Indoor whiteness: a new internationally standardized concept: a new ISO standard measures whiteness under conditions that simulate the indoor environment where papers are sold, bought and used.

A recently approved ISO (International Standards Organization) standard introduces the concept of "indoor whiteness." This new standard provides a method of determining paper whiteness under conditions which may be regarded as realistic.

The new standard is ISO 11476, "Paper and board-Determination of CIE whiteness, C/2[degrees] (indoor illumination conditions)." This new standard is a complement to an earlier standard, ISO 11475:1999, "Paper and board Determination of CIE whiteness, D65/10[degrees] (outdoor daylight)," which was published a couple of years ago.

Both methods specify a routine for adjusting the ultraviolet (UV) content of the illumination in the instrument in order to achieve reproducibility when papers containing UV-sensitive fluorescent whitening agents (FWA) are measured. The main difference between the two methods lies in the intensity of the UV component of the illumination falling upon the sample. The adjustment to the CIE illuminant C[UV(C)] specified in the new standard, ISO 11476, gives a much lower relative UV content than the adjustment to the CIE illuminant D65 [UV(D65)] specified in ISO 11475.

The development of a method to determine an "indoor whiteness" value was entrusted to the ISO/TC6/WG3 working group as an official work task at the same time as the work task for D65 whiteness was initiated. However, ISO 11475 was published first because it was easier to solve the technical problems involved in the adjustment of the UV content of the instrument illumination to the D65 illuminant.

Work on the standardization of whiteness measurements began a decade or more ago. Since then, there has been a strong opinion that measurements under D65 conditions are unrealistic. It is argued that the UV content of" the illumination under such conditions is much higher than that generally experienced in an indoor environment, where paper is normally sold, bought, and used. Whiteness can be raised by adding FWA, but there are other ways of achieving whiteness, such as by bleaching, by adding blue dyes, and by using pigments with a high scattering coefficient, such as precipitated calcium carbonate. Although these techniques may give a visually white paper under normal usage conditions, they are unable to compete with the use of FWA in D65 measurement conditions, where the high UV content associated with the CIE illuminant D65 means that extremely high whiteness values can be obtained simply by adding more and more FWA.

CHOICE OF "INDOOR" ILLUMINANT

Despite the urgency, work on the development of a method for determining whiteness under indoor conditions was delayed because of the lack of a well-defined indoor illuminant. Eventually, it was realised that the CTE illuminant C would best meet the requirements, and that there are good metrological and historical reasons for adopting this as fine desired illuminant. Jordan and O'Neill (1) showed the merits of the illuminant in practical tests, and Bristow and Karipidis (2) later proposed its adoption after a number of comparative assessments. The use of the CIE illuminant C to represent indoor conditions has now been standardized in the new ISO 11476 and for the measurement of ISO brightness in ISO 2470 (3).

However, there is another major difference in the routine adopted for adjusting the UV level in the instruments. In ISO 11475, the UV(D65) adjustment is made with the help of a fluorescent reference standard with an assigned CIE whiteness (D65/10[degrees]) value. In the recently adopted ISO 11476, the UV(C) adjustment is not based on a whiteness value. Instead, the new procedure uses a fluorescent reference standard with an assigned ISO-brightness value, as described in 1SO 2470. This decision is based on the recommendation of the ISO/TC6 working group, which concluded that a single UV(C) adjustment would provide the best possible reproducibility in measuring ISO brightness (3).

COMPARISON BETWEEN THE TWO WHITENESS MEASURES

Table I presents data that show the differences between the two whiteness measurements. It shows mean values for 12 samples measured in five different types of instrument. The samples in Table I are sorted according to increasing W(C/2[degrees]) value, and it is clear that the order of the values in the W(D65/10[degrees]) column is different.

If we accept that the W(C/2[degrees]) order is the realistic ranking order for samples 1-12 under normal user conditions, then the corresponding rankings in the W(D65/I0[degrees]) order are samples 1, 3, 2, 6, 5, 7, 4, 8, 10, 9, 11, and 12. In particular, it is clear that sample 6 is ranked too low and sample 4 is ranked too high in the W(D65/10[degrees]) order.

Columns 4-6 in Table 1 provide some explanation of this disparity. Column 4, designated Wo, shows the whiteness values (D65/10[degrees]) in the UV-excluded (420 nm) mode when a 420-nm cutoff filter is introduced into the light beams and all the fluorescence effect is eliminated. Column 6, the DW(D65/10[degrees]) column, shows the difference between the W(D65/10[degrees]) values and the Wo values, i.e., the component of the W(D65/10[degrees]) value that is influenced by the addition of FWA. Sample 6 was evidently bleached to a high whiteness before a small amount of FWA was added, while sample 4 has a low natural whiteness and a large contribution from the added FWA.

Column 5 shows the approximate contribution of FWA to the W(C/2[degrees]) values. The mean ratio of the values in Column 5 [DW(C/2[degrees])] to the values in Column 6 [DW(D65/10[degrees])] is 0.53, which can be interpreted as meaning that the effective UV content of CIE illuminant C is roughly 50% of that in CIE illuminant D65 (4).

Comparative tests show that, in the modern instruments, the reproducibility of the CIE-whiteness C/2[degrees] values is of the same order as the reproducibility of the CIE-whiteness D65/2[degrees] values. There are also clear indications that the agreement between the instruments is much better if the obsolescent Elrepho 2000 is excluded from the comparison.

CONCLUSION

Comparison of the values obtained under the two illuminant conditions (columns 2 and 3 in Table I) demonstrates that, although the higher values obtained under D65 conditions may be attractive, they do not necessarily rank the papers in the order that may be important tot the customer and the user.

Now that a method for determining indoor whiteness has been developed, the ISO technical committee responsible for this work (ISO/TC6) hopes that this method will become widely used. Mills interested in quoting a realistic whiteness value for their papers--and users interested in applying realistic values to their products--should find ISO 11476 to be a useful standard to adopt in their specifications.

Sample   W(C/2[degrees])    W(D65/10[degrees])    Wo

1         60.2               66.4                 53.7
2         80.9              105.1                 56.0
3         81.1               81.3                 81.3
4         95.3              125.0                 62.3
5         96.2              117.0                 73.0
6         98.4              108.9                 82.6
7         99.9              123.0                 72.5
8        100.3              131.4                 63.7
9        111.2              141.1                 75.2
10       111.4              136.5                 81.0
11       116.1              142.4                 84.0
12       121.5              153.5                 81.7

Sample   DW(C/2[degrees])   DW(D65/10[degrees])

1          6.5               12.7
2         24.9               49.1
3         -0.2                0.0
4         33.0               62.7
5         23.2               44.0
6         15.8               26.3
7         27.4               50.5
8         36.6               67.7
9         36.0               65.9
10        30.4               55.5
11        32.1               58.4
12        39.8               71.8

Table 1. Mean CIE-whiteness data for 12 samples

LITERATURE CITED

(1.) Jordan, B. and O'Neill, M. A., Tappi J. 74(5): 93(1991).

(2.) Bristow, J. A. and Karipidis, C, TAPPI J. 82(1): 183(1999).

(3.) Bristow, J. A., TAPPI J. 82(10): 54(1999).

(4.) Bristow, J. A., Color Res. Appl. 19(6): 475(1994).

J. Anthony Bristow is the principal scientific advisor at Bristow Consulting AB, Bernstromsvagen 28, SE-146 38 Tullinge, Sweden. Address Correspondence to Bristow by email at [e-mail]