Method of printing stealth white image, set of substrate and stealth ink, and printing device

1. A method of printing a stealth white image, comprising:

applying a stealth ink containing a red coloring material that emits visible light at exposure to ultraviolet radiation and a green coloring material that emits visible light at exposure to ultraviolet radiation to a substrate which contains a fluorescent brightener to form the stealth white image thereon,

wherein the stealth white image demonstrates an a* value of from −2.0 to 2.0 and a b* value of from −10.0 to 0 at exposure to ultraviolet radiation having a wavelength of 370 nm according to CIE 1976 L*a*b* colorimetric system.

2. The method according to claim 1 , wherein, in the applying, a mass ratio of the red coloring material to the green coloring material in the stealth ink is changed depending on a blue color tone of light the substrate emits at exposure to ultraviolet radiation.

3. The method according to claim 1 , wherein the stealth ink demonstrates two fluorescence emission maxima of from 605 to 645 nm and from 505 to 545 nm at exposure to ultraviolet radiation having a wavelength of 365 nm.

4. The method according to claim 1 , wherein the stealth ink further contains an organic solvent having a mixing SP value of 13.0 (cal/cm 3 ) 0.5 or greater.

5. The method according to claim 1 , wherein the stealth ink further contains a polyether polyol.

6. The method according to claim 1 , further comprising applying a stealth non-w % bite ink.

7. The method according to claim 1 , further comprising drying the stealth ink after the applying.

8. The method according to claim 7 , further comprising applying a stealth non-white ink after the drying.

9. The method according to claim 8 , wherein, in the applying the stealth ink, the stealth ink is applied to one of an entire surface of the substrate and a same region of the substrate where the stealth non-white ink is applied.

10. A set of a substrate and a stealth ink, comprising:

the substrate containing a fluorescent brightener; and

the stealth ink containing a first stealth ink containing a red coloring material that emits visible light at exposure to ultraviolet radiation and a green coloring material that emits visible light at exposure to ultraviolet radiation and a second stealth ink,

wherein printed matter having the substrate with the first stealth ink applied thereto has an a* value of from −2.0 to 2.0 and a b* value of from −10.0 to 0 at exposure to ultraviolet radiation having a wavelength of 370 nm according to CIE 1976 L*a*b* colorimetric system.

11. The set according to claim 10 , wherein the second stealth ink contains at least one member selected from the group consisting of a red coloring material that emits visible light at exposure to ultraviolet radiation, a green coloring material that emits visible light at exposure to ultraviolet radiation, and a blue coloring material that emits visible light at exposure to ultraviolet radiation.

12. The set according to claim 10 , wherein a mass ratio of the red coloring material to the green coloring material in the first stealth ink is changed depending on a blue color tone of light the substrate emits at exposure to ultraviolet radiation.

13. A printing device comprising:

a container containing an ink that contains a stealth ink containing a red coloring material that emits visible light at exposure to ultraviolet radiation and a green coloring material that emits visible light at exposure to ultraviolet radiation;

an ink discharging head configured to discharge the ink to a substrate which contains a fluorescent brightener to form a stealth white image; and

a circulation assembly including a pressure generating device which is configured to circulate the ink,

wherein the stealth white image demonstrates an a* value of from −2.0 to 2.0 and a b* value of from −10.0 to 0 at exposure to ultraviolet radiation having a wavelength of 370 nm according to CIE 1976 L*a*b* colorimetric system.

14. The printing device according to claim 13 , further comprising a second ink which contains a stealth non-white ink.