A luminescent phosphor compound is a compound that is capable of emitting detectable quantities of radiation in the infrared, visible, and/or ultraviolet spectrums upon excitation of the compound by an external energy source. A typical luminescent phosphor compound includes at least a host material (e.g., a crystal lattice), an emitting ion (e.g., of a rare earth metal), and in some cases, a “sensitizing” ion (e.g., of a transition metal or of a different rare earth metal that can absorb and transfer the energy to the emitting rare earth metal ion). The production of radiation by a phosphor compound is accomplished by absorption of incident radiation by the emitting ion(s) or by either or both the host material and the sensitizing ion(s), followed by energy transfer from the host material/sensitizing ion(s) to the emitting ion(s), and radiation of the transferred energy by the emitting ion(s).
The selected components of a phosphor compound may cause the compound to have particular emission properties, including spectral emission that is at a wavelength greater than an excitation wavelength. Not every ion will produce emissions in all host materials, however. There are many examples in which radiation that has the potential for emission is quenched, or the energy transfer from the absorbing ions or the host material to the emitting ions is so poor that the radiation effects are barely observable. In other host materials, the radiation effects can be very large and with quantum efficiency near unity.
For a specific phosphor compound that does produce observable emissions, the spectral position(s) of the higher spectral energy content (or luminescent output) in its emissions (i.e., its “spectral signature”) may be used to uniquely identify the phosphor compound from other compounds. Primarily, the spectral signature is due to the rare earth ion(s). However, spectral perturbations may be present due to the influence of the host material on the various emitting ions, typically through crystal field strength and splitting. This holds true for the temporal behavior of the emissions, as well.
The unique spectral properties of some phosphor compounds make them well suited for use in authenticating or identifying articles of particular value or importance (e.g., banknotes, passports, biological samples, and so on). Accordingly, luminescent phosphor compounds with known spectral signatures have been incorporated into various types of articles to enhance the ability to detect forgeries or counterfeit copies of such articles, or to identify and track the articles. For example, luminescent phosphor compounds have been incorporated into various types of articles in the form of additives, coatings, and printed or otherwise applied features that may be analyzed in the process of authenticating or tracking an article.
An article that includes a luminescent phosphor compound may be authenticated using specially designed authentication equipment through known authentication techniques. While such authentication techniques are highly effective at detecting and thwarting relatively unsophisticated forgery and counterfeiting activities, they do exhibit drawbacks. For example, individuals with the appropriate resources and equipment may be able to employ spectrometry techniques in order to determine the components of some phosphor compounds. The phosphor compounds may then be reproduced and used with unauthentic articles, thus compromising the authentication benefits that may otherwise be provided by a particular phosphor compound. Further, many phosphor compounds have relatively high densities, such as greater than or equal to about 5 grams per cubic centimeter. As a result, the phosphor compounds are difficult to disperse in many liquid mediums, including inks, and settling can result in uneven concentrations of the phosphor compounds upon application of the inks.
Accordingly, although a number of phosphor compounds have been developed to facilitate article authentication in the above-described manner, it is desirable to develop additional compounds, which may render forgery and counterfeiting activities more difficult, and/or which may prove beneficial for identifying and tracking articles of particular interest. Further, it is desirable to develop phosphor compounds that have a density of less than 5 grams per cubic centimeter and, therefore, can be effectively dispersed within liquid mediums such as ink. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.