Forgery-preventing means of products is broadly divided into means for making it impossible to copy products themselves and means for attaching an unreproducible label to products as forgery-preventing means so that true and correct products (authentic products) can be identified. Herein, “product” is a generic name of a produced item such as an article, a commodity and goods. In particular, the latter means is frequently used, because it is more generally versatile than the former means, which rather needs to be individually dealt with.
The latter means may be further divided into two techniques. One is a technique in which anyone can always identify the existence of forgery-preventing means, and a well known technique includes a hologram. The other is a technique in which forgery-preventing means is ordinarily undetectable, and only persons who know the existence of forgery-preventing means can detect it with special means to determine whether the product is authentic or not. A technique, in which authenticity is identified by observing, with a polarizing plate, a latent image formed using a phase difference medium in which an optical axis is patterned, is known (see, for example, JP-A-2008-137232 (“JP-A” means unexamined published Japanese patent application) and JP-A-2008-129421). However, there are problems in that the thus-visualized latent image is monochromatic when viewed from the front, and further authenticity can not be identified unless the polarizing plate is rotated, which makes authentication cumbersome and complicated.