NASA Technical Memorandum 104566, Vol. 31

SeaWiFS Technical Report Series

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

Volume 31, Stray Light in the SeaWiFS Radiometer

Robert A. Barnes, Alan W. Holmes, and Wayne E. Esaias

July 1995
NASA Technical Memorandum 104566, Vol. 31

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 31, Stray Light in the SeaWiFS Radiometer

Robert A. Barnes

ManTech, Inc.

Wallops Island, Virginia

Alan W. Holmes

Santa Barbara Research Center

Goleta, California

Wayne E. Esaias

NASA Goddard Space Flight Center

Greenbelt, Maryland

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

1995
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Abstract

Some of the measurements from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) will not be useful as

ocean measurements. For the ocean data set, there are procedures in place to mask the SeaWiFS measurements of clouds and ice. Land measurements will also be masked using a geographic technique based on each

measurement’s latitude and longitude. Each of these masks involves a source of light much brighter than the

ocean. Because of stray light in the SeaWiFS radiometer, light from these bright sources can contaminate ocean

measurements located a variable number of pixels away from a bright source. In this document, the sources

of stray light in the sensor are examined, and a method is developed for masking measurements near bright

targets for stray light effects. In addition, a procedure is proposed for reducing the effects of stray light in the

flight data from SeaWiFS. This correction can also reduce the number of pixels masked for stray light. Without

these corrections, local area scenes must be masked 10 pixels before and after bright targets in the along-scan

direction. The addition of these corrections reduces the along-scan masks to four pixels before and after bright

sources. In the along-track direction, the flight data are not corrected, and are masked two pixels before and

after. Laboratory measurements have shown that stray light within the instrument changes in a direct ratio to

the intensity of the bright source. The measurements have also shown that none of the bands show peculiarities

in their stray light response. In other words, the instrument’s response is uniform from band to band. The

along-scan correction is based on each band’s response to a 1 pixel wide bright source. Since these results are

based solely on preflight laboratory measurements, their successful implementation requires compliance with

two additional criteria. First, since SeaWiFS has a large data volume, the correction and masking procedures

must be such that they can be converted into computationally fast algorithms. Second, they must be shown to

operate properly on flight data. The laboratory results, and the corrections and masking procedures that derive

from them, should be considered as zeroeth order estimates of the effects that will be found on orbit.

1. INTRODUCTION

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

is the successor instrument to the NIMBUS-7 Coastal

Zone Color Scanner (CZCS). The design requirements for

SeaWiFS have been based on the National Aeronautics

and Space Administration’s (NASA) experience with the

operation of CZCS. Among other characteristics, CZCS

often contained obvious anomalies in ocean measurements

that immediately followed measurements of clouds. For

CZCS measurements, and also for those from SeaWiFS,

clouds are several times brighter than the ocean surface.

In the CZCS measurements, the output from each of the

six CZCS bands would overshoot the ocean surface levels

on their return from the brighter cloud levels. This would

give ocean measurements, on the down-scan side of clouds,

that were too low. The recovery from this overshoot would

take up to 100 pixels, that is, up to 100 measurements in

the CZCS scan line (Mueller 1988). This problem resulted

from a faulty design of the CZCS electronics. When CZCS

viewed clouds, the analog amplifiers for the CZCS bands

saturated. It is believed that these saturating amplifiers

pulled down the output from their power supplies. After the instrument had scanned past the bright clouds, it

would require up to 100 pixels or more for the amplifiers

and their power supplies to stabilize again. The bright target recovery (BTR) performance specification for SeaWiFS

was prepared with this CZCS problem in mind.

For CZCS, the problems from cloud interference with

ocean measurements occurred only down scan of the cloud.

These problems are considered to have been electronic in

origin. CZCS experienced no problems from cloud interference up scan of the clouds, and for CZCS, there were no

apparent problems with light from clouds contaminating

measurements of adjacent ocean pixels; hence, there were

no apparent problems with stray light.

CZCS was designed with an 8-bit digital resolution.

The minimum value for its measurements was 1/256 of

full scale. SeaWiFS has been designed with 10-bit digital resolution. Since it has comparable full scale readings,

SeaWiFS is four times more sensitive than CZCS. It can

detect effects, both in the water-leaving radiances and in

the instrument itself, that could not have been noticed in

previous measurements.

In March 1993, during testing of the assembled SeaWiFS instrument, light from bright sources was found to

contaminate SeaWiFS measurements, both up scan and

down scan from these bright sources. The down-scan response of SeaWiFS approached, but was not within, the

BTR specification for instrument performance. The effects

of stray light up scan of bright sources were of the same

magnitude of those down scan. At the SeaWiFS Preship

Review (SPR), an instrument performance appraisal held

at the Santa Barbara Research Center (SBRC) in April

1993, it was found that the radiometer did not meet the

BTR performance specification. At the same meeting, it

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Stray Light in the SeaWiFS Radiometer

was decided to investigate instrument changes that would

reduce the effects of stray light in the instrument.

In May 1993, SBRC, the SeaWiFS manufacturer, presented a set of recommendations for changes to ameliorate

stray light in the instrument—the SeaWiFS Stray Light

Signal Paths (SSLSP). In August 1993, the Ocean Color

Contract was changed to allow modification of the sensor. After completion of the instrument modifications and

subsequent testing, a second appraisal of the radiometer’s

performance—the SeaWiFS Post-Modification Preship Review (SPMPR)—found the instrument to meet all specifications (Barnes et al. 1994a).

1.1 Local and Global Area Coverage

SeaWiFS scans across the Earth’s surface from west

to east. Within the instrument, the scanner rotates in a

circle. Each scan of the Earth from the instrument contains 1,285 contiguous pixels over a 116.6◦ scan centered at

nadir, and each pixel is nominally square with a side length

of 1.6 mrad (Barnes et al. 1994a). For nadir measurements

at the nominal spacecraft altitude of 705 km, the length of

the sides of the pixels on the Earth’s surface is 1.13 km.

The scan lines are nominally perpendicular to the motion

of the spacecraft, which moves from north to south. The

spacecraft orbit and the spacecraft rotation rate have been

designed so that the pixels from adjacent scan lines lie next

to each other at nadir. Thus, at nadir the SeaWiFS instrument produces measurements of the Earth’s surface in a

two-dimensional mosaic of square pixels.

