Information detecting devices for detecting information of sheet materials have recently been attracting attention in various technical fields.
An example of those devices is disclosed in U.S. Pat. No. 6,097,497, in which a sheet material is marked in advance with a number code or symbol (hereinafter the method is referred to as marking method) and a sensor provided in a printer reads the number code and other information to set the optimum printing mode.
In this marking method, information that sheet materials can carry is limited to the kind that will not change after marking, such as the name of the manufacturer of the sheet material and the sheet material size.
Sheet materials in general are changed in water content by an environmental change (a change in humidity of atmospheric gas), and properties (for example, Young's modulus and other mechanical properties) of individual sheet material are varied depending on its water content. The marking method is incapable of detecting the water content of sheet materials and therefore cannot detect accurate sheet material information.
Also, it is impossible to obtain information from an unmarked sheet material with the marking method to begin with.