The Spectral Response of the Landsat-8 Operational Land Imager

2.1. Component Level Measurements

The spectral response of each component in the OLI optical path was measured independently: the transmission of each filter wafer, the reflectance of the four mirrors, the response of the detectors, and the transmission of the focal plane window. The relative responses of all of the components other than the spectral filters are shown in Figure 2.

The spectral transmission of each filter wafer was measured at nine positions by the filter provider, Barr Associates (now part of Materion). Measurements were made at ambient temperature and with collimated light; models were used to predict the thermal and angular shifts when used in the instrument. The reflectance of each mirror’s witness sample was measured at three positions by Ball Aerospace using a Cary 5000 spectrometer. Relative spectral responses of 6 VNIR witness detectors and 32 SWIR witness detectors were measured in vacuum at 210 K, by the detector provider, Raytheon Vision Systems (RVS). The spectral transmission of a witness sample of the focal plane assembly window was measured at five positions by Sonoma Photonics. A system-level response for each module of each band was estimated by averaging all the measurements of each component and combining them (Figure 3). The measurements of each component covered the spectral range of 350–1100 nm for the VNIR bands and 800–2600 nm in the SWIR bands. While not a complete out-of-band assessment, this range allowed for the initial inspection of the out-of-band response (the response to radiance outside of the prescribed spectral wavelength range) (Figure 4).

The predicted system-level response based on the component measurements indicated that the out of band response was low enough and that the spectral response across the focal plane was uniform enough to proceed with the development of the instrument.

2.2. Out-of-Band Measurements

The OLI out-of-band responses were measured at the FPM level, when the detectors and spectral filters were mated. At this level of integration, the optical components controlling the out-of-band response are measured directly and the smaller contributors, the telescope optics and focal plane window (Figure 2), can be analytically combined with little addition to the uncertainty. The telescope mirrors are highly reflective (total > 90%) and essentially flat from 450–2500 nm and have almost no impact on the out-of-band response across that range. Below 450 nm, the mirror reflectance drops off. The Focal Plane Array (FPA) window is flat and highly transmissive from 420–700 nm, 820–900 nm and 1300–2400 nm. Its transmission is highly structured below 420 nm. The window transmission combined with the mirrors’ reflectance gives reduced sensitivity and the common structure to all bands below 420 nm. The FPA window also contributes to the reduced response between 900 and 1300 nm.

To measure the out-of-band response, each flight FPM was placed in an evacuated Dewar and cooled to the nominal operational temperature (210 K) (Figure 5). The FPM was illuminated through a window in the chamber with monochromatic light. A single monochromator with appropriate order sorting filters illuminated by a tungsten halogen lamp provided the light. The light exiting the monochromator was set at a distance to flood illuminate the FPM through a baffle and diffuser at the correct f-number. The VNIR bands were measured from 330–1100 nm at 10 nm increments (both sampling and bandwidth) for the out-of-band spectral regions. A calibrated silicon detector provided the reference for the monochromator output. The SWIR bands were measured from 800–2700 nm at 20 nm increments (both sampling and bandwidth) for the out-of-band regions. In-band responses were also measured at 2 nm increments for the VNIR and 4 nm increments for the SWIR. The in-band measurements allowed for normalization of the out-of-band response to the system level in-band response. Calibrated germanium and lead-sulfide detectors provided the references for the SWIR measurements. The resulting response for each detector was corrected for the transmission of the Dewar window and the variation in the monochromator output with wavelength and time using the reference detectors, and normalized to the peak in-band response. The responses were averaged across the full FPM and then across all 14 flight FPMs. To predict the full instrument out-of-band response, the FPM measurements were multiplied by the reflectance of the telescope mirrors and the transmission of the Focal Plane Window (Figure 2).

The band average out-of-band responses based on FPM-level measurements are shown in Figure 6. The data for the individual band average responses, the component level out-of-band predictions and the variability between the 14 FPMs are available in the spreadsheet posted at http://landsat.gsfc.nasa.gov/?p=8829.

There are features in the data (Figure 6) that result from the test conditions as opposed to the instrument spectral response. These features are detailed below. In general, the FPM-level measurements should be considered an upper bound for the true out-of-band response and the component-level roll-up a lower bound (Figure 6).

