Paper and board for printing commonly contains optical brightening agents that fluoresce in the presence of UV radiation, making the prints appear brighter and bluer. If the relative amount of optical brightening agents (OBAs) is not consistent between proof and print, and the UV content of measurement and the viewing illumination difers, there will be a colorimetric diference between proof and print. Colour management workflows are normally media-relative, and visual adaptation to the media white can conflict with the goals of colorimetric matching. The impact of UV content in measurement and viewing illumination on the visual acceptability of colour reproductions was investigated in a series of psychophysical experiments, using substrates with varying OBA concentrations and a media-relative colour reproduction workflow. The results showed that visual acceptability was largely unafected by UV content of measurement or viewing illumination, suggesting that visual adaptation to media white discounts the efect of fluorescence.
For consistency in colour reproduction, D50 has long been the standard illuminant in graphic arts colorimetry and proof viewing. Prior to 2009, the spectral power from 300nm to the beginning of the measurement range at around 380nm was loosely defined. This led to dificulties in colour matching, since the great majority of printed substrates contain optical brightening agents (OBAs) also known as lfuorescent whitening agents that cause energy in the UV region to be re-emitted in the visible, such that the spectral reflectance depends on the relative amount of UV energy emitted by the lamps used in the spectrophotometer and in the viewing cabinet.
The graphic arts measurement and viewing standards, ISO 13655 [
This approach made it possible to consistently match the CIELAB L*, a* and b* values between proof and print. However, where the proof and print substrates contained diferent OBA concentrations, this was at the cost of printing blue or yellow on the unprinted substrate to align the chromaticities of the white points. This tended to lead to dissatisfaction with the end result, particularly when the print was viewed or evaluated in end use conditions or ofice environments rather than professional viewing cabinet.
The more recent adoption of LED lamps in lighting systems has exacerbated the problem, since the widely used blue-pumped white LEDs emit significant amounts of UV, leading to greater mis-matches between end use viewing and the ISO 3664-compliant viewing booth.
An important principle in colour management is scaling the white point colorimetry so that the match is media-relative rather than strictly colorimetric, and it is well established that this gives more pleasing results. The primary reason is that the visual system tends to adapt to the media white point, regardless of its colorimetry.
In an attempt to mitigate the mis-matches in colour reproduction, it has recently been proposed to introduce a D50noUV condition in a revised ISO 3664. This would match the spectral power of D50 only within the visible range, and suppress UV energy outside this range.
In order to investigate the issue further and determine how significant the efect of UV content are in measurement and viewing, a series of psychophysical experiments were performed on the acceptability of colour reproductions when diferent combinations of measurement and viewing conditions are used, and using substrates with a wide range of OBA concentration.
A study by Changlong et al. [
In an other study [
In previous research, various methodologies have been developed to estimate the UV content of
illumination sources and to measure and model paper fluorescence for enhanced colorimetric
characterization of printing processes. For instance, Green and Chang [
A comprehensive psychophysical investigation was undertaken to assess the impact of OBAs on the acceptability of colour matching. The methodology employed for this assessment utilised a six-point categorical judgment scale, enabling a nuanced evaluation of perceived diferences. The experimental design considered two main variables: the illuminant and the substrate. The substrate was especially significant, as it constituted the primary source of ultraviolet (UV) content influencing the colour reproduction process. This study aims to provide deeper insights into how OBAs afect visual perception under varying lighting conditions, contributing to the broader understanding of colour science and its applications in printing and imaging technologies.
For our experiment, we selected viewing illuminations with diferent levels of UV presence. At NTNU in Norway, we conducted the visual experiment in three distinct viewing environments. Additionally, we used two viewing conditions in the Netherlands (NL): • Combined Ofice and Daylight, NTNU (referred to here as: ’Of+Day (NTNU)’) • Daylight only at noon, NTNU (’Daylight (NTNU)’) • D50 simulator, NTNU (’D50 (NTNU)’) • D50 simulator No-UV, Netherlands (’D50 No-UV (NL)’) • D50 simulator with-UV, Netherlands (’D50 with-UV (NL)’)
The visual experiments at NTNU, which included three distinct viewing conditions, were conducted in one of the laboratory facilities. The room is equipped with windows that can either allow daylight to enter or exclude it using sun blades. Additionally, the lab features adjustable ceiling illumination (combination of fluorescent tubes OSRAM L 58W/950 and tungsten light) with an adjustable colour temperature ranging from approximately 2800K to 5000K. This setup enabled the simulation of three viewing conditions: ’natural daylight only at noon,’ ’combined ofice and daylight’, and ’D50 simulator’ exclusively. For each viewing condition, the room’s illumination intensity was calibrated to the ISO 3664 standard, specifically the P2 settings, at 500 lux, using a Konica-Minolta CL-200A chroma meter.