When SeaWiFS samples away from nadir, the pathlength between the instrument and the Earth increases.

The pixel size at the Earth’s surface increases as the instrument’s measurement angle increases from nadir. Along

each scan line, the pixels remain contiguous, but they are

larger at the scan edges than at nadir. As a result, the areal

coverage of the Earth from each scan line resembles a bow

tie. As explained above, the orbit for SeaWiFS has been

designed so that, at nadir, pixels from adjacent scans lie

next to each other. Away from nadir, the increasing size of

the pixels causes them to overlap in the direction of spacecraft motion. Earth scenes in this format are provided from

the SeaStar satellite as local area coverage (LAC). These

LAC scenes are provided in two ways. First, as they are

obtained, the scenes are transmitted directly to the ground

as the instrument passes over local receiving stations. LAC

scenes from some of the ground stations will be forwarded

to the NASA Goddard Space Flight Center (GSFC) and

placed into the SeaWiFS archive. The forwarding process

is relatively slow, and the data will become available over

a period of weeks to months after transmission from the

satellite. Second, specific scenes will be recorded on board

the spacecraft for transmission to the ground along with

the global SeaWiFS data. These recorded LAC scenes will

be sent directly from the satellite to GSFC twice daily. A

summary of these two types of LAC data were provided in

the overview of the SeaWiFS mission (Hooker et al. 1992).

In order to limit the required onboard data storage and

data transmission rate, the mission was designed by NASA

to give global area coverage (GAC) by recording every

fourth pixel from every fourth scan line for transmission to

the ground. These data are transmitted twice daily with

the recorded LAC. As a result, each GAC measurement is

four pixels removed from its nearest neighbor in both the

along-scan and along-track directions.

For stray light in SeaWiFS, LAC and GAC data are

distinctly different. SeaWiFS will measure dark ocean radiances adjacent to very bright clouds. The brightness

and proximity of those clouds will affect the amount of

stray light within the radiometer. The two-dimensional

mosaic of LAC measurements will provide that information. For GAC measurements, the 3 pixel gap between

measurements severely limits knowledge of the size and

brightness of adjacent clouds. This lack of information will

reduce the quality of any GAC stray light correction, relative to LAC. Proposed procedures for on-orbit stray light

corrections are presented in Sections 11 and 12. These procedures must be tested using actual flight measurements,

particularly for the GAC corrections. LAC scenes will be

used to test the GAC correction. These LAC scenes will

contain complete sets of information on the location and

brightness of clouds and other bright sources. They will be

subsampled to provide a GAC product. Downlinked GAC

data will also be available for any LAC scene. Fixed pixel

subsampling has been shown to produce the best statistical agreement between synthesized GAC products and the

LAC measurements from which they are obtained (Justice

et al. 1989 and McClain et al. 1992). Such equivalence is

a prerequisite for testing GAC stray light corrections with

LAC scenes.

1.2 Stray Light Modifications

The effects of stray light in SeaWiFS were discovered

after the construction of the instrument. The radiometer

was developed with an engineering design unit that did

not completely duplicate the flight instrument’s optical

characteristics. The SeaWiFS engineering unit was constructed to verify the instrument’s basic mechanical and

electrical design, particularly the telescope and half-angle

mirror combination, and detector combinations (discussed

in Section 2), in addition to providing a complete electrical interface for testing the SeaStar spacecraft. The

fast-track, 22 month construction schedule for SeaWiFS

precluded the fabrication of a complete, flight-like engineering unit; full-scale testing; and then the production

and testing of the flight instrument. For SeaWiFS, the engineering and flight units were developed nearly in parallel

with each other. As a result, the effects of stray light in

SeaWiFS were discovered after the completion of the instrument. This development effort resulted in a radiometer

that met all of the prelaunch requirements (Barnes et al.

1994a) except for BTR, which was slightly outside of the

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 1. The SeaWiFS scanner assembly. The scanner mounts to the payload shelf using the four mounting

points at the top of the figure.

specifications. Stray light effects were found both before

and after bright sources and are a part of BTR. As discussed in Section 5, there were electronic recovery tails in

the instrument’s bright target responses. The corrections

for the electronic tails were simple, and with those repairs,

the instrument met the BTR specification.

The instrument manufacturer provided to the SeaWiFS

Project, a description of the radiometer’s stray light paths

and their associated corrections (in the SSLSP). These

paths are described in Section 6. Risk to the completed

flight instrument was the primary consideration in the decision of whether or not to adopt instrument modifications.

In particular, modifications to the focal plane assemblies,

with their interference filters and detectors, were deemed

to be at high risk. The focal planes were difficult to access, and the filters, once fixed in place with epoxy, were

too fragile to be removed and replaced. In addition, corrections requiring major modifications to the instrument

were rejected because of cost and schedule restrictions.

1.3 Document Overview

This document illustrates the magnitude of residual

stray light (bright target response) in the completed SeaWiFS flight instrument. It provides procedures for reducing the effects of that stray light in the flight data, thereby

reducing the size of the stray light masks, and increasing the amount of usable data from the instrument. In

Sections 2 and 3, a description of the instrument and a

summary of the specification for BTR are presented. In

Sections 4 and 5, two electronic modifications to the instrument to improve BTR response are discussed: the addition of bilinear gains to allow the detection of clouds,

and the very large reduction in the electronic portion of

the BTR tail. In Section 6, several stray light sources

within the radiometer, and the modifications to the instrument that reduced their effects, are described. Section 7

provides a description of the post-modification stray light

measurements. Section 8 uses the along-scan and alongtrack responses from Section 7 to create an informal stray

light budget. Section 9 describes the procedure for the

along-scan stray light correction based on laboratory measurements. Section 10 applies this procedure to laboratory

scans of bright targets. And finally, in Sections 11 and 12,

proposed schemes are given for applying the corrections to

LAC and GAC scenes from SeaWiFS.

2. INSTRUMENT DESCRIPTION

The SeaWiFS instrument (Fig. 1) is designed to measure Earth-exiting radiances. The sensor’s instantaneous

field-of-view (IFOV) is 1.6 ×1.6 mrad per pixel, with one

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Stray Light in the SeaWiFS Radiometer

Table 1. Constants used in the calculation of the BTR limit in counts. The Lcloud and Ltypical values

come from the performance specifications (see Barnes et al. 1994a, Section 16). The slopes (or sensitivities

for each band) come from the radiometric calibration of the instrument (Barnes et al. 1994b).

