NASA Technical Memorandum 104566, Vol. 23

SeaWiFS Technical Report Series

Stanford B. Hooker, Elaine R. Firestone, and James G. Acker, Editors

Volume 23, SeaWiFS Prelaunch Radiometric

Calibration and Spectral Characterization

Robert A. Barnes, Alan W. Holmes, William L. Barnes,

Wayne E. Esaias, Charles R. McClain, and Tomas Svitek

October 1994
NASA Technical Memorandum 104566, Vol. 23

SeaWiFS Technical Report Series

Stanford B. Hooker, Editor

NASA Goddard Space Flight Center, Greenbelt, Maryland

Elaine R. Firestone, Technical Editor

General Sciences Corporation, Laurel, Maryland

James G. Acker, Technical Editor

Hughes STX, Lanham, Maryland

Volume 23, SeaWiFS Prelaunch Radiometric

Calibration and Spectral Characterization

Robert A. Barnes

ManTech, Inc.

Wallops Island, Virginia

Alan W. Homes

Hughes Santa Barbara Research Center

Santa Barbara, California

William L. Barnes

Wayne E. Esaias

Charles R. McClain

NASA Goddard Space Flight Center, Greenbelt, Maryland

Tomas Svitek

Orbital Sciences Corporation

Dulles, Virginia

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

1994
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Abstract

Based on the operating characteristics of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), calibration

equations have been developed that allow conversion of the counts from the radiometer into Earth-exiting radiances. These radiances are the geophysical properties the instrument has been designed to measure. SeaWiFS

uses bilinear gains to allow high sensitivitymeasurements of ocean-leaving radiances and low sensitivitymeasurements of radiances from clouds, which are much brighter than the ocean. The calculation of these bilinear

gains is central to the calibration equations. Several other factors within these equations are also included.

Among these are the spectral responses of the eight SeaWiFS bands. A band’s spectral response includes the

abilityof the band to isolate a portion of the electromagnetic spectrum and the amount of light that lies outside

of that region. The latter is termed out-of-band response. In the calibration procedure, some of the counts from

the instrument are produced byradiance in the out-of-band region. The number of those counts for each band is

a function of the spectral shape of the source. For the SeaWiFS calibration equations, the out-of-band responses

are converted from those for the laboratorysource into those for a source with the spectral shape of solar flux.

The solar flux, unlike the laboratorycalibration, approximates the spectral shape of the Earth-exiting radiance

from the oceans. This conversion modifies the results from the laboratoryradiometric calibration by1–4%,

depending on the band. These and other factors in the SeaWiFS calibration equations are presented here, both

for users of the SeaWiFS data set and for researchers making ground-based radiance measurements in support

of SeaWiFS.

1. INTRODUCTION

In addition to its role as an ocean color experiment, the

Sea-viewing Wide Field-of-view Sensor (SeaWiFS) serves

as a satellite procurement experiment for the National

Aeronautics and Space Administration (NASA). In a standard procurement, NASA provides the instrument builder

with a detailed specification for the design of the sensor.

In this procedure, NASA also maintains detailed control

over the construction of that instrument. The builder provides NASA with the satellite sensor on a cost-plus basis.

NASA eventuallyobtains the specified instrument, but at

a price. The price includes a substantial supervisoryoverhead bythe agency, a substantial documentation overhead

on the builder showing compliance with the detailed NASA

specification, and often a substantial increase in the cost

of the instrument as the specifications from NASA change

during the construction of the instrument. This process

is the price that must be paid for the construction of new

satellite sensors which expand the definition of state of the

art.

SeaWiFS does not break radicallynew ground in sensor design. The necessaryimprovements in the SeaWiFS

measurements, over those of its predecessor—the Coastal

Zone Color Scanner (CZCS)—are straightforward technically(Barnes and Holmes 1993 and Hooker et al. 1993).

From NASA’s point of view, SeaWiFS is considered an

improved replacement instrument for its predecessor. Such

an instrument does not require the financial and supervisoryoverheads of a standard NASA procurement. These

overhead items have been replaced with a well defined and

well scrubbed set of performance specifications.

For the SeaWiFS Project, NASA is procuring data,

not a specific instrument. NASA has entered a contractual agreement with Orbital Sciences Corporation (OSC)

to obtain, at a fixed price, an ocean color data set. SeaWiFS is a data buy. OSC has entered into an agreement

with the Hughes/Santa Barbara Research Center (SBRC)

to build, as a subcontractor, the satellite sensor required

to provide this data set. In this arrangement, SBRC has

the freedom to design an instrument to meet the predetermined set of performance specifications. The actual design

of the instrument has been left to SBRC.

Based on SBRC’s design of the SeaWiFS radiometer,

it has been possible to develop a set of radiometric calibration equations for the sensor. These equations, and the

philosophybehind them, are presented below. The equations are presented in a form that allow for their update

as relative changes in the instrument’s radiometric sensitivityare detected during on-orbit operation. In addition,

the set of prelaunch radiometric calibration coefficients is

presented.

This report also presents the spectral response of the

eight SeaWiFS bands, using laboratorymeasurements from

SBRC. On orbit, there is no method for determining spectral shifts in the instrument’s response. The primarylaboratorymethod of determining the spectral response of the

SeaWiFS bands has been accomplished through measurements of individual parts in the instrument’s optical train.

In addition, system level response measurements have been

made using a monochromatic light source. The system

level measurements are used as a confirmation to the piece

part measurements, since the system level apparatus does

1
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

not have the sensitivityto measure the out-of-band responses at wavelengths where the response is extremelysmall.

The piece part results are presented here.

2. INSTRUMENT DESCRIPTION

SeaStar is being built byOSC and is currentlyscheduled to be placed into orbit byan OSC stretch-Pegasus

booster in 1995. The SeaWiFS radiometer is shown mounted on the SeaStar satellite in Fig. 1. The instrument is

located on the top spacecraft shelf between the three antennas. SeaWiFS is the onlyresearch instrument that will

be carried bySeaStar. SeaWiFS consists of a scanner and

an electronics module. The scanner, which contains the

optics, detectors, preamplifiers, and scan mechanisms, is

located on the nadir face of the instrument shelf. The electronics module, which contains signal conditioning, command and telemetry, and power supply components, is located directlyopposite the scanner on the inside surface of

the instrument shelf. The total weight of the instrument

is about 110 pounds.

SeaWiFS is designed to measure Earth-exiting radiances. The sensor’s instantaneous field-of-view (IFOV) is

1.6 by1.6 mrad per pixel, with one scan covering +58.3◦

about nadir. The SeaWiFS scanner can be tilted to +20,

0, or −20◦ relative to nadir to minimize the number of

glint contaminated measurements in the data set. Each

pixel value is digitized to 10 bit accuracy, with a typical

scene producing about 600 counts with about one count of

noise.

The SeaWiFS scanner is illustrated in Fig. 2. Light

first strikes the primarymirror, an off-axis parabola, and

then is reflected from a second surface polarization scrambler and the half angle mirror before reaching the field

stop. The half angle mirror removes the rotation of the

image from the scan of the telescope. The mirror rotates

at exactlyhalf the rate of the telescope and polarization

scrambler, and it uses alternating mirror sides on successive telescope scans. The field stop is actually50% larger

than the detectors, and it restricts straylight through the

system. After the field stop, the light is collimated by

another off-axis paraboloid and directed to the aft optics

assembly. Dichroic beam splitters divert the light into four

focal plane assemblies, each containing two spectral bands

delineated bynarrowband filters in close proximityto the

detector. The optical paths in the aft optics assemblyare

shown in Fig. 3.

Attention in the design of SeaWiFS has been given to

minimizing the sensitivityof the instrument to polarized

light. This consideration is the principal reason for splitting the telescope into two sections, with each rotating

at a different speed. This design minimizes the incidence

angle of light on the mirrors. There are other possible instrument designs which reduce this mechanical complexity,

but theyrequire large incidence angles on one or more mirrors, producing unacceptable polarization variations, particularlyin the blue. In addition, SeaWiFS uses a polarization scrambler (Fig. 2 and 3) to further reduce these

variations. The scrambler eliminates the need for individual compensators to remove residual polarization at each

focal plane assembly. The scrambler consists of two optical wedges that act as a variable wave plate to convert

incident polarized light into several cycles of circular, horizontal, and vertical polarized light across the instrument’s

aperture. The sensitivityof the output of SeaWiFS to polarized light is measured in the laboratory, using a source

producing plane polarized light. The rotation of the polarized light through 360◦ produced changes of less than

0.5%in the eight SeaWiFS bands.

Two instrument bands, which have four detectors each,

form a focal plane. The four detectors in each group are

added using a time delayand integration (TDI) technique

to improve signal-to-noise ratios (SNRs). The signal from

each detector is amplified, processed through a selectable

gain stage, and digitized with a 12 bit analog-to-digital

converter (ADC). The four digital words from a band are

then: delayed, summed to obtain the TDI advantage, truncated to 10 bits, and transmitted to the SeaStar satellite

bus through the electronics module. A solar calibrator is

mounted on the instrument so that, if desired, the optical

system views a solar illuminated diffuser when passing over

the South Pole. The entire spacecraft can be rotated to allow the instrument to view the nearlyfull moon, which is

considered to be a stable calibration source for the purpose

of monitoring the long-term repeatabilityof the SeaWiFS

measurements (Woodward et al. 1993).

3. BILINEAR GAINS

Tests of the instrument at SBRC in the spring of 1993

revealed that SeaWiFS measurements would be contaminated bystraylight from clouds in adjacent pixels on orbit.

These tests also showed the straylight contamination to

be roughlyproportional to the brightness of the adjacent

cloud. As part of the set of instrument improvements,

SBRC has changed the sensor’s electronics to allow onorbit measurements of the radiances from clouds. These

changes also maintain the sensitivityof the SeaWiFS measurements of the ocean as specified in the initial requirements for the instrument.

The new electronic configuration uses the four detector circuits in each SeaWiFS band to create bands with

bi-linear gains. The response for SeaWiFS band 1,412 nm,

is shown in Fig. 4. The channel has a high sensitivity,

i.e., considerablyless than 1 mW per count, over threequarters of the band’s dynamic range. Above this point,

the channel’s sensitivityis reduced, allowing the measurement of cloud radiances up to 60 mW (Fig. 4, top). For

ocean measurements, the band will have the same response

as before, except over a reduced number of counts (Fig. 4,

bottom).

The operation of the four channels in a SeaWiFS band

can be illustrated using the radiance levels for band 1.

The values for this band are given in Tables 1–4. The

2
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 1. The SeaStar spacecraft. The SeaWiFS instrument is mounted on the top payload shelf between the

three antennas.

Fig. 2. The SeaWiFS scanner assembly. The scanner mounts to the payload shelf using the four mounting

points at the top of the figure.

3
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 3. The SeaWiFS aft optics assembly.

values are given for Science Gain 1, the standard gain for

SeaWiFS ocean measurements. The values are given using

the output from all four channels for this band. This is the

standard detector configuration for the instrument. The

input radiances, counts, and zero offsets (Table 1) come

from measurements of the radiometric calibration of the

instrument. With the zero offsets removed, the net counts,

along with the sphere radiance, are used to calculate the

sensitivityfor each channel (Table 1). Channel 1, with low

sensitivity, allows measurement of the high radiances from

clouds. Channels 2, 3, and 4, with high sensitivities, allow

measurements of the low radiances from oceans. These

sensitivities, in mW of radiance per count, are fundamental

to the calibration of SeaWiFS. Theyallow the conversion

of the counts from the instrument into radiances at the

instrument’s optical input.

Tables 1–4 describe, in stepwise fashion, the determination of knee and endpoint locations for the bilinear gains

for SeaWiFS band 1. The calculations are made for Science Gain 1. Details of the stepwise calculation algorithm

are given with the tables.

The SeaWiFS channels are digitized at 10 bits. The

output from each channel range from 0–1,023 counts. When

the zero offsets are removed, the saturation counts for the

four channels in SeaWiFS band 1 range from 1,000–1,005

counts (Table 2). From these saturation counts and the

sensitivities for the channels, it is possible to calculate the

saturation radiances for each of the channels. These values

are given in Table 2. For radiances greater than the saturation radiances, the output from the SeaWiFS channels

will remain at their saturation count levels.

The saturation radiances in Table 2 give the three knee

radiances and the maximum radiance for band 1 (gain 1).

The minimum radiance is zero for zero counts, i.e., for zero

counts after the removal of the offset. Using the saturation

radiance levels and the sensitivities, it is possible to calcu4
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 4. Instrument response for SeaWiFS band 1 with a bilinear gain. The top panel shows the response from

0 to 1,000 counts. The slope from 0 to 800 counts shows greater sensitivity, i.e., fewer radiance units per count.

The bottom panel shows the response from 0 to 800 counts. This is the working range for ocean measurements

from band 1.

5
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Table 1. In the following tables (1–4), the process of calculating the knee and endpoint locations for the bilinear

gains of SeaWiFS band 1 is shown. These calculations are made for Science Gain 1. Table 1 gives the input

values and calculated sensitivities for the four channels of band 1. The sensitivities are calculated from the

sphere radiances and the net counts.

Channel Radiance Measurement Offset Net Counts Sensitivity

[mW] [counts] [counts] [mW/count]

1 9.246 175 21 154 0.060039

2 9.246 871 23 848 0.010903

3 9.246 859 18 841 0.010994

4 9.246 871 21 850 0.010878

Table 2. Saturation counts and saturation radiances for the four channels. The saturation radiances are

calculated from the saturation counts and the sensitivities. The offset has been removed from both the zero and

the saturation counts.

Channel Zero Saturation Counts Saturation Radiance

[mW]

1 0 1,002 60.159

2 0 1,000 10.903

3 0 1,005 11.049

4 0 1,002 10.899

Table 3. Calculated instrument output at the saturation radiances for the four channels. The counts at the

knees are calculated from the knee radiances and the sensitivities. The counts for each channel cannot exceed

the saturation counts.

Zero Knee 1 Knee 2 Knee 3 Saturation

Counts—Channel 1 0.00 181.54 181.60 184.03 1,002.00

Counts—Channel 2 0.00 999.64 1,000.00 1,000.00 1,000.00

Counts—Channel 3 0.00 991.39 991.75 1,005.00 1,005.00

Counts—Channel 4 0.00 1,002.00 1,002.00 1,002.00 1,002.00

Sum of Counts 0.00 3,174.57 3,175.35 3,191.03 4,009.00

[Sum of Counts]/4 0.00 793.64 793.84 797.76 1,002.25

Radiance 0.00 10.90 10.90 11.05 60.16

Table 4. Knees and endpoint locations for the bilinear gains. These are the values in the last two rows of

Table 3.

Location Radiance [mW] Counts

Zero 0.000 0.00

Knee 1 10.899 793.64

Knee 2 10.903 793.84

Knee 3 11.049 797.76

Saturation 60.159 1,002.25

late the number of counts from each channel at the three

knees and the two endpoints of the bilinear gains (Table 3).

The counts from the four channels are summed and

divided byfour in Table 3. This duplicates the process

within SeaWiFS. On orbit, the output from the channels,

as selected bythe instrument’s electronics based on commands from the ground, will be summed. The result will

be sent from SeaWiFS to the SeaStar spacecraft. This output will be sent minus its two least significant bits, which

means that the output will be sent from the instrument to

the spacecraft after division byfour.

The radiances and counts at the three knees and the

two endpoints are given in Table 4. The counts at zero

radiance are zero. For these calculations, and for on-orbit

operations, the zero offsets are removed at the start of

the calculations. This initial step opens up a direct relationship between counts and radiances for ocean measurements. Below the first knee in the radiance region for

ocean measurements, Table 3 gives the information for the

calculation of the sensitivityof band 1: (10.899/793.64),

or 0.013773 mW per count. Above the third knee in the

radiance region for cloud measurements, the sensitivity

6
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

is (60.159-11.049)/(1002.25-797.76), or 0.240158 mW per

count. The ocean portion of the bilinear gain is 17.5 times

more sensitive than the cloud portion. This difference

in sensitivities becomes greater in sequence for bands 2

through 8.

The effect of slightlydifferent sensitivities in the three

high sensitivitychannels is shown in Fig. 5. In this case,

the knee is not sharp, but has two internal segments, rather

than just one segment in and one segment out. This increases the region of uncertaintyin the transition between

the ocean and cloud sensitivityregimes.

