When compact discs were mass produced, screen printing was used to decoratively label a large run of compact discs. Screen printing is impractical for single discs or small production runs. Therefore, label applicators, ink jet printers and wax-resin thermal printers have been developed which can print labels onto single compact discs in a more practical manner.

Applying a separate label to compact discs may create undesirable imbalances of the disc, especially as the rotational speed of discs is increased to increase data transfer rate. Labeling a disc with a separately applied member is also a time consuming and cumbersome operation.

Two different types of disc printers have been introduced which overcome the problems with attaching a label to a disc. One of these is wax-resin thermal transfer printer. In this process a thermally activated printhead is used to selectively apply heat and pressure to a carrier ribbon sandwiched between the disc and the printhead. The ribbon has a thin coating of colored wax-resin which bonds to the surface of the compact disc

and creates an image. The wax resin coating is available in different colors for color printing. If more color variability is desired, a dithering process is combined with the primary colors to create the illusion of more colors. The wax resin is somewhat durable but it can be scratched from the disc surface.

Ink jet printing utilizes a printhead which sprays small droplets of ink onto a specially coated disc surface. Like wax resin thermal transfer, ink jet requires dithering techniques to create different colors.

A printing method which provides true color reproduction has been developed and implemented in other applications, namely dye diffusion thermal transfer printing (D2T2). D2T2 printing employs a thermal printhead and ribbon like wax resin thermal transfer, but instead of applying the resin in a binary (on or off) fashion, dye diffusion is capable of applying varying shades of each primary color. This allows true color reproduction and true photographic print quality from a digital printing process.

SUMMARY OF THE INVENTION The present invention combines the photographic reproduction capabilities of D2T2 printing with the capability to print onto a single disc or substrate at a time. The result is a product and process that gives true color reproduction with greater durability on a coated compact disc in particular.

Photographic quality images can be printed on compact discs with higher quality and faster speeds than ink jet printing, with higher quality color depth and better durability than wax-resin thermal transfer printing.

The present invention uses thermal control of heaters to vary the energy level at selected locations

on the printhead length so the dye is diffused into the coating on a disc at different rates. The different rates of diffusion can be controlled as the disc is moved under the printhead to provide for true grey scale capability and photographic color output.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a typical printer used for the present invention; Figure 2 is a schematic cross sectional view of the printer and functions; Figure 3 is a side view of a printhead used i the printer of Figure 1; and Figure 4 is an enlarged schematic view of a section of the printhead showing individual heaters used in printing.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS A typical embodiment of the printer 10 is shown in Figure 1, and it includes an outer housing 12, with a tray 14 for carrying a substrate or disc 16, which can be a DVD, a CD, a mini-compact disc, a recordable or read-write CD, or a recordable or read- write DVD. The tray is operated to move from a position where the disc can be loaded, to a position where the disc is under the printhead. Figure 2 is a schematic sectional view of the printhead and ribbon assembly shown in Figure 1. The printhead 20 is mounted in a suitable manner relative to the printer frame 12, as is known, and includes a row of heat elements 22 that have portions which engage a ribbon 24, and when heated under control of a controller 27 will transfer material from the ribbon 24 onto the disc 16 or other substrate. The carrier 14, as shown, is driven in a suitable manner to move the disc in the direction as indicated by the arrow 26. The ribbon 24 is provided on a supply roll 28

driven by a motor 28A under control of the center control system or controller 27 and a ribbon takeup roll 30 is driven by a motor 30A also controlled by the central controller 27. The disc 16 is coated with a dye receptive coating and the row of heat elements 22 from the printhead will transfer the dye from the ribbon 24 to the coating on the disc 16. The heat elements 22 are positioned side by side on the printhead and are individually controlled as to heat output.

A platen 36 is rotatably mounted on the frame 12, and is driven with a motor 38 to provide a load to urging the carrier 14 and the disc 16 against the ribbon, and then against the printhead with the desired force. The platen creates an intimate contact between the printhead, ribbon and disc.

As the disc moves, the ribbon will be also moved to desired locations so that suitable colors can be applied to the disc.

Images are created on the disc through the control of the individual heat elements that are shown at 40 in Figure 4, which comprise resistance heaters 42 that are individually controlled from the controller 27, through suitable lines. The heaters establish a print line across the disc.

The individual resistors will provide a heat output that is proportional to the current through the controlled resistor. The heat output will selectively apply the imaging compound on the ribbon 24 to specific areas on the substrate or disc 16. The individual heat elements or heaters 40 are individually controlled by controlling the power (voltage) to the respective resistors 42. By varying the energy level at each thermal element or heater 40, that is, by controlling the individual resistor 42, the dye from the ribbon 24

is diffused into the coating on the disc at different rates. This control provides a true grey scale capability in results in photographic color output. By combining variable levels of the primary colors (yellow, magenta and cyan), the color"gamma"is increased significantly and a high quality photographic image is created on a compact disc. The heaters 40 are controlled by the controller 27 under a program which provides variable energy control in accordance with the heat needed for the proper color of the image. The individual heaters 40 extend all across the printhead 20, as shown in Figure 3.

The controller 27 will be programmed with the appropriate print pattern depending on the image, using known software techniques in a print program 46, so that for a particular color at any particular line of printing, the appropriate resistors 42 will be heated to the desired level. The energy level that is provided can be varied between adjacent heaters 40 and across an entire line if desired to provide the appropriate color transfer from the ribbon to the substrate or disc.

The dye sublimation images will diffuse as a function of the variable energy control to each of the resistors 42, and the program controls for the controller will permit the color printing to be directly applied to the coating on the compact disc. Printing techniques that will permit diffusion of colors directly onto the discs, which are generally polycarbonate, can use the same method for obtaining true reproduction of color.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.