Method of authenticating a polymer film by thickness measurement with a white light interferometer

Abstract

A method of authenticating a polymer film comprises measuring the thickness of a layer therein by white light interferometry and/or measuring the birefringence of a layer therein. The method, and devices to carry out the method, may be used in security applications, for example to test for counterfeit bank notes.

1 . A method of authenticating a polymer film comprising measuring the thickness of a layer therein by white light interferometry. 
     
     
         2 . The method according to  claim 1 , wherein the measurement is made on a core layer of the film. 
     
     
         3 . (canceled) 
     
     
         4 . A method of authenticating a polymer film comprising measuring the thickness of a layer therein by white light interferometry and/or measuring the birefringence of a core layer therein. 
     
     
         5 . The method according to  claim 4 , wherein the measurement of thickness is made on a core layer of the film. 
     
     
         6 . The method according to  claim 1 , wherein measurement is carried out on more than one layer. 
     
     
         7 . The method according to  claim 1 , wherein measurement is carried out using a pulsed light source. 
     
     
         8 . The method according to  claim 1 , wherein measurement is carried out using a detuned light source. 
     
     
         9 . The method according to  claim 1 , wherein measurement is carried out using an LED light source. 
     
     
         10 . The method according to  claim 1 , wherein the film comprises at least one clarifying agent. 
     
     
         11 . The method according to  claim 1 , wherein the film is prepared by a bubble process. 
     
     
         12 . The method according to  claim 1 , wherein the film comprises a layer of biaxially oriented polypropylene (BOPP). 
     
     
         13 . The method according to  claim 1 , wherein the film is a monoweb of three or more polymeric layers. 
     
     
         14 . The method according to  claim 1 , wherein the film is a double web of five or more polymeric layers. 
     
     
         15 . The method according to  claim 1 , wherein the method differentiates between films made a bubble process and films made by a different process. 
     
     
         16 . A method of authenticating an object comprising a polymer film, wherein authentication is carried out by authenticating the polymer film according to  claim 1 . 
     
     
         17 . The method as claimed in  claim 16 , wherein the object is:
 a bank note;   a governmental document;   a non-governmental document;   an identity document;   a passport;   a security thread;   an Optical Variable Device (OVD); or   a wrapper of or an attachment to an item.   
     
     
         18 . A method of identifying an authentic film or object on the basis of the film or object meeting a specified pre-defined criterion of thickness and/or birefringence, utilizing a method according to  claim 4 . 
     
     
         19 . A method of identifying a non-authentic or counterfeit film or object on the basis of the film or object not meeting a specified pre-defined criterion of thickness and/or birefringence, utilizing a method according to  claim 4 . 
     
     
         20 - 21 . (canceled) 
     
     
         22 . A detection unit adapted to carry out the method the method of  claim 4 , comprising a white light interferometer and a birefringence detector . 
     
     
         23 . (canceled) 
     
     
         24 . The method according to  claim 4 , wherein measurement is carried out on more than one layer. 
     
     
         25 . The method according to  claim 4 , wherein measurement is carried out using a pulsed light source. 
     
     
         26 . The method according to  claim 4 , wherein measurement is carried out using a detuned light source. 
     
     
         27 . The method according to  claim 4 , wherein measurement is carried out using an LED light source. 
     
     
         28 . The method according to  claim 4 , wherein the film comprises at least one clarifying agent. 
     
     
         29 . The method according to  claim 4 , wherein the film is prepared by a bubble process. 
     
     
         30 . The method according to  claim 4 , wherein the film comprises a layer of biaxially oriented polypropylene (BOPP). 
     
     
         31 . The method according to  claim 4 , wherein the film is a monoweb of three or more polymeric layers. 
     
     
         32 . The method according to  claim 4 , wherein the film is a double web of five or more polymeric layers. 
     
     
         33 . The method according to  claim 4 , wherein the method differentiates between films made a bubble process and films made by a different process.