Band Lcloud/ 0.5% Ltypical

Number Lcloud† Ltypical† Ltypical Slope ‡ [counts]

1 60.0 9.10 6.59 0.0137 3.3

2 66.2 8.41 7.87 0.0134 3.1

3 68.2 6.56 10.4 0.0105 3.1

4 65.6 5.64 11.6 0.00920 3.1

5 65.2 4.57 14.3 0.00746 3.1

6 53.8 2.46 21.9 0.00425 2.9

7 43.0 1.61 26.7 0.00301 2.7

8 34.0 1.09 31.2 0.00215 2.5

† mW cm−2 sr−1 μm−1 ‡mW cm−2 sr−1 μm−1count−1

cross-track scan covering ±58.3◦ about nadir. The scanner

can be tilted to +20◦, 0◦, or −20◦ relative to nadir to minimize the number of glint-contaminated measurements in

the data. Each pixel value is digitized to 10-bit resolution.

In the instrument, light first strikes the primary mirror,

an off-axis parabola, and is then 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 due to the rotation of

the telescope. This mirror rotates at exactly half the rate

of the telescope and polarization scrambler, and uses alternating mirror sides on successive telescope scans. The

field stop, located at the entrance to the aft optics, is 50%

larger than the detectors and restricts stray light through

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

another off-axis paraboloid and directed through the aft

optics. Dichroic beam splitters divert the light into four

focal plane assemblies, each containing two spectral bands

delineated by narrow band filters in close proximity to the

detector. The optical paths in the aft optics assembly are

shown in Fig. 2.

Attention was given in the SeaWiFS design to minimizing the sensitivity of the instrument to polarized light.

This is the main reason for the rotating telescope, rather

than a rotating entrance mirror. There are other possible

instrument designs with less mechanical complexity, but

they require large incidence angles on one or more mirrors, producing unacceptable polarization variations alongscan, particularly for the SeaWiFS wavelengths in the blue

(412 nm and 443 nm) spectrum. The SeaWiFS design minimizes the angle of light incident on its mirrors. In addition, SeaWiFS uses a polarization scrambler (see Figs. 1

and 2) 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 scrambler’s aperture. The sensitivity of

the output of SeaWiFS to polarized light was measured in

the laboratory, using a source producing plane-polarized

light. The rotation of the polarized light through 360◦

produced changes of 0.25% or less in the eight SeaWiFS

bands (Barnes et al. 1994a).

Two instrument bands, each with four detectors aligned

in the scan direction, form a focal plane. The four detectors in each group are added using a time delay and

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 (A/D) converter. The four

digital words from a band are then appropriately delayed,

summed to obtain the signal-to-noise advantage, truncated

to 10 bits, and transmitted to SeaStar through the electronics module. A solar calibrator is mounted on the instrument, allowing the optical system to view a solar illuminated diffuser when passing over the South Pole. The

entire spacecraft can be rotated to allow the instrument

to view the nearly full moon, which is considered to be

a stable calibration source to monitor the long-term repeatability of the SeaWiFS measurements (Woodward et

al. 1993).

3. BRIGHT TARGET RECOVERY

The BTR specification has been described in the SeaWiFS Prelaunch Acceptance Report (Barnes et al. 1994a).

Radiometric data should be relatively free of the effects

of overshoot and ringing when the IFOV scans across a

steep gradient in radiance, from a maximum of Lcloud to

a minimum of Ltypical. For this radiance step change, the

output signal should settle to within 0.5% of its final value

(Ltypical) within 10 pixels. This radiance limit, using values from the specifications, is given in counts in Table 1. It

is given to one-tenth of a count since the values are small,

relative to the quantization of the data.

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 2. Schematic diagram of the SeaWiFS aft optics assembly.

During the April 1993 SeaWiFS Preship Review, it was

determined that the instrument did not meet the BTR

specification. At that time, it was decided that the instrument manufacturer would rework SeaWiFS to improve its

stray light characteristics. The results in Table 2 come

from laboratory measurements, after the instrument modificatio, using the output of the SBRC integrating sphere

(Barnes et al. 1994a). The measurements were made in

four sets, one for each focal plane, using a 3 pixel wide

slit. Color glass filters were placed over the output from

the sphere to give a spectral shape approximating that of

the sun over the wavelength range of the bands on each of

the four focal planes.

The results in Table 2 give the distance in pixels required for the instrument to settle to less than 0.5% of

Ltypical using the counts given in Table 1. For bands 1–5,

the results give the number of pixels required to settle to 3

counts above background. For bands 6–8, the results give

the number of pixels required to settle to 2 counts. Table 2

also gives the number of pixels required for the instrument

to settle to zero counts after illumination by the slit. All

eight bands settle to within the specification limit within

10 pixels.

Table 2. SeaWiFS BTR characteristics. The table

gives the number of pixels after the slit (Pslit) for the

instrument to return to the residual counts allowed

in the specification. In addition, the table gives

the number of pixels required for the instrument to

settle to a level of zero residual counts (Pzero). This

material is taken from Barnes et al. (1994a).

Band 0.5% Ltypical Pslit Zero Resid. Pzero

No. [counts] [pixels] [counts] [pixels]

1 3.3 6 0 10

2 3.1 5 0 9

3 3.1 7 0 10

4 3.1 5 0 15

5 3.1 9 0 15

6 2.9 7 0 9

7 2.7 9 0 11

8 2.5 7 0 10

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Stray Light in the SeaWiFS Radiometer

4. BILINEAR GAINS

Tests of the instrument have revealed that SeaWiFS

measurements will be contaminated by stray light from

clouds in adjacent pixels. These tests also showed the

stray light contamination to be essentially proportional to

the brightness of the adjacent cloud. In order to perform

post-launch corrections, the brightness of the clouds must

be measured; therefore, SBRC changed the sensor’s electronics to allow on-orbit measurements of cloud radiances.

These changes maintain the sensitivity of the SeaWiFS

ocean measurements, as specified in the requirements for

the instrument.

