Abstract
Research in color science includes studies on the spectral properties of light and the perception of color by the human eye and the brain. Color spaces allow the description of the position of a color by using three basic attributes, namely lightness, chroma and hue. Photochromic pigments used as a component of ultraviolet sensors can help to define color changes in time through the measurement of color differences in exposition and decay phases. The change of color can be evaluated in terms of the light source employed, reversion and the kinetics of photochromic effects.
Commercial spectrometers are not suitable for this type of measurement. These instruments cannot measure the kinetics of exposure and the decay phase of photochromic materials. A unique spectrophotometer and a methodology for measuring colorimetric and spectral data of photochromic pigments in selected color spaces are presented. This work is the first step towards standardization of the measurement of color-changing materials containing functional dyestuffs, such as photochromic pigments.
Photochromic pigments and other functional dyestuffs are used nowadays as a component of smart materials (textiles), which are able to indicate environmental variations by color changes. In the case of photochromic sensors the color change indicates the amount of ultraviolet (UV) irradiation.1,2 Many of them are based on pair comparison. There are mainly two types, namely color stable and color changeable. Color changes are evaluated using various standard spaces. These color spaces are developed for presenting color relationships, calculating color differences and understand color appearance phenomena. These spaces include CIE XYZ, CIE L*a*b*, CIE L*c*h* and CIECAM02. In the case of photochromic sensors such as smart materials, there is a need to design a stable material that exhibits high-fatigue resistance against UV irradiation.
In the creation of sensorial system, it is necessary to use not only one comparative segment defining a UV level, but also to use at least three comparative segments with a constant level of color perception. If at least three standards are used causing no detectable color change during variations of UV intensity, then color evaluation and visual evaluation of change in intensity of UV irradiation is correct and sufficiently accurate. In this case color matching can be used as demonstrated in Figure 1.
Scheme of the sensorial photochromic system. UV: ultraviolet.
From the scheme in Figure 1 it is evident that the left part of the sensorial system has a light tint or the left part is white. In the case of low-level UV irradiation, a color match is found between the changeable part of the sensor and reference standard A. For a medium level of UV irradiation a color match is observed after comparing the changeable part of the sensor and reference standard B and, finally, for a high level of UV irradiation, a color match between the changeable part of the sensor and reference standard C is found.
The stable parts are graded according to the intensity of pigment coloration. The first level (low) corresponds to UV index 2, meaning that the person can stay under direct sun without limitation. The second level (middle) corresponds to UV index 4, indicating that when the sensorial (changeable) part is colored in this shade intensity, the person can remain under direct sunlight for a limited time. The third level is designed to give an UV index of 6 and higher. Under these conditions the exposure to sunlight should be limited to as short a time as possible.
The stable section is used as reference scale to which the changeable photochromic part can be compared. The sensorial part is divided into four segments; three segments are stable comparative parts and the fourth is a photochromic changeable segment. However, not all photochromic pigments are colored in the same shade or hue and for various reasons the shade or the hue is shifted in tint. This is visible in Figure 2, wherein an evident shift from a greenish to intensive blue tint is observed. In Figure 11, curves of photochromic color change are visible during the decay phase. Curves for individual pigments appear similarly as a set of concentration of standard dyestuffs, where it is possible to recognize change of hue angle with increasing concentration. In the data presented, pigment P2 with a purple shade is more reddish in the beginning than in the halftime of decay. The consequent effect is connected to chroma, where the increase and subsequent decrease of chroma is visible. The tint in Figure 2(a) is shown as a trend of pigment P1 data in a graph in Figures 5 and 11, which is out of the achromatic point. That means the color changeable part of the sensor has its own shade in the inactive form of pigment P1 in comparison to the solution of pigment P1, which is without shade for the inactive form of pigment P1. Light scattering on the particles of pigment P1 probably causes this effect in a state of weak opalescence, which brings a green yellowish tint.
