When viewing a given reflective object, the perceived color is the result of the spectral reflectance of that object, the spectral power distribution of the illuminant (widely referred to as SPD), and the sensitivity of the observer. Changes to any of these factors can become a source of variation in image reproduction in color critical workflows. Initiatives to control for these in industry include the widespread use of CIE standard observers, and the standardization of the SPD of the illuminant: most commonly D50 as defined by ISO 3644:2009.
While the visible region of the electromagnetic spectrum is generally described as light with wavelengths of approximately 380nm - 760nm, it is important to recognize that a phenomenon known as fluorescence can influence perceived color. Fluorescence occurs when an object absorbs radiation in the Ultra Violet (UV) range of the spectrum (below 380 nm) and re-emits this radiation in the "near UV" visible range (generally, 380-450nm). In commercial color reproduction, fluorescence is realized with the use of Optical Brightening Agents (OBAs) in the manufacture of substrates and colorants. In the case of paper substrates, for example, these OBAs increase the perceived whiteness of a sheet without the more costly and less-environmentally friendly process of bleaching (Vogt & Keif, 2012). Critical to examining OBAs is the recognition that the relative effect is dependent not only on the presence of OBAs in the material, but on the amount of UV radiation present in the illumination source. This represents yet another source of variation in color workflows.
When spectrophotometric instruments are utilized to measure color, the characteristics of the SPD in the respective instrument illuminants needs to be recognized. Historically, CIE Illuminant A, which represents tungsten lighting at 2856 Kelvin has been used in the majority of spectrophotometers (Cheydleur & O'Connor, 2012). This contrasts with the ISO 3664 specification for visual inspection, which specifies CIE Illuminant D50. As Illuminant D50 includes more spectral power in the UV range when compared to Illuminant A (GTI Technote, 2011), it is recognized that when materials containing OBAs are utilized inconsistencies between instrumental and visual evaluations can occur, even when standardized viewing conditions are strictly enforced.
In response, ISO 13655 further refined the measurement conditions for the illuminants utilized in instrument manufacture. The measurement condition known as M1 mandates a close match to D50, including the UV portion (McDowell, 2006). A 'legacy' condition, known as M0, recognizes the wide population of instrumentation used in the field: Cheydleur and O'Connor state: "M0 is limited in its definition and does not fully define either the measurement illuminant condition or the UV content of the sources. This is because M0 is also meant as a broad definition to included historical instruments of all types that do not fit into any of the other M conditions." As the UV content of measurement condition M0 is not defined, it is generally not recommended for color workflows were OBAs are present in the substrates and colorants. A further delineation of the M1 condition separates such instruments into those where the spectral illumination of the instrument light source matches D50, known as M1 Part One, and those that utilize a compensation method and a controlled amount of UV in the light source, known as M1 Part Two.
The present study analyzes instrumentation commonly used in graphics reproduction workflows using both M0 and M1 measurement conditions. Using five 'legacy' spectrophotometers measuring utilizing the M0 condition, and three spectrophotometers capable of measuring both the M0 and M1, four different paper substrates containing various levels of OBAs are analyzed. To evaluate the effect of solid colorants on the chosen substrates, eight commonly used lithographic printing inks are applied to the substrates using a proofing device with the goal of simulating production ink film thicknesses. These samples are then measured with each instrument and measurement condition. These readings are analyzed to note any differences in the various instruments/measurement conditions.
An additional goal of the study is to evaluate how each instrument/measurement condition reads change in the substrate and substrate/ink combination. Therefore, the samples were subject to accelerated aging in a frequently used fade test which utilized a Xenon-Arc test chamber, and then those same samples were re-measured. It is widely recognized that fade testing not only affects both the color of the ink and paper, but also serves to lessen the effect of the OBAs. As such, it is deemed reasonable by the researcher to employ the fade method in the analysis of change in both color and OBA effect.
Rather than utilize tri-stimulus or colorimetric values to evaluate the chosen instruments, spectral curves were generated and the area under the respective curves were analyzed in the 400 - 460nm region. This region was chosen as this is where the OBA effect would be recognized: Herold (2013) states that the spectral histogram is "...a sure indicator of the presence of OBAs...' ( p. 9). Resulting reflectance data are entered into a spreadsheet, and spectral curves are generated. These curves are then fit with trend lines using second order polynomials with the goal of obtaining R2 values over 0.95. In instances where these R2 values were not realized, third order polynomials were utilized to obtain a better curve fit.
The areas under the curves were obtained using Reimann Sum Trapezoidal Rule. To check the validity of this method, ten percent of the resulting equations representing the curves were entered into WolframAlpha(tm) to calculate the definite integral. When compared to the Reimann Sum Trapezoidal Rule, it was determined that in this case the Trapezoidal Rule represented a reasonable method for comparison.
In instances where the spectral curves both before and after accelerated aging were analyzed, and those curves crossed, WolframAlpha(tm) was again utilized to determine the point of intersection by setting the curve equations equal to each other. In these cases, the differences in the areas under the respective curves could be better calculated.