The new electronic configuration uses the four detector

circuits in each SeaWiFS band to create bands with bilinear gains. The response for SeaWiFS band 1 (442 nm)

is shown in Fig. 3. The channel’s high sensitivity, i.e., a few

mW cm−2 sr−1 μm−1count−1, extends over three-quarters

of the band’s dynamic range. Above this point, the channel’s sensitivity is reduced, allowing the measurement of

cloud radiances up to 60 mW cm−2 sr−1 μm−1 (see Fig. 3a).

For ocean measurements, the band will have the same response as the original linear gains over a reduced number

of counts, i.e., over a reduced dynamic range (see Fig. 3b).

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 Table 3. In Tables 3–6,

the process of calculating the knee and endpoint locations

for the bilinear gains of SeaWiFS band 1 is shown. The

values are given for science gain 1—the standard gain for

SeaWiFS ocean measurements—and are given using the

output from the band’s four channels. This is the standard detector configuration for the instrument. The input

radiances, counts, and zero offsets (see Table 3) come from

radiometric tests of the instrument found in the SeaWiFS

Calibration and Acceptance Data Package (SCADP). The

SCADP was generated by SBRC in the course of the characterization and calibration of the SeaWiFS instrument.

With the zero offsets removed, the net counts, along with

the sphere radiance, are used to calculate the sensitivity,

or slope, for each channel (Table 3). 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. They convert the counts

from the instrument into radiances at the instrument’s optical input.

The SeaWiFS channels are digitized at 10 bits, and

each channel’s output ranges 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 (see Table 4). From these saturation counts and

the sensitivities for the channels, it is possible to calculate

the saturation radiances for each channel (Table 4). 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 4 give the three knee

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

The knee radiance is the point at which the high sensitivity channels saturate, and the saturation radiance is the

point at which the low sensitivity channel saturates. 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

calculate the number of counts from each channel at the

three knees and the two endpoints of the bilinear gains

(Table 5). The counts from the four channels are summed

and divided by four in Table 5; this models the process

performed within SeaWiFS. On orbit, the output from the

channels (as selected by the instrument’s electronics, based

on commands from the ground) will be summed; the result

will be sent from SeaWiFS to the SeaStar spacecraft. This

digital output is sent, minus its two least significant bits;

in other words, it will be sent from the instrument to the

spacecraft after division by 4.

The radiances and counts at the three knees and the

two endpoints are given in Table 6. For these calculations

(and as is recommended 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 5 gives the information for calculating the sensitivity

of band 1: 10.899/793.64, or 0.013773 mW/count. Above

the third knee (in the radiance region for cloud measurements), the sensitivity is 0.240158 mW/count, calculated

as (60.159 − 11.049)/(1002.25 − 797.76). 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 2–8.

The effect of slightly different sensitivities in the three

high sensitivity channels is shown in Fig. 4. In this case,

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

rather than just one segment before and one segment after. This results in a region of uncertainty in the transition

between the ocean and cloud sensitivity regimes. The revised electronic configuration that gives SeaWiFS the ability to detect clouds also defines the method in which the

knees for the bilinear gains are calculated (Tables 3–6). If

the detector configuration for a band is changed, then the

knees for the bilinear gains must be recalculated.

5. ELECTRONIC RECOVERY TAIL

Each SeaWiFS band has an electronic offset of approximately 20 counts. This zero offset is necessary to guarantee that the instrument can measure down to zero light.

A schematic drawing of the input circuitry for each A/D

converter is shown in Fig. 5. In this circuitry, the zero

offset (called the bipolar offset) and the input signal enter the A/D converter on different pins. Within the A/D

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 3. The instrument response for SeaWiFS band 1 with a bilinear gain. a) The response from 0–1,000 counts

is displayed. The slope from 0–800 counts shows greater sensitivity, i.e., fewer radiance units per count. b) The

response from 0–800 counts is displayed. This is the working range for ocean measurements from band 1.

7
Stray Light in the SeaWiFS Radiometer

Table 3. The input values and calculated sensitivities for the four channels of band 1. The sensitivities are

calculated from the sphere radiances and net counts.

Channel Radiance Measurement Offset Net Sensitivity

Number [mW] [counts] [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 4. 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

saturation counts.

Channel Zero Saturation Saturation Radiance

Number Counts [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 5. Calculated instrument output at the saturation radiances for the four channels, in counts. The counts

at the knees are calculated from the knee radiances and the sensitivities. The total counts for each channel

cannot exceed the saturation counts.

Channel Number Zero Knee 1 Knee 2 Knee 3 Saturation

1 0.00 181.54 181.60 184.03 1,002.00

2 0.00 999.64 1,000.00 1,000.00 1,000.00

3 0.00 991.39 991.75 1,005.00 1,005.00

4 0.00 1,002.00 1,002.00 1,002.00 1,002.00

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

Sum/4 0.00 793.64 793.84 797.76 1,002.25

Radiance [mW] 0.00 10.90 10.90 11.05 60.16

Table 6. Knees and endpoint locations for the bilinear gains. These are the values in the last two rows of

Table 5.

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

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 4. An expanded view of the change in slope of the bilinear gain for band 1. The figure shows the effect of

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

are created as the two higher sensitivity channels saturate in turn. For real world measurements, the change in

slope does not occur at one radiance level only.

converter, however, the two voltages are summed at the

input to the converter’s sample and hold circuit. This

summation point couples the responses of the two circuits

with each other. In the original design for the SeaWiFS

electronics, the zero offset was 5 counts, instead of the 20

counts in the final design. The size of the offset in the A/D

circuitry is determined by the size of resistor R115 (see

Fig. 5). The original design also had a capacitor (C40) in

parallel with resistor R115. This resistor-capacitor (RC)

pair comprises a standard type of electronic filter to reduce

high frequency noise. The circuit was designed with a capacitance for C40 of 1 μF. The residual stray capacitances

in the circuit are on the order of picofarads, smaller by 4–5

orders of magnitude than C40. For SeaWiFS, this circuit

was originally included to remove possible high-frequency

noise in the zero offset, even though the zener diode was

expected to produce a quiet voltage level.

