Method for examining rotationally symmetrical objects

#The invention pertains to a method for examining a series of objects that are symmetrical in reference to a rotational axis, e.g., circular disks. The invention pertains, in particular, to the optical examination of circular data carriers, e.g., CDs, DVDs, CD-Rs and the like. Although the following description primarily pertains to one of these data carriers, the invention is not limited to objects of this type.

#It is known to carry out a 100% examination of optical data carriers during or after the manufacturing process of the optical data carriers. When examining CDs, CD-Rs or DVDs, optical methods are utilized in which the objects to be examined are turned about their rotational axis and scanned. The surface to be examined, e.g., the flat side having a data area, is scanned and examined with respect to possible defects. Methods of this type are, for example, known from DE 44 34 473 A1, DE 44 34 474 A1 and DE 44 34 475 A1. WO 97/20312 describes a method in which a circular object is always aligned in the same angular position with reference to a marking provided and illustrated on the object.

#The illustration of the examined sections of the object usually takes place on a monitor of a data processing system, in which the optical scanning is evaluated. The illustration may be in the form of tables and coordinate data and/or in the form of a pictorial representation of the object to be examined, in which possible defects are distinguished or made visible.

#The invention is described below with reference to the schematic figure, FIG. 1, which shows a top plan view of a CD.

#The Compact Disk (CD) 11 shown in the figure is conventionally realized in the form of a circular disk with a central hole 12. This means that the CD is entirely symmetrical in reference to the rotational axis 13. The central hole 12 is usually bordered by an inner coaxial edge region 14 for holding the CD in the corresponding player. A coaxial region 15 that contains the identification code, namely the ID code 16, is situated adjacent to the aforementioned edge region. This coaxial region 15 is surrounded by the data area 17. Depending on the data quantity, a lead-out 18 or the outer edge region 19 is situated adjacent to the data area.

#It is quite apparent that such a CD, in particular, the data area 17, always appears identical to an optical scanning element of an examination device independently of the rotation if no additional measures are provided. This is the reason why defects which, for example, always occur at the same position referred to the radial distance and the angular position due to a defective stamper used in the pressing process can, in principle, not be detected and classified as such.

#An orientation of the CD in the rotating direction can only be attained with markings that are not symmetric in the rotating direction. For example, the ID code 16 in the form of a combination of numerals and letters is not symmetrical in the rotating direction. The ID code may also contain a bar code or another pictorial coding that is also not circularly symmetrical. Consequently, it is possible to select an arbitrary symbol 20 or the beginning or the end of the ID code as the marking. This marking makes it possible to define a certain geometric region 21 in a defined angular position about the rotational axis 13 referring to the marking as well as at a corresponding distance from the rotational axis.