The revised electronic configuration that gives SeaWiFS

the abilityto detect clouds, also defines the method in

which the knees for the bilinear gains are calculated (see

Tables 1–4). If the detector configuration for a band is

changed, then the knees for the bilinear gains must be recalculated. If the sensitivities of the high sensitivitychannels for a band are changed, i.e., bychanging the gain for

the band, then the knees for the bilinear gains must be recalculated. However, the sensitivityof the cloud channel,

in this case channel 1 of band 1, remains fixed. It does

not change with gain changes in the instrument. Onlythe

sensitivities of the three high sensitivitychannels for each

band can be changed. For this reason, the upper endpoint

in the bilinear gains does not change when the band’s gains

are changed.

4. MIRROR SIDES

4.1 Scan-to-Scan Differences

For the purpose of instrument calibration, SeaWiFS

is to be considered as two separate, but nearlyidentical

instruments: mirror side 1 and mirror side 2. This separation is necessarysince the two sides of the half angle mirror (Fig. 2) have slightlydifferent reflecting properties and

slightlydifferent alignments in the sensor’s optical path.

Mirror side differences translate into scan to scan differences since the two mirror sides are used alternately, i.e.,

from scan to scan in the SeaWiFS measurements. For this

report the authors have not considered the geometric scanto-scan differences generated bythe small differences in the

two mirror sides. The concern here is the radiometric effects of the differences. As shown below, these differences

are a few tenths of a percent.

For SeaWiFS, there will be two sets of radiometric

calibrations, identical in form but with slightlydifferent

calibration coefficients. Prelaunch calibration coefficients,

tailored for the two mirror sides, are provided bySBRC,

the instrument’s manufacturer. After launch, however, tailored information of this sort will not be available. For the

most part, SeaWiFS measurements of the moon will check

onlythe average response of the two mirror sides, since the

reflectivityof the lunar surface is not constant across its

face. However, with diffuser measurements, it will be possible to monitor scan-to-scan (mirror side to mirror side)

differences in the instrument’s output (Section 4.2). In addition, it will be possible to check scan-to-scan differences

on orbit using Earth-exiting radiances from statistically

uniform bodies of water, that is, from clear water regions.

These conditions lead to the following philosophyfor

monitoring the long-term repeatabilityof the radiometric

sensitivityof SeaWiFS. On-orbit and ground-based measurements will be used to monitor the change, on average,

of the eight instrument bands. Thus, neither of the two

mirror sides is considered as prime, and neither as secondary. Measurements of the average radiometric calibration coefficients combined with measurements of scan-toscan differences will be used to calculate the coefficients

for the two mirror sides, in a manner that the values for

the mirror sides are equallydistant from the average. In

addition to the measurements presented, ocean measurements on orbit can be used to check the magnitude of scan

line-to-scan line differences.

4.2 Scan-to-Scan Difference Equation

Side-to-side differences in the half-angle mirror will be

characterized bythe manufacturer before launch. There

maybe changes in the side-to-side characteristics of the

half-angle mirror during the course of the mission. The

average reflectance for the two mirror sides is an inherent

part in the 20 scans that cover the surface of the moon in

a lunar measurement and in the 480 scan lines of a diffuser

measurement. However, the magnitude of the side-to-side

differences in the half-angle mirror can be tracked during

standard measurements of the instrument’s diffuser. During each diffuser measurement, there are 240 pairs of halfangle measurements between the two mirror sides to be

used to calculate the differences. These differences transform into scan line-to-scan line differences in the ocean

measurements. The magnitude of the scan to scan differences will be tracked as part of the onboard calibration

information from SeaWiFS and will be used in the determination of the two sets of calibration coefficients. Strictly

speaking, these differences are not part of the long-term

radiometric calibration for the instrument.

Mirror side differences for a given SeaWiFS band are

treated independentlyin the following equation:

R1 = (C1 + C1)/(C1 + C2), (1)

where R1 is the radiance value measured with side 1 of the

half-angle mirror, relative to the average radiance from the

two sides, with a value veryclose to unity; (C1+C1)/(C1+

C2) is the calculation used to obtain R1, with a value very

close to unity; and C1 and C2 are measured values of the

flight diffuser for sequential scan lines, as described above,

in counts. R1 and R2 (defined immediatelybelow) are

dimensionless quantities.

The value for the radiance measured with side 2 of the

half-angle mirror, relative to the average radiance from the

two sides, can also be calculated directly:

R2 = (C2 + C2)/(C1 + C2), (2)

7
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 5. An expanded view of the change in slope of the bilinear gain for band 1. This shows the effect of

differences in the sensitivities of the three high sensitivitychannels. The two segments between 793 and 798 nm

are created as the two higher sensitivitychannels saturate in turn. For “real world” measurements, the change

in slope does not occur at one radiance level only.

where R2 is the radiance value measured with side 2 of the

half-angle mirror, relative to the average radiance from the

two sides, with a value veryclose to unity.

Both R1 and R2 are relative values, multipliers that

will be applied to the calibration constants, below, to give

the values for each of the mirror sides. These two constants

are calculated from laboratorymeasurements bythe manufacturer (Table 5).

5. CALIBRATION EQUATIONS

As with the equations for mirror side differences, the

radiometric calibrations are identical in form for each of

the SeaWiFS bands. Thus, the descriptions given here are

representative of each band. The prelaunch radiometric

calibrations for SeaWiFS are based on measurements by

the manufacturer. On orbit, there will be no mechanism

for monitoring changes of individual components within

the radiometer. SeaWiFS carries no onboard calibrators.

Among other things, there will be no means of determining

spectral shifts in the instrument’s interference filters after

launch. In addition, there will be no means of isolating

transmission changes from other elements in the SeaWiFS

optical train. On orbit, it will be possible to monitor only

relative changes in the output of the radiometer’s bands.

Information on the type of internal changes within those

bands will not be available. Before launch, there are absolute quantities in the prelaunch radiometric constants for

the instrument that come from laboratorymeasurements.

There is a program in progress to check the transfer of the

prelaunch radiometric calibration of SeaWiFS to orbit at

the start of on-orbit operations (Biggar et al. 1993). After

that check is made, measurements on orbit will give only

the changes in the instrument’s sensitivityrelative to that

at launch.

5.1 Calibration Equation Factors

There are several factors that must be considered in

the determination of the radiometric sensitivityof the SeaWiFS radiometer. Laboratorymeasurements of the instrument’s sensitivityhave been made with a fixed set up of the

8
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

commandable variables for the radiometer. For example,

these measurements have been made at Science Gain 1,

and the instrument’s sensitivityat the other three gains

has been determined relative to Science Gain 1. However, on orbit the long-term repeatabilityof the SeaWiFS

measurements will be made, using scans of the moon at

the lunar gain. The principal gain for monitoring instrument operation on orbit is different from the principal gain

for laboratorymeasurements. For practical considerations,

the knees and endpoints for the eight SeaWiFS bands have

been calculated in terms of the radiometric sensitivityof

the four channels in each band.

Table 5. Prelaunch mirror side factors. R1 and R2

are dimensionless quantities.

Band Center R1 R2

Wavelength [nm]

1 412 1.002 0.998

2 443 1.001 0.999

3 490 1.001 0.999

4 510 1.001 0.999

5 555 1.002 0.998

6 670 1.002 0.998

7 765 1.001 0.999

8 865 1.003 0.997

The term gain has been present in discussions of the operation of the instrument since the inception of the Ocean

Color Specifications. It is preferable to refer to the gains as

sensitivityfactors, with units of radiance per count. The

higher the gain for a measurement, i.e., the greater the

number of counts per unit radiance, the lower the saturation radiance, and the higher the sensitivityper count.

This format for the calibration equations reflects the

on-orbit conditions for SeaWiFS. The SeaWiFS instrument

has no onboard mechanism to monitor the absolute accuracyof the measured radiances. During the anticipated

5–10 year lifetime of the radiometer, the SeaWiFS Project

will be able to detect relative changes in the sensitivity

of the instrument only. After launch, lunar measurements

will be used to monitor these changes as described below,

and onboard measurements will be used to detect relative

changes among the four electronic gains for each SeaWiFS

band and among the four photodiodes for each band.

The relationships between individual gains are measured with a common electronic voltage, a calibration pulse,

added to the photodiode output while the instrument views

the inner surface of the back of the instrument. This process is discussed in more detail in Woodward et al. (1993).

Relative changes among the individual detectors in a SeaWiFS band will be monitored byhaving the detectors

view the instrument’s diffuser sequentiallyseveral times

while the diffuser is illuminated bythe sun. This can be

done, since each possible detector combination can be commanded from the ground. The relationship among the high

sensitivitydetectors will be treated comparatively, i.e., examining the change with time in the output of anyone of

the high sensitivitydetectors in comparison to the grouping comprised of the other detectors. Sufficient anomalous

behavior of a single detector will cause the removal of that

detector from the measurement set. Again, this process is

discussed in more detail in Woodward et al. (1993).

Other factors in the radiometric calibration equations

are not expected to change over time. These include the

dependence of the radiometric sensitivityon the temperature of the focal planes and the dependence of the radiometric sensitivityon the scan angle of the measurement.

Although there will be thermistors to measure the temperature of each focal plane during the SeaWiFS mission,

there will be no on board means of measuring changes in

the temperature dependence of the output from the detectors. In a like manner, no means has been found of

checking, after launch, changes in the instrument’s radiometric sensitivityat the edge of the SeaWiFS scan relative

to the sensitivityat nadir. These factors are determined

solelyfrom prelaunch measurements bythe manufacturer,

as explained in a review of the CZCS calibration byEvans

and Gordon (1994).

5.2 Long-Term Sensitivity

Long-term changes in the radiometric sensitivityof the

eight SeaWiFS bands will be made using measurements of

the reflected solar flux from the surface of the moon. These

measurements will be complimented byvicarious, groundand ocean-based measurements. The sun is considered as

a stable, non-changing light source, and the surface of the

moon is considered as unchanging over periods that are

short on a geologic time scale. Variations in the solar flux

incident on the lunar surface due to changes in the Earthsun distance can be removed (Woodward et al. 1993). In

a similar manner, variations in the reflectance of the moon

due to small differences in the lunar phase angle and due

to small changes resulting from lunar libration can also be

removed. With these corrections the effective reflectance

from the surface of the moon can be made essentiallyconstant. Changes in the SeaWiFS measured values for the

lunar radiance will be used directlyto detect changes in

the sensitivityof the instrument.

For solar measurements with the SeaWiFS diffuser, it

is not possible to separate changes in the instrument sensitivityfrom changes in the diffuser reflectance in such a

direct manner. All that can be derived from the diffuser

measurements themselves is the product of the change in

the instrument and of the change in the diffuser. There is

an assumption, however, that will allow the diffuser and

the lunar measurements to be tied together. Basically, the

change in the reflectance of the diffuser is assumed to be

linear over time periods of approximatelyone month (Cebula et al. 1988 and Herman et al. 1990). Experience with

diffusers on previous satellite instruments has led to the explanation that diffuser degradation has been caused bythe

9
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

coating of the diffuser with solarized organic and siliconbased materials that have outgassed from the spacecraft.

The accumulation of thin layers of these materials does

not lead to sharp step functions in the reflectivityof the

diffuser over time.

Over longer time periods of one to several years, the

change in the reflectance of the diffuser is expected to become an exponential function of time, asymptotically approaching zero change with time as the SeaWiFS mission

ends. However, this 5–10 year long exponential function

can be treated as a series of manylinear segments that are

a month in duration.

Using nearlysimultaneous lunar and diffuser measurements, it is possible to separate changes in the sensitivity

of the instrument, determined from lunar measurements,

from changes in the reflectance of the diffuser. When the

time-series of diffuser reflectances is normalized bylunar

measurements, it is possible to use the assumption of a linear change in diffuser reflectivityto identifystep changes in

the instrument sensitivitythat mayoccur between lunar

measurements. Such changes were found in CZCS measurements (Evans and Gordon 1994).

5.3 Long-Term Sensitivity Equation

The long-term sensitivityequation for the radiometer

contains onlyfactors that can change during the lifetime of

the SeaWiFS mission. These include two command variables that can be changed from the ground; the selection of

the electronic gains, and the detector configuration. They

also include time-dependent variables whose change can be

monitored during the mission, the temperatures of the focal planes as measured bythermistors, and the long-term

radiometric sensitivities of the instrument bands as determined through lunar and diffuser measurements.

The long-term sensitivityequation does not include the

scan angle dependence of the radiometric sensitivityof the

radiometer (also called scan modulation), i.e., the sensitivityof the instrument for measurements at the limb relative to those at nadir. These values are determined by

the manufacturer and cannot be independentlyverified on

orbit. The effect of scan modulation will be added to the

overall radiometric sensitivityequation in the next section.

In addition, the long-term sensitivityequation does

not include changes in the linearityin the detector response with input radiance. Again, these values are determined solelythrough measurements bythe manufacturer

and cannot be checked after launch. A discussion of the

manufacturer’s measurements is given below.

The on-orbit equation for the time dependent radiometric sensitivityof each SeaWiFS band, at nadir, has the

following form:

Lnadir = K1(t) × K2(gs, ds)

× (1 + K3(T−Tref))

× (Cout−Cdark).

(3)

The variables in (3) are defined as follows:

1. Lnadir is the measured radiance at nadir, in mW

cm−2 sr−1 μm−1.

2. K1, which is a function of the time, is the primary

instrument sensitivityfactor expressed as the inverse of the number of days post-launch, and therefore equal to unityat the start of the SeaWiFS mission. This factor applies equallyto each of the four

gains in a SeaWiFS band.

3. t is the time, in days, after the start of on-orbit

measurements bySeaWiFS.

4. K2 is the gain factor, in mW cm−2 sr−1 μm−1 per

count.

5. gs is the gain selected bycommand from the ground

and present in a datum from the spacecraft engineering telemetrythat precedes each scan line.

6. ds is the detector configuration selected bycommand from the ground and present in a datum from

the spacecraft engineering telemetrythat precedes

each scan line.

7. (1 + K3(T−Tref )) is the focal plane temperature

factor, a dimensionless value veryclose to unity.

8. K3 is the temperature dependence of the output

from the detector, in terms of inverse degrees Celsius, with a value veryclose to zero, as measured

bythe manufacturer.

9. T is the measured temperature of the focal plane

assembly, in degrees Celsius, calculated from a datum in the engineering telemetrystream associated

with each scan line.

10. Tref is the reference temperature for the temperature dependence, 20◦ C, set bythe manufacturer.

11. Cout is the instrument output, in counts.

12. Cdark is the instrument dark restore value, in counts.

The calculation of the focal plane temperatures (T)

from the data in the spacecraft engineering telemetryrequires several steps. As a result, the presentation of the

temperature calculations is deferred to Section 8.0, where

the eight values for K3, one for each band, are also listed

(Tables 10 and 11).

The primaryinstrument sensitivityfor each SeaWiFS

band, K1(t), is calculated from a combined set of lunar

and diffuser measurements. There are also plans to derive

this factor independentlyusing ground based calibrations

once the radiometer is collecting data on orbit. It is anticipated that the time series of K1(t) values for periods

through each lunar measurement will be updated and sent

to the SeaWiFS stations approximatelytwo weeks after

that measurement. As the ensemble of lunar and diffuser

measurements grows, improved time histories of the K1(t)

values will be supplied. In addition, as the rate of change of

the K1(t) values becomes understood, it will be possible to

give improved, educated guesses regarding future changes

in the K1(t) values. Until reasonable time series for K1(t)

10
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

values are established, these K1(t) values will be treated

on orbit as constants during the time periods between the

most recent and the next lunar measurement and then will

be updated using the next lunar measurement.

Since all four gains for each SeaWiFS band—the four

values of K2(gs)—will be used frequentlyduring the radiometer’s mission, calibration coefficients for all of the

SeaWiFS bands will be contained in a periodicallyupdated

list that will be provided to each of the ground stations.

These values will be given in a table of the knees and endpoints for the bilinear gains, using the form of Table 4.

In addition, one word showing the command variable that

determines the choice of the gain will be present in the engineering telemetrythat precedes each scan line from the

satellite.

The relative values for these gains will be monitored

regularlyduring the mission (Woodward et al. 1993). The

value for the lunar gain for each SeaWiFS band, i.e., the

radiance per count for the lunar gain, will not change with

time during the SeaWiFS mission. Since the change in the

sensitivityof the instrument is determined through measurements of the moon, changes in the lunar gain will be

found in K1(t) for each band only. Changes in the values

of the other gains relative to the lunar gain will be updated

as determined. Changes in the gain ratios are anticipated

to be small over time, if detectable at all. However, time

series for the gain ratios for the eight instrument bands

will be provided bythe Project.