This RC pair also has a time period for response to

sharp voltage changes. This time constant is determined

by the product of the resistance and the capacitance in

the RC circuit. The constant gives the time period for the

voltage in the RC circuit to adjust to [1 − e−1], or 63%, of

the voltage difference (Diefenderfer 1972). For SeaWiFS,

such a voltage change occurs when the instrument scans

from a bright cloud to the dark surface of the ocean. During measurements of the bright cloud, the large voltage of

the input signal into the A/D converter is coupled to the

RC circuit through the A/D bipolar offset. The capacitor

charges to that voltage over several time constants. When

the input voltage drops as the instrument scans onto the

comparatively dark ocean, the capacitor then discharges

to the original level over several time constants. For the

original design, the resistor was set to give a voltage equivalent to 5 counts. Along with C40, this gave a time constant that is 0.25 of the measurement period for a SeaWiFS

pixel. That measurement period is 42×10−6 s (Barnes et

al. 1994a). After one measurement period, or four time

constants, the RC circuit in the original design would have

a voltage within 2% of the input voltage from the comparatively dark ocean. After two measurement periods, or

eight time constants, the voltage would settle to 0.03% of

the ocean value.

When the instrument was modified to give a 20 count

offset, the resistance of resistor R115 was increased by a

factor of four. During this modification, the capacitor was

unchanged; as a result, the RC time constant was multiplied by 4. Thus, during initial testing of the radiometer in

the SPR, there were very small, but noticeable, electronic

9
Stray Light in the SeaWiFS Radiometer

tails from the RC circuits in the BTR test results. For

example, for SeaWiFS band 8 the residual electronic tail

at 10 pixels from the bright source was approximately 1

count; in this case, the tail was about 40% of the allowed

BTR response. As part of the stray light modifications

in the SPMPR, capacitor C40 was clipped from all of the

zero offset circuits in the instrument. This change has

reduced the electronic portion of the instrument’s bright

target response to an insignificant level. Although other

capacitances remain in the circuit, the electronic recovery

tail has been reduced by more than two orders of magnitude. Measurements of the system noise in SeaWiFS have

not indicated any problems from the removal of the C40

capacitors; the SNRs for all bands significantly exceed the

specification requirements (Barnes et al. 1994a).

There is an additional interesting characteristic of the

zero offset circuits. There are 32 detectors in SeaWiFS,

but only 16 offset circuits; thus, each band has two offset circuits for four detectors. In each band, the endmost

detectors—detectors 1 and 4—share the same offset circuit

with the adjacent detectors, detectors 2 and 3, respectively.

This causes a coupling of pairs of detectors through the

offset circuit—an intraband electronic cross talk. The effect is short range, covering only 1–2 pixels, and is part

of the system level response to performance tests, such as

the line spread functions used to determine the modulation

transfer function (MTF) (Barnes et al. 1994a). Intraband

electronic cross talk is a component of the MTF and does

not play an independent role in the instrument’s BTR.

6. STRAY LIGHT SOURCES

In the SSLSP, SBRC presented a description of the

stray light paths within the SeaWiFS radiometer and a

set of recommended changes to reduce stray light in the

instrument. The SBRC analysis showed several stray light

paths located in the instrument’s along-scan direction. In

addition, there was one significant stray light path in the

direction perpendicular to the instrument’s scan, i.e., in

the along-track direction. This source (secondary reflections from the polarization scrambler) is discussed in Section 6.2.

Individual sources of stray light are discussed here in

Section 6. The instrument’s stray light response, based on

the response to a 1 pixel wide bright source, is given in Section 7. Those results are used as the basis for an informal

stray light budget (Section 8) that gives the ranges (in pixels) over which the principal stray light sources contribute

to the instrument’s response.

The response of the instrument, along with the stray

light sources in the along-scan direction, is shown in Fig. 6.

The figure shows the instrument response before modifications were made to ameliorate the bright target response

in the instrument; thus, this figure also shows the effect

of the electronic tail that was described in the preceding

section. Figure 6 shows the stray light signal paths for the

four even bands in the instrument. As described earlier,

SeaWiFS has four focal planes, each focal plane containing two bands. With regard to the scan direction of the

instrument, the odd bands on each focal plane are located

before the even bands, i.e., light from a ground or ocean

target will strike the odd band on a focal plane before it

strikes the even band. This creates an asymmetry in the

stray light response for the even and odd bands on a focal

plane. The odd bands experience more stray light after,

i.e., down scan of, a cloud than they do before the cloud.

The additional stray light comes from interband (between

band) optical cross talk from the even band on the focal

plane. Conversely, the even bands experience more stray

light before, i.e., up scan of, a cloud than after; again, this

is due to optical cross talk. The effects of cross talk are

described in more detail in Section 6.5.

Figure 7 gives an example of the stray light paths for

one of the even SeaWiFS bands. As discussed above, most

of the stray light paths in an even band contribute to the

response before a bright source. The use of an even band

has an advantage for the example in Fig. 6, since the effect of the premodification electronic tail is isolated after

the bright source, so it, and the stray light effects, can be

shown separately.

The data in Fig. 6 are the recorded values from the

instrument as it viewed a bright slit 6 pixels wide. These

data have not been taken from the digital output of the

sensor, as the resolution of the standard digital output

does not have the sensitivity to distinguish the very small

radiances that exist several pixels away from the source.

They lie below the quantization limit of the digital data.

For the test results in Fig. 6, the output from the band

was measured with a lock-in amplifier and a voltmeter.

Using several sensitivity ranges for the amplifier, it was

possible to obtain the dynamic range required to measure

the band’s output over five orders of magnitude.

Each of the stray light paths in Fig. 6 is discussed in

the sections that follow. In addition, one section includes

a discussion of the secondary reflections from the polarization scrambler. The discussion sequence duplicates the

path of the radiance transmitted through the instrument,

from the primary mirror into the focal plane assemblies.

6.1 Primary Mirror Scatter

Primary mirror scatter is a minor contributor to stray

light over the range of pixels shown in Fig. 6. To the right

(down scan) of the bright source, the premodification electronic tail provides the major portion of the residual signal. With the removal of the electronic tail, the other stray

light sources still dominate over primary mirror scatter, as

they do to the left (up scan) of the bright source. For this

reason, it was decided to retain the original primary mirror in the radiometer, rather than to install a mirror with

reduced scatter. Primary mirror scatter, however, remains

a contributor to the optical blur near the bright source in

10
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 5. Zero offset circuitry for each SeaWiFS channel. There are 16 of these circuits in the SeaWiFS electronics.

