US 5,331,140 A · Code reading systems

Abstract

Bar code reading systems having enhanced detection capabilities are disclosed. The systems are particularly useful for invisible bar codes. The bar code is irradiated with radiation that is sine wave or square wave modulated at one or more frequencies. A detector is sensitive to a frequency related to the modulation frequencies, preferable to a sum or a difference of at least two modulating frequencies, produced by intermodulation of the modulating frequencies. Such intermodulation can result from the non-linear fluorescing characteristics of dyes incorporated into the bar code markings. The modulation can also be sinusoid or square wave at a single frequency. Detection of coded information on sheets containing whiteners or other dyes capable of fluorescing in response to the radiation utilized by the code reader is improved by employing radiation absorbing components in the code markings.

Claims33

Dependent claims

A method for reading a fluorescent code on the surface of an article comprising the steps of:

directing radiation toward the code to cause the code to fluoresce, the radiation being modulated at two frequencies, which frequencies are non-harmonically related;
and
detecting a radiation signal resulting from fluorescence of the code, which radiation signal is of a frequency that is one of a sum of the two frequencies and the difference of the two frequencies.

Dependencies are inferred from the claim text.

Code reading systems

Life of this patent

Code reading systems

Life of this patent

Abstract

Claims33