A Simplified Method for the Colorimetric Characterization of Fluorescent Inks

According to Gruml luminescence is "the phenomenon of the emission by matter of electromagnetic radiation that for certain wavelengths or restricted regions of the spectrum is in excess of that due to the thermal radiation fiom the material at the same temperature." We encounter several different types of luminescence in everyday life such as fluorescence, phbsphorence, cathodoluminescence and chemiluminescence. Fluorescence occurs when light absorbed in one wavelength band generates emissions in longer wavelength bands. This shift towards longer wavelengths is called Stoke's shift. Fluorescence is specially prevalent in printed materials. It is commonly associated with the optical brightners used to whiten paper and the dyes used in the printing inks. Paper has a strong tendency to absorb in the blue region of the visible spectrum (that is why grocery bags are brown.) Optical brightners work by absorbing in the near UV region, where we do not perceive the decrease in reflectance, andre-emitthe energy as blue light, which offsets the paper natural y ellowishness. The most common fluorescent dyes are red and magenta. They work by absorbing radiation in the blue green regions, which makes them look more saturated, and re-emit energy in the red end of the spectrum, further increasing the colorimetric purity. Fluorescent yellow dyes are also veIy common. Fluorescence has major implications for calibrating color reproduction systems because the spectral reflectance of fluorescent surfaces is dependent on the illumination. Unlike non-fluorescing surfaces, whlch can be described by a n-dimensional vector of spectral reflectance factors, fluorescing surfaces require at least three vectors. They are called here the diffuse reflectance, the stimulation spectrum and the emission spectrum. Conversely, the fluorescent surface can be described as an (n x n) lower diagonal matrix where the diagonal contains the conventional non-fluorescent reflectance and the off-diagonal elements contain the contribution due to fluorescence. Figure 1 illustrates such matrix for a magenta printer ink. For scanner calibration fluorescence represents a major obstacle. Colorimetric calibration can only be accurately performed for a fixed illuminant, and depending on the combination of detector, light and filters one might fmd that such illuminant has undesirable white point and metameric properties. The calibration of printers is no less problematic. While diffke surfaces can be characterized with simple spectrophotometry, and tristimulus computed later for any desired illuminant, fluorescent surfaces must be characterized with a source that closely resembles the desired illuminant, or by measuring the entire reflectance matrix. Figure 1. ~eflecta&ematrix for a magenta printing ink