Optics: measuring and testing – Inspection of flaws or impurities – Having predetermined light transmission regions
Patent
1997-05-15
1998-10-13
Font, Frank G.
Optics: measuring and testing
Inspection of flaws or impurities
Having predetermined light transmission regions
356371, G01N 2100, G01B 1130
Patent
active
058220542
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a lighting apparatus for visual inspection for defects of a reflective surface.
BACKGROUND
A wide range of manufactured products are visually inspected by human inspectors in order to locate and assess surface defects. For effective inspection a surface must be illuminated in such a way that significant defects can be easily seen and distinguished from other features. The characteristics of the light, such as its intensity and direction, strongly affect the inspection performance.
An example of prior art in this field is U.S. Pat. No. 5,237,404, assigned to Mazda Motor Corporation (`the Mazda patent`). This patent is for an automatic inspection system consisting of a light radiation means, an imaging means and a discriminating means. The light from the light radiation means has a `predetermined change pattern` which is, optionally, such that `a luminous intensity is gradually changed from a high level to low level along a predetermined direction`.
The present invention is distinct from the Mazda patent in that the present invention is directed exclusively towards human visual inspection, and not towards automatic inspection. The distribution of light from the present invention is specific to human visual inspection, so the present invention would be inappropriate for automatic inspection.
Thus, hitherto, when the paint on car bodies is inspected in order to find defects to be repaired, such as dirt, arrays of fluorescent lamps are typically used as the lighting system. Human inspectors view the reflection of the lamps in the paint in order to see the defects. This technique, however, suffers from a number of drawbacks. The visibility of defects is different at different positions in the field of view, it is difficult to assess the severity of defects and It is hard to distinguish defects which should be repaired from surface features such as orange peel.
The present invention seeks to provide an apparatus which is less subject to these drawbacks.
ESSENTIAL TECHNICAL FEATURES
According to the present invention there is provided a surface inspection lighting apparatus for use in the visual inspection by a human inspector of a reflective test surface for surface defects, having a region from which light emanates when the apparatus is in use, the intensity of the light emanating from the said region varying gradually substantially as an exponential function of a sinusoidal function of the distance along an imaginary line across the said region, so that the specular reflection of light from said region in a flat, defect free, reflective test surface is perceived by a human inspector to have a substantially sinusoidal intensity distribution.
Preferred attributes and features of the invention are described hereafter.
The inspector looks at the specular reflection of the apparatus in the test surface. If the test surface is reflective, smooth and free of defects then the test surface acts as a mirror and the inspector perceives a smooth, sinusoidal intensity distribution and can judge that the surface is of good quality.
A surface defect on a reflective surface typically distorts the level of the surface. Light specularly reflected by the defect is therefore reflected in a direction different from that of the light reflected by the good quality surface that surrounds it. When the inspector views the reflection of the apparatus in the test surface, at the location of the defect he sees light reflected from a position on the apparatus that is remote from the positions on the apparatus from which light is reflected to the inspector by the good quality surface surrounding the defect. As the intensity of the light emanating from the apparatus varies gradually across it, the defect is perceived to have an intensity different to that of its surrounding area. If the severity and size of the defect are sufficient the inspector sees a sharp change in the intensity at the location of the defect and this can be distinguished from the gradual changes in the intensity due to the
REFERENCES:
patent: 4629319 (1986-12-01), Clarke et al.
patent: 5367378 (1994-11-01), Harding et al.
Font Frank G.
Merlino Amanda
Surface Inspection Limited
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