The procedure described in the “Methodology” section (Fig. 1) was performed for each of the textile samples, to determine the dose causing a JND as a function of prior exposure. From these calculations, a relationship between Jp and p was defined for each test material. Figure 3a shows the trend of increasing lightfastness for the residual colour as the prior dose increases for Hofmann’s Violet RRR. In this example, the dose causing a JND at no prior exposure (p = 0) is 0.12 Mlx·h, while the value increases to 3.6 Mlx·h for the residual colour at p = 20 Mlx·h. This is approximately equivalent to a shift from BW1 to BW3 sensitivity, as shown by the horizontal lines denoting Blue Wools 1–4. Note that the Blue Wool comparisons use generalized values of dose to JND, J0, in Mlx·h for a light source without ultraviolet (UV) energy, according to the CIE 157 Technical Report [20]. The transition between BW ratings was defined as the mid-point between dose to JND values on a plot of log(J0) versus BW number. For reference, Table 1 gives the approximate dose to JND for BW1–3 exposed to a light source without UV, as listed in CIE 157. The calculated number of years to a JND is also given when the annual dose is limited to 0.015 Mlx·h/y for high responsivity materials [205].

Table 1 Approximate dose causing a JND for BW1–3 (no UV), and the corresponding years to JND (at 0.015 Mlx·h/y)
Fig. 3
figure 3

Future dose to JND for residual colours as a function of prior exposure. Approximate Blue Wool (no UV) scale overlaid for reference. a Hofmann’s Violet RRR on wool; b Erythrosin on cotton

When interpreting the results in Fig. 3, it is important to also consider the condition of the colour with respect to the original. At a prior exposure of 16 Mlx·h, the residual Hofmann’s Violet colour is at BW3 sensitivity; however, at this point the colour change from the original is quite large (i.e. ΔE00 = 25 at a dose of 16 Mlx·h in Fig. 1a). Figure 3b shows the results for Erythrosin on cotton [21], which was the most fugitive colourant in the dataset (J0 = 0.028 Mlx·h). The curve shows a sharp increase in Jp with prior exposure and very little colour remains when the lightfastness progresses to BW3 and higher.

Figure 4 shows a summary of results for the full dataset. The plot in Fig. 4a gives the lightfastness of residual colour at p = 10 Mlx·h versus the initial value at p = 0. The angled dashed line indicates unity, where the values are unchanged. Points on the line reflect no change after 10 Mlx·h, while those above indicate an increase in lightfastness. An example is again highlighted for the Hofmann’s Violet RRR sample, showing the transition from its initial sensitivity to the value after 10 Mlx·h. Note that some highly fugitive samples do not appear on this plot since they are highly faded at 10 Mlx·h and the value of J10 is beyond the experimental data (cf. Erythrosin data in Fig. 3b). The plot in Fig. 4b presents similar data; however, the y-axis is now the lightfastness enhancement at 10 Mlx·h, J10/J0. These data show the expected strong effect that 10 Mlx·h has on fugitive materials, while the influence is smaller for the less sensitive samples. The point for Hofmann’s Violet is similarly shown for reference as the purple square.

Fig. 4
figure 4

a Light dose causing a JND for the residual colour after 10 Mlx·h of light exposure versus the value for the original colour; b Lightfastness enhancement of the residual colour after 10 Mlx·h versus dose causing a JND for the original colour

A further analysis was performed to determine the degree of colour change that occurred for each sample once the light sensitivity of the residual colourant diminished to half the initial value. In other words, when the dose that will cause a JND, Jp, is twice the initial value, J0. Figure 5 summarises the findings as a histogram showing the distribution of colour difference values (relative to the original material) binned by ΔE00 values of one unit. The median value for the samples was ΔE00 = 1.4, which is approximately a JND. The findings lead to an interesting generalization, or rule-of-thumb: when a dyed textile progresses to one JND from its initial color, the light sensitivity of the residual colour is roughly halved.

Fig. 5
figure 5

Distribution of colour difference when the light dose causing a JND for the residual colour progresses to double the initial value

Blue wool ratings

The ultimate goal of this analysis was to determine at which prior dose values, p, the residual colourants are no longer in the higher sensitivity ranges (i.e. BW 1, 2, and 3). Figure 6a shows the number of samples that are less than or equal to each BW rating as a function of prior exposure. At p = 0, more than a third of the samples rank as BW1; however, none of the residual colours are in this category when prior dose approaches 3 Mlx·h. They have all progressed to BW2. Similarly, none of the samples rank ≤ BW2 when p ≥ ~ 17 Mlx·h, or ≤ BW3 when p ≥ ~ 45 Mlx·h. These data are also presented in Fig. 6b, where the number of samples is calculated for discrete BW bins. The curve for the number of samples rated as BW1 quickly decays to zero with a relatively small prior dose. As p increases, there is briefly a rise in the number of BW2 materials as the former BW1 samples degrade and progress to the next level of fastness. A similar trend is evident with the number of BW3 materials, showing an initial increase (former BW2 progressing to BW3) followed by a decrease to zero as they eventually progress to BW4. The overall results clearly show the shifting distribution of light sensitivity for the large collection of dyed textile samples. Similar trends are expected for other types of coloured materials in collections; however, this remains to be investigated in future work.

