Barsin, Julia A.

Markahm, Brian L.

Pedelty, Jeffrey A.

NASA Technical Reports Server

National Aeronautics and Space Administration 
www.nasa.gov 
NASA GSFC / USGS EROS 
 www.usgs.gov 
The Operational Land Imager: 
Spectral Uniformity 
Julia A. Barsi 
Science Systems and Applications, Inc. 
 
Brian L. Markham, Jeffrey A. Pedelty 
NASA/Goddard Space Flight Center 
 
 
August 23, 2011 
SPIE                      
Earth Observing 
Systems XVI 
https://ntrs.nasa.gov/search.jsp?R=20120016942 2019-08-30T23:24:08+00:00Z
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Outline 
 Spectral requirements 
 Focal plane 
Component-level testing 
Out-of-band testing 
 Instrument-level testing 
 Spectral Uniformity 
2 
OLI 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Operational Land Imager Spectral Requirements 
 Spectral band edges  
Center wavelength tolerance 
Minimum bandwidth (bands 7, 8) 
 Average in-band response 
Minimum in-band response 
 Flatness between 0.8 response points 
 Slopes of band edges (two cases) 
Non-integrated out-of-band response 
 Integrated out-of-band response 
 Spectral bandpass uniformity 
Radiometric uniformity 
 
 
3 
Band 
Number Band Name 
Center 
wavelength 
(nm) 
Bandwidth (nm) 
1 Coastal/ Aerosol 443 20 
2 Blue 482 65 
3 Green 562 75 
4 Red 655 50 
5 NIR 865 40 
6 SWIR 1 1610 100 
7 SWIR 2 2200 200 
8 PAN 590 180 
9 Cirrus 1375 30 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
OLI Focal Plane 
 Pushbroom design 
 Focal Plane Array 
 Consists of 14 modules to cover the 15-degree field of view 
 6919 detectors per multi-spectral band (~13000 for Pan band)  
 Focal Plane Module (FPM) 
 494-detector array for each multi-spectral band (988 for Pan band) 
 Silicon PIN detectors for VNIR bands, HgCdTe detectors for SWIR bands 
 Butcher-block filter assemblies cover the detector arrays 
FPM2 
FPM1 FPM3 
FPM4 
FPM5 
FPM6 
FPM7 
FPM8 
FPM9 
FPM10 
FPM11 
FPM12 
FPM13 
FPM14 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Filter Assemblies 
 Spectral filter wafers manufactured in large sheets 
– Cut into sticks for incorporation onto the OLI focal plane 
Due to yield, most bands include filter sticks from more than one wafer 
 Spectral differences between wafers will lead to spectral differences between 
modules 
 Table indicates the source wafer for the filter stick for each module. 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Component-level spectral measurements 
 Spectral measurements of each component of the optical path were taken 
before assembly 
– Witness samples of the four mirrors of the telescope 
– Witness sample of the Focal Plane Array window 
– Many samples of each detector type (VNIR and SWIR) 
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SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Component-level spectral measurements 
 Spectral measurements of each component of the optical path were taken before 
assembly 
– Nine measurements of each spectral filter wafer 
 All component-level measurements were convolved together to produce an estimated 
system-level response 
 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Module-level Out-of-Band measurements 
 At the module-level (assembled detector arrays and filter assemblies), tests were run on 
out-of-band response  
– 330-1100nm for VNIR bands 
– 800-2600nm for SWIR bands 
 OLI requirement:  No detector shall have a response greater than 0.001 at 50nm beyond 
the specified in-band wavelength (100nm for the Pan band). 
– Four detectors in the Red band did not meet this requirement. 
8 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Module-level Out-of-Band measurements 
 Limited out-of-band response could be derived from the previously show component-level 
measurements. 
 Module-level measurements revealed a spectral cross-talk feature in the SWIR bands. 
– Since the feature does not appear in the component-level measurements, the cross-
talk is likely due to internal reflections in the modules. 
– Note that, thought apparent in these plots, the feature does not fail requirements. 
9 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Instrument-level measurements 
 The assembled OLI was placed in a thermal vacuum chamber for extensive 
characterization 
 For spectral characterization, the OLI looked at a double monochromator, placed outside 
the chamber 
 60 detectors on each module were tested in every band 
10 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Instrument-level requirement:  
bandpass uniformity 
 OLI Requirement on bandpass uniformity: the variation in the full-width, half maximum 
shall be less than +/-3% across the focal plane. 
11 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Spectral Uniformity 
 Given the large number of detectors, variation in spectral response between pixels could 
result in apparent radiometric differences across the focal plane. 
 Calibration process with flat-field the detectors on the solar diffuser panel  
– But spectral structure in the reflectance of actual targets will still result is spectral 
radiance differences 
 Assess remaining spectral nonuniformity by quantifying spectral radiance differences 
after flat-fielding: 
 
 
 
 Where: the targets are vegetation and bare desert and the calibration is based on the 
solar radiance 
 
12 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Spectral Uniformity 
 OLI requirement: The variation across 100 contiguous pixels cannot be greater that 
0.25% 
 In most bands, the maximum difference in spectral radiance across module boundaries is 
0.2% 
– The Cirrus band does not meet this requirement, but the Cirrus band will not be used 
to differentiate surface types. 
 
13 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Conclusions 
 Spectral Response Function has been characterized at three different levels during 
instrument development 
 Component-level characterization: Did not reveal any issues meeting spectral 
requirements 
 Out-of-band characterization: Four detectors out of ~70000 did not meet the out-of-band 
requirement 
 Instrument-level characterization: All bands pass the bandpass uniformity requirement 
and all earth-viewing bands pass the spectral uniformity (banding) requirement. 
 
 Spectral response curves will soon be published to http://ldcm.gsfc.nasa.gov/ 
 
 
 Thanks to the team at Ball Aerospace that ran all tests and was the first to assess the 
characterization data. 
 
 
14 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Conclusions 
15 
CA Blue Green 
Red NIR SWIR1 
SWIR2 Pan Cirrus 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Instrument-level measurements 
 An assembled OLI was placed in a thermal 
vacuum chamber for extensive 
characterization 
 For spectral characterization, the OLI looked 
at a double monochromator, placed outside 
the chamber 
 60 detectors on each module were tested in 
every band 
16 
SPIE Earth Observing Systems XVI 
NASA GSFC / USGS EROS 
 
Comparison: Instrument-level RSR vs. 
Component-level RSR 
 For two specified targets, the integrated spectral radiance was calculated for each FPM 
to assess the difference between the two RSRs. 
 Spectral radiance differences are generally less than 0.5% 
 Final published RSRs will likely come from the instrument-level measurements 
– Following uniformity analysis is from instrument-level results 
17 


The Operational Land Imager: Spectral Uniformity

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