A method of forming a security document, comprises providing a thread comprising at least one region of heat-transformable material, this material having an optical effect which is transformable in response to applied heat. The thread is integrated with a document substrate material during a substrate forming process, so as to produce a security document substrate having at least one windowed region in which the thread is exposed at the substrate surface, and at least one bridge region in which the thread is not exposed. A patterned heat process is then applied, causing a structural modification in the heat-transformable layer of the thread so as to produce an observable optical effect representative of the heat pattern at one or more predetermined positions with respect to one or more of the said windowed region or bridge region. Security documents bearing threads with such optically transformable materials are also disclosed.

A method of forming a security document, comprises providing a thread comprising at least one region of heat-transformable material, this material having an optical effect which is transformable in response to applied heat. The thread is integrated with a document substrate material during a substrate forming process, so as to produce a security document substrate having at least one windowed region in which the thread is exposed at the substrate surface, and at least one bridge region in which the thread is not exposed. A patterned heat process is then applied, causing a structural modification in the heat-transformable layer of the thread so as to produce an observable optical effect representative of the heat pattern at one or more predetermined positions with respect to one or more of the said windowed region or bridge region. Security documents bearing threads with such optically transformable materials are also disclosed.