Code reading systems

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.