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Abstract
Reading with a bit of yellowish or greenish paper, as compared to white paper, is thought to be more comfortable and friendly, and can help decrease eye fatigue to some degree. In this work, we try to map the light of different colors on a given paper within a region of interest to alter the colors presented by the paper and consequently influence the reading experience. We conducted an ergonomic experiment to study the comfort and clarity under consistent illuminance levels. We adopted 6 color series(red, yellow, green, cyan, blue, and magenta), 5 chroma levels(0, 10, 20, 30, 40), and 4 types of paper with the same hue(yellow) but different lightness(the white, light yellow, yellow, and dark yellow), and conducted pairwise selection experiments within each light color series. Results show that white and low chroma (≈10) color characteristics contribute to comfort, while higher chroma blue(30∼40) color benefits clarity. Referring to white, low chroma greenish and yellowish color characteristics are preferred in terms of comfort and clarity. This work proposes the spectrum mapping technology to endow the paper with new color effects and verifies that although spectrum compositions might differ, people’s preferences and comfort perception are consistent with the same object color.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Lighting plays a crucial role in human learning, working environments and circadian rhythm, and it significantly affects comfort, performance, mood, and overall well-being [1,2]. Poor lighting environments might bring a series of negative effects on individuals, including but not limited to visual fatigue, disrupted circadian rhythm, mental stress, reduced work efficiency, and so on [3–6]. The development of advanced lighting technologies enables more precise control over spectral composition, necessitating a further exploration of the impacts of the lighting spectrum on human visual performance and psychological perception in various environments [7,8].
Numerous studies have investigated the effects of lighting quality on reading behavior, including comfort, visual fatigue, and working efficiency [9,10]. Related research shows that the performance of non-deductive reasoning tasks improved with increasing illumination levels, but the visual fatigue also significantly raised under high-illumination conditions [11]. Color temperature is also a crucial indicator, and high color temperature light significantly improves work efficiency, concentration, and vitality [12]. In 2021, a comprehensive visual comfort model was proposed, which is significantly influenced by twelve examined factors, including glare sensation and perceived lighting level. Notably, lighting level holds the greatest significance while view satisfaction is of the least impact [13].
Legibility and visual comfort, as crucial indices for reading experiences, are mainly related to brightness, contrast and color effect. Different printing and paper colors can cause significant differences in readability, with the fundamental reason attributed to the brightness contrast between symbols and background [14]. A greater brightness contrast can cause a clearer visual perception. Brightness contrast and chroma may be main factors affecting visual preference and reading speed, and the effect of color on visual preference and reading speed is inconsistent, color combinations that result in higher reading speeds can cause lower visual preferences as well [15]. Recent studies on negative polarity interface design have also shown that bright text generates better subjective preferences in terms of aesthetics, readability, and visual comfort. By contrast, different background colors with the same brightness and saturation do not have a significant effect on subjective preference [16].
Furthermore, the spectrum of lighting also contributes to the alleviation of eye strain and the improvement of overall visual comfort. Symptoms of reading difficulties can be alleviated by using spectral overlays (colored plastic sheets) on a page of text during reading [17,18]. However, this method also has the drawback of causing inconvenience during reading and writing tasks. Colored lenses were also confirmed to reduce visual stress [19], researchers have observed that children with reading disabilities showed a significant improvement in reading ability after wearing yellow filters for 3 months, thus proving that yellow filters can permanently enhance the function of magnocellular [20,21].
People not only need a comfortable reading environment on using printed books, but also a similar need on using electronic devices. With the advancement of display technologies, electronic devices and screens such as LCDs, e-ink, and OLEDs have been designed to mimic the appearance and feel of paper through specific spectral modes, offering users a reading experience similar to that of traditional paper while also enhancing reading convenience. Nonetheless, an extensive body of research indicates that compared to reading on screens, people experience less visual fatigue and achieve higher reading efficiency when reading printed books [22,23].