It is anticipated that the standard detector selection,

ds, will not change during the SeaWiFS mission. For a

given instrument band, that standard selection uses the

sum of the output of the four detectors divided byfour.

Using the comparative method of the previous section, it is

anticipated that none of the detectors will exhibit anomalous behavior and have to be removed from the measurement set. Should this occur, however, a new table of knees

and endpoints for the bilinear gains will be supplied by

the Project. In addition, a datum showing the command

variable that determines the detector configuration will be

present in the engineering telemetrythat precedes each

scan line from the instrument.

The focal plane temperature factor, 1 + K3(T−Tref),

contains two constants that are supplied from measurements bythe radiometer’s manufacturer: K3 and Tref. For

all eight SeaWiFS bands, the constant Tref is set to 20◦ C.

The calculation of the temperature of the two detectors

on each focal plane requires the conversion of engineering

telemetryfrom the SeaStar spacecraft, in counts, into an

analog signal from SeaWiFS, in volts, and then into temperature, in degrees Celsius. The conversion from volts to

temperature requires a series of algebraic manipulations.

These manipulations are not conceptuallydifficult, but

theydo require several steps of computation. The series

of calculations required to obtain the focal plane temperatures is found in Section 8.0. This section also contains

the eight values that are required for the calculation of the

temperature correction.

Thermal models of the operation of the SeaWiFS instrument indicate that the anticipated day-night temperature differences for a single orbit are on the order of 1◦ C.

However, as the instrument’s thermal blankets age over

the 5–10 year duration of the SeaWiFS mission, the average temperatures of the focal planes are anticipated to

increase slowlyat the rate of 1–2◦ Celsius per year.

5.4 Off-Nadir Measurement Differences

The primarylaboratorymeasurements of the radiometric response of the SeaWiFS instrument have been made

in the laboratoryat nadir, i.e., pixel 643. Although not explicitlystated in the SeaWiFS performance specifications,

it is important to know how the measurement at pixel 643

can be transferred to the other pixels in the scan. SBRC

ran such tests during its characterization of the instrument.

In the presentation of the test results, SBRC referred to

the effect as scan modulation.

The data from these tests were taken approximately

at nadir and at 100 pixel intervals above and below nadir.

Data were taken for both mirror sides, and the off-nadir responses for the two mirror sides were identical at the 0.2%

level, which is the resolution limit for the laboratorymeasurements. Overall the measurements can be summarized

as follows:

1. All bands show changes in their responses with pixel

number. These changes are small, ranging from 1%

to 2% over the 1,285 pixels of a scan line.

2. The scan modulations for odd bands (bands 1, 3, 5,

and 7) show a marked similaritywith each other, as

do the scan modulations for the even bands. The

odd and the even bands, however, show distinctly

different responses from each other.

3. The scan modulations can be fitted to quadratic

equations easily. It is possible to account for the

scan modulation in all eight bands with two equations, one for the even bands and one for the odd

ones.

4. The measurements bySBRC show no mirror side

differences in the scan modulation.

The data provided bySBRC did not include error bars

for the individual radiometric measurements. From the

results, however, it appears that the data have a scatter

at one standard deviation of about ± 0.3%. This scatter

falls above and below the linear results versus wavelength

discussed above. The average values for the responses of

the odd and even SeaWiFS bands are given in Fig. 6. The

quadratic curves in Fig. 6 show the off-nadir corrections

from the manufacturer for the odd and even bands.

The cause of the scan modulation has been explained

as vignetting (shadowing) of the detectors during portions

of the scan. The detectors can be considered as having

11
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 6. Scan modulation measurements for the odd and even SeaWiFS bands. The results are given as

relative differences from the nadir (pixel 643) value. The symbols show the average values from the laboratory

measurements of the odd and even bands. The quadratic curves show the scan modulation corrections—see text

for details. The top panel shows odd bands, and the bottom panel shows the even bands.

12
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

the image of the field stop imposed upon them. Anyimperfection of the alignment of the instrument’s optics will

cause the image of the field stop to shift relative to the

detectors. To minimize straylight entering the aft optics,

the image of the field stop is 50% greater than the size

of the detectors for each band. In the direction normal

to the scan plane, this gives onlya one-quarter pixel leewayon either side of the detectors. Imperfect alignment

of the optics causes the image of the field stop to move relative to the focal plane assemblies. Increasing the size of

the field stop, or cocking it slightly, could eliminate much

of this effect. These changes were not part of the stray

light improvements bySBRC. However, a portion of the

straylight repairs to SeaWiFS involved work on the focal

plane assemblies. The repositioning of the focal planes during their reinstallation did affect the final scan modulation

values slightly.

The quadratic response curves, calculated bySBRC,

are based on measurements of the output of the SBRC integrating sphere over a period of about 30 minutes. To

view the sphere, the instrument was rotated to seven angular positions, and the radiances from the sphere were

recorded. The positions for the measurements were approximately220 pixels apart and covered the range from

1–1,285 pixels. Small changes in the output of the sphere

mayhave added a scatter of a few tenths of a percent to

the measurements bythe instrument.

The scan modulation response curves equal unityat

nadir. For other pixels, the curves derived bySBRC give

the difference of the instrument measurements, in relative

terms, from the value at nadir. The corrections to the calibration equations are the inverse of the calculated instrument responses from SBRC. The SBRC response curves

shows the output of odd bands at pixel 1 to be 1% lower

than those at pixel 643. To correct for this, 1% must be

added to the values from the odd bands at pixel 1.

This is the scan modulation correction:

K4(Pxl) = 1

1 + A0(Pxl −643) + B0(Pxl −643)2 , (4),

where Pxl represents the pixel number, and where A0 =

3.115 × 10−6 and B0 = −1.929 × 10−8 for odd bands, and

A0 = 1.713×10−5 and B0 = −1.456×10−8 for even bands.

For all bands, K4(Pxl) is equal to unityat nadir. As a

result, this factor can be applied to (3) to cover all pixels

in each scan. For a SeaWiFS band, the values over a scan

line are given as

Lscan = K1(t) × K2(gs, ds)

× (1 + K3(T−Tref))

× K4(Pxl) × (Cout−Cdark),

(5)

where Lscan is the measured radiance at anypixel in a scan,

in mW cm−2 sr−1 μm−1.

Equation (5) does not consider the mirror side differences in the instrument. Except for the variable, K1, none

of the variables have anymirror side dependence. Based on

considerations in the methods for determining long-term

changes in the instrument’s sensitivity, K1 has also been

defined without a mirror side effect.

5.5 Full-Up Equations

The use of the radiometric calibration equations on orbit will require the consideration of the two sides of the

half-angle mirror. This factor has been described above,

including the factor R1 for mirror side 1 and R2 for mirror

side 2. When these two factors are included, (5) can be

modified to produce two equations with the form:

LS1 = R1 × K1(t) × K2(gs, ds)

× (1 + K3(T−Tref))

× K4(Pxl) × (Cout−Cdark),

(6)

and

LS2 = R2 × K1(t) × K2(gs, ds)

× (1 + K3 × (T−Tref))

× K4(Pxl) × (Cout−Cdark).

(7)

The variables in (6) and (7) are defined as follows:

1. LS1 is the measured radiance for mirror side 1, in

mW cm−2 sr−1 μm−1.

2. R1 is the dimensionless multiplier for mirror side 1

from (1).

3. LS2 is the measured radiance for mirror side 2, in

mW cm−2 sr−1 μm−1.

4. R2 is the dimensionless multiplier for mirror side 2

from (2).

Equations (6) and (7) are the two on-orbit radiometric

calibration equations for SeaWiFS.

5.6 Instrument Response Linearity

Equations (6) and (7) are based on the assumption that

the measured radiance is a linear function of the number of

counts in the instrument’s output. The SeaWiFS performance specifications call for the output to be linear with

radiance at the 1% level or better. For the six shortest

wavelength SeaWiFS bands (412, 443, 490, 510, 555, and

670 nm), the measurements at SBRC show the linearityto

be at the 0.5% level. For bands 7 and 8 (765 and 865 nm),

the laboratorymeasurements show linearities that are better than the specifications, but not at the levels of the other

six bands.

The measurements of the linearityof SeaWiFS were

made using an integrating sphere. The radiances from

the sphere were determined using a laboratoryradiometer

that compared the sphere’s output to that from a standard

lamp that has been calibrated bythe National Institute of

Standards and Technology(NIST). The radiances required

for measurements of SeaWiFS at 765 and 865 nm are very

13
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

small when compared to the output from the standard

lamp at these wavelengths. For the transfer radiometer,

these levels were so low that noise in the transfer measurement approached 1%.

From this evidence, the conclusion is that the relatively

poor linearityin the SeaWiFS laboratorymeasurements

at 765 and 865 nm is due to limits in the qualityof the

characterization of the sphere itself. It is also concluded

that the readings for all eight SeaWiFS bands are linear

with input radiance at the level of 0.5% (Barnes et al.

1994). There is no term in the instrument’s calibration

equations for non-linear response.

5.7 Polarization Sensitivity

The degree of polarization of the eight SeaWiFS bands

is measured in the laboratoryusing a polarizer that is rotated through 360◦ in 22.5◦ intervals. Over this angular

range, the instrument bands should show two cycles of

response. The response from SeaWiFS band 1 is shown

in Fig. 7. There is little indication of the predicted twocycle response. It is more reasonable to conclude that the

variations in the instrument output shown in Fig. 7 are

indicative of changes in the transmission of the polarizer

with angle. Barnes et al. (1994) used Fourier analysis to

expose the two-cycle signal. Their results indicate that

polarization in the instrument is 0.25% or less.

The measurements in Fig. 7 are taken at pixel 633, near

nadir. The response in this figure is also representative

of the instrument responses at 30◦ and 55◦ from nadir.

From the polarization measurements at these three angles,

the polarization of SeaWiFS remains at 0.25% or less over

a scan. There is no term in the instrument’s calibration

equations for polarization sensitivity.

6. GAIN FACTORS

The relative differences in the four sensitivitycoefficients for each band result from different selectable gains

in one of the intermediate amplifiers in the instrument’s

electronics. The SeaWiFS specifications give the gain requirements for gains 2, 3, and 4 relative to gain 1. Gain

1 is the principal science gain. Gain 2 is the secondary

science gain with a radiometric sensitivitytwice that for

gain 1, that is, one count from Science Gain 2 represents

half the radiance for one count from Science Gain 1.

6.1 Gain Ratios

6.1.1 Individual Channels

The gains in the intermediate amplifiers have also been

set up to give a three-quarters full scale output for lunar

measurements and for solar measurements, using the flight

diffuser. Details of the lunar and solar measurements can

be found in McClain et al. (1992) and in Woodward et al.

(1993). The gain values for the 32 SeaWiFS channels are

listed in Table 6 and are based on laboratorymeasurements

bySBRC. The gain ratios are given with respect to Science

Gain 1.

The gain ratios in Table 6 show little channel-to-channel

variability. The four channels in band 1 can be taken as

an example. For channel 1, the gain ratios for the four

gains in Table 6 are all equal to unity. Channel 1 is the

cloud channel, and those values should all be unity. In the

calibration data from SBRC, the measured gain ratios for

band 1–channel 1 are 1.016 (G2/G1), 1.005 (G3/G1), and

1.010 (G4/G1). These differences are assumed to come

from noise in the measured counts for each gain. That

noise amounts to 2 or 3 counts out of 220. The data

for each gain have a single calibration datum and a single zero. On orbit, there will be the capacityto take 25

calibration points and 25 zeros from each calibration pulse

scan. This will improve the on-orbit gain ratios significantlywhen compared with the prelaunch measurements.

As a result, the gain ratios for the cloud channels have

all been set to unityto eliminate the effects of noise in

the measurements. In addition, there has been an averaging of gain ratios in the high sensitivitychannels. For

band 1–gain 2, the three high sensitivitybands all have the

same gain ratio in Table 6 (1.988). In the calibration data

from SBRC, the measured gain ratios are 1.987 (channel

2), 1.982 (channel 3), and 1.995 (channel 4). As with the

measurements of the cloud channel, there is noise in these

gain ratios. In the prelaunch data, the gain ratios have

been set to the average of the three channels to remove

noise. On orbit, it will be possible to measure these ratios with greater precision. Until then, it is not possible

to distinguish between noise in the gain measurements of

the high sensitivitychannels and small differences in their

gains.

6.1.2 Combinations of Channels

The gain ratios in Table 6 are given for individual channels. It is possible to calculate these ratios for combinations of channels. Such ratios can be calculated for the

standard SeaWiFS detector configuration, 4:1 TDI, where

the output is the sum of the four channels. The calculations are based on the procedure outlined in Tables 1–4.

Theycan be explained byusing the values from SeaWiFS

band 1. The gain ratios for the high sensitivitychannels

for band 1–gain 2 are 1.988 (Table 6). This means that

a channel will produce 1.988 counts per unit radiance at

gain 2 if the channel gives one count per unit radiance at

gain 1. Conversely, if gain 1 has a sensitivity of one unit of

radiance per count, gain 2 will have a sensitivityof 1/1.988

unit of radiance per count.

For gain 2 calculations, 1/1.988 is the fractional multiplier applied to the sensitivities for channels 2, 3, and 4

from Table 1. For gain 2, the sensitivities of these channels

are approximately0.005 mW per count (Table 1). With

these three sensitivities and the sensitivityfor channel 1,

14
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Table 6. Gain ratios for the SeaWiFS channels, relative to Gain 1. The gains for the cloud detection channels

for each band do not change with the gain selection.

Band Channel Gain 1 Gain 2 Gain 3 Gain 4

(G2/G1) (G3/G1) (G4/G1)

1 1 1.000 1.000 1.000 1.000

2 1.000 1.988 1.320 1.681

3 1.000 1.988 1.320 1.681

4 1.000 1.988 1.320 1.681

2 1 1.000 1.989 1.319 1.682

2 1.000 1.989 1.319 1.682

3 1.000 1.989 1.319 1.682

4 1.000 1.000 1.000 1.000

3 1 1.000 1.000 1.000 1.000

2 1.000 1.989 0.896 1.681

3 1.000 1.989 0.896 1.681

4 1.000 1.989 0.896 1.681

4 1 1.000 1.989 0.789 1.682

2 1.000 1.989 0.789 1.682

3 1.000 1.989 0.789 1.682

4 1.000 1.000 1.000 1.000

5 1 1.000 1.000 1.000 1.000

2 1.000 1.989 0.642 1.595

3 1.000 1.989 0.642 1.595

4 1.000 1.989 0.642 1.595

6 1 1.000 1.989 0.364 0.665

2 1.000 1.989 0.364 0.665

3 1.000 1.989 0.364 0.665

4 1.000 1.000 1.000 1.000

7 1 1.000 1.000 1.000 1.000

2 1.000 1.987 0.311 0.575

3 1.000 1.987 0.311 0.575

4 1.000 1.987 0.311 0.575

8 1 1.000 1.991 0.261 0.499

2 1.000 1.991 0.261 0.499

3 1.000 1.991 0.261 0.499

4 1.000 1.000 1.000 1.000

which has not changed, it is possible to work through the

calculations in the manner of those in Table 2 and 3. The

results give three new knees for the bilinear gains, both in

radiance and in counts. The upper endpoint remains the

same, since the sensitivityfor channel 1 has not changed.

Using the zero values plus those from the first knee in

the bilinear gain, it is possible to calculate the sensitivity

of band 1 in the radiance region for ocean measurements.

The ratio of the sensitivities for gain 1 and gain 2 give

the gain ratio for these gains with the standard detector

configuration (Table 7). Table 7 also lists the gains that are

used for lunar and solar measurements. These lunar and

solar gains agree with the gains required bythe SeaWiFS

performance specifications.

From this discussion, it follows that the calculation

method in Tables 1–4 is fundamental to the operation of

the bilinear gains. If the gain of a channel or combination

of channels changes, then new knees must be calculated

using the procedure from Tables 1–4.

6.2 Lunar Measurements

Once on orbit, the monitoring of the long-term repeatabilityof SeaWiFS measurements will be done byviewing

the moon. On orbit, the changes in the sensitivities of the

science gains will be based on measurements of the moon

at the lunar gains. The lunar gains will be treated as a

constant throughout the SeaWiFS mission. Changes in

the measured lunar radiance, with the real lunar radiance

remaining constant, will result in corresponding changes

to the variable K1, also described in (3). The lunar gains

in these interchannel measurements will be kept constant,

15
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 7. Polarization sensitivityof SeaWiFS band 1 (412 nm).

and the gain ratios for the other gains will be allowed to

vary. These are important considerations for an instrument whose long-term repeatabilitywill be monitored by

lunar, rather than terrestrial, measurements. Among other

things, these considerations require a change in viewpoint

from terrestrial to lunar measurements bythose tracking

changes in the instrument’s sensitivityon orbit.