(1)

Order-sorting filter effects: For the VNIR bands, the order-sorting filters were changed between 450 and 460 nm, 590 and 600 nm and 940 and 950 nm. The common discontinuities in the apparent out-of-band responses at these wavelengths are due to these filter changes, e.g., Green, Red, NIR and Pan between 450 and 460 nm; CA and Blue between 590 and 600 nm and Green, Red and Pan between 940 and 950 nm. The order-sorting filters inserted for measurements at 450 nm and below also blocked light above 500 nm (as well as below 250 nm). This significantly reduced the apparent out-of-band response due to spectral stray light in the single monochromator set up for bands with the band pass above 500 nm. Similarly, the order-sorting filter, inserted for measurements at 950 nm and above, filters out light below 705 nm causing bands with bandpasses below this wavelength to show reduced response (particularly green, red and pan). For the SWIR bands, the order sorting filter effects are not as visually evident, with the exception of between 1740 and 1760 nm in the Cirrus band.

(2)

In-band to out-of-band measurement effects: As indicated, the in-band measurements were taken at a finer spectral resolution than the out-of-band ones. The in-band and out-of-band data sets are merged together at approximately the 1% response points. Due to the differences in the bandwidths, these measurements do not always merge smoothly, producing discontinuities in the apparent resulting response.

All three SWIR bands in Figure 6 show higher out-of-band response in the spectral ranges corresponding to the other two SWIR bands regions than in surrounding spectral regions. For example, the SWIR1 band response (Figure 7b) approaches 1e−3 in the range corresponding to SWIR2, compared to about 1e−4 in surrounding regions. Additional pre-launch testing and analysis indicated that this was crosstalk between the bands, most likely optical crosstalk within the detector material. Subsequent post-launch imagery and analysis discussed later in this paper also showed the crosstalk.

For the OLI pushbroom architecture, there are significant differences in the impact of out-of-band response due to crosstalk as opposed to filter out-of-band response, when the sensor is viewing a non-uniform scene. This is because at any instant different bands are viewing different regions of the scene. Thus out-of-band response originating from crosstalk is both out-of-field as well as out-of-band, whereas out-of-band signal due to imperfect filter cutoffs is purely out-of-band.

2.3. Instrument Level Measurements

After the OLI was assembled, it was characterized in a thermal vacuum chamber. The spectral characterization was performed using a double monochromator located outside the chamber (Figure 8a). A tungsten halogen lamp illuminated the input slit of a double monochromator. At the output slit of the monochromator a beam splitter sent part of the beam to a monitor detector and part through a collimator and a window in the thermal vacuum chamber to the OLI (Figure 8b). The OLI was pointed using ground support equipment so that the collimated beam covered 16 different locations for each band, one position at the center of each FPM and one location each at the ends of the two extreme cross-field FPMs. The size of the beam was such that there was sufficient signal to characterize approximately 60 detectors at each location. At each location that OLI data were collected, the monochromator stepped through the OLI spectral bandpass at the wavelength intervals given in Table 3. The characterization was performed across a fixed wavelength range for each band that was designed to achieve responses down to at least the 0.005 relative spectral response point. Each OLI detector’s digital response was offset corrected, normalized for temporally and spectrally dependent variations in the illuminating radiance, and adjusted for the transmission of the path optics:

The FPM-average relative spectral responses of sample bands are shown in Figure 9, along with the uncertainty in the measurement. Repeatability measurements were made on the center 60 detectors of a single module and used to estimate uncertainty of the response. The differences between the FPMs’ responses are primarily due to the spectral differences between the source wafers. The Red band is shown as an illustration of a band where all the filter sticks were cut from the same filter wafer while the CA filters come from two wafers.

During this test, estimates of the in-band spectral characteristics were made: spectral band edges, center wavelength, average response, minimum response, and bandpass uniformity (Table 4). The bandpass uniformity is the difference between the per-detector full-width, half-maximum band edges. A sample of the bandpass uniformities is shown in Figure 10.

The band average results of this instrument level test are being provided as the official relative spectral response (RSR) of OLI. These are available on the Landsat-8 web site at http://landsat.gsfc.nasa.gov/?p=5779.

4.1. On-Orbit Evidence of Spectral Crosstalk

The Cirrus band is unique on OLI in that in many cases the in-band signal is very weak when there are strong VNIR and SWIR in-band signals. The Cirrus signal, when present, most often comes from clouds that appear bright on the dark atmosphere background. As such, it is the band where out-of-band response is likely to be most apparent, i.e., the radiance in the out-of-band regions is often much higher than the in-band radiance. Prelaunch measurements indicated some spectral crosstalk (Figure 6) and early on-orbit observations revealed that the Earth surface was sometimes weakly visible in the Cirrus band where it was not expected, i.e., under moist atmospheric conditions.

