It really does depend on how you look at it.

This case involved the presentation of what was alleged to be a fraudulently created till receipt.  We examined the receipt to see if we could detect the most likely difference between a genuine and a forged receipt - the use of ink by the forger, rather than the heat-sensitive method used to produce some of the text on the genuine till receipt. 

Printing on till receipts of the type produced in this case is created on heat-sensitive paper.  The nature of this sort of printing is such that it absorbs infra-red light, meaning that the text is invisible when examined under infra-red light.  

This is a different process to that used when inks or laser printing are used. 

The principles are illustrated by these pictures of a rail ticket showing how the contemporary data (e.g. date, destination, class) which is printed at the time of purchase can be seen against the background of the pre-printed (ink) text.

When infra-red light is used, the contemporary text vanishes while the ink remains visible.

In this way, we showed that to produce a fraudulent till receipt the forger would need the same kind of equipment as the till to produce similar absorbance of light in the text.  

The prosecution withdrew the charge in the 'light' (sorry!) of this evidence.

This technique and variations on it can be used to identify fraudulent changes to documents such as cheques, letters, and tickets. 

In examining an item to establish whether it is the same as other items it is never possible to say scientifically that they are certainly so because there may be minute differences that have not been detected by the techniques used in the examination.  Each technique will have a discriminating power dependent on the circumstances of use.  The more tests in which the items are the same then the more confident will be the assertion that they are not in fact different.