Table 7. SeaWiFS gain values for the standard detector configuration. These values are given relative

to gain 1. The nominal value for gain setting 2 is 2.

The gains used for the lunar measurement and for

the solar diffuser measurement are shown. These

gains cover the measurement regions from zero radiance to the first knees in the bilinear gains.

Band Gain 1 Gain 2 Gain 3 Gain 4

1 1.000 1.931§ 1.302 1.642†

2 1.000 1.940 1.303† 1.648§

3 1.000§ 1.951 0.900† 1.655

4 1.000§ 1.955 0.796† 1.658

5 1.000§ 1.961 0.652† 1.579

6 1.000 1.969 0.376† 0.671§

7 1.000 1.969 0.323† 0.583§

8 1.000 1.975 0.272† 0.507§

§ Gain used for solar diffuser measurement.

† Gain used for lunar measurement.

The SeaWiFS calibration plan (McClain et al. 1992)

includes a program of ground based measurements, primarilymeasurements of water-leaving radiances with field

instruments, in conjunction with SeaWiFS measurements

on orbit. This calibration plan has been developed to make

both the vicarious and satellite measurements traceable to

a single set of laboratorystandards so that ground and on

orbit measurements can be reconciled. For the vicarious

measurements, there is an ongoing program of round-robin

intercalibrations of field instruments using NIST-traceable

standards (Mueller 1993). The reconciliation of vicarious

and on-orbit measurements also requires knowledge of the

atmospheric transmission of light at the SeaWiFS wavelengths. For the SeaWiFS mission, this knowledge is derived from a set of models of the atmosphere that include

light scattering bymolecules of air, Rayleigh scattering,

and bysmall particles, Mie scattering (H. Gordon, pers.

comm.). These models will playa fundamental role in

the reconciliation of the on-orbit radiance measurements

from SeaWiFS and the water-leaving radiance measurements from field instruments.

The on-orbit monitoring scheme for the long-term repeatabilityof SeaWiFS measurements as described here,

and also found in Woodward et al. (1993), comprises a

closed set of measurements made bythe satellite instrument only. The set of ground based measurements proposed in the SeaWiFS calibration plan (McClain et al.

1992) will be performed in conjunction with SeaWiFS measurements during satellite overpasses of the ground, or

ocean, sites. The ground based measurements will complement the set of satellite based, long-term repeatability

measurements described here. Ultimately, the combined

16
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Table 8. Input values and calculated sensitivities for the eight SeaWiFS bands. The radiances, the measurement

counts, and the offset counts come from laboratorydata. The sensitivities are calculated from the radiances and

the net counts. The values are given for Science Gain 1, the standard gain for SeaWiFS ocean measurements.

Band Channel Radiance Measurement Offset Net Counts Sensitivity

[mW] [counts] [counts] [mW/count]

1 1 9.246 175 21 154 0.060039

2 9.246 871 23 848 0.010903

3 9.246 859 18 841 0.010994

4 9.246 871 21 850 0.010878

2 1 9.122 883 18 865 0.010546

2 9.122 887 21 866 0.010533

3 9.122 878 16 862 0.010582

4 9.122 153 18 135 0.067570

3 1 7.216 127 21 106 0.068075

2 7.216 899 22 877 0.008228

3 7.216 905 21 884 0.008163

4 7.216 903 19 884 0.008163

4 1 5.970 856 21 835 0.007150

2 5.970 855 20 835 0.007150

3 5.970 856 19 837 0.007133

4 5.970 111 21 90 0.066333

5 1 4.692 98 26 72 0.065167

2 4.692 840 22 818 0.005736

3 4.692 837 22 815 0.005757

4 4.692 828 17 811 0.005785

6 1 1.682 540 21 519 0.003241

2 1.682 538 17 521 0.003228

3 1.682 544 33 511 0.003292

4 8.058 168 21 147 0.054816

7 1 9.885 253 23 230 0.042978

2 2.057 915 20 895 0.002298

3 2.057 913 21 892 0.002306

4 2.057 922 27 895 0.002298

8 1 1.063 671 20 651 0.001633

2 1.063 670 24 646 0.001646

3 1.063 671 18 653 0.001628

4 10.283 320 20 300 0.034277

set of satellite based and ground based measurements will

form the basis for the understanding of the SeaWiFS data

set.

7. CALIBRATION CONSTANTS

7.1 SeaWiFS Radiometric Calibration

The laboratoryresults of the SeaWiFS radiometric calibration are given in Table 8. The 32 listings in the table

show the input radiances and output counts for each channel of each band from the laboratorymeasurements. The

values are given for Science Gain 1. The sensitivities for

the channels are calculated from the radiances and the net

counts. The values from Table 8 are sufficient to calculate the knees and endpoints for each band for all detector

combinations at Science Gain 1. The procedure outlined in

Tables 1–4 can be applied to anycombination of channels.

For example, it is possible to combine channel 1 of band

1, taken once, with channel 2 of that band, taken three

times, to create a trulybilinear response, a response with

onlyone knee.

As discussed in Section 6.1, the gain ratios can be applied to the sensitivities in Table 8 to calculate the knees

for each band for gains 2, 3, and 4. The endpoints for these

gains will not change. The zero remains zero, and the upper endpoint is a function of the sensitivityof the cloud

channel, which also does not change. Both the radiance

17
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

levels and count values for the knees are a function of the

gain setting since the sensitivities of three of the channels

change, while the cloud channel remains constant.

7.2 Out-of-Band Correction

There is an artifact of the radiometric calibration that

has not been discussed in the calibration data from SBRC.

The radiometric calibration of the instrument was performed with an integrating sphere as the source of radiance. The sphere has a wavelength dependent output that

can be approximated bya 2,850 K blackbody. This source

has a peak output near 1,000 nm (Section 9.8, Fig. 17). On

orbit, SeaWiFS will measure input radiances with spectral

shapes that are similar to that from a 5,900 K blackbody,

which has a peak output near 490 nm.

The bandwidths for the SeaWiFS bands are 20 nm wide

for bands 1 through 6, and 40 nm wide for bands 7 and 8.

For these bandwidths, the spectral shape of the source has

little effect (Section 11). The SeaWiFS bands, however,

have small out-of-band responses (Section 9). The photodiodes in each SeaWiFS band have a slightlydifferent

output when illuminated bya 2,850 K or 5,900 K source.

This output difference is of the order of a few percent or

less (Section 9.10). This output difference is also the same

for each channel of a band. The cloud and high sensitivity

channels for a band all have the same fractional change

in output when measuring these two sources. The out-ofband effect changes the sensitivities of all four channels in

a band identically.

7.3 Bilinear Gain Calibration Constants

The calibration constants for the SeaWiFS bands are

listed in Table 9. The table gives the endpoints and the

knees for each band and gain. These values are given for

the standard detector combination, i.e., for the sum of the

four channels in each band.

Using the radiance levels and count values in Table 9,

it is possible to calculate the radiance corresponding to

each count for each band. Table 9 is the basis for the

calculation of the gain factors (K2 values) used in (3). For

computational purposes, it is also possible to prepare a

lookup table for each band and for each gain. The lookup

table would have approximately32,000 entries, with about

1,000 entries for each of the four gains for each of the eight

bands. This lookup table applies onlyto the standard

detector configuration. Anychanges to that configuration

would require the recalculation of Table 9. It would also

require the creation of a new lookup table, if one was used

in the on-orbit data reduction.

The knees and endpoints for band 1–gain 1 in Table 9

are not the same as those in Table 4. The radiances for

channel 1–gain 1 in Table 9 are 3.7% greater than the corresponding radiances in Table 4. This is the result of the

application of the out-of-band correction discussed in Section 9.10. There is a smaller output from the photodiodes

for band 1 when it is exposed to a 5,900 K source, as measured on orbit, than when it is exposed to a 2,850 K source,

measured in the laboratory. The counts per unit radiance

of this band will be smaller on orbit than theyare in the

laboratory.

It will take a 3.7% greater radiance to saturate the

channels of band 1 on orbit than is required to saturate

them in the laboratory. Thus, the radiances at the knees

and the upper endpoint for band 1–gain 1 in Table 9 are

3.7% greater than those in Table 4. These are the radiances

at which each channel of the band saturates.

The fractional change from the out-of-band correction

has onlychanged the radiances at the knees and the upper endpoint. The count values have remained the same.

This results from the fact that the out-of-band correction

affects the sensitivityof each channel in a band identically.

For band 1, each channel changes by3.7%. For the other

bands, the fractional change is different (Section 9.10, Table 12). The out-of-band corrections have been applied to

all bands and all channels in Table 9.

7.4 Out-of-Band Contributions

The out-of-band responses for the eight SeaWiFS bands

are parts of the instrument’s radiometric calibration. In

that calibration, the instrument views a broad area of

known radiance, and records the output from the bands in

counts. The counts from each band include the out-of-band

contribution. Those out-of-band contributions are functions of the spectral shape of the source that is measured.

The SeaWiFS laboratorycalibration has the out-of-band

correction for a 2,850 K source factored into its results. If

the instrument measures a source with this spectral shape,

these measurements automaticallycontain the appropriate

out-of-band corrections.

The prelaunch calibration equations for SeaWiFS contain correction terms that convert the out-of-band responses from a 2,850 K source to a 5,900 K source. As a result,

the SeaWiFS calibration equations now have the out-ofband correction for a 5,900 K source factored into them.

The 5,900 K spectral shape closelyduplicates the spectral

shape for SeaWiFS ocean measurements. The errors that

arise from the use of the 5,900 K out-of-band corrections

for ocean measurements are estimated to be small, i.e., a

few tenths of a percent. If an alternate out-of-band correction is to be used, then the 5,900 K correction must be

removed from the measurement results, and a new out-ofband correction inserted in its place.

8. TEMPERATURE FACTORS

8.1 Dependence Coefficients

The information about the temperature sensors for the

focal plane assemblies includes the temperature dependencies of the output of the eight SeaWiFS bands, the K3

18
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Table 9. Prelaunch calibration constants for the SeaWiFS bilinear gains. These values have been corrected for

out-of-band response (see Table 12 for the conversion factor). These values are given for the standard detector

configuration (each channel used once). For all bands and at all gain settings, the instrument was calibrated to

give zero counts at zero radiance.

Band Gain Knee 1 Knee 2 Knee 3 Saturation

Radiance Counts Radiance Counts Radiance Counts Radiance Counts

1 1 793.64 11.313 793.84 11.317 797.76 11.469 1,002.25 62.445

2 771.09 5.691 771.27 5.693 774.89 5.769 1,002.25 62.445

3 782.64 8.571 782.83 8.547 786.60 8.688 1,002.25 62.445

4 775.26 6.731 775.45 6.734 779.12 6.824 1,002.25 62.445

2 1 789.10 10.734 791.34 10.778 792.93 10.837 1,004.75 69.062

2 769.69 5.397 771.85 5.420 773.33 5.449 1,004.75 69.062

3 779.67 8.140 781.86 8.174 783.40 8.218 1,004.75 69.062

4 773.27 6.382 775.44 6.408 776.94 6.444 1,004.75 69.062

3 1 779.55 8.343 780.61 8.360 782.00 8.401 1,002.25 69.576

2 764.62 4.194 765.64 4.202 766.96 4.224 1,002.25 69.576

3 783.05 9.315 784.11 9.333 785.52 9.379 1,002.25 69.576

4 767.39 4.963 768.42 4.973 769.74 4.998 1,002.25 69.576

4 1 778.79 7.175 779.00 7.178 779.28 7.185 1,002.75 66.599

2 765.37 3.607 765.57 3.609 765.84 3.612 1,002.75 66.599

3 786.02 9.098 786.23 9.101 786.52 9.110 1,002.75 66.599

4 767.85 4.267 768.05 4.268 768.32 4.272 1,002.75 66.599

5 1 769.72 5.794 771.63 5.815 774.33 5.872 1,001.25 65.556

2 758.77 2.913 760.64 2.923 763.23 2.952 1,001.25 65.556

3 782.02 9.029 783.97 9.062 786.80 9.152 1,001.25 65.556

4 761.50 3.631 763.38 3.645 766.00 3.681 1,001.25 65.556

6 1 763.41 3.211 763.48 3.212 764.36 3.223 1,000.00 54.322

2 756.05 1.615 756.12 1.615 756.97 1.620 1,000.00 54.322

3 789.32 8.831 789.40 8.832 790.36 8.861 1,000.00 54.322

4 770.87 4.829 770.95 4.830 771.85 4.846 1,000.00 54.322

7 1 759.48 2.300 763.07 2.317 763.69 2.323 1,000.25 43.193

2 752.87 1.158 756.41 1.166 757.01 1.169 1,000.25 43.193

3 788.94 7.390 792.73 7.442 793.43 7.460 1,000.25 43.193

4 769.31 3.999 772.97 4.027 773.61 4.036 1,000.25 43.193

8 1 762.22 1.618 762.77 1.620 763.74 1.626 1,002.50 34.001

2 756.28 0.813 756.82 0.813 757.77 0.817 1,002.50 34.001

3 796.06 6.206 796.64 6.213 797.74 6.237 1,002.50 34.001

4 774.20 3.243 774.76 3.246 775.78 3.258 1,002.50 34.001

values. The reference temperature for the temperature dependence (Tref) has been set to 20◦ C bySBRC. The variable K3 and the constant Tref are used in (3), (5), (6), and

(7). The K3 values for the eight SeaWiFS bands are listed

in Tables 10 and 11.

The temperature corrections require knowledge of the

temperatures of the focal plane assemblies. The conversion of the engineering information in the scan lines from

SeaStar into focal plane temperatures requires two steps

(Sections 8.2 and 8.3). Since there is onlyone temperature

sensor per focal plane assembly, the derived temperatures

for bands 1 and 2 will be identical, as will those for bands

3 and 4, and so forth. Thus, the coefficients for the calculation of these pairs of temperatures should be identical in

the following sections.

8.2 Data Conversion

In this section, the term voltages is used to refer to

voltages from SeaWiFS.

8.2.1 SeaStar Counts to Voltage

The voltages for everyanalog telemetryoutput from

SeaWiFS range from 0–5.11 volts. These voltages are trans19
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

mitted to the ground bySeaStar as 8 bit binarywords.

Since the counts from the SeaWiFS radiance measurements

are transmitted in 10 bit binarywords, the analog telemetryvalues are also packaged in 10 bits. However, for the

analog telemetryvalues, onlyeight of those 10 bits per

word are used.

Table 10. Values in the temperature dependence

equations. This table contains the temperature dependencycoefficients (K3) and the coefficients to

convert OSC telemetrycounts to temperature sensor output volts (K5 and K6).

Band K3 K5 K6

[(◦ C)−1] [volts/count] [volts]

1 0.000901 0.020 0.0

2 0.000585 0.020 0.0

3 0.000420 0.020 0.0

4 0.000390 0.020 0.0

5 0.000391 0.020 0.0

6 0.000151 0.020 0.0

7 0.000106 0.020 0.0

8 0.000078 0.020 0.0

Table 11. Current output for the current source

diodes (K7). These data are given for a temperature of 20◦ C. A correction must be applied to these

data at other temperatures. Values are given for

the prime and backup temperature sensors for each

focal plane assembly. The backup sensor for bands

3 and 4 is inoperative.

Band K7 (prime) K7 (backup)

[mA] [mA]

1 0.493 0.484

2 0.493 0.484

3 0.492 Inoperative

4 0.492 Inoperative

5 0.491 0.497

6 0.491 0.497

7 0.486 0.492

8 0.486 0.492

The SeaStar spacecraft will convert the analog voltages

from SeaWiFS using an ADC with a 5.12 volt reference

voltage. This will eliminate the need for scaling amplifiers

between SeaWiFS and the ADC. Since an 8 bit binarysignal ranges from 0–255 counts, each count will have a value

of 0.02 volts, with 0 counts equal to 0 volts. This corresponds to temperature sensor voltages that range from 0–

5.10 volts. This is the basis for the coefficients K5 and K6

in Tables 10 and 11. The conversion equation for SeaStar

telemetrycounts to SeaWiFS temperature sensor voltages

is linear:

VT = K5 × Ctemp + K6. (8)

The variables in (8) are defined as:

1. VT is the output from the focal plane temperature

sensor, in volts.