Closer investigation revealed that this surface visible in the Cirrus band was often misaligned from where the surface, if present, would occur in the Cirrus band (Figure 13). In particular, at land-water boundaries (Figure 13a), the odd and even FPMs in the geometrically corrected Cirrus band appear to be misaligned. Recognizing that this signal could be crosstalk, the Cirrus band images were reprocessed and treated geometrically as if the signal was coming from the location of the SWIR1 band. As shown in Figure 13b, the alignment was corrected. This is clear evidence that a good portion of Cirrus band signal over the land in this case is crosstalk from the SWIR1 band. The profiles show that the land-water contrast is about 0.1 W/m² sr μm in the Cirrus band and about 31 W/m² sr μm in the SWIR1 band. When converted to reflectance units, the effect is that about 0.2% of the SWIR1 signal leaks into the Cirrus band, which is roughly consistent with the pre-launch estimates of Cirrus out-of-band response shown in Figure 6.

Observations of Cirrus images in very dry atmospheres indicate a surface signal that is properly aligned (Figure 14) and is therefore in-band. In Figure 14, the ice is approximately 1.4 W/m² sr μm and the island is approximately 0.4 W/m² sr μm at the top of the atmosphere, both brighter than the water and land (0.05 and 0.15 W/m² sr μm, respectively) in Figure 13, when the crosstalk was apparent.

4.2. Simulation of Out-of-Band Response Contribution

In order to confirm that the pre-launch measured spectral response is consistent with the observed on-orbit crosstalk and to look for other cases of potential crosstalk that may not be readily visible in the imagery, simulations were performed using MODTRAN [5]. The full out-of-band RSR was derived from the FPM-level measurements combined with the OLI optical component measurements (Figure 4). A selection of atmospheres with a range of water content was processed through MODTRAN Version 4.1.1 (Table 6); three were standard atmospheres, the fourth was based on meteorological reanalysis data. A cirrus cloud was added within MODTRAN to one of the standard atmospheres, in order to quantify the effect of cirrus in the atmosphere. Four different surface targets out of the MODTRAN spectral library were run through each of the atmospheres (Figure 15). Most other MODTRAN options were set to the default except for the solar zenith angle, which was 45° in all cases. Integrated spectral radiances for the Cirrus band were calculated based on the MODTRAN output:

The SWIR crosstalk radiances (L_λ,SWIR1 and L_λ,SWIR2) were calculated with SWIR in-band wavelength ranges (Table 7) and the remaining out-of-band radiance (L_λ,OOB) is calculated from the other contributions.

The spectral radiances for the components contributing to the total signal are shown in Figure 16. The simulation results were generally consistent with the on-orbit observations. In typical, cirrus-free atmospheres, as in both of the mid-latitude cases, there is very little in-band signal (green bars) reaching the sensor from the ground; even over bright targets, at least half of the in-band radiance is from the atmosphere (blue bars). For the vegetated and desert surfaces, 50% of the total radiance is from out-of-band and most is crosstalk from the SWIR1 band (red bars). Over the dark ocean there is little out-of-band radiance as the water is very dark in the other SWIR bands and over 90% of the in-band radiance is due to the atmosphere.

In very dry, cirrus-free atmospheres, as in the Greenland case, the in-band response over land increases due to reflected light from the surface reaching the sensor. Though there is still a cross talk component, it is less than 5% of the total response. It is important to note that under some conditions (water vapor ≤ 0.5 cm) the Cirrus band will see the ground.

In the case of an atmosphere containing a cirrus cloud, the in-band radiance is primarily from the atmospheric path; the in-band radiance from the ground contributes less than 0.5% of the in-band signal. The SWIR1 crosstalk is less than 2% of the total signal even over targets that are bright in the SWIR bands.

This crosstalk should have little effect for the user except that in operational processing, the cirrus cloud detection algorithm is a simple threshold algorithm; if the cirrus reflectance is greater than 0.02, then a pixel is flagged as cirrus in the quality band. The weak crosstalk could make an otherwise non-cirrus pixel get flagged as a cirrus pixel by pushing it over the threshold. Thus, the cirrus cloud mask will flag more cirrus clouds over targets bright in the SWIR1 band (e.g., soil and vegetation) than over targets dim in the SWIR1 (e.g., water and snow).

Similar crosstalk analysis was done for the SWIR1 and SWIR2 bands. The in-band signal for all cases is greater than 99% of the total SWIR1 signal and greater than 99.9% of the total SWIR2 signal. The contribution from crosstalk to the SWIR1 band is approximately 0.15%, though other out-of-band response reached as much as 0.5%.