When reading a book, the visual effect is decided by both the light sources and reflectance of the printed materials. To create a tunable reading environment, we propose the spectrum mapping technology, namely an illumination strategy that simulates eye-protective paper by utilizing differentiated lighting spectra from the light source, achieving a favorable reading experience for any printed materials. In our experiments, we investigated the impact of different spectra and colors of light on the perceived comfort and clarity for humans when interacting with papers that have varying reflectance curves. Moreover, systematic clustering method is employed to analyze and select the most comfortable illuminating light color and its spectral composition under various modes.
This work offers a novel perspective for the design of reading light environments with spectral mapping technology. Compared to traditional simulation technologies and methods, spectral mapping technology offers flexibility, precision, and broad applicability, making it more in line with the development trends in illumination technology.
2. Method
In this work, we altered the reading experience by adjusting the illumination on a given paper within the reading area and conducted ergonomic experiments to investigate the comfort and clarity at consistent illuminance levels. We employed six light color schemes (referring to the hue), including red, yellow, green, cyan, blue, and magenta, five chroma levels(0, 10, 20, 30, 40), which is based on the CIE1976 L*a*b* color space formula, and four types of paper with the same hue(yellow) but varying reflectance and lightness. We conducted pairwise selection experiments within each color scheme. Two projectors(EPSON EF100-B, EF100-W) were used as light sources to achieve precise zonal illumination as shown in Fig. 1. Lighting regions of interest(ROIs) were composed of two distinct components, outer ROI and inner ROI. The outer ROI, covering the desktop background area, is illuminated by white light, while the inner ROI, covering the reading material area, is illuminated by colored light with tunable colors and chromas to simulate colored paper.
Fig. 1. Schematic diagram of the experimental environment. During the experiment, external lighting is isolated to ensure that the illumination on the desktop comes only from the two projectors.
2.1 Subjects
A total of 44 subjects were recruited to participate in the experiment, including 20 males and 24 females. Their mean age was 22.5 year (SD 3.22, range 18-38). All the subjects had normal vision or corrected vision to normal levels and were unaware of the purpose and expected results of the experiment. All participants passed the Ishihara test to ensure that the experimental results would not be affected by color vision deficiencies.
2.2 Experiment design
2.2.1 Variables and Indicators
The experiment involves a total of six variables, including four independent variables: hue, chroma, paper color, and gender, as well as two dependent variables: clarity and comfort. Among them, hue and chroma refer to the color characteristics presented by the illuminant on the standard diffuse whiteboard, while paper color indicates the color characteristics it presents under equal energy white light. Clarity refers to the extent to which text can be read accurately and quickly without blurring or distortion, while comfort involves the overall sensation of relaxation and ease experienced during the reading process, associated with factors such as visual fatigue. These two aspects are independent dimensions. The values and corresponding meanings of the variables are defined in Table 1.
Table 1. Experimental variable tablea
2.2.2 Control groups and experimental groups
For each color series, there are five different chromas, white color with chroma close to 0 serves as the control group, while colors with different hue and chroma acts as the experimental group. Pairwise comparisons are performed between colors of the same hue but different chroma, such as green color with a chroma of approximately 10 on the left side and green color with a chroma of approximately 20 on the right side. Colors with different hue were not compared with each other.
2.2.3 Experimental settings and conditions
The experimental environment consists of symmetrically arranged desks, two projectors, reading papers, an observation platform, black opaque curtains, and two laptops for controlling the projectors. The opaque curtains are used to separate the lighting environments on the left and right sides to prevent potential influence on the subjects. The subjects stand at the observation platform in the middle to fill out questionnaires and can move left and right to experience the lighting environments on both sides.
Projector calibration: Although the two projectors used in this study are of the same model, there are still some differences in color output due to factors such as production batch variations. In order to obtain equivalent light output, it is necessary to perform color and brightness calibration on both projectors. Luminance and chromaticity presented on the diffuse whiteboard on the desk were measured by using a spot luminance meter (TOPCON SR-UL1R), as Fig. 2 shows, and the illuminance on the desk was measured with an illuminance meter. Data of the two projectors under four color modes (dynamic, bright cinema, natural, and cinema, respectively) were collected separately, thus determining the corresponding color gamut area.
