Assessment of the effect of Vacuum Ultra Violet (VUV) irradiation on the Bidirectional Reflectance Distribution Function (BRDF) of Spectralon is presented in this paper. The sample was a 99% white Spectralon calibration standard irradiated with VUV source positioned at 60o off the irradiation direction for a total of 20 hours. The BRDF before and after VUV irradiation was measured and compared at number of wavelengths in the UV, VIS and IR. Non-isotropic directional degradation of Spectralon diffuser under ionizing radiation was detected at different BRDF measurement geometries primarily at UV spectral range. The 8o directional/hemispherical reflectance of the same sample was also measured and compared from 200nm to 2500nm. Index Terms BRDF, Reflectance, Multiangular, Spectralon, Remote Sensin

Butler, J. J.

Ding, L.

Georgiev, G. T.

Kowalewski, M. G.

NASA Technical Reports Server

  
DIRECTIONAL DEGRADATION OF SPECTRALON DIFFUSER UNDER IONIZING 
RADIATION FOR CALIBRATION OF SPACE-BASED SENSORS 
 
G.T. Georgieva, J.J. Butlerb, M.G. Kowalewskic, L. Dinga 
a Sigma Space Corp., Lanham, MD 20706, e-mail: georgi.t.georgiev@nasa.gov 
b NASA Goddard Space Flight Center, Greenbelt, MD 20771 
c Universities Space Research Association, Columbia, MD 21044 
 
 
ABSTRACT 
 
 Assessment of the effect of Vacuum Ultra Violet 
(VUV) irradiation on the Bidirectional Reflectance 
Distribution Function (BRDF) of Spectralon is presented 
in this paper. The sample was a 99% white Spectralon 
calibration standard irradiated with VUV source 
positioned at 60o off the irradiation direction for a total of 
20 hours. The BRDF before and after VUV irradiation 
was measured and compared at number of wavelengths in 
the UV, VIS and IR. Non-isotropic directional 
degradation of Spectralon diffuser under ionizing 
radiation was detected at different BRDF measurement 
geometries primarily at UV spectral range. The 8o 
directional/hemispherical reflectance of the same sample 
was also measured and compared from 200nm to 
2500nm. 
Index Terms— BRDF, Reflectance, Multiangular, 
Spectralon, Remote Sensing 
 
1. INTRODUCTION 
 
 The goal of the current study is to better understand 
the non-isotropic changes of Spectralon calibration 
standards caused by ionizing VUV irradiation. Although 
there are a number of studies on the Spectralon use as a 
diffuser standard for pre-flight and on-orbit calibration, 
including polarization properties [1], long-term studies 
[2], UV stability [3] and low-level irradiation, [4] so far 
little attention has been devoted to the case of non-
isotropic degradation under ionizing radiation in 
laboratory conditions. The current study was initiated to 
address the requirements of ongoing on-orbit calibration 
of MODIS (Moderate Resolution Imaging 
Spectroradiometer), a key instrument aboard the Terra 
(EOS AM) and Aqua (EOS PM) satellites by 
experimenting with irradiating Spectralon with VUV 
radiation and we will see if we can directionally induce 
dependent reflectance degradation. The MODIS solar 
diffuser pre-flight contamination witness assembly made 
from space-grade carbon-loaded Spectralon is shown in 
Fig.1.a. 
 
2. MEASUREMENTS 
 
 The Spectralon, which is one of the trademarks of 
polytetrafluoroethylene (PTFE), is not a perfect 
Lambertian diffuser but it is a best known material used 
as a reflectance standard in the UV-VIS-NIR spectral 
range for remote sensing instrument calibration. The data 
was obtained at Goddard Space Flight Center’s 
Radiometric Calibration Facility, a secondary calibration 
facility with reflectance measurements traceable to those 
made by the Spectral Tri-function Automated Reference 
Reflectometer (STARR) facility at the National Institute 
of Standards and Technology (NIST). The sample was a 
25.4 mm diameter, 99% white Spectralon target 
fabricated by Labsphere. The 10mm tick Spectralon puck 
is shown in Fig.1.b. 
 