The bipolar offset creates a constant voltage that gives a 20 count offset in the output from the A/D converter.

Capacitor C40 in each circuit created an electronic tail in each channel’s response to bright targets. As part of

instrument modifications to improve the sensor’s response to stray light, this capacitor has been removed from

all of the SeaWiFS circuits.

Fig. 6. Stray light paths in the SeaWiFS radiometer. For the even band in this example, the dominant stray

light sources occur before the bright source. For odd bands, the dominant sources occur after the bright source.

11
Stray Light in the SeaWiFS Radiometer

Fig. 6, and contributes to scatter from other stray light

sources farther out.

Additional measurements of stray light, not shown here,

were made at distances of up to 35 pixels from the bright

source. These so-called deep scans in the along-scan and

along-track directions show residual scattered light from

the primary mirror to be five orders of magnitude below

the bright source radiance. From these measurements, it

is possible to calculate the bidirectional reflectance distribution function (BRDF) for the primary mirror:

RB = 1.93e−61.5α, (1)

where RB is the BRDF, and α is the off-axis angle, in

radians, as documented in the SSLSP.

For large area bright targets outside the instrument’s

field of view (FOV), the scattered light from the mirror

sums and can become a substantial stray light source.

There is an aperture at the instrument’s optical inlet (see

Fig. 2). This aperture attenuates radiance at the primary

mirror for angles equal to 60 SeaWiFS pixels and greater.

The effect of light, well outside the FOV from clouds,

should resemble the effect of light within the instrument’s

FOV that has been scattered from atmospheric aerosols.

In flight, these instrumental and atmospheric effects may

be very difficult to separate.

6.2 Polarization Scrambler Reflections

The polarization scrambler consists of two crystalline

magnesium fluoride (MgF2) wedges with a reflective coating on the lower surface. The scrambler acts as a second

surface mirror in the SeaWiFS optical train. The MgF2

wedges in the scrambler create several cycles of varying

polarization in the reflected light across its aperture. The

incorporation of this scrambler into a design based on a

rotating telescope has reduced the residual polarization in

SeaWiFS to 0.25% or less for all eight bands (Barnes et al.

1994a).

The original design of the polarization scrambler, as

described in the SeaWiFS Critical Design Review (SCDR)

held at SBRC, produced two secondary reflections that

straddle the image in the along-track direction. The secondary reflections from that design were located about 5

pixels from the center, and had an intensity about 0.5%

of that in the central pixel, as documented in the SSLSP.

These along-track secondary reflections are shown in Fig. 7.

Figure 7 also shows the response of the polarization scrambler plus the response of a mirror that was substituted for

the scrambler.

As part of the instrument modifications to remove stray

light, the original polarization scrambler was replaced with

a modified unit. The front surface of the modified polarization scrambler was given a very slight wedge, relative to

the mirrored back surface of the scrambler. The geometry

of the new scrambler collapses the secondary reflections

onto the image from the central pixel at the optical stop

in the optical inlet for the aft optics (Fig. 8). Although it

may not be immediately obvious from the figure, ray traces

predicted the effect in the improved geometry. In addition,

post-modification testing using a laser light source showed

the reflections to converge onto the central beam as a cardboard target was moved away from the scrambler. The

post-modification testing also showed no secondary reflections from the scrambler in the along-scan direction.

6.3 BG26 and BG39 Filter Reflections

There are two broadband glass color filters in the SeaWiFS aft optics assembly (Fig. 2). These filters (Schott

catalog numbers BG26 and BG39) play an important role

in the laboratory calibration of the radiometer (Barnes et

al. 1994b). Reflections from these filters (referred to as

ghosts in Fig. 6) combine with both in-band optical cross

talk and the reflections from the final focusing lenses in

a region located 5–12 pixels from the edge of the bright

source. The reflections from the color filters are thought to

be far out of focus and very diffuse at the detectors—diffuse

to the point that they do not have the characteristics of an

image. Although referred to as ghosts, they are actually

more characteristic of stray light. Lens reflections and optical cross talk are discussed in the following sections. The

sum of these three stray light sources gives a signal that

is about 0.2% the magnitude of the bright source. In the

laboratory testing of stray light in SeaWiFS, there was no

effort to separate the stray light from these sources.

The ghosts from the broadband filters are reflections

from their optical surfaces of the upwelling light from the

focal plane assemblies (see Fig. 2). The source of this upwelling light is reflections off the interference filters on the

focal planes. These interference filters are all of dichroic

design without components that absorb light. They either

transmit or reflect the incoming radiation (Barnes et al.

1994b). Thus, the path for these ghosts is reflection from

the interference filters, reflection from the broadband filters, and transmission back through the interference filters.

As part of the instrument changes to ameliorate stray

light in the instrument, modifications were made to the

mounting rings for the two broadband absorptive filters.

The new mounting rings tilted the filters by about 5◦ from

the optical path, moving the reflections away from the focal planes. Since the responses of the filters are very broad

(Barnes et al. 1994b), this modification created no significant effect in the spectral response of the radiometer.

6.4 Focusing Lens Reflections

A diagram of one SeaWiFS focal plane assembly is

shown in Fig. 9. This figure includes the focusing lenses

associated with each assembly. The path for ghosts from

these lenses is similar to that for the broadband filters,

with reflections off the interference filters, to the lenses,

12
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 7. Secondary reflections from the scrambler in the along-track direction. The secondary reflections (dark

symbols) are displaced in the along-track direction. These stray light sources have magnitudes about 0.5% of

that from the bright source. With the substitution of a mirror (open symbols), the secondary reflections in the

along-track direction disappear. There is no sign of secondary reflections from the scrambler in the along-scan

direction.

Fig. 8. Modification to the polarization scrambler. The original geometry for the scrambler produced two

secondary reflections in the along-track direction. The improved geometry incorporates a slight wedge in the

along-track direction. This wedge causes the reflections to collapse onto the central image at the optical stop at

the entrance to the aft optics. The reflections, called ghosts in the figure, are diffuse and unfocused.

13
Stray Light in the SeaWiFS Radiometer

and back through the interference filters to the detectors.

