The development of an instrumental setup for measuring reflectance of diffusing materials in the infrared region from 2 to 15 μ and a standard for reflectance measurements in that range are described. Magnesium oxide, which generally is used as a standard for reflectance measurements in the visible range, has a very low reflectance beyond 2.7 μ. Flowers of sulfur, containing an estimated amount of approximately 30% mu form of sulfur, or other sulfurs which contain more than 30% mu sulfur, such as vaporized and quenched high-polymer sulfur, have been studied as a reference material in the determination of the infrared reflectance of diffusely reflecting surfaces; and the calculations have been shown for its application in reflectance measurements. The selection of the mu sulfur compounds has been discussed, based on comparison with other sulfur forms which lack the required characteristics for a suitable reference standard in infrared reflectance work.
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Slitwidths used with single-beam, double-pass infrared spectrometer with Stupakoff glower as light source.
Wavelength range (μ)
Slitwidth (μ)
Wavelength Range (μ)
Slitwidth (μ)
0.545–0.6
200
4.5–5.5
49
0.600–0.7
140
5.5–6.45
70
0.7–0.85
90
6.45–7.95
90
0.85–0.9
55
7.95–9.7
130
0.9–1.02
35
9.7–12.6
200
1.02–2.4
25
12.6–13.7
400
2.4–2.9
25
13.7–14.4
650
2.9–4.0
29
14.4–15.0
1400
4.0–4.5
38
Table II
Spectral reflectance of the rhodium mirror used to calculate correction factors.a
Wavelength (μ)
Reflectance (%)
Wavelength (μ)
Reflectance (%)
0.4
72.5
5.0
94.1
0.6
76.3
6.0
94.5
0.8
81.0
7.0
94.8
1.0
82.0
8.0
95.1
1.2
84.6
9.0
95.3
1.4
86.4
10.0
95.5
1.6
87.6
11.0
95.5
1.8
88.4
12.0
95.5
2.0
90.5
13.0
95.5
2.4
91.4
14.0
95.5
3.0
92.0
15.0
95.5
3.5
92.5
4.0
93.0
Data from 0.4 to 4.0 μ obtained from the National Bureau of Standards. Data from 5 to 15 μ obtained from the U. S. Army Engineer Research and Development Laboratories, Ft. Belvoir, Virginia. (The ERDL data from 0.4 to 4.0 μ differed no more than 0.7% from the NBS data).
Table III
Correction of spectral-reflectance measurements made on the ERDL spectrophotometer with glass hemisphere.
The absolute reflectance of magnesium oxide according to the data of Sanders and Middletona (average of three specimens one day old) used to calculate correction factors.
Ratio of absolute reflectance to fractional reflectance. The correction factors for surfaces having a large specular component based on the rhodium mirror data from Table II.
Wavelength (μ)
Factor
Wavelength (μ)
Factor
0.616
1.00
1.285
1.01
0.619
1.01
1.36
1.01
0.621
1.01
1.45
1.00
0.624
1.03
1.56
1.00
0.680
1.04
1.69
1.00
0.710
1.05
1.86
1.00
0.725
1.05
2.04
1.00
0.740
1.06
2.21
1.01
0.760
1.06
2.42
1.01
0.780
1.06
2.67
1.02
0.800
1.07
2.87
1.02
0.820
1.07
3.02
1.03
0.845
1.06
3.21
1.03
0.870
1.06
3.62
1.03
0.909
1.06
4.01
1.04
0.940
1.05
4.05
1.04
0.975
1.04
4.18
1.04
1.005
1.03
4.30
1.05
1.05
1.02
4.50
1.05
1.095
1.02
4.70
1.05
1.14
1.01
4.95
1.06
1.20
1.01
Table VI
The correction factors for a diffusely reflecting surface, based on the magnesium oxide data of Table IV.
Wavelength (μ)
Factor
To 0.845
1.81
0.870
1.80
0.900
1.79
0.940
1.76
0.975
1.74
1.005
1.70
1.050
1.69
1.095
1.68
1.140
1.67
1.200
1.67
1.285
1.67
1.360
1.67
1.450
1.66
1.560
1.66
1.690
1.66
1.860
1.67
2.040
1.67
2.210
1.68
2.420
1.69
2.670
1.70
2.870
1.72
3.020
1.80
Tables (6)
Table I
Slitwidths used with single-beam, double-pass infrared spectrometer with Stupakoff glower as light source.
Wavelength range (μ)
Slitwidth (μ)
Wavelength Range (μ)
Slitwidth (μ)
0.545–0.6
200
4.5–5.5
49
0.600–0.7
140
5.5–6.45
70
0.7–0.85
90
6.45–7.95
90
0.85–0.9
55
7.95–9.7
130
0.9–1.02
35
9.7–12.6
200
1.02–2.4
25
12.6–13.7
400
2.4–2.9
25
13.7–14.4
650
2.9–4.0
29
14.4–15.0
1400
4.0–4.5
38
Table II
Spectral reflectance of the rhodium mirror used to calculate correction factors.a
Wavelength (μ)
Reflectance (%)
Wavelength (μ)
Reflectance (%)
0.4
72.5
5.0
94.1
0.6
76.3
6.0
94.5
0.8
81.0
7.0
94.8
1.0
82.0
8.0
95.1
1.2
84.6
9.0
95.3
1.4
86.4
10.0
95.5
1.6
87.6
11.0
95.5
1.8
88.4
12.0
95.5
2.0
90.5
13.0
95.5
2.4
91.4
14.0
95.5
3.0
92.0
15.0
95.5
3.5
92.5
4.0
93.0
Data from 0.4 to 4.0 μ obtained from the National Bureau of Standards. Data from 5 to 15 μ obtained from the U. S. Army Engineer Research and Development Laboratories, Ft. Belvoir, Virginia. (The ERDL data from 0.4 to 4.0 μ differed no more than 0.7% from the NBS data).
Table III
Correction of spectral-reflectance measurements made on the ERDL spectrophotometer with glass hemisphere.
The absolute reflectance of magnesium oxide according to the data of Sanders and Middletona (average of three specimens one day old) used to calculate correction factors.
Ratio of absolute reflectance to fractional reflectance. The correction factors for surfaces having a large specular component based on the rhodium mirror data from Table II.
Wavelength (μ)
Factor
Wavelength (μ)
Factor
0.616
1.00
1.285
1.01
0.619
1.01
1.36
1.01
0.621
1.01
1.45
1.00
0.624
1.03
1.56
1.00
0.680
1.04
1.69
1.00
0.710
1.05
1.86
1.00
0.725
1.05
2.04
1.00
0.740
1.06
2.21
1.01
0.760
1.06
2.42
1.01
0.780
1.06
2.67
1.02
0.800
1.07
2.87
1.02
0.820
1.07
3.02
1.03
0.845
1.06
3.21
1.03
0.870
1.06
3.62
1.03
0.909
1.06
4.01
1.04
0.940
1.05
4.05
1.04
0.975
1.04
4.18
1.04
1.005
1.03
4.30
1.05
1.05
1.02
4.50
1.05
1.095
1.02
4.70
1.05
1.14
1.01
4.95
1.06
1.20
1.01
Table VI
The correction factors for a diffusely reflecting surface, based on the magnesium oxide data of Table IV.
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