(a) Smart sensor before irradiation. (b) Smart sensor after irradiation. (Color online only). (a) 1,3,3-trimethylspiro[indolino-2,3’-(3H)naphtho (2,1-b)(1,4)-oxazine] – pigment P1. (b) 5-chloro-1,3,3-trimethylspiro [indoline-2,3’-(3H) naphtho(2,1-b) (1,4)-oxazine] – pigment P2. (c) 3,3-diphenyl-3H-naphtho[2,1-b]pyran – pigment P3. Simplified optical scheme of Laboratory of Color and Appearance Measurement Photochrom. Prototype of the Photochrom measuring system, which was developed at the Laboratory of Color and Appearance Measurement, Faculty of Textile Engineering, Technical University of Liberec.
Due to the problems of measuring kinetic photochromic color change during the exposure phase, a special device named PHOTOCHROM was developed at LCAM (Laboratory of Color and Appearance Measurement, Technical University of Liberec, Czech Republic). The PHOTOCHROM spectrophotometer allows the measuring of the colorimetric and the spectral characteristics of photochromic textiles as photochromic sensors and also the fatigue test for the control of color change stability.3,4 This concept of colorimetric and spectral parameters also allows one to find the dependence of color change on intensity of UV irradiation and temperature. This article presents the advantages of this measuring device when obtaining colorimetric data and their kinetic behavior in basic color spaces.
Material and methods
For the experiment, three commercial photochromic pigments, P1–P3, with the following chemical structures were used:
pigment P1: 1,3,3-trimethylspiro[indolino-2,3’-(3H)naphtho (2,1-b)(1,4)-oxazine] CAS: 27333-47-7, Figure 3(a);
pigment P2: 5-chloro-1,3,3-trimethylspiro[indoline-2,3’-(3H) naphtho(2,1-b) (1,4)-oxazine], CAS: 27333-50-2, Figure 3(b);
pigment P3: 3,3-diphenyl-3H-naphtho[2,1-b]pyran, CAS: 4222-20-2, Figure 3(c).
Such pigments were applied on a polyethylene terephthalate (PET) substrate by the method of screen-printing. A PET substrate was used according to standard ISO 105-F04:2001 in the form of plain weave. Pigment concentration was 100 g per 1 kg of printing paste. After printing, all samples were dried for 10 minutes at 105℃ and then cured for 2 minutes at a temperature of 150℃.
Measurement of the colorimetric and spectral parameters of photochromic textiles was conducted on an original measuring device, LCAM Photochrom, for two reasons. Without hardware adaptation of standard reflectance spectrophotometers, it is impossible to measure the growth phase of photochromic color change. As a result, a second problem was found. Measured data is influenced by a delay between the activation of photochromic materials by the external light source and the initial series of measurements with a constant time interval. This problem affects the validity of the measured data. Standard reflectance spectrophotometers in “time measurement” mode are limited by a time delay of 5 s. That means if the decay part of the photochromic color change is faster than this interval, it is impossible to measure kinetic data correctly. The above-mentioned original LCAM Photochrom measuring system solves these problems via complex construction. 3 This device allows the studying of color photochromic kinetics, the influence of exposure time and the thermal sensitivity if the spectral sensitivity of the excitation monochromator is included, as shown in Figures 4 and 5. 4 The dual light source construction of the spectrophotometer with a shutter over the exciting light source makes the continuous measurement of photochromic color change possible during reversion after switching off the exciting light source. A Xenon discharge lamp with continuous discharge is used. The pulsed discharge lamp allows using such a system as a fatigue tester for photochromic systems with a halftime of photochromic color change around 500 ms. For textile samples, which are frequently slower, it is recommended to use an electric shutter and a continuous discharge lamp for the fatigue tests.