Fig. 2. (a) Comparison of full-mode illuminance. When inputting the same RGB signal, the maximum illuminance difference between the outputs of the two projectors reached 26.07%. The illuminance levels of different colors vary greatly, ranging from 173 lx to 11,280 lx. (b) Color gamut of dual projectors. There are also notable differences in the color gamut of the projectors under various color modes, and discrepancies also exist in the color gamut range between the two projectors when operating in the same color mode.
Apart from the differences in color coordinates, when inputting a same RGB value, the illuminance produced by the output light of the two projectors on the desktop also varies. An illuminance meter was placed directly below the projectors on the desktop to measure the illuminance, with the results illustrated in Fig. 2.
To maintain similar color coordinates and brightness presented by two projectors for comparative experiments, polynomial fitting was conducted on the RGB input signals from the computer and the tristimulus values of the light from two projectors reflected by diffuse white board, respectively. The Bright Cinema mode was selected as the fixed color mode, and all subsequent experiments were conducted under this mode.
Equation (1) represents the input-output conversion matrix for projector EF100-B, while Eq. (2) represents that for projector EF100-W. In these equations, the input values are the RGB values, and the output values are the tristimulus values and brightness. Through these conversions, the luminance and chromaticity information could be accurately mapped.
Illuminance: The actual illuminance generated by the projectors on the desktops is 859.09$\pm$20.30 $lx$, which falls within the comfortable illuminance range for human eyes [3,24]. As Fig. 3(a) shows, the illuminance difference with a same color is strictly controlled with a maximum of 5.7%, which could guarantee that the reading experience of the subjects is affected only by color and chroma. The emission spectra are illustrated in Fig. 3(c), corresponding to 25 selected colors as shown in Fig. 3(b).
Fig. 3. (a) Comparison of desktop illuminance output from dual projectors (Illumination Height 0.5m). (b) Positions of the 25 selected colors in the CIE1931 chromaticity diagram. (c) Spectra of the lights for 6 hue angles and 4 chroma levels. The white light spectrum is represented as a gray line for comparison, presented alongside each hue group.
Hue and chroma: In this investigation, twenty-five distinct chromatic conditions were selected for experimentation as shown in Fig. 3, which encompassed both white color and color series in 6 hues(red, green, blue, cyan, magenta, and yellow). Each color series was further divided into 4 chroma levels(10, 20, 30, 40). The calculation of chroma is based on the CIE1976 L*a*b* color space formula (3).
The selecting algorithm process is illustrated in Fig. 4, which follows three objectives. First, the color coordinates are positioned along the six line segments depicted in Fig. 5, with each segment representing a distinct color. Second, the chromas are close to 10, 20, 30, and 40. Third, the brightness level remains uniformly stable. The chromas of the actual reflected lights are shown in Fig. 6.
Fig. 5. (a) Selection of 6 different hue. Line segments are generated through connecting the coordinate points of the D65 light source in the CIE 1931 chromaticity diagram with the color coordinates of the light presented when the projector inputs RGB signals of (255, 0, 0), (0, 255, 0), (0, 0, 255), (255, 255, 0), (255, 0, 255), and (0, 255, 255). (b) Measured chroma of the actual reflected light. Due to luminance limitations, green color with a chroma of 30 could not be exactly obtained; therefore, the chromas of green colors are close to 10, 20, 40, and 50. The labels in the figures are named using the initial letter of the corresponding color and the chroma level (from 1 to 4, increasing sequentially, with 0 representing white color). For example, "R1" represents red color with a chroma level of 1, and "Y3" represents yellow color with a chroma level of 3.
Fig. 6. (a) Four paper colors’ locations in the CIE 1976 L*a*b* chromaticity diagram and their reflectance curves. In the diagram, only the colors within the sRGB color space are displayed. (b) The reflectance curves of experimental papers. The color saturation of the four types of paper increases progressively, with the aim of investigating whether the "eye-protecting paper" available on the market has an impact on people’s preference for the lighting spectrum. (c) The chroma shifts of 25 lights presented on 4 types of paper.