A further phase of the experiment was performed in the Netherlands using D50 simulator illuminants with and without UV in a standard light booth. The illumination intensity in the light booth was set according to the ISO 3664 standard, with P2 settings at 500 lux, and the self-luminous white point of the screen was 160cd/m2.
To ensure the consistency of UV levels in the viewing environment over time, three reference
substrates ’Ultra’, ’Alpine’ and ’Filter’ containing diferent amounts of OBA [
We chose four commercial substrates from diverse families, each with varying levels of OBAs. These
families comprised paper, ’Yupo’, ’Textile’, ’Hexis vynil’, and ’Blockout PP’. The quantity of OBAs
present is indicated by the white point measurement of each substrate under M1 and M2 measurement
conditions, as shown in Table 2. Additionally, Figure 1 presents the spectral reflectances of the four
substrates measured under both M0, M1 and M2 modes. As observed, the substrates ’Textile,’ ’Hexis,’
and ’Blockout,’ when measured under M1 mode, exhibit a fluorescence peak around 437nm, enhancing
brightness and imparting a slight bluish tint to the paper. The extent of this fluorescence depends on the
amount of UV present during both the excitation and viewing of the paper. Chaikovsky and Garrison
(2012) [
In contrast, the M2 measurement mode suppresses the UV radiation. The degree of this fluorescence is influenced by the amount of UV in the excitation region in both measurement and viewing of the paper. For substrates without any OBAs, such as ’Yupo’, the choice of measurement mode does not afect the reading.
The specification ISO 13655:2017 provides four diferent measurement modes M0, M1, M2, and M3.
The correlated colour temperature of the illuminant defined in the M0 measurement condition does not
define the UV information [
In our experiment to create an ICC profile, we printed an ECI2002 characterization chart [
A supplementary measurement condition was calculated to define an intermediate level of UV content between M1 and M2. This intermediate condition, termed "M1.5", was derived by linearly interpolating spectral reflectances between M1 and M2 measurements.
As a result, three ICC printer profiles were created using the characterisation data M1, M2, and M1.5 for each of the four substrates.
In the experiment, we utilised five distinct sets of CMYK standard colour image data (CMYK/SCID) sourced from the ISO standard ISO 12640-1:1997 [14], which encompassed natural scenes, as depicted in Figure 2.
N1A
The CMYK/SCID images N1A Portrait, N3A Fruit Basket, N4A Wine and Tableware, N5A Bicycle, and N6A Orchid were printed on the Latex560 printer using previously created ICC profiles with measurement modes M1, M2, and M1.5, respectively. The untagged CMYK/SCID images were assigned with standard CMYK encoding and converted to the destination colour space of the four target substrates using media relative rendering intent including black point compensation. Then, the five target images were printed onto each of the four substrates. Each substrate has three measurement modes (M1, M2, and M1.5) resulting in a total of 60 printed images.
The reference image was presented on a calibrated display, with a display white point corresponding to D50, a luminance level of 160.00 cd/m2, and a Gamma value of 2.2. To visualize the target images on the display, the original CMYK/SCID images were reassigned with a standard CMYK profile (ISO Coated v2 (ECI)) and a soft-proof was created using the destination colour space of the four target substrates, including the display option to simulate paper colour. Positioned in front of the display was the corresponding printed hard-copy. Subsequently, observers were tasked with assessing the visual coherence between the two stimuli, as shown in Figure 3.
While the display calibration remained consistent, the visual experiment was conducted under three distinct viewing environments at NTNU and two distinct environments in the Netherlands. At NTNU, these environmental changes were continuously monitored and verified using a Konica-Minolta CS2000A telespectroradiometer.
The visual experiments were conducted in both Norway and the Netherlands. A total of 31 participants took part in each stage of the experiment, 16 in the Netherlands and 15 in Norway. The participants, who varied in age (22-60 years) and nationality, included both novice and experienced observers in colour perception. Intra-observer and inter-observer repeatability tests were conducted with each participant to evaluate the consistency of their responses over time and the results are shown in Table 3.
Participants were instructed to visually compare printed samples to a reference image displayed on a screen and to provide a categorical assessment of the acceptability of the reproduction. Each observer conducted the experiment under diferent viewing conditions: three distinct environments were utilized for observers at NTNU, while two viewing environments were used for those in the Netherlands. Prior to the start of the experiment, participants received training samples to familiarize themselves with the task and the judgment criteria. Subsequently, reference images were displayed on a calibrated screen positioned at a 45-degree angle next to the printed samples (as depicted in Figure 3), and participants were asked to assess the diferences using a scoring system. For each distinct viewing condition, the experiment was repeated and prior to the experiment the observers could visually adapt to the new viewing environment.
