An additional filter of borosilicate crown glass having a nominal transmittance of 92% (transmission density 0.036) is included in the set.

The filter sets were prepared in three sizes in order to accommodate different instruments. The largest filters are mounted in 51 × 51 mm holders with a filter aperture of 38 mm diameter. The intermediate size filters are mounted in 51 × 38 mm holders with a filter aperture of 25 mm. The smallest filters are mounted in cuvette holders and provide a filter aperture of 30 × 8 mm.

The preparation of the filters from the stock glass was done in the NBS Optical Shop. The tolerances on parallelism of the two faces of the filters was maintained at approximately 0.01°. They were polished to a flatness of 3 fringes or better.

Calculations of the required thicknesses were made from internal transmittance data supplied by the manufacturer. The internal transmittance of 1 mm path length for the three glass types is as follows:

3.5

3.0

(500 nm) NEUTRAL GLASS

FIGURE 1. Transmission density versus thickness of Schott NG-4, NG-9, and NG-11 neutral glasses as derived from internal transmission data for 1 mm and 2 mm thicknesses.

3. Transmission Analysis

3.1 Light Scattering

Scattering of a collimated sample beam transmitted through the neutral filters was quantitatively assessed by the ASTM recommended method [3] for measuring haze percentage. The measurements were made on the reference hazemeter [4] using the ASTM recommended geometry and methods. This instrument has a well collimated circular incident beam having a color temperature of approximately 6800 K. A visual response filter at the detector modifies its response to give the hazemeter a peak sensitivity at approximately 550 nm. The instrument measures total forward scattering relative to the total transmission within the definition of this response and for the ASTM recommended geometry. The haze percentage determined for a set of filters was found to be as follows:

The purpose of a survey study of the neutral filter transmittance was to assess their over-all spectral properties in the visible spectrum and select a specific wavelength for detailed analysis and calibration. Since the filters are not perfectly neutral, it was desirable that a wavelength be selected for calibration, at which the transmittance values of the filters would be least sensitive to variation in passband and wavelength scale errors associated with various spectrophotometers.

With the exception of the borosilicate crown glass filter, the filters in the set are of a similar type glass with a common dye in three different concentrations for the NG-4, NG-9, and NG-11 glass types. The spectral transmission density curves of these filters in the spectral range between approximately 440 nm and 620 nm shows that the absorptions appear at the same wavelengths for the different concentrations associated with these glass types. Figures 2 through 8 illustrate the results of a 1-nm interval survey of the spectral transmittance of the filters between 440 and 620 nm. These measurements were made with a commercial spectrophotometer. The slitwidth was maintained at approximately 0.1 mm (0.37 nm passband). Further measurements were made of the 10% filter with slit-widths of 1.0, 2.0, and 3.0 mm. These slit-widths correspond to passbands of 3.7, 7.4, and 11.1 nm, respectively.

There are four wavelengths within the wavelength range 440 to 620 nm corresponding to peaks and valleys that could be used for the purpose of establishing a calibrated set of photometric scale standards. However, only one wavelength is needed and the absorption peak at approximately 548.5 nm was selected as being the most suitable. The spectral peak at approximately 464 nm and the absorption valleys at approximately 510 and 591 nm do not show a significant change in transmittance for the range of passbands used in this survey. A

TRANSMITTANCE

TRANSMITTANCE

.85

27

.150 400

450

500 550 WAVELENGTH (nm)

.105

FIGURE 5. Spectral transmittance of Master Filter 1-4.

.00110

.100

.095

FILTER 1-5

.085 400

450

500 550 WAVELENGTH (nm)

600

FIGURE 6. Spectral transmittance of Master Filter 1-5. .0105

.0100

.0095

.0090

.0085

FILTER 1-6

650

FIGURE 8. Spectral Transmittance of Master Filter 1-7.

3.3 High Accuracy Transmittance Measurements Calibration of the spectral transmittance of the master set of filters and three other sets was done on the NBS Reference Spectrophotometer [5,6,7] for measuring high accuracy transmittance. The measurements were made with a passband of 1.5 nm and a beam diameter of 10 mm. Measurements were made for perpendicularly (s) and horizontally (p) polarized incident sample beams at 1.5 nm intervals from 545.5 to 557.5 nm. Each filter was scanned for uniformity at 547 nm. The uniformity measurements were made at three locations; center, 2 mm horizontally from center, and 2 mm below center. Transmittances for master set #1 filters are listed in table 1. The results of the uniformity scan at 547 nm are listed in table 2. The transmittance values listed in table 1 are an average of the two polarizations. The values for both polarizations are listed for the uniformity measurements made at 547 nm. The instrumental uncertainties in the values listed in tables 1 and 2 are estimated to be ±0.04% for filter 1-1 to 0.0005% for filter 1-7. A complete description of the measurement sequence and data reduction for the high accuracy measurements and an analysis of the errors associated with such measurements is given in refs. [5-7].

4. Statistical Analysis

A statistical analysis of the data obtained on the spectral transmittance of the neutral filters was used to determine the magnitude of change in transmittance values due to changes in passband and also the magnitude of

change due to location of the sample beam (filter uniformity).

Preliminary data from the commercial instrument indicated that the central wavelength of interest was at approximately 547 nm. However, a further analysis of the data from the high accuracy instrument indicated that 548.5 nm was a better choice.

Data are presented in table 3 to illustrate the effect of passband on the transmittance at the central wavelength for the master set #1 filters. These values are based on calculation for a triangular passband, using continuous integration. Figures 9 through 15 show these results plotted with the standard error limits. The data listed in table 4 illustrate the magnitude of change due to position location for the master set of filters.

Remeasurement of the spectral transmittances of the neutral filters was made one year after initial calibrations. The results of this second calibration for master set #1 are compared with the original calibration and listed in table 5 for wavelength 548.5 nm. Differences in spectral transmittances of the filters shown in table 5 are too small to be clearly interpreted as changes with the possible exception of filter 1-1. Here the apparent change in transmittance was -0.0009. Further measurements at longer time intervals will be required to confirm any real changes in spectral transmittance due to aging.

Additional details of the statistical analysis are given in appendix A.