Fig. 6
figure 6

Summary of results showing the number of samples with given lightfastness ratings as a function of prior dose. a results binned as values less than or equal to each BW rating up to BW4; b samples binned by discrete BW ratings

An additional consideration for the results in Fig. 6 is the effect of the illuminant spectral power distribution (SPD). The results specifically relate to irradiance from the LED light source used in the study, which is representative of modern indoor gallery lighting. Previous studies [22,23,24] have shown the influence of different spectral regions on fading, highlighting some trends in addition to a large degree of variability. For example, the work of McLaren [23] indicates a decreasing proportional influence of the UV region on total fading (with respect to visible) as the ISO BW rating decreases. There is, however, considerable scatter in the results. For the Blue Wools and a small selection of artist pigments, Saunders and Kirby [24] also show the varied influence of wavelengths through the visible range on colour change.

Of the many possible illuminant spectra, two comparisons are most relevant with the LED: (1) Traditional incandescent lamps at ~ 2700–3000 K; and (2) Daylight through glass (with/without UV filtered). Incandescent lamps have been in use since the early 20th century, and the importance of daylight does not need mention. In a discussion of LED lighting for museums, Michalski and Druzik [25] review lightfastness data and note a similar rate of colour change for a warm-white LED (high CRI, 3000 K, blue pump) source in comparison to incandescent. For daylight through glass, the rate of damage may be nearly the same or up to ~ 3X faster for some materials that were considered.

Practical considerations

In order to address the practical impact of this study, the results obtained from Fig. 6 were compared with published measurements of annual light dose for different exhibit settings. The cumulative dose versus time is shown in Fig. 7 for several conditions that were considered. These include the work of Thomson [10] at the National Gallery, London in 1967, and recent research at English Heritage and the National Trust [11, 26]. For the latter work, Fig. 7 shows the average annual dose for an English country house used in a study of the prior exposure of paintings at the Yale Center for British Art [11]. Also included is the logged annual exposure at an office exhibit space in Ottawa, which gave 1.85 Mlx·h/y (Irene Karsten, personal communication, 2022 June 16). For comparison, Thomson’s proposed approximation of indoor daylight illuminance-hours gives ~ 2.4 Mlx·h/y for Ottawa on average. We calculated this value using daily NASA weather data (horizontal solar irradiance in kWh/m2/d) from RETScreen [27] for the period of 1984–2021 and converted to illuminance hours by the factor 1 W/m2 ≈ 120 lx [28]. Thomson’s approximation involves multiplying this value by 1.5% for a representative indoor value.

Overlaid on the graph in Fig. 7 are horizontal lines indicating the dose values where residual colours ceased to exhibit sensitivities of BW1, BW2 and BW3 for textile samples in the present study. At larger exposure conditions (labelled a, b, and d), none of the residual colours would be classified as BW1 after approximately two years, and none would be ≤ BW2 after 10 years. The line labelled ‘f’ is shown to represent the modern conservative limit of 0.015 Mlx·h/y in the CIE 157 report [20] for preservation of high responsivity materials (BW1–3). Given the summary in Fig. 7, one may consider how likely it is for an object more than 100 years old to retain a sensitivity of BW1, 2 or 3 given its history. A similar plot could be constructed to illustrate the dose that would accumulate for a textile garment worn outdoors in daylight using general illuminance values: e.g., direct sunlight, midday (100 klx); daylight from clear sky (20 klx); overcast sky (10 klx); thick overcast, grey sky (5 klx) [29].

Fig. 7
figure 7

Cumulative light dose for different exhibit scenarios. Horizontal lines show threshold values where BW1, BW2, and BW3 sensitivities were no longer present for residual colours in the present study (cf. Fig. 6)

A particular scenario is worth considering with respect to the dose limit for ‘high responsivity’ materials in the CIE 157 document. The recommended limit is 0.015 Mlx·h per year for this class of material, which typically includes textiles dyed with early synthetic colorants [1, 8, 20, 30]. Classification of material sensitivity in grouped ranges (e.g. high: BW1–3, medium: BW4–6) is typically necessary due to the large degree of uncertainty in risk assessment unless specialized instrumental techniques are available. When a material is broadly classified as ‘highly sensitive’, Table 1 shows the years to JND (no UV) for the individual Blue Wools as: BW1 (20 years); BW2 (67 years); BW3 (200 years).

Now consider a hypothetical situation where the assessment of an object or collection history leads to the conclusion that the material(s) have experienced more than ~ 15 Mlx·h of light exposure. A review of the results in Fig. 6 indicates that it is unlikely that the residual colours are in the BW1–2 sensitivity range. It is now possible to use the risk-management strategy described by Michalski [1] to reassess the annual dose limits. For example, based on the significance [4] of the object or collection, and mandate of the institution (e.g. access), is 200 years to a JND appropriate? If 20 years to a JND were acceptable, it would increase the dose limit by a factor of 10 to give 0.15 Mlx·h per year. This value is roughly in-line with the requirements for full-year exhibition at 50 lx (8 h a day, every day of the year = 0.15 Mlx·h). Alternatively, higher light levels could be used for a shorter period to enhance visibility (e.g. for older visitors), while staying within a prescribed dose limit. Ford and Smith [3] give a related example that considers significance in the management of light exposure using MFT test results. For objects at about BW3–4 sensitivity, the authors explore an option that would allow illumination at 50–150 lx (lowest possible for good display) with an exhibition period based on significance: (a) five years per decade (high significance); (b) life of the exhibition, up to ten years (average significance).

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