The categorical scale reflected the perceptible diference from the reference image on the display and the corresponding hard-copy and served as an indicator of the acceptability threshold. The scale ranged from: • 1 - Not perceptibly diferent • 2 - Barely perceptible diference (but acceptable) • 3 - Perceptible diference but acceptable • 4 - Barely acceptable diference • 5 - Barely unacceptable diference • 6 - Unacceptable diference
We transformed the mean opinion score (MOS), which represents the average category score, into a Z-score, reflecting how many standard deviations a data point deviates from the mean of the distribution.
Typically, a mean opinion score is used to capture the average subjective rating provided by participants for a particular item. In our study, this involved evaluating the visual diference between the reference image displayed on the screen and the physical reproduction on the substrate. For instance, if participants rated the acceptable diference on a scale from 1 to 6, where 1 signifies "Not perceptibly diferent" and 6 indicates "Unacceptable diference," the MOS would be the average of these ratings.
Overall, we received responses from fifteen observers in Norway and sixteen in the Netherlands, who assessed five images on four substrates with three colour encoding across five diferent viewing environments.
The efect of diferent levels of UV in the viewing illumination on the visual acceptability of colour reproductions on papers with diferent degrees of OBAs was investigated. Five viewing conditions (three at NTNU and two in the Netherlands (NL)), four paper substrates, three measurement modes (M1, M2, and M1,5), and five test images were used to investigate how fluctuations in UV radiation influence the acceptability of colour matching between soft-proofs and prints.
Inter-observer and intra-observer repeatability were estimated during the experiments, which involved ifve distinct viewing conditions. Under each condition, three hard copies of diferent CMYK/SCID images were assessed using three measurement modes. Each assessment was repeated twice to ensure accurate estimations of repeatability across diferent observers and within the same observer.
Table 3 presents the mean and standard deviation (SD) values for the inter- and intra-observer repeatability. A lower SD indicates that the values are close to the mean (less variability), while a higher SD indicates more spread-out values (greater variability). Since the ratings are on a scale of 1 to 6, an SD of approximately ±1.00 suggests that most ratings are within 1.00 points of the mean rating. Given that the scale range is 5 points (from 1 to 6), an SD of 1.00 reflects a reasonable level of agreement among observers (inter-observer repeatability).
The intra-observer repeatability was slightly lower, suggesting that individual observers are more consistent with themselves than with each other.
The results of the psychophysical experiment predicting perceptible diference using categorical scale resulted in a frequency matrix from which Z-scores were calculated. The obtained Z-scores reflect the relative positioning of data points compared to the mean and standard deviation of the dataset. The error bars are set at the 95% confidence interval.
The Figure 5 shows the Z-scores for all five images (N1A, and N3A-N6A), colour-encoded according to measurement modes M1, M1.5, and M2, were evaluated on four diferent substrate properties under ifve dissimilar viewing conditions.
Below we review the efect of the diferent variables in the study.
Four types of substrates have been used in the experiment containing diferent amount of OBA’s. As expected, the substrate without any OBA (’Yupo’) does not efect the appearance significantly under any viewing conditions, with or without UV radiation. Although the ’Textile’ substrate shows moderately higher Z-scores, we can observe that neither the measurement modes (M1, M1.5, and M2) nor the five diferent viewing conditions significantly afected the observers’ judgments. Additionally, the three viewing conditions containing UV radiation (e.g., ’Daylight (NTNU)’ or ’D50 with-UV (NL)’) didn’t impact the appearance compared to non-UV radiation viewing conditions.
Analyzing the data for a single image (e.g. N1A) reveals that while viewing conditions containing UV radiTaetsitoimnagheaNv1eAsaonmdveieewf eincgtcoonnditthioen Daacyclieghptt(aNbTiNlUit)ytoothferleepft raondduviecwtiinognctohndeitidoinfesrDe5n0twsituh-bUsVtr(NaLte)tso uthseerdighht ave a considerably greater efect than the viewing conditions.
Test image N1A and viewing condition Daylight (NTNU)
Test image N1A and viewing conditions D50 with-UV (NL)) s e r o c s Z
As can be seen in Figure 5, the measurement conditions (M1, M1.5, and M2) have minimal or no efect on substrates without OBAs. However, the M2 mode suppresses fluorescence of the substrate regardless of the OBA amount. Except for the viewing condition ’D50 with-UV (NL)’, (see Figure 4), the measurement conditions do not significantly influence the acceptability of reproduction between the reference and the hard-copy. Similarly, the intermediate measurement condition ’M1.5’ did not show a significant impact on the acceptability of colour reproduction.