2. K5 is the spacecraft analog-to-digital (AD) conversion factor, in volts per count.

3. Ctemp is an 8-bit digital word in the SeaStar telemetry, in counts.

4. K6 is the offset in the spacecraft AD conversion, in

volts.

8.2.2 Voltage to Focal Plane Temperature

The temperature detection circuit for each focal plane

assemblyuses a precision thermistor in parallel with a

16.2 Kohm resistor. The current through the thermistorresistor pair is provided bya current source diode that has

a nominal output of 0.48 mA. The actual output of the current sources at 20◦ C are given in Tables 10 and 11. The

output from the temperature sensor is the voltage across

the thermistor-resistor pair as caused bythe current from

the diode. With knowledge of this voltage drop and of

the current from the source, it is possible to calculate the

effective resistance for the thermistor-resistor pair. With

knowledge of the value of the resistor, 16.2 Kohm, it is

then possible to calculate the actual resistance of the thermistor. From the thermistor resistance, the temperature

of the focal plane assemblyis derived using a conversion

equation that is specific for the type of thermistor in the

temperature sensor.

There is a small, additional complication to this calculation. The output from the current source diode has a

small temperature dependence. The correction for current

source diodes requires an approximate temperature (T C)

for the focal plane assembly. T C is good to about 2◦ C

over the range from 5◦ C to 45◦ C. The details of these

calculations will be presented in a series of steps below.

The calculated focal plane temperatures are accurate to

0.3◦ C. This includes the uncertaintyin the digitization of

the temperature sensor voltages bythe SeaStar spacecraft.

1. Calculate the approximate temperature, with

T C = (5−VT ) × 40/3, (9)

where T C is the approximate focal plane temperature, in

degrees Celsius.

2. Calculate the current from the current source diode,

including its temperature correction, with

ICS = K7−(0.0013 × (T C−20)), (10)

where ICS is the current from the current source diode,

in mA; K7 is the current from the diode at 20◦ C, in mA

(Tables 10 and 11); and (0.0013 × (T C−20)) is the temperature correction for the diode source, in mA.

3. Calculate the effective resistance for the thermistorresistor pair, with

RE = VT /ICS, (11)

20
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

where RE is the effective resistance for the thermistorresistor pair, in Kohms.

4. Calculate the resistance for the thermistor, with

RT = (16.2 × RE)/(16.2−RE), (12)

where RT is the resistance of the thermistor, in Kohms.

5. Calculate the focal plane temperature from the thermistor resistance using the following conversion equation.

T = −341 + 5398.94/(ln(254898 × RT )), (13)

where T is the focal plane temperature, in degrees Celsius,

and the constants are given values for the type of thermistor used in the temperature sensors.

The variable T, given for each SeaWiFS band, is used

in equations (3), (5), (6), and (7). The working range for

these temperature calculations is about +5◦ C to +45◦ C,

which roughlycorresponds to the range 1.7–5.0 volts, or

about 85–250 counts.

9. SPECTRAL RESPONSE

SBRC’s primarymethod for determining the spectral

response of SeaWiFS is based on the measurement of the

individual piece parts in the instrument’s optical train.

The transmission and reflection characteristics of the piece

parts and the responsivityof the detectors are multiplied

together to give the optical throughput of the eight SeaWiFS bands at 1 nm intervals. The results of the calculations are given as amperes of electrical current from the

photodiode per watt of radiant flux at each 1 nm interval.

This throughput must be combined with a radiance source

having the spectral shape of the sun to give the total output of each band for the full range of wavelengths.

SBRC’s secondarymethod is based on a system level

measurement using a monochromatic light source. This

source employs a single monochromator to provide light input with a spectral width of about 1 nm and with a known

radiant flux at each wavelength. In this method, the output of the photodiode is measured, in amperes, for a known

radiant flux from the monochromator, in watts, at each

1 nm interval, allowing the calculation of the throughput

of the instrument. This throughput is equivalent to that

from the piece part measurements. For SBRC, the system

level measurements give a double check of the piece part

calculations. The piece part values give the prime measure

of spectral response.

The system level measurements work reasonably well

in the wavelength range where there is at least a minimal

output from the band’s photodiode. There are, however,

regions outside of the pass bands of the radiometer’s optical components where the throughput of the system is sufficientlyclose to zero, such that essentiallyno measurable

current comes from the photodiode. In these wavelength

regions, noise in the system level measurements dominates

the results. The piece part results give much better values,

since theyare based on measurements of individual components, each having a small throughput. At the system

level, the measurements in these regions include the product of these small throughput contributions, which creates

an overall throughput at the system level that is immeasurablysmall.

The following sections contain discussions of the spectral responses of the individual parts for each SeaWiFS

band. These individual parts are shown in the aft optics schematic (Fig. 3). It is important to note that the

piece part measurements in the following sections have

been made of interference filters taken from the production run for the SeaWiFS flight filters. The piece part

measurements have not been made with the actual flight

filters.

9.1 Mirrors

There are five mirrors in the optical train for SeaWiFS.

Four of these are shown in Fig. 3. Theyare the primary

mirror, the polarization scrambler, the half-angle mirror,

and the collimator. The fifth mirror, a folding flat mirror

(not shown) is located after the collimating mirror (Fig. 17

of Woodward et al. 1993). Each of these mirrors uses silver as the reflecting surface, and four of these mirrors have

essentiallythe same spectral response. The reflectivityof

one of these four nearlyidentical mirrors has been measured at several wavelengths in the laboratorybySBRC.

Values between these wavelengths have been calculated by

linear interpolation. Figure 8 (top) shows the reflectivity

of one of the silver mirrors.

As part of the modifications to reduce straylight in SeaWiFS, the polarization scrambler was reworked to move

ghosts, in the direction normal to the scan plane, onto the

primaryimage. In this modification, the front surface of

the optical plate over the silver mirror in the scrambler

was tilted slightly, relative to the mirror. This change has

given the scrambler a slight wedge shape. A spectral scan

was made bySBRC of the reflectivityof the reworked polarization scrambler. This spectral response is also shown

in Fig. 8 (top).

The model for the combined reflectances of the five mirrors, including that for the polarization scrambler, is shown

in Fig. 8 (bottom). At each wavelength, the reflectance for

the silver mirror in Fig. 8 (top) is multiplied byitself four

times and then multiplied bythe reflectance of the polarization scrambler. This calculation accounts for the net

reflectance of these five mirrors in series in the SeaWiFS

optical train.

9.2 Lenses

There are four sets of lenses in the SeaWiFS aft optics (Fig. 3). These sapphire lenses are spectrallyflat over

wavelengths from 300–1,000 nm, covering the region of the

21
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 8. SeaWiFS mirror reflectances. The top panel shows measured values for a standard silver mirror and

for the polarization scrambler. The bottom panel shows the reflectance model for the four mirrors and the

polarization scrambler in series in the SeaWiFS optical train.

22
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

SeaWiFS measurements. The lenses have transmittances

of 99%. Theyare not included in the spectral response calculations. The lenses focus the collimated light that has

passed through the dichroics onto the focal plane assemblies. The angle with which this light converges must be

considered in the measurements of the spectral response of

the narrowband interference filters.

9.3 Dichroics

The SeaWiFS radiometer uses three dichroics. In the

SeaWiFS application, these dichroics transmit light for

wavelengths above a reference wavelength in the dichroic’s

design and reflect light below the reference wavelength.

The three SeaWiFS dichroics perform a prefiltering of the

radiance from the Earth, limiting the range of wavelengths

that reach the interference filters on each focal plane.

The dichroics and focal plane assemblies in SeaWiFS

are shown in schematic form in Fig. 3. In the schematic,

the light to the focal plane assemblyfor bands 7 and 8

(765 and 865 nm) passes in a straight line through two

dichroics. These dichroics are numbered 1 (nearest the

collimator) and 3. The light for bands 5 and 6 (555 and

670 nm) is transmitted bydichroic 1 and then reflected by

dichroic 3. In Fig. 3, the focal plane assemblyfor bands 5

and 6 is located on the far right.

The focal plane assemblyfor bands 1 and 2 (412 and

443 nm) is located behind the BG39 filter in Fig. 3. The

light for this focal plane is reflected off dichroics 1 and 2.

Dichroic 2 is located to the left of dichroic 1 in Fig. 3. The

remaining focal plane assembly, for bands 3 and 4 (490

and 510 nm), receives light reflected from dichroic 1 and

transmitted bydichroic 2. This focal plane assemblyis

located on the left most side of Fig. 3.

For each focal plane assembly, and its pair of SeaWiFS bands, light is either transmitted or reflected by

two dichroics. These dichroics playa major role in forming

the shapes of the spectral responses of the eight SeaWiFS

bands. This is particularlytrue in establishing the out-ofband response of the instrument. The performance specifications sent from SBRC to its filter manufacturer call for

the use of the dichroics to remove the transmission spikes

in regions well away from the pass band of the interference

filters. This has limited the number of filter elements required for the interference filters and has helped increase

the transmission of the interference filters to almost 100%

at their center wavelengths.

Figure 9 gives the spectral responses for the two dichroics in the optical train for band 1. Figure 9 (top) gives

the reflectance curve for dichroic 1. The cutoff for reflection bythis dichroic falls between 510 nm (band 4) and

555 nm (band 5), separating the four shortest wavelength

bands from the four longest. When viewing Fig. 9 (top),

it should be remembered that a dichroic either transmits

light or reflects it. The dichroics and the interference filters

in SeaWiFS are designed without light absorbing components. In regions where the reflectance is near 100%, the

transmittance is near 0%.

Figure 9 (bottom) gives the reflectance curve for dichroic 2. Its cutoff wavelength is between 443 nm (band 2)

and 490 nm (band 3). Dichroics 1 and 2 keep most of the

longer wavelength radiation from the Earth from reaching bands 1 and 2, which limits the wavelength range over

which the interference filters need to work.

For wavelengths above 1,140 nm, the SBRC measurements of dichroic 1 (Fig. 9, top) gave reflectances lower

than the resolution of the laboratoryequipment. For these

wavelengths, the laboratoryreadings were zero. The values from the dichroics contribute to spectral responses presented in the following sections. These results are presented as a set of logarithmic plots. As a result, the zero

values for dichroic 1 in this technical memorandum have

been replaced with amounts that are equal to the lowest

reflectance in the rest of the dichroic measurements. Similar measures have been taken at other places where the

SBRC data set has values of zero.

9.4 Broadband Filters

There are two pieces of Schott color glass filters in the

SeaWiFS aft optics assembly(Fig. 3). These broadband

filters were not part of the original SeaWiFS optical design.

The two filters (BG39 and BG26 in the figure) were added

to the instrument after the SeaWiFS critical design review.

Both broadband filters transmit in the blue-green region

of the visible spectrum and reflect in the red.

The transmission curve for the BG39 filter is shown in

Fig. 10 (top). This filter was added to the focal plane assemblyfor bands 1 and 2 to provide additional suppression

of transmission spikes from the interference filters in the

red. These spikes do not present a significant problem for

the 412 and 443 nm bands on orbit. The broadband filter

was added to aid in the radiometric calibration of bands 1

and 2 in the laboratory.

The light source used for the radiometric calibration of

the instrument does not have the same spectral shape as

the sun. The sun, with the approximate spectral shape of

a 5,900 K blackbody, has a peak flux output at a wavelength of 491 nm. The integrating sphere in the SBRC

laboratoryhas the spectral shape of a 2,850 K blackbody

and a maximum light output at 1,017 nm. This is true of

all integrating spheres since all use tungsten lamps as light

sources. Blackbodyradiation is described in Section 9.8.

In relative terms, the laboratorylight source produces

considerablymore light in the red portion of the spectrum

than in the blue-green. In relative terms, this excess of

red light from the integrating sphere accentuates the outof-band transmission spikes from the interference filters.

When using the integrating sphere, the so-called red leak

for band 1 (412 nm) is about 30% of the transmitted light

through the filter’s pass band.

23
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 9. Reflectance curves for SeaWiFS dichroics 1 and 2. These dichroics are in the optical train for bands

1 and 2 (412 and 443 nm). The top panel shows the reflectance curve for dichroic 1. This element separates

bands 1 through 4 from bands 5 through 8. The bottom panel shows the reflectance curve for dichroic 2. This

element separates bands 1 and 2 from bands 3 and 4.

24
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

The sun (or a 5,900 K blackbody) produces much more

radiance flux in the blue-green region than in the red. For

sunlight, the red leak through the 412 nm interference filter

is less than 1%. As a result, the BG39 filter has little use

to SeaWiFS on orbit. The filter has been of great utility

in the laboratorymeasurements which are required before

the instrument is placed in orbit.

The BG26 filter has been added to the instrument to

reduce the amount of light in the two longest SeaWiFS

bands (765 and 865 nm). This reduced optical signal has

helped the straylight characteristics for these bands. Although the BG26 filter has not been added to change spectral shapes in the instrument, its spectral effects must still

be considered.

The transmission spectrum for the BG26 filter is given

in Fig. 10 (bottom). Both colored glass filters have been

measured in the laboratoryat SBRC. The measurements

have been made at several wavelengths. Values between

these wavelengths have been calculated bylinear interpolation. With its position at the entrance to the focal plane

assembly, this filter affects the transmission of all eight SeaWiFS bands. Figure 10 (bottom), however, shows that the

curve has verylittle spectral shape over the pass bands of

the eight SeaWiFS interference filters. The BG26 broadband filter is spectrallyflat over SeaWiFS band 1 and adds

next to nothing to its spectral shapes.

9.5 Photodiode Responsivity

The responsivityof the photodiode used in SeaWiFS,

expressed in terms of mA of current out per mW of radiant

flux in, has been measured at SBRC. These measurements

are made for one of the diodes in the production batch

used for the instrument. SeaWiFS employs 32 photodiodes, and it is impractical to measure the spectral response

of each. As a result, there has been onlyone measurement

at SBRC. The diode’s response curve shows little structure

over the 10 and 20 nm half-widths of the eight SeaWiFS

bands (Fig. 11, top).

There is, however, structure in Fig. 11 (top) around

the 412 and 443 nm wavelengths. This results from noise

in the measurement at SBRC and not from the wavelength dependence of the efficiencyof the photodiode. The

SBRC measurements of the spectral response curve were

performed using the same equipment as the system level

test of the wavelength dependence from SeaWiFS. Light

from a monochromator was used to illuminate a NIST calibrated photodiode and then to illuminate the SeaWiFS

diode. The response curve in Fig. 11 (top) was calculated

using these data and the NIST calibration curve for the

reference photodiode. The monochromator used a tungsten lamp for its source, and the measurements had low

light levels in the blue wavelengths and increased noise at

wavelengths below 500 nm.

This artificial structure at the blue end of Fig. 11 (top)

was removed using a second order polynomial fit of the

data from 406–490 nm. These fitted results replace the laboratorymeasurements over wavelengths from 380–490 nm.

The measured values and the fitted curve are shown in

Fig. 11 (bottom). The extrapolation of the fitted curve to

380 nm removes unwarranted structure in the out-of-band

response of the bands below 400 nm, although the extrapolation creates some additional error in the instrument level

spectral response curves below 400 nm. This error, however, is a small part of values that are alreadyclose to

zero. The spectral response calculations in this technical

memorandum use the fitted curve from 380–490 nm.

Elements of the SeaWiFS optical train, other than the

photodiodes, transmit or reflect a fraction of the input radiance. In the calculation of the optical throughput of the

instrument, theyare dimensionless. It is the photodiodes

that convert mW of radiant flux into mA of electrical current. The diodes integrate the radiant flux over the wavelength range where the optics have a non-zero response.

For each channel, the electronics transform this current

into voltage, amplifythe voltage, and change the voltage

into counts with an ADC. This process gives the fundamental radiometric conversion factor for SeaWiFS, mW of

radiance flux per count.

9.6 Detector-to-Detector Differences

For the ocean color instrument, the performance specifications address spectral uniformityfor an instrument

with multiple detectors. SeaWiFS has four detectors per

band which are cut from a single piece of silicon substrate and lie in a single line. The individual detectors are

0.025 cm square with a 0.005 cm saw cut in the substrate

between them. The total length of the detector arrayis

0.117 cm. All four detectors lie under the same narrowband interference filter. Assuming a reasonable uniformity

in the silicon substrate and in the interference filters, the

spectral responses for the four detector elements in each

band are assumed to be essentiallyidentical. The response

curve for each band is dominated bythe band’s interference filter, with the photodiode adding little to the band’s

spectral response. The specifications call for the central

wavelength of each channel to be within +0.5 nm of the

central wavelength of all four channels. The prelaunch acceptance report (Barnes et al. 1994) shows the SeaWiFS

instrument to meet this specification.

