Abstract

Laboratory measurements were performed to characterize the geometrical effects in the calibration of the NASA's cloud absorption radiometer (CAR). The measurements involved three integrating sphere sources (ISSs) operated at different light levels and experimental setups to determine radiance variability. The radiance gradients across the three ISS apertures were 0.2%–2.6% for different visible, near-infrared, and shortwave infrared illumination levels but <15% in the UV. Change in radiance with distance was determined to be 2%–20%, being highest in the UV. Radiance variability due to the edge effects was found to be significant; as much as 70% due to the sphere aperture and <10% due to the CAR telescope's secondary mirror.

© 2007 Optical Society of America

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References

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2003 (1)

C. K. Gatebe, M. D. King, S. Platnick, G. T. Arnold, E. F. Vermote, and B. Schmid, "Airborne spectral measurements of surface-atmosphere anisotropy for several surfaces and ecosystems over southern Africa," J. Geophys. Res. 108, doi: (2003).
[CrossRef]

2001 (1)

H. Yoon and C. Gibson, "Understanding your calibration sources is the key to making accurate spectroradiometric measurements," OE Mag. 48, (2001).

1998 (3)

B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smirnov, "AERONET - a federated instrument network and data archive for aerosol characterization," Remote Sens. Environ. 66, 1-16 (1998).
[CrossRef]

W. L. Barnes, T. S. Pagano, and V. V. Salomonson, "Prelaunch characteristics of the moderate resolution imaging spectroradiometer (MODIS) on EOS-AM1," IEEE Trans. Geosci. Remote Sens. 36, 1088-1100 (1998).
[CrossRef]

C. J. Bruegge, V. G. Duval, N. L. Chrien, R. P. Korechoff, B. J. Gaitley, and E. B. Hochberg, "MISR prelaunch instrument calibration and characterization results," IEEE Trans. Geosci. Remote Sens. 36, 1186-1198 (1998).
[CrossRef]

1997 (1)

K. Thome, B. Markham, J. Barker, P. Slater, and S. Biggar, "Radiometric calibration of Landsat," Photogramm. Eng. Remote Sens. 63, 853-858 (1997).

1996 (1)

M. D. King, W. P. Menzel, P. S. Grant, J. S. Myers, G. T. Arnold, S. E. Platnick, L. E. Gumley, S.-C. Tsay, C. C. Moeller, M. Fitzgerald, K. S. Brown, and F. G. Osterwisch," Airborne scanning spectrometer for remote sensing of cloud, aerosol, water vapor, and surface properties," J. Atmos. Ocean. Technol. 13, 777-794 (1996).
[CrossRef]

1995 (1)

S. Janz, E. Hilsenrath, J. Butler, D. F. Heath, and R. P. Cebula, "Uncertainties in radiance calibrations of backscatter ultraviolet (BUV) instruments," Metrologia 32, 637-641, (1995/96).
[CrossRef]

1991 (1)

J. H. Walker, C. L. Cromer, and J. T. McLean, "A technique for improving the calibration of large-area sphere sources," Proc. SPIE 1498, 224-230 (1991).
[CrossRef]

1987 (1)

C. R. N. Rao, "Pre-launch calibration of channels 1 and 2 of the advanced very high resolution radiometers, NOAA technical report," NESDIS 36 (1987).

1986 (1)

M. D. King, M. G. Strange, P. Leone, and L. R. Blaine, "Multiwavelength scanning radiometer for airborne measurements of scattered radiation within clouds," J. Atmos. Ocean. Technol. 3, 513-522 (1986).
[CrossRef]

1983 (1)

1965 (1)

"Observations from the Nimbus I meteorological satellite," NASA Spec. Publ. 89 (Goddard Space Flight Center, Greenbelt, MD, 1965), 90 pp.

1963 (1)

F. E. Nicodemus, "Radiance," Am. J. Phys. 31, 368-377 (1963).
[CrossRef]

1931 (1)

1892 (1)

W. E. Sumpner, "The diffusion of light," Proc. Phys. Soc. London 12, 10 (1892).
[CrossRef]

Am. J. Phys. (1)

F. E. Nicodemus, "Radiance," Am. J. Phys. 31, 368-377 (1963).
[CrossRef]

Appl. Opt. (1)

IEEE Trans. Geosci. Remote Sens. (2)

W. L. Barnes, T. S. Pagano, and V. V. Salomonson, "Prelaunch characteristics of the moderate resolution imaging spectroradiometer (MODIS) on EOS-AM1," IEEE Trans. Geosci. Remote Sens. 36, 1088-1100 (1998).
[CrossRef]

C. J. Bruegge, V. G. Duval, N. L. Chrien, R. P. Korechoff, B. J. Gaitley, and E. B. Hochberg, "MISR prelaunch instrument calibration and characterization results," IEEE Trans. Geosci. Remote Sens. 36, 1186-1198 (1998).
[CrossRef]

