Marking by phase change

If a crystalline polymer is heated and cooled rapidly, it may be converted into its amorphous form. If the amorphous form is heated to just below the melting point for sufficient time, it can anneal back to the crystalline 

The final problem is that of controlling the crystallite size. If it is too small ( 0.1 / m), it will be indistinguishable from the amorphous state when using light of 0.8 //m wavelength. But if the crystallite size is too large ( 0.2//m), the definition of a written spot (—l//m) becomes blurred and the read-out signal becomes noisy. 

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