9.7 Narrowband Filters

The spectral shape for each SeaWiFS band is dominated bythe band’s narrowband interference filter. The

response of this filter gives the principal definition of the

following for each SeaWiFS band: the center wavelength;

the band edges, from the full width at half maximum; and

the extended band edges, from the 1% power points. The

transmission curve for the 412 nm narrowband interference

filter from 380–580 nm is shown in Fig. 12 (top). This

25
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 10. Transmission curves for the SeaWiFS color glass filters. The top panel shows the transmission curve

for broadband filter BG39. This filter forms a part of the optical train for bands 1 and 2 (412 and 443 nm). The

bottom panel shows the transmission curve for broadband filter BG26. This filter is part of the optical train for

all eight SeaWiFS bands.

26
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 11. Spectral response curve for the SeaWiFS photodiodes. The top panel shows the measured response

curve for the diodes from 380 to 1150 nm. The bottom panel shows the measured values for the photodiodes

and the fitted curve from 380 to 550 nm. The spectral response calculations in this technical memorandum use

the fitted curve from 380 to 490 nm.

27
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 12. Transmission curve for the 412 nm narrowband interference filter. Vertical scale in linear units. The

top panel shows the transmission curve from 380–580 nm covering the pass band of the filter and the out-ofband response near 500 nm. The bottom panel shows the transmission curve for the filter from 380 to 1150 nm

showing the complete set of out-of-band responses.

28
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 13. Transmission curve for the 412 nm filter. The vertical scale is given in logarithmic units. This

accentuates the out-of-band response of the filter.

curve, with its linear ordinate, shows the features and the

shape of the pass band of the filter. Figure 12 (top) also

shows the out-of-band transmission leak near 500 nm.

Figure 12 (bottom) shows the response of the filter from

380–1,150 nm. The curve for the same filter is also given in

Fig. 13 with a logarithmic ordinate. Figure 13 shows the

large and the small out-of-band components of the filter.

Some of these peaks are substantial. The SeaWiFS optical

train for bands 1 and 2, however, contains dichroics and a

BG39 broadband color glass filter to reduce these out-ofband components.

The SeaWiFS radiometer uses a scrambler to remove

anypolarization that maybe present in the input radiance

to the instrument, although polarization is not a consideration in the measurement of the interference filters. The

lenses between the dichroics and the focal plane assemblies

(Fig. 3) convert the collimated light into converging light

with a relative aperture (f-number) of 2. For this f-number,

the distance between the lenses and the focal plane assemblies is twice the diameter of the opening in the lens assemblies. This converging angle has been included in the

measurements of the narrowband interference filters in the

laboratory.

The measurements of the SeaWiFS interference filters

at SBRC have been made with laboratoryequipment having a resolution of 0.0001 transmission units. There are

data points in the test results where the transmission of

the filters is less than that threshold. In the laboratory

results, these points have been given transmission values

of zero. For this report, the zero values have been replaced

with one-half of the resolution of the measurements, i.e.,

at 0.00005 transmission units. This allows for plots of the

results on a logarithmic axis.

For each SeaWiFS band, the dichroics substantiallyreduce the out-of-band transmission spikes. Figure 14 (top)

shows the net result for the interference filter for band 1

(from Fig. 12, top) combined with the response curves for

the band’s two dichroics (from Figure 9, top and bottom).

This figure uses a logarithmic ordinate to emphasize the

characteristics outside of the filter’s pass band. As shown

in Fig. 14 (top), the out-of-band response is reduced significantlywhen compared with Fig. 13.

A BG39 filter has been added to the instrument to further reduce the out-of-band response for the 412 nm channel. The inclusion of the BG26 and BG39 filter responses

to Fig. 14 (top) give the result in Fig. 14 (bottom). The

29
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

out-of-band response of band 1 above 600 nm is more than

four orders of magnitude below the transmission peak at

412 nm.

Figure 15 shows the addition of the mirror and the

photodiode responses to Fig. 14 (bottom). Figure 15 gives

the spectral response of SeaWiFS band 1 (412 nm) to a

spectrallyflat light source. At each wavelength from 380–

1,150 nm, this theoretical source provides a radiance of 1

mW cm−2 sr−1 μm−1. At each wavelength, the photodiode

converts this radiance into picoamperes of current. The integral of this current over wavelength gives the total output of the photodiode, in nanoamperes, for such a source.

Figure 15 provides an intermediate step in the spectral response calculations, since SeaWiFS views sources that have

specific spectral shapes, both in the laboratoryand on orbit. In addition, Fig. 15 marks the point in the spectral

calculations where the measurements of the band edges,

i.e., 50% power points, and extended band edges, i.e., 1%

power points, are made to show compliance with the SeaWiFS specifications.

The inclusion of the photodiode and the mirror responses to the data set in Fig. 15 has the effect of increasing

the out-of-band response for band 1 relative to the response

for the data sets in Fig. 14 (top and bottom). This can be

seen in the relative size of the peak near 650 nm in Fig. 15

(bottom). The diode and mirror responses have enhanced

this peak in Fig. 15 (bottom) between four and five times

relative to its size in Fig. 14 (top and bottom). The 650 nm

peak lies below the resolution of Fig. 14 (bottom), and is

four orders of magnitude below the transmission peak in

Fig. 15 (bottom).

9.8 Blackbody Radiation

For an idealized blackbodyradiator, the radiance can

be given as a function of temperature and wavelength. For

the purposes of calculations, the equation can be used in

the following form (Wyatt 1978):

L(λ) = 1.191066 × 108

λ5(e1.43883×104/λ T − 1), (14)

where λ is the wavelength, in μm; T is the temperature,

in kelvins; and L is the radiance, in W m−2 sr−1 μm.

The radiance values from (14) can be converted directly

into units of mW cm−2 sr−1 μm−1, the units for SeaWiFS

measurements. The radiance values from this equation increase at all wavelengths with increasing temperature, and

the peak of the curve shifts toward shorter wavelengths

with increasing temperature. These two effects are significant, since the laboratorymeasurements for SeaWiFS are

made with light sources that use tungsten lamps having

blackbodytemperatures near 2,850 K, while the sun approximates a 5,900 K source for wavelengths in the blue

and the visible (Allen 1973 and Warneck 1988). For this

reason, the SeaWiFS performance specifications require

that the out-of-band response for the SeaWiFS bands be

determined for a source with the spectral shape equivalent

to the sun.

There are xenon arcs and other laboratorysources that

mimic the wavelength dependent shape of the blackbody

shape of the sun better than tungsten lamps. These laboratorysources do not have the inherent stabilityof integrating spheres using sets of tungsten lamps. In addition, these other sources cannot be easilyadjusted over

the range of intensities necessaryfor the calibration of

the instrument. As a result, laboratoryradiometric measurements of the SeaWiFS bands have been made using

sources with tungsten lamps and then calculated for a

solar-equivalent spectral shape.

A 5,900 K blackbodycloselyapproximates the spectral

shape for the maximum anticipated radiances from the

Earth found in the SeaWiFS performance specifications

(Barnes et al. 1994). When the 5,900 K curve is normalized

to the eight cloud radiances in the specifications (Fig. 16),

the relative standard deviation of the curve values from

the cloud radiances is 5%, with the values from the curve

falling low in bands 1, 2, and 6. A fine tuning of the temperature of the solar equivalent blackbodytemperature to

other temperatures near 5,900 K will have no significant

effect on the spectral response calculations for SeaWiFS.

Figure 17 (top) shows the response of SeaWiFS band 1,

412 nm, to a source with the shape of a 5,900 K blackbody.

The flux from the blackbodycurve has been normalized at

412 nm to the typical saturation radiance for band 1 from

the SeaWiFS performance specifications, 13.63 mW cm−2

sr−1 μm−1 (Barnes et al. 1994). Figure 17 (bottom) shows

the same response but to a 2,850 K blackbodynormalized

at 412 nm.

In addition, the vertical scale for Fig. 17 (bottom) is

15 times greater than that for Fig. 17 (top). The radiation curve for the 2,850 K blackbodypeaks at 1,017 nm,

far to the red of the normalization wavelength, 412 nm.

The normalized 5,900 K curve in Fig. 17 (top) would appear as a small hump in Fig. 17 (bottom) The illumination

of SeaWiFS band 1 at the SeaWiFS saturation radiance

level with the SBRC integrating sphere also illuminates

this band with considerable light in the red region. At

1,000 nm, the illumination is about 16 times greater than

the illumination at the band’s center wavelength. This

is a factor that must be considered in the transfer of the

SeaWiFS radiometric calibration in the laboratoryto the

calibration on orbit.

The system response curves in Fig. 15 are given for a

spectrallyflat source with a radiance of 1 mW cm−2 sr−1

mum−1. The two blackbodyradiance curves in Fig. 17

use the same units. This allows a direct calculation of the

response of SeaWiFS band 1 to 2,850 K and 5,900 K sources

that have a radiance of 13.63 mW at 412 nm. Figure 18

shows the differences between the output of SeaWiFS band

1 when exposed to 2,850 K and 5,900 K blackbodysources.

30
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 14. Combined transmission curves for the interference filter, dichroics, and broadband filters in SeaWiFS

band 1. The top panel shows the combined curve for the interference filter and dichroics. Compared with

Fig. 10, the out-of-band response has been reduced byabout two orders of magnitude. The bottom panel shows

the combined curve for the interference filter, dichroics, and broadband filters. The out-of-band response above

600 nm has been further reduced bytwo orders of magnitude.

31
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 15. System response curve for SeaWiFS band 1. This includes the optical components and the photodiode

response to a spectrallyflat light source with a radiance of 1 mW cm−2 sr−1 μm−1. The top panel shows the

system response on a linear scale. The out-of-band response near 500 nm is barely noticeable. The bottom panel

shows the system response on a logarithmic scale.

32
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 16. Radiance curve for a 5,900 K blackbodynormalized to the cloud radiances from the SeaWiFS performance specifications.

The spectral response characteristics for the other seven

SeaWiFS bands are calculated in a manner that is the

same as the method that has been described here for band

1. The spectral responses of all eight bands to a 5,900 K

source are shown below (Figs. 19–24). The integrated output of the instrument for the 2,850 K and 5,900 K blackbodytemperatures must be used in ratios to convert the

laboratorymeasured radiance calibration for SeaWiFS to

the in-flight calibration. For SeaWiFS band 1, the laboratorymeasurements will give a slightlygreater number of

counts per unit radiance than will the measurements on orbit. The results of these calculations for all eight SeaWiFS

bands are given below.

The performance specifications for SeaWiFS allow outof-band responses that are less than 5% of those within

the extended band edges, which are defined bythe 1%

power points. These out-of-band response specifications

cover the response of the instrument to a radiance source

that mimics the sun. Calculations for all eight bands, using

the 5,900 K blackbody, are also given in Section 12. In

addition, the band edges and extended band edges for all

of the SeaWiFS bands are tabulated in Section 11. These

values are calculated for a spectrallyflat light source, a

5,900 K blackbody, and for a 2,850 K source.

9.9 Absolute Throughput

Using additional information, including the collection

aperture for the instrument and the instrument’s field of

view, SBRC has calculated the absolute throughput for the

eight SeaWiFS bands. Using these data and the output of

the instrument when illuminated byan integrating sphere,

SBRC has calculated the sphere radiance based ultimately

on the piece part curves. The worst comparison between

the calculated and measured sphere radiances was for band

1 (412 nm). For this band, the output of the instrument

was about 50% greater than expected in the calculations.

Absolute instrument throughput is not investigated here.

9.10 Spectral Response Curves

In previous sections, the spectral responses of SeaWiFS

band 1 for three light sources have been given: spectrally

flat, 5,900 K blackbody, and 2,850 K blackbody. The radiances from the two blackbodylight sources were normalized to the expected saturation radiance for band 1 at

33
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 17. Blackbodyradiance curves normalized to the saturation radiance at 412 nm from the SeaWiFS

specifications. The top panel shows the curve for a 5,900 K blackbodythat approximates the solar spectrum.

The bottom panel shows the curve for a 2,850 K blackbodythat approximates the SBRC integrating sphere.

34
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 18. Instrument output for the two blackbodies. The differences in the instrument’s responses to the

laboratoryand flight sources are a factor in the on-orbit radiometric calibration of the SeaWiFS. The top panel

shows the instrument output using a linear scale. This demonstrates the in-band differences from the two light

sources. The bottom panel shows the instrument output using a logarithmic scale. This shows the out-of-band

differences.

35
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 19. Instrument output for the SeaWiFS bands. These are the responses of the instrument to a 5,900 K

blackbody. The linear scale shows the in-band responses. The top panel shows Bands 1 and 2 (412 and 443 nm).

The bottom panel shows Bands 3 and 4 (490 and 510 nm).

36
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 20. Instrument output for the SeaWiFS bands. These are the responses of the instrument to a 5,900 K

blackbody. The linear scale shows the in-band responses. The top panel shows Bands 5 and 6 (555 and 670 nm).

The bottom panel shows Bands 7 and 8 (765 and 865 nm).

37
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 21. Instrument output for the SeaWiFS bands. These are the responses of the instrument to a 5,900 K

blackbody. The logarithmic scale shows the out-of-band responses. The top panel shows Band 1 (412 nm). The

bottom panel shows Band 2 (443 nm).

38
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 22. Instrument output for the SeaWiFS bands. These are the responses of the instrument to a 5,900 K

blackbody. The logarithmic scale shows the out-of-band responses. The top panel shows Band 3 (490 nm). The

bottom panel shows Band 4 (510 nm).

39
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 23. Instrument output for the SeaWiFS bands. These are the responses of the instrument to a 5,900 K

blackbody. The logarithmic scale shows the out-of-band responses. The top panel shows Band 5 (555 nm). The

bottom panel shows Band 6 (670 nm).

40
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 24. Instrument output for the SeaWiFS bands. These are the responses of the instrument to a 5,900 K

blackbody. The logarithmic scale shows the out-of-band responses. The top panel shows Band 7 (765 nm). The

bottom panel shows Band 8 (865 nm).

41
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

the band’s nominal center wavelength, 412 nm. For the

overview of the responses of the eight SeaWiFS bands in

this section, the response of the eight bands to a 5,900 K

blackbodysource, a source that approximates the solar

spectrum will be shown. For each of the bands, the 5,900 K

radiance has been normalized to the expected saturation

radiance at that band’s nominal central wavelength.

Figure 19 (top) gives the in-band responses for the 412

and 443 nm channels, and Fig. 21 gives their out-of-band

responses. The plots in Fig. 21 show that the interference

filters for the two bands exhibit similarities, especiallyevident in their out-of-band transmittances near 500–560 nm.

The similarities between the filters seem reasonable, since

their peak transmittances are only30 nm apart. For bands

1 and 2, the components in the optical train—other than

the interference filters—are the same, so the similarities in

Fig. 21 come from the filters, themselves.

For band 1, and to a lesser extent for band 2, there has

been a reduction in the blue side of the in-band response

of the filter. This can be seen most easilyin a comparison

of the size of the left side transmission peak in the band

1 interference filter near 406 nm in Fig. 12 (top) with the

corresponding system response peak in Fig. 15 (top) Both

the mirror reflectances (Fig. 8, bottom) and the photodiode output (Fig. 11, top) act to trim the blue side of the

output from the interference filter for band 1. Combined

with the shape of the 5,900 K blackbodyresponse near

412 nm (Fig. 17, top), these elements are responsible for

the reduction in the portion of the peak nearest the blue

end of the spectrum of band 1 in Fig. 19 (top). To a lesser

extent, these elements are also responsible for the reduced

blue side peak in the response of band 2. For both bands 1

and 2, the system responses are about one-half nanometer

more to the red side than the responses of the interference

filters.

The in-band responses for SeaWiFS bands 3 and 4 are

shown in Fig. 19 (bottom), and the out-of-band responses

are shown in Fig. 22. As was seen for bands 1 and 2, the

out-of-band responses for bands 3 and 4 show marked similarities (Fig. 22). This is particularlytrue for the transmission curves for the 490 and 510 nm interference filters

(not shown) above 800 nm. In this region, the interference

filters transmit 70% to 80% of the incident radiation. However, the two dichroics in the optical path for these bands

effectivelyremove most of this flux before it reaches the

filters.