J. Atmos. Ocean. Technol. (2)

M. D. King, W. P. Menzel, P. S. Grant, J. S. Myers, G. T. Arnold, S. E. Platnick, L. E. Gumley, S.-C. Tsay, C. C. Moeller, M. Fitzgerald, K. S. Brown, and F. G. Osterwisch," Airborne scanning spectrometer for remote sensing of cloud, aerosol, water vapor, and surface properties," J. Atmos. Ocean. Technol. 13, 777-794 (1996).
[CrossRef]

M. D. King, M. G. Strange, P. Leone, and L. R. Blaine, "Multiwavelength scanning radiometer for airborne measurements of scattered radiation within clouds," J. Atmos. Ocean. Technol. 3, 513-522 (1986).
[CrossRef]

J. Geophys. Res. (1)

C. K. Gatebe, M. D. King, S. Platnick, G. T. Arnold, E. F. Vermote, and B. Schmid, "Airborne spectral measurements of surface-atmosphere anisotropy for several surfaces and ecosystems over southern Africa," J. Geophys. Res. 108, doi: (2003).
[CrossRef]

J. Opt. Soc. Am. (1)

Metrologia (1)

S. Janz, E. Hilsenrath, J. Butler, D. F. Heath, and R. P. Cebula, "Uncertainties in radiance calibrations of backscatter ultraviolet (BUV) instruments," Metrologia 32, 637-641, (1995/96).
[CrossRef]

NASA Spec. Publ. 89 (1)

"Observations from the Nimbus I meteorological satellite," NASA Spec. Publ. 89 (Goddard Space Flight Center, Greenbelt, MD, 1965), 90 pp.

OE Mag. (1)

H. Yoon and C. Gibson, "Understanding your calibration sources is the key to making accurate spectroradiometric measurements," OE Mag. 48, (2001).

Photogramm. Eng. Remote Sens. (1)

K. Thome, B. Markham, J. Barker, P. Slater, and S. Biggar, "Radiometric calibration of Landsat," Photogramm. Eng. Remote Sens. 63, 853-858 (1997).

Proc. Phys. Soc. London (1)

W. E. Sumpner, "The diffusion of light," Proc. Phys. Soc. London 12, 10 (1892).
[CrossRef]

Proc. SPIE (1)

J. H. Walker, C. L. Cromer, and J. T. McLean, "A technique for improving the calibration of large-area sphere sources," Proc. SPIE 1498, 224-230 (1991).
[CrossRef]

Remote Sens. Environ. (1)

B. N. Holben, T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak, and A. Smirnov, "AERONET - a federated instrument network and data archive for aerosol characterization," Remote Sens. Environ. 66, 1-16 (1998).
[CrossRef]

Other (6)

J. W. T. Walsh, Photometry (Constable, 1958), p. 258.

http://daac.gsfc.nasa.gov/guides/GSFC/guide/CZCS_Sensor.gd.shtml.

C. R. N. Rao, "Pre-launch calibration of channels 1 and 2 of the advanced very high resolution radiometers, NOAA technical report," NESDIS 36 (1987).

Cloud Absorption Radiometer: http://car.gsfc.nasa.gov/subpages/index.php?section=Instrument&content=Schematics.

S. Brown, NIST, Gaithersburg, MD (personal communication, 2007).

E. W. Weisstein, "Circle-Circle Intersection, Equation 14," A Wolfram Web Resource. http://Mathworld.wolfram.com/Circle-CircleIntersection.html.

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Figures (13)

Fig. 1
Fig. 1

Schematic of the CAR Optical System. The CAR has 14 narrow spectral bands between 0.34 and 2.30 μ m , six of which ( 1.5 2.3 μ m ) are defined on the filter wheel and share one detector, while the rest have separate detectors as shown on the diagram. The CAR scans through a 190° plane with an IFOV of 1 ° . Full illustration of the CAR instrument can be seen in King et al. [4] and Gatebe et al. [5].

Fig. 2
Fig. 2

Pictures showing setup for Slick sphere and CAR for determining (a) responsivity across the scan range, and (b) change in radiance with distance of separation, with the CAR at the closest distance from the Slick sphere.

Fig. 3
Fig. 3

(Color online) Spectral radiance output from the Hardy sphere at six-lamp level as measured by the CAR at 0.5° intervals across the 190° scan range, averaged over 360 scans. The signal is relatively flat at the peak as Hardy sphere fills the IFOV.

Fig. 4
Fig. 4

(a) Radiance measured in channel 4 ( 0.682 μ m ) across the three ISS apertures and normalized to their maximum peak radiance. The three ISSs were operated at full power with the Hardy sphere and Slick sphere internally illuminated by 16 lamps and the Uncle sphere by four lamps. (b) Relative response at 342   nm normal to the Uncle sphere exit aperture plane viewed with Ocean Optics mini-spectrometers with a 2° FOV. The sphere was illuminated at full power (four-lamp level). (c) Relative response at 382   nm normal to the Uncle sphere exit aperture plane viewed with Ocean Optics mini-spectrometers with a 2° FOV. The sphere was illuminated at full power (four-lamp level).

