Creating Twin Dyes in the Lighter Gray Tonal Range for Infrared-Responsive Black and White Photographs on Textile Materials

This study presents an extension of the Infraredesign® approach to grayscale imaging and black and white photography based on the concept of twin dyes in the gray tonal range. Twin dyes are defined as visually indistinguishable color formulations in the visible spectrum that exhibit intentionally differentiated behavior in the near-infrared (NIR) range. In the proposed method, grayscale photographic images are generated using CMY-only formulations without black ink in the visible layer, particularly in light and mid-tone regions. Hidden information is embedded through spectrally differentiated CMYK twin formulations, in which minimal reductions of CMY components are compensated by the controlled addition of black ink, limited to a maximum of 15%. Multispectral analysis was conducted to characterize the visible and NIR absorption and reflectance properties of selected twin dye pairs and to confirm their visual equivalence and infrared separability. The validated twin dyes were applied to black and white photographic images and evaluated through textile printing experiments. The results demonstrate that visually identical grayscale images (ΔE00 < 2) can reveal distinct information layers under infrared illumination, with consistent spectral separation observed up to 1000 nm, while preserving the perceived monochromatic quality in the visible spectrum. Unlike existing approaches primarily focused on chromatic or high-contrast regions, the proposed method enables information embedding within perceptually sensitive light and mid-tone areas of grayscale imagery. The proposed approach transforms black and white photographs into dual-layer information carriers and opens new possibilities for textile printing, image authentication, artistic expression, and security-oriented applications.