Figure 20 (top) gives the in-band responses for the 555

and 670 nm channels, and Fig. 23 gives their out-of-band

responses. Figure 20 (top) shows an out-of-band leak between 690 and 700 nm. This peak and a second out-of-band

peak around 450 nm are more clearlyseen in Fig. 23. In relative terms, the out-of-band response of band 5 is just over

twice that for band 6. The calculations of the out-of-band

responses for the eight SeaWiFS bands are summarized in

Section 12.0.

The in-band responses for SeaWiFS bands 7 and 8 are

shown in Fig. 20 (bottom), and the out-of-band responses

are shown in Fig. 24. These bands have been designed

with twice the spectral widths of the other SeaWiFS bands.

The small scale structure, i.e., the lack of smoothness in

the peak of the curve for band 7 (765 nm) is an artifact of

the measurement of the band’s interference filter bySBRC.

Interference filters of the type used in SeaWiFS exhibit a

much smoother spectral response. Band 8 (865 nm) shows

the greatest out-of-band leakage for the SeaWiFS set, both

in absolute and relative terms. However, its 3.7% out-ofband response is well within the 5% maximum value in the

SeaWiFS performance specifications.

Figures 21–24 shows the output of the photodiodes for

each SeaWiFS band in picoamperes per nanometer (from

380–1,150 nm). The actual current from the photodiode

is the sum of the band output over the entire wavelength

range. The photodiode serves to integrate the instrument’s

spectral response. The integrals, i.e., summations, of the

spectral responses of the eight SeaWiFS bands are given

in Table 12. These summations are given for the responses to a 5,900 K blackbody(Figs. 21–24) and to a 2,850 K

blackbody(not shown). In the calculations for each band,

both sources give the saturation radiance for the band at

the band’s nominal center wavelength (for band 1, 13.63

mW at 412 nm; for band 2, 13.25 mW at 443 nm; and so

forth).

Table 12 also provides factors which allow conversion

between the 2,850 K light source used in the laboratory

and a 5,900 K source that mimics the sun. For band 1

(Table 12), the output of the photodiode in response to

a 5,900 K source is almost 4% less than the output in response to a 2,850 K source. Correction factors, similar to

the ones in Table 12, must be used in the transfer of the

laboratoryradiometric calibrations to orbit.

The out-of-band responses for the eight SeaWiFS bands

are part of the instrument’s radiometric calibration. In

that calibration, the instrument views a broad area of

known radiance, and records the output from the bands

in counts. The counts from each band include the outof-band contributions which are functions of the spectral

shape of the source that is measured. The SeaWiFS laboratorycalibration has the out-of-band correction for a

2,850 K source factored into its results. If the instrument

measures a source with that spectral shape, those measurements automaticallycontain the appropriate out-of-band

corrections.

The prelaunch calibration equations for SeaWiFS contain correction terms that convert the out-of-band responses from those for a 2,850 K source to those for a 5,900 K

source. As a result, the SeaWiFS calibration equations

now have the out-of-band correction for a 5,900 K source

factored into them. The 5,900 K spectral shape duplicates the spectral shape for SeaWiFS ocean measurements

closely. The errors that arise from the use of the 5,900 K

42
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Table 12. Integrated instrument responses for the eight SeaWiFS bands. The results are given for responses in

nanoamperes to 2,850 K and 5,900 K blackbodyradiance sources. The radiances for the blackbodysources are

normalized to the expected saturation radiance for each band at the nominal center wavelength for each band.

The conversion factor gives a fractional multiplier to convert the laboratoryinstrument response using a 2,850 K

blackbodyto the response on orbit using a 5,900 K source. These results are calculated over the wavelength

range from 380 –1,150 nm.

Band Response to Response to Conversion Normalization Normalization

2,850 KSource 5,900 KSource Factor Wavelength Radiance

[nA] [nA] [nm] [mW cm−2 sr−1 μm−1]

1 2.275 2.190 0.963 412 13.63

2 3.493 3.435 0.983 443 13.25

3 4.424 4.336 0.980 490 10.50

4 4.624 4.613 0.998 510 9.08

5 3.749 3.717 0.991 555 7.44

6 2.069 2.092 1.011 670 4.20

7 2.875 2.859 0.995 765 3.00

8 2.249 2.274 1.011 865 2.13

out-of-band corrections for ocean measurements are estimated to be small, or a few tenths of a percent. If an alternate out-of-band correction is to be used, then the 5,900 K

correction must be removed from the measurement results

and a new out-of-band correction inserted in its place.

10. SYSTEM LEVEL RESPONSE

System level measurements of the spectral response of

SeaWiFS were made using a 0.5 m monochromator, illuminated with a 100 W halogen lamp, as a light source.

The slits on the monochromator were adjusted to give a

spectral resolution for the source (full width at half maximum) of 0.9 nm at 546 nm. The wavelength accuracyof

the monochromator was checked using five emission lines,

in first and second order, from a mercurylamp.

The relative output energyfrom the monochromator

was measured using a photodiode with a known quantum

efficiency, i.e., with a known number of mA of current out

per mW of radiant flux in. The output of this photodiode, after amplification and conversion to mW of radiant

energy, is shown in Fig. 25. No effort was made to determine the complete set of geometric (goniometric) factors

in the illumination of the calibrated photodiode, such as

the area of illumination of the diode and the solid angle

of the illuminating light. Figure 25 gives relative values,

only. Since the comparisons in this section are of spectral

responses normalized to 100%, the calculation of absolute

radiances is not necessary.

In previous sections, the calculated piece part results

for the SeaWiFS bands were convolved with the spectral

shapes of blackbodysources to give the responses of the

instrument to laboratoryand solar spectra. For the comparisons in this section, the shape of the halogen lamp and

monochromator light source must be removed from the

system level measurements of the SeaWiFS bands. This is

done bydividing the output of the bands bythe values in

Fig. 25 and normalizing the result to 100%. This gives the

system level response to a light source that is spectrally

flat over the wavelength range from 380–1,100 nm. For the

piece part measurements, the responses to a spectrallyflat

source have also been calculated (Fig. 15).

Data from the SeaWiFS instrument in the system response tests has been taken from the output of the preamplifiers for each band. These voltages have been amplified with a lock-in amplifier and digitized with a 12 bit

ADC. This modification to data acquisition gives the tests

a sensitivityto low light levels that is orders of magnitude

better than the resolution of the standard digital output

from the sensor. Piece part calculations, however, still give

results that are one to two orders of magnitude more sensitive than the minimum detectable limit of the system level

measurements.

The piece part and the system level spectral response

curves for the eight SeaWiFS bands are shown in Figs. 26

and 27. The system level measurements have been made

over 60 nm wavelength ranges, centered for each band’s

nominal pass band. In Figs. 26 and 27, the piece part

wavelength ranges have been trimmed to the same ranges

as the system level results. For this reason, the spectral

curves for bands 7 and 8 float above the horizontal axis

in Fig. 27 (bottom). The system level measurements for

bands 7 and 8 do not come down to the abscissa.

The band edge values from the piece part and system

level measurements are given in the next section. All of the

band edges, which give the full width at half maximum for

the spectral response, are within 1 nm for the two sets of

measurements here—with the exception of two, the lower

band edges for band 3 and band 7 (Fig. 26, bottom, and

Fig. 27, bottom). In all cases, however, each band edge

measurement is within the performance specifications for

the instrument.

The shapes of the peaks for the two measurement sets

show differences. Requirements for these shapes are not

43
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 25. Spectral shape of the light source for the system level spectral response Measurements. The curve

gives the output of a photodiode with known quantum efficiencywhich is illuminated bythe output slit of

the monochromator. Geometric (goniometric) factors, such as the illuminated area of the photodiode, are

not included in the calculation of the curve, so absolute radiance values are not given in the ordinate for the

graph.

part of the SeaWiFS specifications. Both the piece part

and the system level results are within the requirements of

the specifications, and the measurement sets are final and

will not be repeated. The most reasonable explanation for

the differences is that the system level measurements have

been made with the flight filters, and the piece part measurements have been made with filters from flight spares

that have come from the same lot as the flight parts.

During the thermal vacuum testing of SeaWiFS, in the

spring of 1993, the focal plane assemblyfor bands 5 and 6

was replaced due to an electrical problem. When the interference filter for band 5 was removed from the old focal

plane assembly, it was chipped. A replacement filter was

placed in the new assembly. The system level measurements for bands 5 and 6 were not repeated after thermal

vacuum testing. As a result, the system level measurements for band 5 no longer represent the actual flight unit

but represent a band with an interference filter that has

come from the same lot as the flight part, like the piece

part measurements.

For band 1 (Fig. 26, top), the shape of the system level

spectral response resembles the transmission curve for the

412 nm interference filter (Fig. 12, top). It seems likely

that the model for the mirror reflectances (Fig. 8, bottom)

shows too sharp a reduction in reflectance at the lowest

wavelengths. Such an effect would remove too much of the

blue side of the piece part results in Fig. 26 (top).

There are differences between the piece part and the

system level response curves. The piece part results are

considered prime, and the system level measurements are

considered backup checks. The differences between the two

sets of measurements are sufficientlysmall, and the system

level results verifythe piece part results. The comparison of the out-of-band measurements for the eight bands

are given in Figs. 28–31. The comparisons are given for

the responses to a spectrallyflat radiance source. This is

the arrangement used to calculate the band edges to check

agreement with the SeaWiFS performance specifications.

Figures 28–31 show the limited dynamic range for the system level measurements. To improve the sensitivityof the

44
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 26. Spectral responses for the SeaWiFS bands. The curves show the response of the bands to a spectrally

flat source as measured at the system level and as calculated from piece part measurements. The responses are

normalized to 100%. The top panel shows Bands 1 and 2 (412 and 443 nm). The bottom panel shows Bands 3

and 4 (490 and 510 nm).

45
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 27. Spectral responses for the SeaWiFS bands. The curves show the response of the bands to a spectrally

flat source as measured at the system level and as calculated from piece part measurements. The responses are

normalized to 100%. The top panel shows Bands 5 and 6 (555 and 670 nm). The bottom panel shows Bands 7

and 8 (765 and 865 nm).

46
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 28. Out-of-band measurements for the SeaWiFS bands. The curves show the response of the bands to

a spectrallyflat source as measured at the system level and as calculated from piece part measurements. The

responses are normalized to 100%. The top panel shows Band 1 (412 nm). The bottom panel shows Band 2

(443 nm).

47
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 29. Out-of-band measurements for the SeaWiFS bands. The curves show the response of the bands to

a spectrallyflat source as measured at the system level and as calculated from piece part measurements. The

responses are normalized to 100%. The top panel shows Band 3 (490 nm). The bottom panel shows Band 4

(510 nm).

48
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Fig. 30. Out-of-band measurements for the SeaWiFS bands. The curves show the response of the bands to

a spectrallyflat source as measured at the system level and as calculated from piece part measurements. The

responses are normalized to 100%. The top panel shows Band 5 (555 nm). The bottom panel shows Band 6

(670 nm).

49
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Fig. 31. Out-of-band measurements for the SeaWiFS bands. The curves show the response of the bands to

a spectrallyflat source as measured at the system level and as calculated from piece part measurements. The

responses are normalized to 100%. The top panel shows Band 7 (765 nm). The bottom panel shows Band 8

(865 nm).

50
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

system measurements, the lock-in amplifier was set to saturate at about 10% of the full-scale output for each band.

The lower limit for the system level measurements, for instance, the output from 600–1,100 nm in Fig. 28 (bottom),

has been set at one count in these figures. Where the actual output of the lock-in amplifier was zero or negative in

the measurements, theyhave been set to one count in this

presentation. One count sets the resolution limit for the

system level measurements.

Above the resolution limit for the system level measurements, there are structures in the piece part results that

are not seen in the output from the lock-in amplifier. In

general, these differences are at levels that are four to five

orders of magnitude below the peak transmission of the

bands. Again, the differences in the two sets of measurements are sufficientlysmall that the system level results

verifythe piece part results.

11. BAND EDGE WAVELENGTHS

The band edge measurements are given for the wavelengths at which the instrument response equals half that

at the peak. The extended band edges give the points at

which the output is 1% of the peak. The results (Table 13)

come from the data sets that make up the sets of figures in

this memorandum. The results from the piece part calculations are given for three source spectral shapes which are

a spectrallyflat source, a 5,900 K blackbody, and a 2,850 K

blackbody.

Table 13 includes the band edge calculations for the

piece part measurement of each interference filter, plus the

values from the system level measurements. For the system

level measurements, the spectral shape of the monochromator light source has been removed. Thus, the system

level measurements are equivalent to the measurement of

piece part values using a spectrallyflat light source. Since

the data in the measurement sets are tabulated at one

nanometer intervals, the values in Table 13 have been obtained bylinear interpolation.

12. OUT-OF-BAND RESPONSE

The values in Table 14 are based on the measurements

shown in Figs. 21–24. The locations of the upper and lower

extended band edges come from Table 13. All of the values in Table 14 come from the piece part calculations for

the SeaWiFS bands as illuminated bya 5,900 K blackbody.

This is the arrangement set forth in the SeaWiFS performance specifications (Barnes et al. 1994).

Figures 21–24 give the response of the instrument in

picoamperes per nanometer. Table 14 gives the following

sums of these responses over three wavelength ranges: from

380 nm to the lower extended band edge; from the lower

to the upper extended band edge; and from the upper extended band edge to 1,150 nm. The SeaWiFS specifications

require that out-of-band responses be no more than 5% of

the response between the extended band edges. The instrument is within these specifications for all eight bands.

13. SUMMARY

There are aspects to the SeaWiFS calibration equations

that are specific to the spacecraft instrument. For example,

there are small differences in the output of SeaWiFS from

side to side of the half angle mirror. Most ground based

instruments do not scan in the same manner as SeaWiFS.

In addition, SeaWiFS uses bilinear gains to allow high sensitivitymeasurements of ocean-leaving radiances and low

sensitivitymeasurements of radiances from clouds, which

are much brighter than the ocean (Section 3.0).

There are, however, manycalibration factors that are

common to SeaWiFS and to other radiometers. The application of these factors to the SeaWiFS calibration equations has been presented both for users of the data set

from the satellite instrument and for researchers making

ground-based radiance measurements in support of SeaWiFS. Ground based radiometric measurements must account for manyof these calibration factors.

In particular, there has been a detailed discussion of the

spectral responses of the eight SeaWiFS bands. The discussions have been developed to show the integrated spectral responses within SeaWiFS as functions of the shape

of the radiant source measured bythe instrument (Section

9.10). These out-of-band responses of the bands contribute

to the laboratorycalibration of SeaWiFS. For the instrument’s calibration coefficients presented here, the out-ofband response for the SBRC integrating sphere has been

replaced with the out-of-band response for a source with

the spectral distribution identical to the solar flux (Sections 7.2 and 7.3). Such spectral considerations mayalso

be important for experimenters making ground based radiance measurements in support of SeaWiFS.

Acknowledgements

The authors and editors wish to acknowledge the editorial assistance of Barbara A. Leroux, who worked on this document

during her participation in the NASA/Goddard Prince George’s

County Teacher Intern Program.

Glossary

AD Analog-to-Digital

ADC Analog-to-Digital Converter

CZCS Coastal Zone Color Scanner

IFOV Instantaneous Field-Of-View

NASA National Aeronautics and Space Administration

NIST National Institute of Standards and Technology

OSC Orbital Sciences Corporation

SBRC Santa Barbara Research Center

SeaWiFS Sea-viewing Wide Field-of-view Sensor

SNR Signal-to-Noise Ratio

51
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Table 13. Band edges (half maximum wavelengths) and extended band edges (1% wavelengths) for SeaWiFS bands

1–4. The center wavelength is calculated from the upper and lower band edges. Results are given for three light

sources: spectrally flat, 5,900 K blackbody, and 2,850 K blackbody. Results are also given for the interference filter

only, and for the system level measurement using the monochromator as a light source.