 
Fig.1.a: Spectralon solar diffuser pre-flight witness 
assembly 
 
 The BRDF is defined in radiometric terms as 
reflected surface radiance in a given direction divided by 
the incident surface irradiance from another direction. 
https://ntrs.nasa.gov/search.jsp?R=20120013611 2019-08-30T21:48:44+00:00Z
  
The incident irradiance is the radiant flux incident on the 
surface. The reflected surface radiance is the light flux 
reflected through solid angle Ω per projected solid angle: 
ri
r
P
P
BRDF cos
    (1) 
where Pr is the reflected radiant power, Ω is the solid 
angle determined by the area of detector aperture, A, and 
the radius from the sample to the detector, R. The solid 
angle can be computed as Ω = A/R2. Pi is the incident 
radiant power, and θr is the reflected zenith angle. 
 
 
Fig.1.b: 99% white Spectralon test diffuser 
 
 
Fig.2: Irradiation monitoring system in the vacuum chamber 
 
 The irradiation was performed in a custom built 
vacuum chamber shown in Fig.2. McPherson 632 
Deuterium Vacuum Ultra Violet (VUV) lamp was used 
for irradiation the sample 2 times by 10hours for a total 
of 20hours. One hour of irradiation time in the Lab 
corresponds to about one hour irradiation on orbit. The 
sample was tilted at 60o off the irradiation direction. The 
lamp characteristics are shown in Fig.3. The intensity of 
the lamp on the sample was monitored with monitoring 
system, which consists of preamplifier photodiode 1 – 
full range and photodiode 2 – with deposited filter. 
 
 A xenon lamp/monochromator tunable assembly with 
a well-defined incoherent illumination was used at this 
study. The sample was tested before and after irradiation 
by measuring the Bidirectional Reflectance Distribution 
Function (BRDF) at 250nm, 275nm, 300nm, 350nm, 
400nm, 500nm, 700nm and 900nm at incident angles of 
0o, 45o and 60o, and at viewing angles from 0o to 80o, step 
5o, Fig.4. The BRF results were then compared to a long-
term data set of GSFC and NIST Spectralon calibration 
measurements. The 8o directional-hemispherical 
reflectance from 250nm to 2.5 µm was also measured 
and compared pre- and post-irradiation. A xenon 
lamp/monochromator assembly with a well-defined 
incoherent illumination was used for BRDF 
measurements and Perkin-Elmer 150mm integrating 
sphere was used for 8o directional/hemispherical 
measurements. The total combined uncertainty for BRDF 
and 8o directional/hemispherical measurements is less 
than 1%. 
 
 
Fig.3: Calibration data of McPherson Deuterium lamp from 
113nm to 167nm 
 
3. RESULTS AND DISCUSSIONS 
 
 The BRDF ratio after/before irradiation with VUV 
lamp is presented for angle of incidence 45o, wavelengths 
of 250nm, 300nm, 400nm, and 700nm for 10 hours 
irradiation, Fig.3 and 20 hours irradiation wavelengths of 
250nm, 300nm, 350nm, 400nm, 500nm and 900nm, 
Fig.4. The measured data is also fitted for better 
presentation. The BRDF change shows non-isotropic 
directional degradation of Spectralon after irradiation of 
10 hours at 250nm, and 300nm and also degradation at 
250nm, 300nm and 350nm after irradiation of 20 hours. 
Although there is a small directional change in BRDF at 
longer wavelengths they are into the measurement 
uncertainty. The measured directional BRDF difference 
  
after 10 hours irradiation is -6.35% at 250nm, and -4.79% 
at 300nm. The same difference after 20 hours irradiation 
increases to -16.52% at 250nm, -9.28% at 300nm, -
3.42% at 350nm and -1.93% at 400nm. We studied also 
the directional dependence of BRDF when measured at 
angle of incidence 60o, shown on Fig.5 for wavelength of 
275nm as a BRDF ratio after/before 10 hours and 20 
hours irradiation.  
 The 8deg directional/hemispherical reflectance was 
also measured before and after 10 hours and 20 hours of 
VUV irradiation in addition to the BRDF 
characterization. The degradation of the Spectralon 
diffuser is better pronounced in the UV spectrum, but 
also in the visible spectrum after 20hours of irradiation, 
Fig.6, could be as high as 38% at 200nm and as low as 
2% at 450nm. 
 
