scanning the symbol can be accomplished by means of a laser spot scanner as used for non contact emissivity measurements as known in the art such instruments can detect emissivity differences on the order of 0 05 at a spatial resolution of 0 1 mm or less as required for bar code reading although emissivity measurements and estimates are employed to correct the temperature measurements of very hot objects spectral emissivity measurements can now be made at room temperature scanning is accomplished by means of a scanner which can detect and measure the emissivity of a particular spot the preferred embodiment of the scanner is comprised of the following elements a laser scanning mirror or prism mirror prism deflection motor and controller photo detector for example active laser pyrometer technology is disclosed in u s pat no 4 417 822 issued on nov 29 1983 to alexander stein such instruments obtain an accurate temperature measurement by discounting the emissivity of a surface in u s pat no 4 840 496 issued on jun 20 1989 elleman et al disclose a narrow laser beam contactless pyrometer capable of scanning a small area and determining the emissivity and temperature fig1 shows a machine readable mark created of any desired size and shape on a surface 1 the mark contains blank areas of unmarked surface 1 and a pattern of areas of varying emissivity 2 3 the pattern may be a bar code or other machine readable code or may contain a human readable character or symbol in a preferred embodiment the pattern is printed on a surface using a black colored carbon.