Illumination and OpticsCommon Image FormatsPixel RepresentationsImage File HeaderChromatic AberrationDepth of FieldDiffuse IlluminiationDiffuse ReflectionsSpecular ReflectionDistortionf-stop or f-number12: Coaxial IlluminationPowerPoint Presentation11: Diffuse Front IlluminationSlide 1513: Grazing IlluminationSlide 1717: Back Lighting of Matt Opaque ObjectsSlide 1919: Back Illumination of Shiny ObjectsSlide 2120: Dark-Field IlluminationSlide 2321: Dark-Field IlluminationSlide 2525: Viewing Small ObjectsSlide 2728: Telecentric LensSlide 29Polarized LightSlide 31Front and Back IlluminationSlide 33Types of illuminationIllumination and Opticsand just a bit on image formatsCommon Image Formats•Bmp:Windows standard format is clumsy•tiff: “Universal” format (tifflib)•jpg: Excellent compression but has limitations•gif: good graphics format•pbm/pgm/ppm: simple formats (open source)•png: portable net graphics (open source)Pixel Representations•1 bit/pixel: binary•8 bits/pixel: grayscale•4/8/15/16 bits/pixel: palette based•24+ bits/pixel: “true color”Image File Header•Image header contain information specifying image size, image format, gamma and other related information•“Raw” images have no headers and applications that can read them need manual input or are programmed only for special sizes and formatsChromatic AberrationAn optical aberration in which the component colours in a point source of white light are brought to a focus at different points; caused by the variation of the refractive index of the glass with wavelength. Machine vision applications can also be affected by infrared radiation!Depth of FieldWidth of that region on either side of the focal plane, where the image produced by an optical system remains in focus. Normally, a large DOF is advantageous. In General, a small lens aperture and bright object illumination will improve the DOF. Depth of focus is the range of image distances over which the image remains sharp for a given object distanceDiffuse IlluminiationIllumination technique, projecting light onto a matt surface first. This acts as the illuminator for the object being viewed. Baffles avoid direct illumination of the object. Avoids sharp shadows and glinting due to specular reflection.A diffuser is made from translucent material, e.g. polypropylene, and used to produce diffuse illumination.Diffuse ReflectionsLight returned from a matt (i.e. non shiny) surface is diffuse; light is scattered in all directions (above the object surface). It is not strongly polarized.Specular ReflectionReflection of light (e.g. from a smooth polished or metallic surface) in which the angle of incidence is equal to the angle of reflection. This gives rise to GlintingDistortionDefect of an optical system in which magnification varies with angular distance from the axis, causing straight lines to appear curved. Common distortion patterns are pincushion and barrel.f-stop or f-numberThe ratio of the focal length of a lens to the diameter of its input aperture. For a given lens, reducing its f-number by closing its aperture, increases the image brightness and reduces the depth of field.12: Coaxial Illumination•Ideal for specular objects•Camera and light source “coincide”OBJECTIVE To provide diffuse illumination without causing glinting.TYPICAL APPLICATIONS Viewing small bright shiny objects, such as coins, electrical connectors, printed circuit boards, EQUIPMENT Small incandescent filament lamp, LED array or fibre-optic illuminator, diffusing plate & beam-splitter. GEOMETRY•Mirror-like surfaces•Complex, embossed, etcMATERIALS •Bright metal surfaces•Reflective plastic•Lacquered / painted surfacesREMARKS Excellent images can be obtained from mirror-like surfaces, which are often difficult to view using other methods. It appears as though the widget is illuminated by light emanating from a small area, corresponding to the front of the camera lens. Since this produces a much narrower illumination beam than Method 11, it is very sensitive to the geometry of the widget surface; it can readily detect embossing, engraving, stippling creasing, dimples, pimples, scratches and pits. This method relies on producing specular reflection. Hence, the widget must have a well-defined shape and be held in fixed orientation, to ensure that glinting is visible to the camera.11: Diffuse Front IlluminationOBJECTIVE To reduce the effects of glinting .TYPICAL APPLICATIONS Inspecting objects with shiny, nearly planar surfaces, to detect scratch es, crack s, pit s, embossing , engraving , etc. shiny object s. Suitable for CD s, coin s, electrical connector s, printed circuit board s, smooth plastic , glass , porcelain , etc.EQUIPMENT Bright flood lamp s (e.g. incandescent filament or fluorescent illuminator with a high-frequency driver) Diffusing screen with viewing aperture. This consists of a flat matt white board that has a small cut in its center. It may be necessary to fit a short tube (painted matt black on its inside surface) into this hole, to prevent light reaching the camera directly from the lamps. A secondary light source and beam-splitter are used to compensate for the dark aperture in the diffusing screen.GEOMETRY Mirror-like surfaces and nearly flat surfaces with embossing, engraving, etc. Can also be useful for more complex structure s, since it does not cast sharp shadows.MATERIALS Bright metal , smooth plastic , glass , porcelain , etc., lacquered / painted surface s.REMARKS Excellent images of defects such as scratches can be obtained from mirror-like surfaces, which may be difficult to view otherwise. The secondary light source and beam-splitter avoid creating a reflection of the camera when viewing a mirror-like surface. In some applications, it is possible to eliminate them altogether.13: Grazing IlluminationOBJECTIVES •To highlight shallow surface features, created by engraving , embossing , rolling , pressing , stamping , casting , moulding , etc•To detect surface imperfections, such as scratch es, crack s, pit s, dirt , dust , grit , etc, on a smooth mirror-like surface .TYPICAL APPLICATIONS Inspecting flat machined metal surfaces, plastic , glass , glossy painted & lacquered surfaces, plastic sheeting, ceramic tiles, etc.EQUIPMENT Collimator. For close-range operation, it may not be necessary to use a collimator . In some applications, a very small ("point") source of light, such as a LED or fibre