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Frequently Asked Questions

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Question:
How do you convert radiance to reflectance?

Answer:
For relatively clear Hyperion and ALI scenes, a reduction in between-scene variability can be achieved through a normalization for solar irradiance by converting spectral radiance to planetary reflectance or albedo. This combined surface and atmospheric reflectance of the Earth is computed with the following formula:

reflectance

Where

	=	Unitless planetary reflectance
	=	Spectral radiance at the sensor's aperture
	=	Earth-Sun distance in astronomical units from nautical handbook or interpolated from values listed in Table 1
	=	Mean solar exoatmospheric irradiances ALI solar irradiances listed in Table 2 Hyperion solar irradiances listed in Table 3
	=	Solar zenith angle in degrees

Julian Day	Distance	Julian Day	Distance	Julian Day	Distance	Julian Day	Distance	Julian Day	Distance
Table 1 Earth-Sun Distance in Astronomical Units
1	.9832	74	.9945	152	1.0140	227	1.0128	305	.9925
15	.9836	91	.9993	166	1.0158	242	1.0092	319	.9892
32	.9853	106	1.0033	182	1.0167	258	1.0057	335	.9860
46	.9878	121	1.0076	196	1.0165	274	1.0011	349	.9843
60	.9909	135	1.0109	213	1.0149	288	.9972	365	.9833

ESUN_λ units are in W/(m².µm)
Table 2: ALI Mean Solar Exoatmospheric Irradiances (ESUN)
Band	E0-1 ALI
b1	1967.6
b1p	1851.8
b2	1837.2
b3	1551.47
b4	1164.53
b4p	957.46
b5	230.03
b5p	451.37
b7	79.61
b8	1747.86

ESUN_λ units are in W/(m².µm)
Table 3: Hyperion Mean Solar Exoatmospheric Irradiances (ESUN)
Download hyperion irradiance band information.