Band Nominal Lower Extended Lower Center Upper Upper Extended Source

Band Edges Band Edge Band Edge Wavelength Band Edge Band Edge

[nm] [nm] [nm] [nm] [nm] [nm]

1 402–422 394.9 403.1 413.2 423.3 433.4 SpectrallyFlat

395.1 403.3 413.3 423.4 433.6 5,900 K

395.9 404.5 414.1 423.7 434.8 2,850 K

393.6 402.4 412.6 422.7 432.3 Filter Only†

393.8 402.3 413.0 423.7 433.8 System Level§

2 433–453 424.0 434.1 443.9 453.7 463.7 SpectrallyFlat

424.1 434.2 444.0 453.8 463.7 5,900 K

424.8 435.1 444.6 454.1 464.3 2,850 K

423.3 433.5 443.5 453.5 464.6 Filter Only†

422.3 433.6 444.1 454.6 463.8 System Level§

3 480–500 470.7 480.8 491.1 501.4 511.8 SpectrallyFlat

470.7 480.8 491.1 501.4 511.8 5,900 K

471.3 481.5 491.6 501.6 512.3 2,850 K

470.1 480.5 490.8 501.2 511.3 Filter Only†

468.1 479.1 490.1 501.1 511.7 System Level§

4 500–520 488.1 498.9 510.1 521.3 530.7 SpectrallyFlat

488.1 498.9 510.1 521.2 530.7 5,900 K

488.9 499.4 510.5 521.5 531.1 2,850 K

487.8 498.7 509.9 521.0 532.9 Filter Only†

487.2 498.6 510.3 522.0 530.9 System Level§

5 545–565 536.4 545.5 554.6 563.8 577.3 SpectrallyFlat

536.3 545.4 554.6 563.8 577.2 5,900 K

536.9 545.8 554.9 563.9 577.9 2,850 K

536.6 545.5 554.6 563.8 577.1 Filter Only†

535.3 544.6 554.2 563.9 577.0 System Level§

6 660–680 646.8 658.3 668.2 678.2 692.7 SpectrallyFlat

646.7 658.3 668.2 678.1 692.5 5,900 K

646.9 658.5 668.4 678.3 692.9 2,850 K

646.8 658.4 668.3 678.2 692.8 Filter Only†

646.2 658.8 668.8 678.8 692.2 System Level§

7 745–785 728.0 744.7 764.9 785.0 814.5 SpectrallyFlat

727.6 744.6 764.6 784.6 812.9 5,900 K

728.4 745.1 765.1 785.1 815.6 2,850 K

725.1 744.3 765.0 785.7 814.2 Filter Only†

‡ 743.3 763.8 784.2 ‡ System Level§

8 845–885 826.7 845.7 866.4 887.0 908.2 SpectrallyFlat

826.4 845.5 866.1 886.7 907.5 5,900 K

826.8 845.7 866.5 887.2 908.4 2,850 K

826.5 845.6 866.2 886.9 908.0 Filter Only†

‡ 845.6 866.4 887.2 ‡ System Level§

† Calculated from measurements of the narrowband interference filter only.

§ Calculated from system level measurements using a monochromator as the light source.

‡ Outside of the range of the system level measurements.

52
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Table 14. Calculated out-of-band responses for the eight SeaWiFS bands. The instrument responses are given

as the output of the photodiode in picoamperes. The 5,900 K radiances in the calculations are normalized to

the expected saturation radiance for each band at the nominal center wavelength for each band. The upper and

lower extended band edges come from Tables 7 and 8. These results are calculated over the wavelength range

from 380 nm to 1150 nm.

Band Lower Lower In-Band Upper Upper Out-of-Band

Out-of-Band Extended Response Extended Out-of-Band Response

Response [pA] Band Edge [nm] [pA] Band Edge [nm] Response [pA] [%]

1 3.38 395.2 2,175.35 433.6 11.77 0.70

2 9.59 424.1 3,418.80 463.7 1.56 0.33

3 6.48 470.7 4,301.14 511.7 28.08 0.80

4 17.32 488.1 4,586.23 530.7 8.96 0.58

5 39.14 536.6 3,631.84 577.2 46.14 2.35

6 12.66 646.7 2,071.19 692.2 7.84 0.99

7 10.17 727.3 2,818.97 813.4 29.58 1.41

8 66.36 826.4 2,191.97 907.5 15.43 3.73

TDI Time Delay and Integration

Symbols

A0 Coefficient for linear term in scan modulation correction equation.

B0 Coefficient for power term in scan modulation correction equation.

C1 Measured value for the flight diffuser on a given scan

line, in counts.

C2 Measured value of the flight diffuser for the scan line

immediately sequential to the first scan line used to

measure the flight diffuser, i.e., S1, in counts.

Cdark Instrument dark restore value, in counts.

Cout Instrument output, in counts.

Ctemp Temperature sensor output, in counts, represented

by an 8 bit digital word in the SeaStar telemetry.

ds Detector configuration datum.

G1 Gain setting 1.

G2 Gain setting 2.

G3 Gain setting 3.

G4 Gain setting 4.

gs Gain selection datum.

ICS Current from the current source diode.

K1 Primary instrument sensitivity factor.

K2 Gain factor.

K3 Temperature dependence of detector output.

K4 Scan modulation correction factor.

K5 Spacecraft analog to digital conversion factor.

K6 Analog-to-digital offset in spacecraft conversion.

K7 Current from the diode at 20◦C.

L(λ) Radiance.

Lnadir Measured radiance at nadir.

LS1 Measured radiance for mirror side 1.

LS2 Measured radiance for mirror side 2.

Lscan Measured radiance at any pixel in a scan.

Pxl Pixel number, i.e., the numerical designation of a

pixel in a scan line.

R1 Multiplier for mirror side 1.

R2 Multiplier for mirror side 2.

RE Effective resistance for the thermistor-resistor pair.

RT Resistance of the thermistor.

T Measured temperature of the focal plane assembly.

T C Approximate focal plane temperature.

Tref Reference temperature for the temperature dependence (20◦ C).

VT Focal plane temperature sensor voltage output.

References

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Barnes, R.A., and A.W. Holmes, 1993: Overview of the SeaWiFS Ocean Sensor. SPIE, 1,939, 224–232.

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1994: Prelaunch Acceptance Report for the SeaWiFS Radiometer. NASA Tech. Memo. 104566, Vol. 22, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 32 pp.

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53
SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization

Hooker, S.B., C.R. McClain, and A.W. Holmes, 1993: Ocean

Color Imaging: CZCS to SeaWiFS, Mar. Tech. Soc. J., 27,

3–5.

McClain, C.R., W.E. Esaias, W. Barnes, B. Guenther, D. Endres, S.B. Hooker, B.G. Mitchell, and R. Barnes, 1992:

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Memo. 104566, Vol. 3, S.B. Hooker and E.R. Firestone,

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Maryland, 41 pp.

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Maryland, 60 pp.

Warneck, P., 1988: Chemistry of the Natural Atmosphere. Academic Press, 757 pp.

Woodward, R.H., R A. Barnes, C.R. McClain, W.E. Esaias,

W.L. Barnes, and A.T. Mecherikunnel, 1993: Modelling of

the SeaWiFS Solar and Lunar Observations, NASA Tech.

Memo. 104566, Vol. 10, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 26 pp.

Wyatt, C.L., 1978: Radiometric Calibration: Theory and Methods. Academic Press, 200 pp.

The SeaWiFS Technical Report Series

Vol. 1

Hooker, S.B., W.E. Esaias, G.C. Feldman, W.W. Gregg, and

C.R. McClain, 1992: An Overview of SeaWiFS and Ocean

Color. NASA Tech. Memo. 104566, Vol. 1, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, 24 pp., plus color plates.

Vol. 2

Gregg, W.W., 1992: Analysis of Orbit Selection for SeaWiFS:

Ascending vs. Descending Node. NASA Tech. Memo.

104566, Vol. 2, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 16 pp.

Vol. 3

McClain, C.R., W.E. Esaias, W. Barnes, B. Guenther, D. Endres, S. Hooker, G. Mitchell, and R. Barnes, 1992: Calibration and Validation Plan for SeaWiFS. NASA Tech.

Memo. 104566, Vol. 3, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 41 pp.

Vol. 4

McClain, C.R., E. Yeh, and G. Fu, 1992: An Analysis of GAC

Sampling Algorithms: A Case Study. NASA Tech. Memo.

104566, Vol. 4, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 22 pp., plus color plates.

Vol. 5

Mueller, J.L., and R.W. Austin, 1992: Ocean Optics Protocols

for SeaWiFS Validation. NASA Tech. Memo. 104566, Vol.

5, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 43 pp.

Vol. 6

Firestone, E.R., and S.B. Hooker, 1992: SeaWiFS Technical

Report Series Summary Index: Volumes 1–5. NASA Tech.

Memo. 104566, Vol. 6, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 9 pp.

Vol. 7

Darzi, M., 1992: Cloud Screening for Polar Orbiting Visible

and IR Satellite Sensors. NASA Tech. Memo. 104566, Vol.

7, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 7 pp.

Vol. 8

Hooker, S.B., W.E. Esaias, and L.A. Rexrode, 1993: Proceedings of the First SeaWiFS Science Team Meeting. NASA

Tech. Memo. 104566, Vol. 8, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 61 pp.

Vol. 9

Gregg, W.W., F.C. Chen, A.L. Mezaache, J.D. Chen, J.A.

Whiting, 1993: The Simulated SeaWiFS Data Set, Version 1. NASA Tech. Memo. 104566, Vol. 9, S.B. Hooker,

E.R. Firestone, and A.W. Indest, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 17 pp.

Vol. 10

Woodward, R.H., R.A. Barnes, C.R. McClain, W.E. Esaias,

W.L. Barnes, and A.T. Mecherikunnel, 1993: Modeling

of the SeaWiFS Solar and Lunar Observations. NASA

Tech. Memo. 104566, Vol. 10, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 26 pp.

Vol. 11

Patt, F.S., C.M. Hoisington, W.W. Gregg, and P.L. Coronado,

1993: Analysis of Selected Orbit Propagation Models for

the SeaWiFS Mission. NASA Tech. Memo. 104566, Vol.

11, S.B. Hooker, E.R. Firestone, and A.W. Indest, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 16 pp.

Vol. 12

Firestone, E.R., and S.B. Hooker, 1993: SeaWiFS Technical Report Series Summary Index: Volumes 1–11. NASA Tech.

Memo. 104566, Vol. 12, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 28 pp.

Vol. 13

McClain, C.R., K.R. Arrigo, J. Comiso, R. Fraser, M. Darzi,

J.K. Firestone, B. Schieber, E-n. Yeh, and C.W. Sullivan, 1994: Case Studies for SeaWiFS Calibration and Validation, Part 1. NASA Tech. Memo. 104566, Vol. 13,

S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 52 pp., plus

color plates.

54
R. Barnes, A. Holmes, W. Barnes, W. Esaias, C. McClain, and T. Svitek

Vol. 14

Mueller, J.L., 1993: The First SeaWiFS Intercalibration RoundRobin Experiment, SIRREX-1, July 1992. NASA Tech.

Memo. 104566, Vol. 14, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 60 pp.

Vol. 15

Gregg, W.W., F.S. Patt, and R.H. Woodward, 1994: The Simulated SeaWiFS Data Set, Version 2. NASA Tech. Memo.

104566, Vol. 15, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 42 pp., plus color plates.

Vol. 16

Mueller, J.L., B.C. Johnson, C.L. Cromer, J.W. Cooper, J.T.

McLean, S.B. Hooker, and T.L. Westphal, 1994: The Second SeaWiFS Intercalibration Round-Robin Experiment,

SIRREX-2, June 1993. NASA Tech. Memo. 104566, Vol.

16, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 121 pp.

Vol. 17

Abbott, M.R., O.B. Brown, H.R. Gordon, K.L. Carder, R.E.

Evans, F.E. Muller-Karger, and W.E. Esaias, 1994: Ocean

Color in the 21st Century: A Strategy for a 20 -Year Time

Series. NASA Tech. Memo. 104566, Vol. 17, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, 20 pp.

Vol. 18

Firestone, E.R., and S.B. Hooker, 1994: SeaWiFS Technical Report Series Summary Index: Volumes 1–17. NASA Tech.

Memo. 104566, Vol. 18, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, (in press).

Vol. 19

McClain, C.R., R.S. Fraser, J.T. McLean, M. Darzi, J.K. Firestone, F.S. Patt, B.D. Schieber, R.H. Woodward, E-n. Yeh,

S. Mattoo, S.F. Biggar, P.N. Slater, K.J. Thome, A.W.

Holmes, R.A. Barnes, and K.J. Voss, 1994: Case Studies

for SeaWiFS Calibration and Validation, Part 2. NASA

Tech. Memo. 104566, Vol. 19, S.B. Hooker, E.R. Firestone,

and J.G. Acker, Eds., NASA Goddard Space Flight Center,

Greenbelt, Maryland, 73 pp.

Vol. 20

Hooker, S.B., C.R. McClain, J.K. Firestone, T.L. Westphal,

E-n. Yeh, and Y. Ge, 1994: The SeaWiFS Bio-Optical

Archive and Storage System (SeaBASS), Part 1. NASA

Tech. Memo. 104566, Vol. 20, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 40 pp.

Vol. 21

Acker, J.G., 1994: The Heritage of SeaWiFS: A Retrospective on the CZCS NIMBUS Experiment Team (NET) Program. NASA Tech. Memo. 104566, Vol. 21, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, 43 pp.

Vol. 22

Barnes, R.A., W.L. Barnes, W.E. Esaias, and C.R. McClain,

1994: Prelaunch Acceptance Report for the SeaWiFS Radiometer. NASA Tech. Memo. 104566, Vol. 22, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard

Space Flight Center, Greenbelt, Maryland, 32 pp.

Vol. 23

Barnes, R.A., A.W. Holmes, W.L. Barnes, W.E. Esaias, C.R.

McClain, and T. Svitek, 1994: SeaWiFS Prelaunch Radiometric Calibration and Spectral Characterization. NASA

Tech. Memo. 104566, Vol. 23, S.B. Hooker, E.R. Firestone,

and J.G. Acker, Eds., NASA Goddard Space Flight Center,

Greenbelt, Maryland, 55 pp.

55
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 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

 October 1994 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

6. AUTHOR(S)

 Robert A. Barnes, Alan W. Holmes, William L. Barnes, Wayne E. Esaias,

 Charles R. McClain, and Tomas Svitek

Laboratory for Hydrospheric Processes

Goddard Space Flight Center 94B00144

Greenbelt, Maryland 20771

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

National Aeronautics and Space Administration

Washington, D.C. 20546–0001 TM–104566, Vol. 23

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

 Robart A. Barnes: ManTech, Inc., Wallops Island, Virginia; Alan W. Holmes: Hughes Santa Barbara Research Center,

 Santa Barbara, California; Tomas Svitek: Orbital Sciences Corporation, Dulles, Virginia; Elaine R. Firestone: General

 Sciences Corporation, Laurel, Maryland; and James G. Acker: Hughes STX, Lanham, Maryland

13. ABSTRACT (Maximum 200 words)

Based on the operating characteristics of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), calibration equations have

been developed that allow conversion of the counts from the radiometer into Earth-exiting radiances. These radiances are the

geophysical properties the instrument has been designed to measure. SeaWiFS uses bilinear gains to allow high sensitivity

measurements of ocean-leaving radiances and low sensitivity measurements of radiances from clouds, which are much

brighter than the ocean. The calculation of these bilinear gains is central to the calibration equations. Several other factors

within these equations are also included. Among these are the spectral responses of the eight SeaWiFS bands. A band's

spectral response includes the ability of the band to isolate a portion of the electromagnetic spectrum and the amount of light

that lies outside of that region. The latter is termed out-of-band response. In the calibration procedure, some of the counts

from the instrument are produced by radiance in the out-of-band region. The number of those counts for each band is a

function of the spectral shape of the source. For the SeaWiFS calibration equations, the out-of-band responses are converted

from those for the laboratory source into those for a source with the spectral shape of solar flux. The solar flux, unlike the

laboratory calibration, approximates the spectral shape of the Earth-exiting radiance from the oceans. This conversion

modifies the results from the laboratory radiometric calibration by 1–4%, depending on the band. These and other factors in

the SeaWiFS calibration equations are presented here, both for users of the SeaWiFS data set and for researchers making

ground-based radiance measurements in support of SeaWiFS.

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SeaWiFS, Oceanography, Radiometer, Prelaunch Radiometric Calibration, 55

Report is available from the Center for AeroSpace Information (CASI),

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NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI Std. 239-18, 298-102

 SeaWiFS Technical Report Series

Volume 23–SeaWiFS Prelaunch Radiometric Calibration and Spectral Code 970.2

 Characterization

 Series Editors: Stanford B. Hooker and Elaine R. Firestone

 Technical Editor: James G. Acker