10. Bright-field illumination and dark-field illumination Bright-field illumination directly lights the specimen with a solid cone of rays and is the simplest method available. Dark-field illumination uses a hollow cone of rays formed by an opaque stop at the center of the condenser large enough to prevent direct light from entering the objective. The specimen is placed at the concentration of the light cone, and is seen with light scattered or diffracted by it, therefore scratches and dents on the specimen surface are illuminated while the rest remains dark. 11. Apochromatic objective and achromatic objective An apochromatic objective is corrected for chromatic aberration at the red, green and blue wavelengths. An achromatic objective is corrected for chromatic aberration at the red and blue wavelengths only. 12. Köhler illumination Köhler illumination overcomes the disadvantages of other schemes by causing parallel rays to light the specimen so that, because they will not be in focus, the image of the specimen will not include an image of the light source.
16. Plan Denotes an objective lens that produces a flat (planar) image by correcting the spherical aberration/curvature of the field of an achromatic lens or an apochromatic lens. All Mitutoyo FS series objectives are plan apochromat. 17. Vignetting This unwanted effect is the reduction of an image's brightness or saturation at the periphery compared to the image center. May be caused by external (lens hood) or internal features (dimensions of a multi-element lens). 18. Flare Lens flare is typically seen as several starbursts, rings, or circles in a row across the image or view, caused by unwanted image formation mechanisms, such as internal reflection and scattering of light. 19. Double image An image degrading a phenomenon in which an image appears as if it is a double image due to redundant light projection and optical interference within the optical system. 20. Pupil diameter and spot diameter of an objective • Pupil diameter Denotes the maximum diameter of a parallel light flux along the optical axis that can enter an objective from the rear. The pupil diameter is calculated according to the following expression.
Aperture diaphragm Condenser lens Fiber-optic cable
Imaging system
Relay lens Field stop
Beam splitter
Pupil diameter
Objective
ømm=2 x N.A. x f 1
Illuminated field of view
13. Telecentric illumination This illuminating optical system is designed so that principal light passes through the focal point. This system has the advantage of retaining the size of the image center even if it is out of focus (although the circumference of the image is defocused). This illumination system provides an even illumination intensity over the entire field of view. 14. Aperture diaphragm This diaphragm adjusts the amount of light passing through and is related to the brightness and resolving power of an optical system. This diaphragm is especially useful in width dimension measurement of cylindrical objects with contour illumination, and provides the highest degree of correct measurement/observation by suppressing diffraction in an optimal aperture. 15. Field stop This diaphragm is used for blocking out unwanted light and thereby preventing it from degrading the image.
Beam spot diameter
• Spot diameter If a beam of light with a uniformly distributed intensity enters an objective from the rear, the beam is focused to a spot of finite size. This size is known as the spot diameter. The approximate value of a spot diameter is calculated from the following expression.
λ N.A.
øµm=1.22x
However, the above expression cannot be applied if the light source is a laser beam of which the intensity forms a Gaussian distribution on the cross section. The diameter of a laser beam is generally indicated by 1/e 2 of the peak value, i.e. 13.5% of the peak value. The spot diameter of a laser beam is calculated from the following expression.
4x λ xf π xD
øµm =
(where λ is in μm; f and D are in mm)
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