1. Field of the Invention
The present invention relates to a defect detector and a method of detecting a defect and more particularly, to a defect detector for inspecting a surface of an electrophotographic photoreceptor used for copiers, printers, and facsimiles.
2. Discussion of Background
In processes of manufacturing electrophotographic photoreceptors used for copiers, printers, and facsimiles, the photoreceptors occasionally have defective surfaces such as scratches, foreign materials, and coating irregularities. As a method of detecting the defects, a visual inspection is typically performed by an inspector. However, since the visual inspection has fluctuations due to individual differences among inspectors, various automatic detecting methods and apparatus have been suggested.
For example, as a prior art apparatus and method, Japanese Laid-Open Publication No. 9-325120 (hereinafter referred to as a Prior Example 1) discloses a detector detecting the following defects by irradiating an electrophotographic photoreceptor from a light source, receiving light reflected from the photoreceptor with a camera, and producing an electrical signal representing the difference between:                (1) a concavo-convex defect having a high rate of change of surface concavity and convexity, such as pin holes, dents, coating scratches, and dusts; and        (2) a blocky defect having a low rate of change of surface concavity and convexity, such as coating irregularities of the photosensitive layers.        
In addition, Japanese Laid-Open Publications Nos. 7-128240(hereinafter referred to as a Prior Example 2) and 7-128241(hereinafter referred to as a Prior Example 3) disclose a detector taking plural photographs of a concavo-convex defect and a blocky defect with a light source and plural charge coupled device (hereinafter “CCD”) cameras or a CCD camera and plural light sources.
However, in an optical system of the Prior Example 1, both the concavo-convex defect and the blocky defect are photographed without distinction, and a decision to pass or fail has to be made in accordance with the defect having a severer acceptance criterion. Therefore, even a normally good photoreceptor is disposed of as a defective one, or a photoreceptor, which the defect detector judges to be rejected, has to be inspected again by an inspector. In addition, there is an advantage that the optical system is easily controlled because of its few controllers, but there is a disadvantage that when sensitivity of photographing either the concavo-convex defect or the blocky defect is increased, sensitivity of photographing the other defect is decreased.
Further, in the Prior Examples 2 and 3 , two light sources or two CCD cameras can increase sensitivities of photographing both the concavo-convex defect and the blocky defect. However, inspection time is long because of its plural times of photographing. In addition, the control operation is complicated because plural CCD cameras are combined with a light source or plural light sources are combined with a CCD camera.
Because of these reasons, a need exists for a defect detector which can efficiently detect a concavo-convex defect and a blocky defect by photographing a defect having a high rate of change of surface concavity and convexity and a defect having a low rate of change thereof with separate optical systems, and which can reduce losses due to excessive inspections.