Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Marking systems where an image of some type is applied on a substrate, such as on a flat piece of paper or plastic or on a 3-dimensional object, are designed to be read by humans and/or by machine scanners and cameras. The optical contrast between the image and non-image areas on the substrate is important for readability, particularly for the readability of encoded information, such as bar codes, by machine readers. Some machine reading is done at visible wavelengths, such as at the typical laser and light-emitting diode (LED) wavelengths of approximately 630, 650, and 680 nm. Other machine reading is done at infrared wavelengths, such as the scanning of ID cards at approximately 900 nm. For machine reading, a uniform reflective background is desirable to provide the required optical contrast to read the image that has a greater absorption and lower reflectance at the wavelength of the machine reader.
For some marking systems, such as security marking of ID cards and of goods for anti-counterfeiting purposes and such as the marking of postal flat pieces for sorting, it would be advantageous to have a visibly transparent layer that is applied to the substrate and provides a uniform reflective background, while still permitting the substrate and any images on the substrate to be seen visually, while being as aesthetically unobtrusive as possible, and while being opaque to the wavelength of the machine reader. This combination of properties in a passive optical layer that does not require any activation or stimulation to provide the needed properties would be especially desirable. A dynamic optical layer that temporarily and reversibly switches from a visibly transparent layer to provide a uniform reflective background with opacity to the machine reader adds complexity and cost to the marking system. An example of a marking system with a dynamic optical layer that reversibly switches to provide a reflective layer is described in U.S. Patent Application Publication No. US 2002/0152928, titled “Contrast Enhancing Marking System for Application of Unobstrusive Identification and Other Markings”, published Oct. 24, 2002, to Lawandy et al.
Thin metallic layers or, alternatively, multiple interference layers where the index of refraction of alternating layers is varied are typically used in applications requiring passive reflective layers. Thin metallic layers often have a relatively flat absorption and reflectance across the visible and infrared regions so that it is difficult to achieve the desired combination of transparency, reflectance, and opacity for visual viewing and for machine reading at visible or infrared wavelengths. Multiple interference layers are complex and costly to manufacture, often requiring 6 or more layers, less than a 1% thickness variation in each layer, and extremely tight control over coating defects. Also, multiple interference layers that are visually transparent as desired in visible marking systems and solar window films often do not have a combination of strong absorption and reflectance in the near-infrared region of 700 nm to 1000 nm that would be useful for some types of marking systems and of solar window films.
It would be advantageous for marking systems and other applications, such as solar window films, optical mirrors, and optical splitters, to have a passive reflective layer comprising a material that is intrinsically reflective and has strongly varying absorption and reflectance properties across the visible and infrared regions that can be matched to the combination of absorption and reflectance properties desired in the particular application, such as matched to the human and machine readers for marking systems.