This invention relates to a method of forming a color image by superposing one on top of the other a photosensitive sheet with one surface coated with microcapsules encapsulating a chromogenic agent capable of producing a specified color and a light-curable agent which hardens upon exposure to light within a specified wavelength range and an image transfer sheet with one surface coated with a developing agent having a chromogenic effect on this chromogenic agent and compressing these sheets together.
Japanese Patent Publication Tokkai 58-88739 disclosed a method of image formation by using a photosensitive sheet having a sheet-like transparent base material coated with microcapsules encapsulating a specified colorless dye (a chromogenic agent) and a light-curable agent which hardens upon exposure to light. If such a photosensitive sheet is exposed to image-forming light such as reflected light from a document, only those of the microcapsules on the photosensitive sheet which are exposed to light are hardened such that a latent image is formed on the photosensitive sheet corresponding to the image on the exposed document. If a photosensitive sheet with a latent image thus formed thereon is superposingly covered by an image transfer sheet with a surface coated with a developing agent having a chromogenic effect on the colorless dye encapsulated inside the microcapsules and if these sheets are compressed together, those of the microcapsules which are not hardened are ruptured, causing the encapsulated colorless dye to flow out thereof. The colorless dye flowing out of the ruptured microcapsules reacts with the developing agent on the image transfer sheet, producing a visible image on the image transfer sheet.
It is also possible to form full-color images by using a photosensitive sheet with a transparent base material coated with microcapsules. For example, use may be made of three kinds of light-curable agent, each having the property of hardening upon exposure to red, green or blue light. Each of these light-curable agents is encapsulated in the same microcapsules with a chromogenic agent capable of producing the color complementary to that to which the light-curable agent is sensitive such that the following three kinds of microcapsules are prepared: the Y-capsules encapsulating a light-curable agent which hardens upon exposure to light having wavelength of blue and a chromogenic agent capable of producing yellow which is complementary to blue, the M-capsules encapsulating a light-curable agent which hardens upon exposure to light having wavelength of green and a chromogenic agent capable of producing magenta which is complementary to green, and the C-capsules encapsulating a light-curable agent which hardens upon exposure to light having wavelength of red and a chromogenic agent capable of producing cyanic which is complementary to red. If a photosensitive sheet is prepared by uniformly coating a sheet-like transparent base material with these three types of microcapsules and if white light is projected onto a document to expose this photosensitive sheet to the reflected light, each light-curable agent inside a microcapsule hardens, corresponding to the amount of light in the corresponding wavelength range, thereby forming on the photosensitive sheet a latent image with color information. Next, such a photosensitive sheet is superposingly covered by an image transfer sheet coated with a developing agent having the property of reacting with each kind of chromogenic agents encapsulated in the microcapsules to thereby produce their intended colors and these sheets are compressed together. This causes those of the microcapsules containing an unhardened light-curable agent to rupture and the chromogenic agents encapsulated in such microcapsules flow out, reacting with the developing agent with which the image transfer sheet is coated and thereby producing colors. As a result, a full-color image corresponding to the one on the original document is formed on the image transfer sheet. In summary, if use is thus made of a photosensitive sheet with three kinds of microcapsules uniformly dispersed for coating its transparent base sheet, there is no need to decompose the full-color image on an original document into light beams in different wavelength ranges (such as blue, green and red) to which the light-curable agents in the microcapsules are sensitive. By this method, therefore, a latent image corresponding to the full-color image on an original document can be formed on a photosensitive sheet merely by projecting white light on the document and exposing the photosensitive sheet to the reflected light.
FIG. 3 shows the spectroscopic sensitivity characteristics of the light-curable agents encapsulated in the three kinds of microcapsules coating such photosensitive sheets. The light-curable agent for the Y-capsules which hardens mainly upon exposure to blue light is sensitive to light in a wavelength range of about 500 nm. The light-curable agent for the M-capsules which hardens mainly upon exposure to green light is sensitive to light in a wavelength range of about 480-600 nm. The light-curable agent for the C-capsules which hardens mainly upon exposure to red light is sensitive to light in a wavelength range of about 570-700 nm. Thus, the wavelength ranges in which the three light-curable agents of different kinds are sensitive to light overlap partially one another. In other words, light of wavelength in such an overlapped range causes more than one type of light-curable agent to harden. For example, both Y-capsules and M-capsules are hardened by exposure to light in the wavelength range of about 480500 nm and both M-capsules and C-capsules are hardened by exposure to light in the wavelength range of about 570-600 nm.
Examples of light source conventionally used for exposing photosensitive sheets for image formation include halogen tungsten lamps and xenon lamps. As can be seen in FIG. 4 which shows the radiation intensity characteristics of a halogen tungsten lamp, the light intensity from a halogen tungsten lamp increases with the wavelength. In other words, a halogen lamp emits light both in the wavelength range where the light-curable agents in Y-capsules and M-capsules are both sensitive and also in the wavelength range where the light-curable agents in M-capsules and C-capsules are both sensitive. As a result, a faithful color reproduction cannot be achieved in the case of an original purely of one of the three basic colors (blue, green and red) or of their complementary colors (yellow, magenta and cyanic) because some unwanted capsules are also hardened.
Another problem with light sources such as halogen tungsten lamps and xenon lamps which are used in forming images on photosensitive sheets is that the efficiency of light emission in the visible range is as low as about 20 Lm/W. Since the photosensitivity of the light-curable agents encapsulated in the microcapsules on ordinary photosensitive sheets is low, a high power is usually required to form an image by means of such a lamp. This results in the production of a large amount of heat and a special cooling device becomes necessary.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention in view of the above to eliminate the problems of prior art image formation methods.
According to a method embodying the present invention by which the above and other objects can be achieved, image-forming light is projected onto a photosensitive sheet with a surface coated with microcapsules each encapsulating one of a plurality of kinds of light-curable agent which hardens upon exposure to light in a wavelength range and a specified chromogenic agent. Thereafter, an image transfer sheet with a surface coated with a developing agent having chromogenic effects on the chromogenic agents is superposed on the microcapsulecoated surface of the photosensitive sheet and these sheets are compressed together. The method of the present invention is characterized in that the photosensitive sheet is exposed to light from a source having its radiation intensity concentrated in each of the wavelength regions where the light-curable agents have their maximum photosensitivities. According to a preferred embodiment of the present invention, use is made of a metal halide lamp as the light source. According to another preferred embodiment of the present invention, the aforementioned light-curable agents are sensitive to light respectively in the wavelength range of red, green and blue which are three basic colors. According to still another preferred embodiment of the present invention, the chromogenic agents encapsulated in the microcapsules each produce the color which is complementary to the color of wavelength range of the light-curable agent contained in the same microcapsule.
By a method of the present invention, a faithful color reproduction is possible even if there is an overlapping among the sensitivity wavelength ranges of the microcapsules coating the photosensitive sheet because the exposure of the microcapsules to light in the overlapping wavelength ranges can be prevented. Moreover, the waste of energy can be reduced because the energy supplied to the light source is concentrated at the centers of the wavelength ranges where the microcapsules of different kinds are most photosensitive. As a result, the light emission efficiency of the light source is improved and the power consumption can be reduced. If the light emission efficiency of the light source is improved, the exposure time can be shortened and the speed of image formation can be increased.