Results
Photochromism in the CIE chromacity diagram
The color in the CIE chromaticity diagram is defined by coordinates x and y, but Helmholtz numbers can also be used. These values inform us about the percentage of clear spectral color corresponding to the relevant wavelength. This description is near to coloristic practice where expressions such as the color is lighter, darker, more pure, more cloudy or the hue is reddish, yellowish are used. The dominant wavelength λ D (in the case of a purple shade λ DK – the complementary dominant wavelength) and excitation purity pE for study of behavior in CIE chromacity diagram was used.5–8
It was found that for pigment P1 the dominant wavelength λ
D
and excitation purity pE were 479 nm and 0.3798, respectively. These values correspond to 38% of pure spectral color shade at a wavelength of 486 nm. Figure 6 shows the change in shade for the pigment P1 in the CIE chromacity diagram and the line of blue shade generation does not point to W, as in the case of pigments P2 and P3 (in this situation for light source D65). However, the shade is shifted in its line to a shade with a green yellowish tint.
CIE xy diagram showing the discoloration of the tested photochromic pigments: time of exposure 120 s, irradiation 89.06 W.m−2 and D65 illuminant (the arrows show the change of colorimetric parameters during testing – deflection from achromatic point W is visible). (Color online only).
This is because the inactivated form of the photochromic pigment P1 shows distinct shade coloration (to green yellowish) compared to the other pigments P2 and P3. This situation is shown in Figure 7, where minimum reflectance is documented at 400 nm, which is independent of the activation of pigment P1.
Dependence of total reflectance on the time of discoloration [s] for pigment P1: time of activation 120 s, irradiation 89.06 W.m−2.
After comparing the spectral curves for the discoloration of pigments P1 and P2 (Figures 7 and 8) the time of pigment P2 is two times slower than for pigment P1. There is also a hypsochromic shift of the reflectance minima for the pigment P2 relative to its blue counterpart P1. The behavior is the effect of the chlorine atom at the fifth position in an indoline molecule. In the case of pigment P2, chlorine acts as auxochrome and results in a deepening of shade, as shown in Figure 8. Since the chlorine is at the para-position relative to the indoline nitrogen, the chlorine shifts the maximum of absorbency in a hypsochromic way about 37 nm. The first reflectance minimum is shifted towards the UV area and exerts influence on the total color perception only to 420 nm. When studying the discoloration time, different decreasing curves of the exciting purity are observed (Figures 9–11).
Dependence of total reflectance on the time of discoloration [s] for pigment P2: time of activation 120 s, irradiation 89.06 W.m−2. Dependency of excitation purity pE on the discoloration time and on intensity of irradiation, pigment P2.
Figures 9–11 show that all investigated pigments have a longer time of discoloration for a higher intensity of irradiation. This is well described in the case of the pigment P1, where the exciting purity comes nearest to the achromatic point W in its minimum. Also, the fixation constant value at a different time can be observed and it can be predicted that a higher exciting purity during exposition means a longer reaction time. The pigment P3 has a time of discoloration of hours. The trend of discoloration for pigment P3 to the achromatic point is shown as a broken yellow line (Figure 6). It is also evident that the inactivated forms of pigment P3 and P2 end in achromatic point W. Other significant differences include the curve of excitation purity for the pigment P1 for the highest value of irradiation, where differences are visible (Figure 10). This is caused by the effect of dependence of shade intensity on concentration from classical dyestuff. The characteristic curvature is visible showing a butterfly effect for pigment P2 (Figure 6).
Dependency of excitation purity pE on the discoloration time and on intensity of irradiation, pigment P1. Dependency of excitation purity pE on the discoloration time and on intensity of irradiation, pigment P3.
Photochromism in the CIELAB color space
The CIELAB space can be used to describe photochromism. This system uses an orthogonal three- axis system for describing colors. The shift in hue is described using the chromatic plane a*b*, while the shift in lightness can be recorded using the L* axis. In Figure 12, the color shift of photochromic pigments on the chromatic plane a*b* is presented. This shift does not give information about the change in lightness. However, there is a visible achromatic white shade in the central position “0” corresponding to the area of white color in Figure 12. The level of lightness decreases, while the intensity of shade increases with exposure time.
