tometer. Two commercial spectrophotometers, CM3630d from Konica Minolta and CT2 from Technidyne, have been simulated in this study. These two instruments use two dif- ferent types of UV-filtering techniques: the CM3630d uses the numerical UV filtering, and the CT2 uses the conven- tional UV filtering with an adjustable UV filter. IR2 Reference Standards and the Total Spectral Radiance Factors Two carefully selected paper pads, one nonfluorescent and one fluorescent, were sent to the Standardizing Labo- ratory at NRC for calibration. These paper pads with the assigned values are the second-level international refer- ence standards (IR2s) in the ISO hierarchy of calibration. According to the ISO standards, the assigned values for the nonfluorescent IR2 are the spectral reflectance factors, whereas for the fluorescent IR2s, three assigned values were given to the same fluorescent paper pad thereby actually three IR2s corresponding to three standard illu- minants, D 65 , D 50 , and C. These assigned values are CIE whiteness (D 65 /10 8 ), CIE whiteness (D 50 /2 8 ), and ISO brightness (illuminant C). In addition, the total spectral radiance factors, denoted as the assigned total spectral radiance factors, corresponding to these assigned values were also given. The NRC facility used for calibrating the fluorescent IR2s is a two-monochromator reference spectrophotometer that is capable of providing high- accuracy total radiance factor measurements. 21,22 The measurement scales of the instruments were cali- brated against the assigned spectral reflectance factors of the nonfluorescent IR2. After the calibration, these appa- ratuses were used to measure the total spectral radiance factors of the fluorescent paper pad corresponding to three distinct UV content levels, that is, R uv-full , R GG395 , and R uvx . Here, R uv-full stands for illumination directly from the lamp with the full UV content, R GG395 for the reduced UV content with full engagement of the GG395 UV filter, and R uvx with the full engagement of the UV cutoff filter up to 420 nm, respectively. Simulations of the Total Spectral Radiance Factor Measurements In this subsection, we explain how physical measure- ments with CT2 or CM3630d can be “performed” by numerical means. We begin with the CT2 apparatus that has an adjustable UV filter (GG395). Assuming that the area percentage of the GG395 filter setting into the opti- cal path is a and the rest 1 2 a (see Fig. 3), the total spec- tral radiance of the incident light that reaches the sample may be written as follows: I in ð l Þ 5 ð 1 2 a Þ I uv - full ð l Þ 1 a I GG395 ð l Þ ; (1) where the variable l stands for wavelength in both UV and visible spectral range, and the symbol k is reserved for visible light only. In the expression, the first term on the right-hand side stands for the portion of light that does not pass through any UV filter hence with the full
As shown in Fig. 1, the UV content of an illumination is controlled by the position of the adjustable filter. For the light beam that passes through the GG395 filter, the UV radiations below 400 nm from the lamp are removed by the filter, whereas the rest (radiations of longer wave- lengths) pass freely through the filter and reach the paper sample. Naturally, the UV content of the light that reaches the paper sample depends on the portion of the light that passes through the GG395 filter. The mathematical expres- sion for this kind of UV content adjustment is given in the “Simulation of the Total Spectral Radiance Factor Meas- urements” section. The spectrophotometers also equip with another UV filter having the cutoff wavelength at 420 nm. When this filter is fully engaged, all the UV radiations from the lamp are absorbed by the filter and only the visi- ble light reaches the paper sample. Fig. 2. Schematic diagram of the numerical UV-filtering technique. A spectrophotometer equipped with three lamps. Light beams from lamps 2 and 3 are filtered by the GG395 and 420 nm cutoff filters. The total spectral radi- ance factors of the paper sample (not shown) are meas- ured under the exposures of each of the light beams.
Numerical UV-Filtering Technique
The numerical UV-adjustment technique was introduced in 1997. One of the spectrophotometer that uses this tech- nique is Minolta CM3630d. Instead of using one lamp and one adjustable UV filter, three xenon lamps are mounted in a row inside the integration sphere. Two of the lamps are completely filtered either by the GG395 UV filter or the UV cutoff filter (420 nm), respectively (see Fig. 2). The lamp with GG395 UV filter emits light of reduced UV content, whereas the one with 420 nm cutoff filter emits light having essentially no UV content. The third lamp has no UV filter coverage. Flashing individually with these lamps, one lamp at a time generates illuminations of full UV content, UV- reduced, and UV-excluded, respectively. Corresponding to these illuminations, one obtains three total spectral reflec- tance factors, R uv-full , R GG395 , and R uvx . The total spectral radiance factor under a specific standard illuminant, such as CIED 65 , is expressed as a linear superposition of two of the spectral radiance factors, as explained below in detail.
METHODS
In this study, we present a method that exactly simulates the UV-adjustment technique with a physical spectropho-
Volume 42, Number 1, February 2017
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