Paper color: In the experiment, we used four different colors of reading paper, namely white, light yellow, yellow, and dark yellow, investigating whether there is a difference in the preferred color for reading so-called "eye protection paper" compared to reading ordinary white paper. The text content presented on the paper was consistent, and all colors were black. The reflectance curves and L*a*b* values of the paper were measured by using a spectrophotometer (Konica Minolta, CM-700d) and plotted their positions in the CIE 1976 L*a*b* color space. The results are shown in Fig. 6(a),(b). Due to variations in reflectance, there are differences in the colors presented by the 25 lights on the 4 types of paper. The chroma shifts are illustrated in Figure Fig. 6(c).
Luminance threshold contrast: Luminance threshold contrast (also known as adaptive threshold contrast) is an indicator that measures the ability of the human visual system to discern the difference between the target and background under specific luminance conditions [2,25]. The larger the value, the higher the contrast between the target and background, making it easier to distinguish. The calculation formula is represented in Eq. (4), where $L_t$ represents the luminance of the target area, and $L_b$ represents the luminance of the background area.
As Fig. 7 shows, the luminance threshold contrast for the four types of paper and 25 different colors ranged between 1.1 and 1.5. As the paper color became darker, the luminance threshold contrast decreased, but all values satisfied the requirements for human eye discernment.
Fig. 7. (a) Luminance threshold contrast(k), the x axis represents for hue and chroma. (b) Luminance ratio.
Luminance ratio: Different text and paper colors can lead to significant differences in readability, with the fundamental cause being the luminance contrast between symbols and the background as shown in Fig. 8. A greater luminance contrast can result in clearer visual perception [14]. To control the impact of this factor on visual clarity, the luminance contrast was maintained within a stable, narrow range across all four experimental stages. The measurement results shown in Fig. 7 indicate that the luminance contrast for different color and chroma combinations is relatively consistent ($21.51 \pm 1.67, 22.36\pm 0.23, 24.70\pm 0.99, 25.73\pm 0.20$).
Fig. 8. Symbol and background. Luminance ratio is calculated by dividing the background luminance by the symbol luminance. The image was captured through the viewport of the spot luminance meter.
2.3 Experiment process
The experimental procedure and the scene of subjects participating in the experiment are shown in Fig. 9. At the start of the experiment, subjects are informed of the experimental procedure and questionnaire completion methods. The experiment is divided into four stages, using reading papers with different chroma. In each experimental stage, subjects are asked to observe sixty groups of different lighting environments on both the left and right sides, and choose the side that performs better under each indicator and record their choice in the questionnaire. The comparison order is randomly shuffled to ensure the reliability of the experiment.
Fig. 9. (a) Experimental scene. (b) Experimental flowchart. The paper color for the first stage is white, the second stage is light yellow, the third stage is yellow, and the fourth stage is deep yellow. The printed text content on the paper remains unchanged.
After each experimental stage, the subjects take a short break to avoid eye fatigue affecting subsequent experiments. During this process, the experiment operator changes the reading papers.
3. Results and discussion
3.1 Data analysis
Descriptive statistics for each variable are shown in Table 2. For detailed definitions of variable values and meanings, please refer to Table 1. To study the impact of different hue on visual perception, Spearman correlation coefficient calculations were performed separately for different hue due to the scores for clarity and comfort did not conform to a normal distribution. To explore the change trend of clarity and comfort with chroma for different color series, curve fitting was performed on the experimental result. Finally, cluster analysis was conducted to screen for hue that performed well in both comfort and clarity indicators.
Table 2. Descriptive statistics for all variables
3.2 Correlation between visual perception and chromaticity
The bivariate Spearman correlation coefficients for the six hue are shown in Table 3. It is clear that the chroma has a significant correlation with both comfort and clarity, and the relationship varies for different colors. For example, the chroma of blue color is positively correlated with clarity, while other hue are negatively correlated. A common pattern is that the chroma of all hue is negatively correlated with comfort. This result indicates that excessively high chroma in lighting can cause visual discomfort, making it unsuitable for use in lighting environments during daily work.