Projection of discoloration on the photochromic plane a*b*: time of activation 120 s, irradiation 89.06 W.m−2.
The change in lightness records for the three pigments under investigation is shown in Figure 13. The difference in kinetics of discoloration for every tested pigment to colorless shade is observed when the lightness reaches a maximum value. The speed of coloration and discoloration are different and the half-life of discoloration depends on the structure of the photochromic molecule and the additives used in the final stages of photochromic dye production. This is also the reason why different commercial photochromic compositions exhibit different light fastness properties.
Dependence of lightness on the discoloration time: time of activation 120 s, irradiation 89.06 W.m−2.
Another important parameter for the evaluation of color constancy in colorimetry is the shift in hue expressed by the color difference dE*. The color difference dE* is also suitable for defining the hue shift of a photochromic reaction in the CIELAB space. As a complex parameter, the color difference dE* includes information about the change in shade intensity and the change of lightness. The color difference dE* can explain the visibility of color change dependent on the physiology of human vision. This parameter is suitable for evaluation of the stability of the photochromic effect by individual subjective evaluation and assessment of photochromic reaction by human eyes.
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Photochromic pigments are also used as a sensorial system for evaluation of UV radiation and its harmful effects. Therefore, an important requirement is the ability to recognize specific color differences during visual evaluation; in other words, how the shade intensity changes during the photochromic reaction on a textile substrate that undergoes a transition from colorless to colored shade. The color difference for individual assessment could be higher than 0.4 DE*. This is the limit for color differences discernible by human eyes. In Figure 14 it can be observed that the color difference for the tested pigment was higher than 40 DE* units. Also, the speed of discoloration is different for the tested pigment, as can be seen from the dependency on the lightness parameter (Figure 13).
Dependence of color difference on the discoloration time: time of activation 120 s, irradiation 89.06 W.m−2.
The CIELAB color space is very well established for the evaluation of color changes by the use of the colorimetric parameters L*a*b*. However, if the photochromic effect shows a color or tint shift, expressions such as more yellowish, reddish or greenish are used. Therefore, for this type of evaluation the cylindrical system CIELCH could be used. The cylindrical system CIELCH uses the parameter h (hue angle) to describe a color shift.
Photochromism in the CIELCH color space
The main disadvantage of mathematical description employed by CIELAB space is the definition of tint and coloration shift. In colorimetry, the cylindrical system CIELCH is employed since it is analogous to the Munsell system. The color shift can be described as the hue angle. The description of the hue angle is used when working in the design of smart photochromic sensors as shown in Figure 1.
In the case of a photochromism study and a photochromism description of differential chromatic planes DC*DH*(Figure 15), the differences between the various pigments tested can be evaluated by comparing hue and chroma differences. The pigment P1 exhibits large hue versus chroma differences relative to other pigments. This is in agreement with the assumption that photochromic pigments mainly experience changes associated with their chroma values.
Change of chroma and hue of photochromic pigments in decay: time of activation 120 s, irradiation 89.06 W.m−2.
The reason behind these differences could be the opalescence caused by the particles fixed on the textile substrate due to the presence of pigment P1. A slight coloration can be observed in the inactivated form, which is usually colorless.
When designing sensorial systems on textile substrates it is necessary to take into consideration the change in hue as a complex factor describing change in hue angle as well. This situation is described in Figure 16, where it is evident that all tested pigments exhibit different curves of DH* in time comparable to the DE* values shown in Figure 14. Figure 14 also shows that pigment P3 was not fully stabilized to a constant value of DE*, while the pigments P1 and P2 are fully changed to the leuco form of the photochromic dyestuff.
Dependency of hue difference on the time of decay: time of activation 120 s, irradiation 89.06 W.m−2.