The effect of reflections from the focusing lenses is potentially much more severe than for the broadband filters; but

for SeaWiFS, the lens reflections are severely out of focus

at the detectors. Indeed, all of the stray light ghosts within

SeaWiFS are well out of focus and extremely diffuse.

This possibility was considered in the initial design of

the radiometer found in the SCDR from SBRC. For that

reason, the focusing lenses were given a general antireflection coating; however, these coatings were not optimized

for individual focal planes. As part of the testing of stray

light within the instrument, there was a spectral scan of

one of the lenses in the same lot as the flight parts. Over

the range of wavelengths measured by SeaWiFS, the reflectances of the lenses were less than 2%. It was decided

that the minor reduction in stray light, which might result from special tailoring of the antireflection coatings of

the lenses, was not worth the risk that would accompany

the removal and replacement of the lenses in the focal

plane assemblies. Changes to the focusing lenses, therefore, were not part of the modifications to ameliorate stray

light within the radiometer.

6.5 Optical Cross Talk Between Bands

The SeaWiFS instrument is designed with two bands

on each focal plane assembly. The bands are placed endto-end with a saw cut between the two detector arrays (see

Fig. 10). The two interference filters that cover each focal

plane are connected over the saw cut with black epoxy. In

the SeaWiFS focal plane assemblies, there are open spaces

between the black epoxy joints and the saw cuts. These

open spaces provide paths for optical cross talk between

bands. The SeaWiFS interference filters do not absorb

light—they either transmit or reflect it. The wires, bond

pads, and the light shield metallization around the detectors in each band, are gold or aluminum and provide shiny

surfaces to reflect light. The active surfaces on the detector arrays are either a dull or glossy black. For the most

part, light that is reflected from the surface of a SeaWiFS

detector array will pass back through the interference filter

(Fig. 10). This is particularly true for wavelengths where

the transmission of the filter is nearly 100%. There are,

however, wavelengths where the transmission of the interference filters are a few percent or less. Any light that

is transmitted through the filter at these wavelengths can

reflect from the bottom of the filter. This reflected light

can bounce between the detector arrays and the filters in

an alternating fashion until it is absorbed by the array or

transmitted through the filter.

In this manner, a very small amount of light can pass

between the bands on a focal plane assembly. Since the two

interference filters on a focal plane transmit at different

wavelengths, light passing between bands can encounter a

near perfect reflecting surface from the filter of the other

band. It is estimated that 15 reflections are required for

specular optical cross talk to occur. The light shield metallization around the detectors is somewhat rough, so the

primary source of optical cross talk is probably a single,

low angle reflection between bands.

Between-band cross talk can be removed by placing a

septum below the junction between the filters. Such a

septum would have been reasonably simple to fabricate

during the initial construction of the radiometer. In this

process, the filters would have been joined with an excess

of black epoxy hanging below the junction between the filters; whereupon the epoxy would have been trimmed so it

would reach from the filter junction to a point just above

the sawcut between the detector arrays (Fig. 10). This

septum would have blocked the optical path between the

two bands on each focal plane. The SeaWiFS interference

filters, however, have long since been epoxied into place.

Their removal is exceptionally difficult and risky. This fact

was demonstrated experimentally during the thermal vacuum testing of SeaWiFS in the spring of 1993. At that

time, the focal plane assembly for bands 5 and 6 failed because of an electrical problem (Barnes et al. 1994b). When

the band 5 interference filter was removed, i.e., dug out

from the focal plane assembly, it was chipped and had to

be replaced. Added cost, and the potential for delays in

fabrication and instrument delivery, as well as a subsequent

launch delay could have resulted from further operations

on the filters. Based on this evidence, it was decided to

leave the status quo. Septa were not added to the focal

plane assemblies.

Optical cross talk between bands on a focal plane assembly is a function of the spectral shape of the measured

radiance, in addition to the stray light characteristics of

the instrument. The bright sources within the set of ocean

measurements are clouds and, in the near-infrared, land

and vegetation. For the purposes here, clouds are considered to be white, i.e., their reflective properties are almost spectrally flat over the wavelengths measured by SeaWiFS; thus, cloud radiances measured by SeaWiFS will

have a spectral shape that very closely approximates the

spectral shape of solar flux. This assumption is central

to the post-modification measurements of residual stray

light within the instrument. These measurements require

the use of a light source that has the spectral shape of

the sun. Since the tests are performed individually for

the two bands on each focal plane assembly, the spectral

shape must be maintained only over the wavelength range

for each pair of bands. Additional spectral source shapes,

such as those anticipated for land measurements, were not

included in the stray light measurements at SBRC.

The selection of the low-sensitivity cloud channel for

each SeaWiFS band was made with the effects of stray

light in mind. SeaWiFS has been designed such that the

outer detectors have reduced gain for cloud sensing. Channel 1 of each odd band is illuminated by a bright source

first, and channel 4 of each even band is illuminated last.

These are the low sensitivity cloud channels in SeaWiFS.

14
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 9. A focal plane assembly and its focusing lenses. This entire structure can be removed from the instrument

in one piece. The focal plane assembly contains two filters and two detector arrays. The focusing lenses are a

minor source of stray light in the radiometer.

Fig. 10. The interference filters and detector array in a focal plane assembly. There are two bands within each

assembly, and each filter covers the four detectors in a band. The odd band is located up scan of the even band.

There is a pathway for optical cross talk between bands, with light reflecting in an alternating fashion between

the underside of the filters and the surfaces of the detector arrays.

15
Stray Light in the SeaWiFS Radiometer

The optical cross talk described in this section dominates

the asymmetric response to bright targets by SeaWiFS.

Other, smaller sources of asymmetry also exist within the

instrument. Successor instruments should be tested for

those secondary asymmetric stray light sources.

6.6 Summary of Proposed Modifications

In the SSLSP, SBRC proposed a set of eight possible

changes to ameliorate stray light in SeaWiFS, along with

the risks to the instrument resulting from each. [These

were presented at the Stray Light Paths Review at GSFC

on 27 May 1993. The decision whether or not to implement these changes was made at the Performance Specification Modification Meeting at Orbital Sciences Corporation (OSC) 3 August 1993.] These possibilities and the

decisions on their implementations are listed in Table 7.

