Laser projection devices (laser markers) that thermally mark characters and symbols on materials are commercially available. Specifically, such laser projection devices generally incorporate a technology in which a material absorbs, on exposure to a laser beam, the laser beam to generate heat and the generated heat changes colors of the exposed portion of material, thereby recording characters and symbols on the material.
Examples of a laser light source of the laser projection device include a gas laser, a solid-state laser, a liquid laser, and a semiconductor laser, and characters and symbols are marked, based on oscillation wavelengths of the exposed lasers, on materials such as metallic or plastic media, heat-sensitive paper, and thermal rewritable media.
In the metallic or plastic media, heat generated upon laser projection engraves or singes surfaces of the media, thereby marking characters and symbols on the media. Meanwhile, in the heat-sensitive paper or thermal rewritable media, heat generated upon laser projection causes recording layers of the media to generate colors, thereby printing characters and symbols on the media.
The heat-sensitive paper may also be used as media on which destinations or names of articles are printed. For example, such heat-sensitive media are attached to plastic containers to be used as tags in factories. Since heat-sensitive media including the heat-sensitive paper have a discoloring property due to heat, characters and symbols are written on the media by a thermal head.
Recently, a rewritable type of heat-sensitive paper has been made available to the public, so that characters and symbols can now be repeatedly recorded on and erased from the rewritable heat-sensitive paper. It is preferable that such heat-sensitive paper attached to a container be rewritable without removing it from the container in a physical distribution service. Japanese Laid-Open Patent Application No. 2004-90026, for example, discloses a technology in which characters are printed on desired media in a non-contact manner by laser projection. In this technology, desired characters are printed on the desired media in a non-contact manner by laser projection that causes the surfaces of the media to generate heat. This application also discloses a relay lens system composed of pluralities of lens systems and flexible joints. In the disclosed relay lens system, images formed by laser projection via one end of the relay lens system are transmitted to the other end.
Note that forming images by laser projection is a well-known technology and disclosed, for example, in Japanese Laid-Open Patent Application No. 2004-341373. Japanese Laid-Open Patent Application No. 2004-341373 discloses an image forming technology in which an original image data unit is divided into plural lines and each line of the divided images are formed on a photosensitive drum by laser projection.
Note that colors on the thermal rewritable media disappear at a certain temperature but reappear by further increasing the temperature. However, when excessive heat is applied to the thermal rewritable media, the properties may be altered to cause degradation, such as a decrease in service life-span of the media or incomplete erasure of characters or images from the media.
Some portions of the medium may, when recording a certain image on the medium, easily acquire the excessive heat. For example, there is a case where a certain enclosed region of the thermal rewritable medium is raster scanned (solidly shaded by scanning). FIG. 26A is a diagram illustrating raster scanning an enclosed region. A laser beam is applied in an enclosed region of the thermal rewritable medium by gradually shifting an origin (i.e., scanning start point), and sweeps horizontally left-to-right at a steady rate. The enclosed region of the thermal rewritable medium exposed to the laser beam generates heat, and the solidly shaded region is obtained. Hereafter, projecting a stroke of laser is also simply called “scanning” or “scanning a stroke”. The enclosed region is solidly shaded by sequentially scanning from the top line to the bottom line without any gap between the lines.
For example, in FIG. 26A, the line II is scanned after the scanning of the line I without a gap between the lines I and II. However, if the line II is scanned immediately after the scanning of I, the line II is being scanned while the line I just scanned still has residual heat of a laser beam. Thus, the temperature of thermally overlapped regions of the lines I and II may exceed the temperature specified by the specification of the thermal rewritable medium. In this case, the structure of the molecules of the thermal rewritable medium may be damaged due to thermal denaturation, thereby exhibiting an adverse effect on the thermal rewritable medium, such as inability to erase recorded matter.
Japanese Laid-Open Patent Application No. 2008-208681, for example, discloses a technology to control the scanning positions of the lines I and II so as not to create the aforementioned thermally overlapped regions of the lines I and II. However, Japanese Laid-Open Patent Application No. 2008-208631 does not disclose a technology to eliminate thermal overlaps between the two parallel strokes. It is difficult to control the scanning positions without creating thermally overlapped regions of the lines I and II and to display colors without creating any gaps between the lines I and II.
Japanese Patent No. 3990891 discloses a technology for preventing the adverse effect due to residual heat, in which time or amount of laser projection is reduced while a laser beam is scanning the line II. However, similar to the case of controlling the scanning positions of the line I and II, the reduction of time or the reduction of amount of laser projection generates a gap between the lines I and II.
Japanese Laid-Open Patent Application No. 2008-62506 discloses a technology to overcome such a drawback by controlling time between the strokes while laser beams are scanning the two parallel lines. Specifically, Japanese Laid-Open Patent Application No. 2008-62506 discloses the technology to control the time between a start of scanning a preceding stroke of the line I and an end of scanning a subsequent stroke of the line II. In this manner, since the subsequent stroke of the line II is scanned after residual heat of the line I caused by the preceding stroke has been cooled, the adverse effect on the thermal rewritable medium may be prevented.
However, there may still be observed the adverse effect on the thermal rewritable medium with the technology disclosed in the Japanese Laid-Open Patent Application No. 2008-62506. FIG. 26B illustrates such a case where the stroke of the line I is shorter than the stroke of the line II in raster scanning. With this technology in which only the time between the scanning of the two strokes is controlled, even though there is a sufficient time in total for scanning the two strokes of the respective line I and II, the scanning of the stroke of the line II may have started without having a sufficient cooling time after the scanning of the short stroke of the line I has been completed. That is, since time to scan the stroke of the line I is short, the line II can be scanned while the region around the scanned line I still has residual heat. Accordingly, there may be the thermally overlapped regions of the lines I and II which exceed the temperature specification of the thermal rewritable medium.
In addition, in this disclosed technology, interference due to residual heat may also occur between proximate strokes, that is, between strokes that are not parallel to each other. FIG. 27 is a diagram illustrating residual heat interference generated in scanning stroke-based fonts. In scanning a font-based stroke in the order and in directions illustrated in FIG. 27, an end point of the stroke “0” and a start point of the stroke “1” are close to each other. Therefore, simply controlling time between the start point of the stroke “0” and the end point of the stroke “1” may not completely eliminate the thermal interference generated due to residual heat between the end point of the stroke “0” and the start point of the stroke “1”.