The present invention provides a method for mending projection and recess defects existing on the surface of a metallic plate, wherein steps (1) to (2) are repeated until it is determined in step (1) that mending of the projection and recess defects on the surface of the metallic plate is no longer necessary. Step (1): A step for shining light onto the surface of the metallic plate, detecting the positions of projection and recess defects on the surface of the metallic plate according to the brightness distribution on the metallic plate, the brightness distribution being obtained from reflected light, quantifying the brightness intensities of the projection and recess defects, and determining whether it is necessary to mend the projection and recess defects. Step (2): A step for mending the projection and recess defects for which it is determined in step (1) that mending is necessary.

There is provided a method that makes it possible to observe fine crystal defects using light of a visible region. The method includes illuminating a substrate with polarized parallel light and evaluating a crystal quality of at least a part of the substrate from an image obtained by light transmitted through or reflected by the substrate. The half width HW, the divergence angle DA, and the center wavelength CWL of the parallel light satisfy conditions given below
3HW100
0.1DA5
250CWL1600 where the center wavelength CWL and the half width HW are expressed in units of nm and the divergence angle DA is expressed in units of mrad.

There is provided a method that makes it possible to observe fine crystal defects using light of a visible region. The method includes illuminating a substrate with polarized parallel light and evaluating a crystal quality of at least a part of the substrate from an image obtained by light transmitted through or reflected by the substrate. The half width HW, the divergence angle DA, and the center wavelength CWL of the parallel light satisfy conditions given below

3HW100

0.1DA5

250CWL1600 where the center wavelength CWL and the half width HW are expressed in units of nm and the divergence angle DA is expressed in units of mrad.

In a method of inspecting a steel strip, at least one surface of the steel strip is illuminated and scanned by at least one camera so as to generate an image record that defines a two-dimensional image of the scanned surface. The image record is sent to an image processing unit, with the image processing unit subjecting the image record to the detection of defects and, upon detection of a surface defect, classifying the detected surface defect. The steel strip is magnetized and the magnetic flux leakage on the surface of the steel strip is detected by at least one magnetic field-sensitive flux leakage sensor in order to detect inhomogeneities in the interior of the steel strip, with the flux leakage sensor generating a flux leakage record that is sent to the image processing unit and that is subjected by the image processing unit to the detection of defects so as to identify inhomogeneities in the interior of the steel strip.

A defect inspection apparatus includes a first slit light source together with a machine base in which a through hole is formed. A second slit light source and a half mirror are provided inside the through hole. First slit light from the first slit light source is directly incident on an object to be photographed (for example, an automobile body). On the other hand, second slit light from the second slit light source proceeds in a direction perpendicular to the direction in which the first slit light proceeds, and thereafter, is refracted by the half mirror, led out from the through hole, and made incident on the object to be photographed.