Photochromism in the CIECAM 02 color space
The reactions of photochromic substances were studied under different light sources showing various spectral power distributions. The experiments were aimed at studying the chromatic adaptation of visual differences and the shift in colorimetric parameters in caused by the use of different illuminants, such as D50, D65, D75 and D90. These sources describe and simulate the spectral power distribution of natural daylight. This simulation corresponds with the effect of using photochromic sensors during an entire day and the shift in colorimetric parameters together with the chromatic adaptation can be evaluated. These differences are caused by changes in spectral power distribution taking place over the day.
The construction of photochromic sensors must include a reference hue and a color changeable part. That will vary depending on the intensity of the exciting radiation.
Since colorimetric parameters depend on the light source employed, it is necessary to take into consideration these influences together with the chromatic adaptation, meaning the change of perception and adaptation of human vision to changes in types of illumination.
Complex models for evaluating color appearance, such as CIECAM02,9–12 allow predicting colorimetric changes under different light sources. Figure 17 shows the shift of colorimetric parameters for pigment P1 under D50, D65, D75 and D90 illumination. Figure 15 shows the shift observed when comparing two data sets corresponding to sunlight and cloudy skies. The shift is approximately 3 DE units, being approximately 10 times higher than the resolution of the human eye to color differences.
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The advantage is the constant shift of colorimetric data for the stable part of the sensorial system and will not cause significant measurement error. Table 1 documents the influence of the light source on color differences of samples corresponding to agreement between the stable and changeable parts of the sensorial system.
Shift of colorimetric parameters on plane ab of CIECAM02 after using different illuminants: pigment P1, time of activation 300 s, irradiation 86.5 mW.m−2. Influence of the illuminant on the level of color difference between the stable and changeable part of the sensor – for medium level
The sensitivity of human eyes to color differences is approximately 0.4 DE units in the CIEDE2000 color difference formula; due to non-uniformity of CIELAB color space the first noticeable threshold varies from 0.4 to 1.614,15 consequently in CIECAM02. Therefore, light source changes may induce color differences, which are higher than the limit of human vision.
It can be discussed that color differences for D65 were not the ideal 0. However, color differences were near the sensitivity limit of human vision. If this value is used for normalizing the measurements conducted using different light sources, then the color differences also remain under the threshold value. This result indicates that in the case of photochromic sensors, the change of illuminant in the temperature range from 5000 to 9000 K has no influence on the evaluation of UV irradiation by the method of color match between two sensor parts (stable and changeable).
Conclusion
The speed of color change is a complex parameter that can be described by the change from achromatic to chromatic hue, when all three colorimetric coordinates are evaluated. In this article, a complex description of photochromic colorants in selected color spaces is provided. Parameters such as excitation purity, color and hue difference and also the shift in lightness provide the main description of photochromic coloration of samples from a visual point of view. The observation and evaluation of photochromic effects together with the stable part of a sensorial photochromic system is carried out. The precise measurement of the colorimetric parameters allows computer match prediction of the stable part of smart UV sensors based on the relevant intensity of UV irradiation. It is also possible to develop specific sensors especially for different skin types using the Minimum Erythemal Dose (MED) scale factor. MED is the Minimum Erythemal Dose, the amount of sun exposure which causes barely perceptible skin sunburn redness (erythema). The base MED is equal to 21 mJ.cm-2 of weighted UV energy. The specific sensors for different skin types will have the photochromic part of a sensor containing different amounts of sensitivity modulator (UV absorbers, etc.). That means the color changeable part of the photochromic sensor should be more sensitive for skin type 1 compared to skin type 4. The results show different kinetics of photochromic color change for the tested pigments and the possible modulation speed of UV sensor reaction. The evaluated standardized color spaces show that the pigment P1 is affected by colored residuals, which affect its colorimetric and photochromic properties.
Footnotes
Funding
The paper was supported in part by the project OP VaVpI “Innovative products and environmental technologies”, registration number CZ.1.05/3.1.00/14.0306.