Table 3. Spearman correlation coefficient table for variables
3.3 Change trend of clarity and comfort with chroma for different colors
To more clearly observe the changing trends of clarity and comfort with chroma of different lighting colors, curve fitting was performed to the data and the results are shown in Fig. 10 and Fig. 11. It is worth noting that although different color with chroma 0 are identical(white), the scores may differ across various color series due to relative comparisons. To ensure comparability, the absolute scores for each hue were divided by the corresponding score of white color, resulting in relative scores which were used to construct the relative trend graph.
Fig. 10. Trend of visual clarity while reading papers of different colors as the chroma of the color changes. In the figure, the left side is the representation of the relative trend with the colors of the paper indicated. On the right side is the depiction of the absolute trend, indicating the variation trend of actual scores.
Fig. 11. Trend of visual comfort while reading papers of different colors as the chroma changes. The overall trend shows that as the chroma increases, visual comfort gradually decreases. Yellow light behaves somewhat uniquely; when the saturation of the reading material is low, the comfort curve first rises and then falls, but when the saturation of the reading material is high, it converges with other hue.
When reading on ordinary white paper, chroma and visual clarity are negatively correlated for all colors except blue color. As the saturation of the reading material increases, The increase in chroma no longer leads to a decrease in clarity as the saturation of the reading material increases. However, within a certain range, an increase in blue color chroma consistently leads to an increase in clarity. This trend holds true across different genders and colors of reading materials.
Curve fitting for the trend of comfort level as chroma changes was also performed, and the results are shown in Fig. 11. High chroma color generally having lower comfort levels except yellow color. However, when the saturation of the reading paper increases, this phenomenon is no longer evident.
3.4 High-quality lighting spectra and color coordinates
In order to filter out the preferred chroma of human eyes under various saturation levels of paper, a systematic clustering was conducted on the scores of clarity and comfort. The number of categories in each dataset was determined by the elbow method. The clustering result is shown in Fig. 12.
Fig. 12. Clustering results of human eye comfort evaluation scores for different chromaticities. The horizontal axis represents the clarity scores, and the vertical axis represents the comfort scores. The four figures correspond to four different types of paper. White points represent centroids, and the gray areas are confidence ellipses.
The clustering results indicate that the 25 colors utilized in the experiment can be categorized into three groups. The first group demonstrates a high level of comfort, the second group performs moderately in terms of both comfort and clarity indicators, while the third group exhibits a high degree of clarity. For ordinary white paper, white light and colors of around 10 in chroma, such as red, green, cyan, blue, and magenta, as well as yellow with chroma around 20, are relatively comfortable. For light yellow paper, white light and colors of around 10 in chroma, such as red, green, cyan, blue, and magenta, along with yellow light with a chroma of 10-30 and green light with a chroma of 10-20, are all considered relatively comfortable light colors. For yellow paper, white light and chromatic lights of around 10 in chroma, such as red, green, cyan, and magenta, as well as yellow light with chroma around 20 are more comfortable. For dark yellow paper, white light and chromatic lights with a chroma of 10-20, such as red, green, cyan, and magenta, yellow with chroma of 10-30, blue with chroma around 10 are more comfortable. Overall, the reading experience is generally more comfortable with white and yellow, as well as other colors with chroma of 10-20. Regardless of the variation in paper saturation, blue color with a chroma between 20 and 40 consistently exhibits the highest level of clarity.
Figure 13 shows the illumination spectra with high clarity and high comfort for different types of paper, all derived from the best-performing cluster in the clustering results. Under the same type of paper, the spectra of the best-performing light colors have a high degree of overlap, for which the chromaticity coordinates of these colors in the CIE 1931 chromaticity diagram were plotted beside to achieve better visualization. As the paper color becomes darker, the brightness of the reflected spectrum decreases, and the intensity of the blue light band decreases. In addition, the blue light content in the spectra with high clarity is significantly higher than that in the spectra with high comfort.
