth of focus becomes shallower, as shown by the expression below:DOF = ll = 0.55μm is often used as the reference wavelength2·(NA)2 Example: For an M Plan Apo 100X lens (NA = 0.7)The depth of focus of this objective is 0.55μm = 0.6μm2 x 0.72Bright-eld Illumination and Dark-eld IlluminationIn brighteld illumination a full cone of light is focused by the objective on the specimen surface. This is the normal mode of viewing with an optical microscope. With darkeld illumination, the inner area of the light cone is blocked so that the surface is only illuminated by light from an oblique angle. Darkfield illumination is good for detecting surface scratches and contamination.Apochromat Objective and Achromat ObjectiveAn apochromat objective is a lens corrected for chromatic aberration (color blur) in three colors (red, blue, yellow).An achromat objective is a lens corrected for chromatic aberration in two colors (red, blue).MagnicationThe ratio of the size of a magnified object image created by an optical system to that of the object. Magnication commonly refers to lateral magnication although it can mean lateral, vertical, or angular magnication.Principal RayA ray considered to be emitted from an object point off the optical axis and passing through the center of an aperture diaphragm in a lens system.Aperture DiaphragmAn adjustable circular aperture which controls the amount of light passing through a lens system. It is also referred to as an aperture stop and its size affects image brightness and depth of focus.Field StopA stop which controls the eld of view in an optical instrument.Telecentric SystemAn optical system where the light rays are parallel to the optical axis in object and/or image space. This means that magnification is nearly constant over a range of working distances, therefore almost eliminating perspective error.Erect ImageAn image in which the orientations of left, right, top, bottom and moving directions are the same as those of a workpiece on the workstage.Field number (FN), real eld of view, and monitor display magnicationunit: mmThe observation range of the sample surface is determined by the diameter of the eyepiece’s eld stop. The value of this diameter in millimeters is called the eld number (FN). In contrast, the real eld of view is the range on the workpiece surface when actually magnied and observed with the objective lens.The real eld of view can be calculated with the following formula:(1) The range of the workpiece that can be observed with the microscope (diameter)Real eld of view = FN of eyepieceObjective lens magnicationExample: The real eld of view of a 1X lens is 24 = 24 1The real eld of view of a 10X lens is 2.4 = 24 10(2) Monitor observation rangeMonitor observation range =The size of the camera image sensor (diagonal length) Objective lens magnication●Size of image sensor FormatDiagonal lengthLengthHeight1/3” 6.04.83.61/2” 8.06.44.82/3”11.08.86.6(3) Monitor display magnicationMonitor display magnication = Objective lens magnication xDisplay diagonal length on the monitorDiagonal length of camera image sensor