 
Fig.3: BRDF ratio after 10hours irradiation measured at 
AOI = 45o  
 
 Currently we are considering a couple reasons for the 
directional degradation of BRDF after VUV irradiation. 
Pure geometrical – illumination path difference, 
shadowing and obscuration effects caused by the 
roughness of the sample, spatially non-uniform 
distribution of outgassing products, and in-depth 
irradiation effects are more pronounced at shorter 
wavelengths. Our initial results indicate that the 
directional degradation is not geometric effect only as the 
degradation is not increasing linearly with the time of 
irradiation. The shadowing and obscuration effects 
contribute to some degree but the initial outgassing 
process should be over by the time the measurements 
were performed. 
 
 
Fig.4: BRDF ratio after 20 hours irradiation measured at 
AOI = 45o  
 
 
Fig.5: BRDF ratio after 10 and 20hours irradiation 
measured at AOI = 60o  
 
 
Fig.6: Directional Hemispherical Reflectance ratio after 10 
and 20 hours irradiation 
  
 
4. CONCLUSIONS AND FUTURE WORK 
 
 Our current results indicate that the often overlooked 
impact of VUV irradiation on Spectralon diffusers used 
on-orbit and at pre-flight calibrations may cause a 
directional dependent degradation that may need to be 
considered as the requirements on accuracy of remote 
sensing data continue to become more stringent. The 
degree of the degradation depends on several factors as 
(i) the irradiation geometry, (ii) the intensity and time of 
the irradiation, (iii) the BRDF measurement geometry, 
and (iv) the wavelength at which the BRDF is measured. 
The Spectralon diffusers show degradation as well after 
VUV irradiation when their 8o directional/hemispherical 
reflectance is compared before and after 10 hours and 20 
hours of VUV irradiation. The results from studies such 
as the one presented here will be of interest to the remote 
sensing community both in developing sensor design 
requirements and in providing constraints for modeling 
and correction efforts of airborne and satellite-based data. 
We are expanding the current work with more irradiation 
hours to confirm the current results and explain the 
possible causes of the observed directional dependent 
degradation due to VUV irradiation. The presented data 
will improve our understanding of processes occurring 
on-orbit and their impact on the solar diffusers. The data 
of this study is also compared to our previous 
characterization efforts of Mie scattering volume diffuser 
[5] 
 
5. REFERENCES 
                                                 
[1] Haner D.A., McGuckin B.T., Bruegge C.J., “Polarization 
characteristics of Spectralon illuminated by coherent light”, 
Appl. Optics, 38, 30, 6350-6356, 1999, 
[2] Georgiev G.T., Butler J.J.: “Long-term calibration 
monitoring of Spectralon diffusers BRDF in the air-
ultraviolet”, Applied Optics, 2007, vol.46, No.32, pp.7892-
7899, 
[3] Stiegman A.E., Bruegge C.J., Springsteen A.W., 
“Ultraviolet stability and contamination analysis of 
Spectralon diffuse reflectance material,” Opt. Eng. 32, 799–
804 (1993). 
[4] W. Moeller, K.-P. Nikolaus, and A. Hoepe, 
“Degradation of the diffuse reflectance of Spectralon under 
low-level irradiation,” Metrologia 40, S212–S215 (2003). 
[5] Heath D.F., Georgiev G.T., “Characteristics of a new 
type of Mie scattering volume diffuser and its use as a 
spectral albedo calibration standard for the solar reflective 
wavelength region”, Proc. SPIE, vol. 8153, 81530V-1-14, 
2011. 


Directional Degradation of Spectralon Diffuser Under Ionizing Radiation for Calibration of Space-Based Sensors

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