Fig. 13. High comfort and high clarity illumination spectra and color coordinates. The figure shows the absolute spectra of different lights, with the four types of paper represented from left to right.
As shown in Fig. 13, to reach ideal visual effect, reflected spectra contained a relatively high proportion of blue light, attributing to the limitation of spectrum compositions from projectors. The narrow band blue light contribute to the visual effect, but may do harm to the non-visual effect, and could be further optimized in the future.
It is worth mentioning that no comparative experiments were conducted with different color series of illumination lights in the experiment. Therefore, the final scores depend on the relative level of the light color within the same color series, providing some reference value but also having limitations. Due to the white ambient light used in the experiment and only low chroma colored light illuminating the ROI, the issue of color adaptation is essentially non-existent. In addition, our experiment did not involve long-term reading experience. Therefore, the experimental results only represent short-term visual perception. Notably, participants were afforded breaks to alleviate eye strain every ten minutes, thus preventing any substantial impact of accumulated eye fatigue on the experiment. Long-term visual task on fatigue research might be a next-step work.
To minimize the potential influence of color adaptation and visual constancy on our experimental results, several precautions were instituted. Color adaptation is characterized by the visual system’s adjustment to prolonged exposure to a specific chromatic environment [26]. However, in our experiment, the region of interest was refreshed at an average interval of 10 seconds, preventing any substantial adaptation by the human visual system. Visual constancy describes the perceived invariance where the visual attributes of an object (such as color, shape, and size) appear consistent across varying observational conditions. In our study, participants discerned variations in visual stimuli across different luminous conditions. Additionally, our experiment The design incorporated a neutrally-hued induction zone, with the central stimulus region presenting a color of low saturation, reducing the likelihood of generating marked color contrast or adaptation effects [27,28]. Furthermore, meticulous alignment was conducted to ensure the rectangular light patches coincided precisely with the edges of the paper. Participant feedback uniformly reported that the transition between the reading zone and the ambient lighting was subtle. While participants noted a discernible change in the color of the paper during the experiment, there was no dominant perception of a shift in the overall lighting color. Hence, the potential biases introduced by color adaptation and visual constancy in our study can be deemed negligible.
Although we have proposed the concept of spectral mapping technology in illumination, the current adjustment of illumination spectra is limited due to experimental light source equipment constraints. Moreover, non-visual parts has not been fully involved. However, with the advancement of technology, the practical implementation of spectral mapping technology remains a promising trend in healthy lighting.
4. Conclusion and outlook
In this work, we constructed a chromatic reading light environment through spectral mapping techniques and conducted ergonomic experiments to study the mechanism of the effects of different variables on eye comfort and clarity. The color characteristics of the illumination light presented on the paper has a crucial impact on human visual perception, specifically manifesting as white light and low chroma($\leq$30) yellow providing better comfort, while the reading comfort significantly decreases as chroma increases. Furthermore, the visual effects produced by different hue also vary, blue with relatively high chroma can provide higher clarity. There are similarities and differences for different genders on visual perception. An increase in chroma leads to a decrease in clarity when reading materials are printed on white paper. However, when reading materials are printed on a bit yellowish paper, the increase in chroma no longer results in a decrease in clarity.
Moreover, the light environment perceived by the human eye depends on the spectral characteristics of the illumination light and the reflective properties of the reading materials. Therefore, replacing reading materials under the same illumination spectrum will produce different visual effects. However, under the conditions of four different chroma reading materials, the preferred light for the human eye still maintains a relatively high consistency, indicating that although the spectral composition may differ, people have consistent preferences for the color of the same object.
This work provides a new perspective on the design of reading light environments. Compared to traditional simulation techniques and methods, spectral mapping technology offers flexibility, accuracy, and broad applicability, aligning more closely with the development trends in lighting technology. Using spectral mapping technology to customize exclusive light environments according to the needs of different groups of people has extensive application prospects in improving work efficiency and reducing visual fatigue. In future work, we will further quantify the effects of color-contrast reading environments on the human visual system and examine their eye protection effects during extended periods of work and reading activities.
Funding
National Natural Science Foundation of China (62275051).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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