Multilayer record carrier and method of manufacturing thereof and recording thereon, with reduced transmission differences in the upper layer

Abstract

A multi-layer record carrier and method of manufacturing thereof and recording thereon, includes data written in units of blocks on tracks of at least two information layers. A first guard field is written at the start of a data block and a second guard field is written at the end of the data block. The power in the focused spot in the deepest of the at least two information layers is maintained substantially at an optimal value, by setting the length of the first and second guard fields such that the end position of the second guard field of a preceding data block is located within the area of the first guard field of the succeeding data block. Thereby, gap portions between the first and second guard fields can be prevented. Alternatively, the length of the first and second guard fields is set to be approximately equal to the sum of half the diameter of a recording beam in the upper one of the at least two information layers when focused on the lowest one of the at least two information layers and a maximum allowed misalignment between the two layers. Thereby, the area through which the beam passes in the information layer is of a uniform nature, when user data is being read or written. The at least two information layers are aligned by optically measuring and aligning alignment marks, such as header spokes, at predetermined measuring points.

1 - 19 . (canceled)  
   
   
       20 . A method of manufacturing a multilayer record carrier provided with at least two substantially parallel information layers, comprising the step of performing an alignment of said at least two information layers by measuring and aligning alignment marks at predetermined measuring points.  
   
   
       21 . The method as claimed in  claim 20 , wherein said method further the step of: 
 performing said measurement and alignment optically by using a camera device.    
   
   
       22 . The method as claimed in  claim 20 , wherein said alignment marks are header areas provided on said at least two information layers.  
   
   
       23 . The method as claimed in  claim 22 , wherein said header areas are arranged to form header spokes, and said measuring points comprise a first point located on an inner diameter and a second point located on an outer diameter of a single header spoke and third point located on a spoke arranged at an angle of 90° with respect to said single header spoke.  
   
   
       24 . The method as claimed in  claim 22 , wherein said header areas are arranged to form header spokes, and said measuring points comprise a first point and a second point located at the outer diameters of two opposite header spokes and a third point located on a spoke arranged at an angle of 90° with respect to said two opposite header spokes.