While the NTNU viewing condition ’Daylight (NTNU)’ include significant amounts of UV (as shown in Figure 6), the results presented in Figure 7 indicate that neither the image content, the presence of UV in D50 with-UV (NL)
D50 NO-UV (NL) Daylight (NTNU)
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compare light sources in terms of their spectral properties and visual appearance. ge of all images
N1A-N6A and viewing conditions
Daylight (NTNU) and
D50 (N ‘Daylight (NTNU)’ ‘D50 (NTNU)’ ‘Off+Day (NTNU)’ (right) and reference substrates Ultra, Alpine and Filter measured and normalized at 560nm.
Textile substrate and viewing condition D50 with-UV (NL)
Textile substrate and viewing condition Daylight (NTNU) N1A
N3A
N4A
N5A
N6A
N1A
N3A
N4A
N5A
N6A s reo M1 c s Z
M1,5
M1 M2
The results discussed thus far demand consideration of visual adaptation. The fact that the reference displayed on the screen is self-illuminated, while the physical samples are printed on diferent substrates, suggests that the human visual system is to a degree adapted to the illumination condition and the white point of the medium. This partial adaptation occurs to the perceived media white, regardless of the environment, as evidenced by the varying white points of the substrates.
Since the physical samples were printed with media-relative adjustment, visually, there are diferences between the reference on the display and the printed samples. This occurs because the human eye adapts to the white substrate, which fluorescence. As noted by Fairchild [15], the diferences between adaptation to display and printed (hard-copy) images have revealed that cognitive mechanisms, which are engaged when viewing familiar objects under known lighting, difer from sensory mechanisms that respond directly to the light’s spectral power distribution.
Other studies by Green [16] and High et al.[17] supports the assumption that for on-screen viewing, the observer’s state of chromatic adaptation is strongly influenced by the colour of the substrate as well as the viewing illumination and the content of the image itself.
Observers scaled the acceptability of hard-copy reproductions relative to a reference soft-proof, using a scale from ’Not perceptible’ to ’Unacceptable.’ The results show no significant diference between the acceptability of the printed samples under any viewing conditions compare to the reference image on the display, regardless of the amount of UV in the viewing environment and the amount of OBA in the substrate. This outcome is somewhat unexpected, as some of the substrates contain a significant amount of OBA, which one might suspect would result in a noticeably diferent appearance compare to the reference on the display or an unacceptable judgment by the observer. It implies that adaptation to the media white is the strongest influence on visual acceptability, even in a side-by-side proof-to-print comparison.
Although most participants were experienced in color perception and received thorough instructions for the experimental task, the combination of perceptibility and acceptability thresholds may have introduced some additional bias into the results. Specifically, in printing applications, acceptance is highly dependent on the use case and the individual’s role in the production chain (a print buyer generally has higher demands than a print service provider). Thus, acceptability questions should be posed only to experts with professional experience and a specific printing application in mind. Consequently, it may be beneficial to conduct a new psychometric experiment using a category scale that focuses solely on descriptions of perceptions.
To investigate adaptation efects in soft-copy versus hard-copy comparisons, future research could focus on controlled studies assessing how extended viewing times and diferent lighting conditions (such as daylight and ofice lighting) influence colour perception. Evaluating colour matching accuracy under various screen calibration settings and paper types including diferent degree of OBAs, and utilizing psychophysical studies to measure visual fatigue, could provide valuable insights. Employing eyetracking technology to monitor gaze patterns and developing a psychometric scale to quantify adaptation efects would further enhance understanding. Additionally, examining the impact of surrounding colours and environmental context on colour consistency between digital and printed media would be beneficial.
The authors extend their gratitude to all participants involved in the psychophysical experiments conducted at NTNU and in the Netherlands. Special thanks are owed to former COSI student Nafia Akter for her invaluable contribution in setting up and conducting the experiment at NTNU. [14] ISO 12640-1:1997(E), Graphic technology. Prepress digital data exchange. Part 1: CMYK standard colour image data (CMYK/SCID), Standard, International Organization for Standardization, Geneva, CH, 1997. [15] M. D. Fairchild, Chromatic adaptation in hard copy/soft copy comparisons, in: Color Hard Copy and Graphic Arts II, volume 1912, SPIE, 1993, pp. 47–61. [16] P. J. Green, B. Oicherman, Reproduction of colored images on substrates with varying chromaticity, in: Color Imaging IX: Processing, Hardcopy, and Applications, volume 5293, SPIE, 2003, pp. 91–100. [17] G. High, P. Green, P. Nussbaum, Content-dependent adaptation in a soft proof matching experiment, Electronic Imaging 29 (2017) 67–75.