Fig. 5
Fig. 5

(Color online) CAR responsivity within the 190° scan range with Slick sphere internally illuminated by four lamps. Each point represents maximum peak radiance at a given scan angle, normalized to the maximum peak radiance from within the 190° scan range in each channel. The UV channels (channels 1 and 2) show the highest sensitivity (4–6%), whereas channels 3–8 have lowest sensitivity ( < 1 % ) .

Fig. 6
Fig. 6

(Color online) (a) Peak radiance as a function of distance in channel 1 ( 0.340 μ m ) with the Slick sphere illuminated by four lamps. The peak radiance initially decreases and then increases beyond 43 cm. The difference between maximum peak radiance at d = 23   cm and d = 490   cm is 12.3%. Similar results were seen in channel 2 (cf. Table 4). (b) Peak radiance as a function of distance in channel 3 ( 0.472 μ m ) with Slick sphere illuminated by four lamps. The peak radiance decreases with distance. The difference between maximum peak radiance at d = 23   cm and d = 490   cm is 6.7%. Similar results were seen in channels 4 to 8 (cf. Table 4).

Fig. 7
Fig. 7

(Color online) Average radiance as a function of distance with the Slick sphere internally illuminated by four lamps, normalized to peak maximum radiance in each channel at each distance. The radiance in channels 3–8 varies slightly with distance, but show spectral dispersion at the longest distance, d N = 490   cm . Channels 1 and 2 appear different with the smallest value at d = 206   cm . (cf. Table 4).

Fig. 8
Fig. 8

(Color online) The Hardy sphere radiance (SR curve; from GSFC's radiometric calibration) and CAR relative spectral response function (RSP curve; from previous measurements as described in Gatebe et al. 2003) in channel 3 (central wavelength at 0.472 μ m ). A convolution of SR and RSP is obtained from a product of sphere radiance (SR curve) and relative spectral response (RSP curve). The integral of the product (combined SR and RSP curve) divided by the integral of the RSP curve gives the band weighted sphere radiance, which is equivalent to the CAR digital counts in channel 3. This procedure is repeated with different sphere output, usually spanning the entire detector dynamic range. A plot of the derived radiance versus corresponding digital counts gives a linear relationship whose coefficients define CAR responsivity and offset. These coefficients are used to convert instrument digital count to radiance as described in Section D.

Fig. 9
Fig. 9

(Color online) Standard deviation derived from the Hardy sphere measurements shown in Fig. 3. The standard deviation peaks correspond to when the Hardy sphere aperture enters or leaves the CAR IFOV. The noise pattern is thought to be unique to scanning radiometers as an ISS aperture moves in or out of the IFOV.

Fig. 10
Fig. 10

(Color online) Mean spectral radiance in channel 3 and its standard deviation. Notice the peaks coincide with the sloping areas of the radiance curve.

Fig. 11
Fig. 11

(Color online) Standard deviation curves from measurements made at selected distances between the CAR and Slick sphere to show the noise pattern dependence on distance. As the Slick sphere becomes more distant, the standard deviation peaks reduces from four to two, whereas the magnitude increases.

Fig. 12
Fig. 12

(Color online) Radiance simulation from a convolution of CAR IFOV (shaded circle) and sphere aperture (open circle). The area of the sphere aperture that is inside the IFOV (area of overlap) plotted as a function of the distances between the centers of both circles, representing the signal strength. Random variations in the distance between the centers of the circles seem to introduce higher standard deviation (solid curve with + symbol) over the area of partial overlap of the two circles; an effect that is observed in our measurements. (N.B.: size of apertures not drawn to scale.)

Fig. 13
Fig. 13

(Color online) Radiance simulation from a convolution of CAR IFOV (shaded circle with grayish central circle denoting a hollow portion—to denote the CAR telescope's secondary mirror) and sphere aperture (open circle). The area of the sphere aperture that is inside the IFOV (area of overlap) is plotted as a function of the distances between the centers of both circles, thereby defining more accurately the signal strength observed in Fig. 10. Random variations in the distances between the centers of the circles introduce a higher standard deviation (solid curve with + symbol) over the area of partial overlap of the two circles, and has peaks similar to those observed in Fig. 9 from real measurements. (N.B.: size of apertures not drawn to scale.)

Tables (5)

Tables Icon

Table 1 Current Cloud Absorption Radiometer and ISS Specifications

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Table 2 Spatial Gradient of Radiance at Points-Across the Hardy Sphere Aperture (in percent) a

Tables Icon

Table 3 Spatial Gradient of Radiance at Points Across Apertures of the Uncle Sphere and Slick Sphere (in percent)

Tables Icon

Table 4 Difference Between Maximum Peak Radiance at Distance d 1 and d N Relative to Radiance at d 1 (Slick Sphere d 1 = 23 cm and d N = 490 cm, and Uncle Sphere d 1 = 30 cm, d N = 421 cm) a

Tables Icon

Table 5 Variation of Standard Deviation Peaks Relative to their Average Radiance as Function of Distance (%)

Equations (1)

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A = r 2 cos 1 ( d 2 + r 2 R 2 2 d r ) + R 2 cos 1 ( d 2 + R 2 r 2 2 d R ) 1 2 ( d + r + R ) ( d + r R ) ( d r + R ) ( d + r + R ) ,

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