Black light (or ultraviolet-reactive) printing, while not new, is usually done on inkjet printers with its application often limited to posters or high-end brochures. High cost-per-copy is an issue hindering wider usage of this printing process. But two years ago, Tokyo-based Kosaido Printing developed Luminous Palette, a CMYK+RGB technology that offers cost-effective black light printing on offset presses.

Publisher PIE International became the first to use it extensively in Yoshiyuki Yoshimura’s Find Monsters, a children’s activity book, in September 2017. On the pages, Japanese youkai monsters hidden in ten daily scenes (at school and home, and on the beach, for instance) are only visible under UV light of a specific wavelength.

So the first issue for PIE International, even before embarking on the book design and editorial development, was to ensure the safety of the product to young readers, who are the target audience. “It took our team about half a year to ensure that the UV light was not dangerous to children or in any way detrimental to their vision,” says Hiromoto Miyoshi, president of PIE International.

Color-matching and color-proofing were the two biggest technical challenges during the production of this book. “Using seven colors by putting two different color gamuts together means working hard to avoid having the colors interfering with one another,” explains Ayako Sano, editor at PIE International. “We realized that the luminosity of the youkai monsters differ greatly even with just a slight variation in the RGB combination. So each color had to be set precisely. If the values of R, G, and B are antagonistic, then it becomes achromatic, resulting in muddy impressions. Throughout the process of making this book, it became critical for us to think differently about adjusting—or even seeing—colors.” The color separation process, naturally, was tedious and exacting.

Since RGB gamut is not visible under normal light, corrections had to be marked on “invisible images”, which was an unusual experience in itself. At the same time, the press proofs had to be checked to ensure that the RGB colors will remain intact and consistent in different environments, such as in wet conditions, at freezing temperatures, and even after cleaning or wiping with cloth and other materials. During the printing process, RGB shown-through and set-off were recurring critical issues that had to be rectified immediately.

The content pages were printed on paper that does not contain any fluorescent particulates, which may affect the luminosity of the RGB gamut. Miyoshi and his team decided to use 157-gsm HS Gao paper from Hokuetsu Corporation for this book.

The expensive RGB inks, however, made the per-copy printing cost relatively high for a normal print run of 5,000 copies. But after seeing the unusual book concept, bookstore pre-orders started to climb. Miyoshi then made a critical decision to print 17,000 copies—an exceptional quantity for such type of book—to bring down the price. The first printing sold out within two months, and as of today, nearly 32,000 copies have been printed. The sequel Find World Monsters, which is set for launch in October, will have a first printing of 20,000 copies; the retail price is 2,700 yen (or approximately $24.40).

The challenge with this series, says Miyoshi, “is less about the CMYK+RGB technology and technical aspects—which have already been tested and proven at Kosaido Printing—but more on the content development, where the ‘monster’ research often went into uncharted territories and required so much of the author’s vivid imagination and creativity. The content also had to be developed to create appropriate scenes where children will derive the most enjoyment out of searching for the youkai monsters using the UV light source supplied with the book.”