Quick Guide to Precision Measuring Instruments

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PG45Quick Guide to MeasurementNumerical Aperture (NA)The NA figure 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 = 0.55μm 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 DistanceThe distance between the mounting position of a microscope objective and the surface of the workpiece at which the sharpest focusing is obtained. Objective lenses mounted together in the same turret should have the same parfocal distance so that when another objective is brought into use the amount of refocussing needed is minimal.Innity Optical SystemAn optical system where the objective forms its image at innity and a tube lens is placed within the body tube between the objective and the eyepiece to produce the intermediate image. After passing through the objective the light effectively travels parallel to the optical axis to the tube lens through what is termed the `innity space’ within which auxiliary components can be placed, such as differential interference contrast (DIC) prisms, polarizers, etc., with minimal effect on focus and aberration corrections.Finite Optical SystemAn optical system that uses an objective to form the intermediate image at a nite position. Light from the workpiece passing through the objective is directed toward the intermediate image plane (located at the front focal plane of the eyepiece) and converges in that plane.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 magnification 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 = 200Example: 10X = 200 200 20Working distanceParfocal distanceWorkingdistanceParfocal distanceObjective lensImage forming (tube) lensLight from point source is focused at the intermediate image planef1f2Magnication of the objective = f2/f1A point-source on the specimenInnity spaceLight from point source is focused at the intermediate image planeMagnication of the objective = L2/L1Objective lensL1L2A point-source on the workpieceMicroscopes