J-16JMicroscopesQuick Guide to Precision Measuring InstrumentsNumerical Aperture (NA)The NA gure is important because it indicates the resolving power of an objective lens. The larger the NA value the ner the detail that can be seen. A lens with a larger NA also collects more light and will normally provide a brighter image with a narrower depth of focus than one with a smaller NA value.NA = n·SinθThe formula above shows that NA depends on n, the refractive index of the medium that exists between the front of an objective and the specimen (for air, n=1.0), and angle θ, which is the half-angle of the maximum cone of light that can enter the lens.Resolving Power (R)The minimum detectable distance between two image points, representing the limit of resolution. Resolving power (R) is determined by numerical aperture (NA) and wavelength (λ) of the illumination.R = l (µm)2·NA l = 0.55 µm is often used as the reference wavelengthWorking Distance (W.D.)The distance between the front end of a microscope objective and the surface of the workpiece at which the sharpest focusing is obtained.Parfocal DistanceDistance between the surface of the specimen and the objective's seating surface when in focus.Innity-corrected Optical SystemAn optical system in which the image is formed by an objective anda tube lens with an 'Innity Space' between them, into which opticalaccessories can be inserted.Depth of Focus (DOF)unit: mmThis 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 = 0.55 µm 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 0.55 µm = 0.6 µm2×0.72Working distanceParfocal distanceFinite-corrected Optical SystemAn optical system in which the image is formed only by an objective lens.Objective lensImage forming (tube) lensLight from point source is focused at the intermediate image planef1f2Magnication of the objective = f2/f1A point-source on the specimenInnity spaceBright-eld and Dark-eld IlluminationIn bright-eld 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 dark-eld illumination, the inner area of the light cone is blocked so that the surface is only illuminated by light from an oblique angle. Dark-eld illumination is good for detecting surface scratches and contamination.Apochromat and Achromat ObjectivesAn apochromat objective is a lens corrected for chromatic aberration (color blur) in three colors (red, green, blue). An achromat objective is a lens corrected for chromatic aberration in two colors (red, blue).MagnicationThe ratio of the size of a magnied 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 StopAn aperture 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 magnication 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.Focal PointLight rays traveling parallel to the optical axis of a converging lens system and passing through that system will converge (or focus) to a point on the axis known as the rear focal point, or image focal point.Focal Length (f)unit: mmThe distance from the principal point to the focal point of a lens: if f1 represents the focal length of an objective and f2 represents the focal length of an image forming (tube) lens then magnication is determined by the ratio between the two. (In the case of the innity-correction optical system.)Objective magnication = Focal length of the image-forming (tube) lens Focal length of the objectiveExample: 1X = Light from point source is focused at the intermediate image planeMagnication of the objective = L2/L1Objective lensL1L2A point-source on the workpiece200200Example: 10X =20020Field 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 work-piece 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 10X lens is 2.4 = 24 10(2) Monitor observation rangeMonitor observation range = The size of the camera image sensor (Length×Height)Objective lens magnicationSize of image sensorFormatDiagonal lengthLengthHeight1/3 in 6.04.83.61/2 in 8.06.44.82/3 in11.08.86.6(3) Monitor display magnicationMonitor display magnication = Objective lens magnication ×Display diagonal length on the monitorDiagonal length of camera image sensor
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