Table 7. Possible stray light modifications to the

SeaWiFS radiometer. Implemented modifications

are denoted with a checkmark (). Those modifications that were not implemented are denoted with

an ×.

Mod. Possible ImplemenNo. Modification tation

1 Tilt BG26 and BG39 filters. 

2 Add septa to focal planes. ×

3 Rotate filters (plus the addition of septa). ×

4 Add custom anti-reflection

coatings on the final

focusing lenses. ×

5 Add wedge to scrambler face. 

6 Replace primary mirror. ×

7 Eliminate electronic tail. 

8 Add cloud sensors. 

Among the possibilities discussed, is exchanging the positions of some of the filters, since the stray light responses

of the even and the odd bands are different (Section 6.5).

One of the principal data products from SeaWiFS measurements is the ratio of the response of band 2 (443 nm)

to band 5 (555 nm). It would be possible for the filters

for bands 5 and 6 to be interchanged, so that bands 2 and

5 would be in the even position on their respective focal

planes. Then, both measurements in the band 2 to band

5 ratio would have similar stray light characteristics.

In a similar manner, the same effect could be obtained

by interchanging the filters in the focal plane for bands

1 and 2, placing band 2 in the odd position on its focal

plane. As discussed above, however, there is difficulty and

risk in digging out the interference filters from their epoxy

mountings; as with the proposal to add septa between the

interference filters, this potential modification was not implemented.

During the period prior to the SeaWiFS Stray Light

Paths Review in May 1993, a large number of scenarios were proposed to reduce the effects of stray light in

the instrument. Many of these scenarios were summarized

in an unpublished internal memorandum to the SeaWiFS

Project (included in this document as Appendix A).

7. POST-MODIFICATION TESTS

Post-modification stray light measurements were performed as part of the stray light modifications to SeaWiFS

(Barnes et al. 1994a). The measurements were made at

several slit widths (in the along-scan direction) and at several slit heights (in the along-track direction). They were

made with two sets of radiances to examine the linearity

of the residual stray light with the intensity of the bright

radiant source (see the SCADP). The measurements used

the SBRC 100 cm spherical integrating sphere as the light

source. Schott optical glass filters, placed in front of the

exit port of the sphere, were used to obtain spectral shapes

for each of the measurements approximating that for solar flux. The test apparatus is described in the following

section.

7.1 Measurement Apparatus

The measurement apparatus for residual stray light in

SeaWiFS is shown in Fig. 11. The apparatus consists of a

target aperture on the exit port of the SBRC integrating

sphere and a collimating lens. The 15.24 cm diameter of

the lens provides a collimated output from the source that

overfills the input aperture for SeaWiFS. The target aperture on the integrating sphere has been designed to hold

both a 5.08×5.08 cm piece of optical colored glass and a

target mask. The sphere is positioned a distance from

the instrument such that a 2.92 mm wide slit in the target

mask subtends an angle of 1.6 mrad, the angular width of

a SeaWiFS pixel.

The vertical dimension of the opening in the target

panel is equivalent to 10 SeaWiFS pixels. Target masks

1, 2, and 3 have widths equivalent to 1, 3, and 10 SeaWiFS pixels. Combined with the vertical dimension of the

opening in the target panel, these masks produce images

of 1×10, 3×10, and 10×10 pixels. Since SeaWiFS was

positioned to scan in the horizontal during these measurements, the slit widths in the target masks determined the

along-scan sizes of the test images, and the slit heights

determined the along-track size.

Mask 4 was used for measurements of the along-track

response of SeaWiFS. This mask produced an image of

1×3 pixels, with the along-track portion of the image 3

pixels in size. For the along-track measurements, a series

of SeaWiFS scans were taken, with adjustments of the vertical position of the integrating sphere made using a screw

crank. In the test procedure, the vertical position of the

sphere was continuously adjusted by a distance equal to

16
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 11. The test apparatus for the measurement of residual stray light in SeaWiFS. The 15.24 cm, F/12 lens

collimates the flux from the integrating sphere.

one-sixth of a SeaWiFS pixel between sets of scans by the

sensor; thus, the along-track residual stray light measurements were oversampled by a factor of six.

7.2 Intensities of the Bright Sources

The SeaWiFS performance specifications require each

SeaWiFS band to settle to a radiance value that is within

a specified radiance step above the background ocean radiance. The size of that radiance step is 0.5% of the typical

radiance (Ltypical) for that band (Barnes et al. 1994a). According to the specifications, the output from the sensor

should meet that radiance level within 10 pixels of the edge

of a bright source. The SeaWiFS BTR specification uses a

percentage of the Ltypical values, rather than an absolute

set of radiances expressed in units of mW cm−2 sr−1 μm−1.

For that reason, the discussions of stray light in this document will refer to the bright radiances from the sources,

and to the residual stray light radiances within the instrument, in terms of their Ltypical values.

The typical ocean radiances (Ltypical) and the maximum cloud radiances (Lcloud) expected for SeaWiFS measurements on orbit are listed in Table 22 of Barnes et al.

(1994a). Those radiances are shown in Fig. 12a. As can

be seen in the figure, the maximum cloud radiances have a

peak value of about 68 mW cm−2 sr−1 μm−1 for SeaWiFS

band 3, and the Ltypical radiances have a peak of about

9 mW cm−2 sr−1 μm−1 for band 1. The nature of the two

sets of radiances is such that the ratio Lcloud/Ltypical increases with SeaWiFS band number (Fig. 12b); thus, the

SeaWiFS cloud radiances vary from about 6.5 times the

value of Ltypical for band 1, to about 31 times the value of

Ltypical for band 8 (see Table 22 of Barnes et al. 1994a).

The fluxes from the SBRC test apparatus were set, as

closely as possible, to provide the radiances equivalent to

Lcloud and 0.5Lcloud; however, the limited output from the

SBRC sphere in the shorter wavelengths severely limited

the quality of this equivalence (Fig. 13). The cloud radiances from Fig. 12b are repeated in Fig. 13 for reference. The

radiances that are 3 and 10 pixels wide come from SeaWiFS measurements of the test images from the sphere.

For band 1 in Fig. 13b, the image from the SBRC test apparatus has a radiance of about 0.5Ltypical. For bands 7

and 8, in Fig. 13a, the radiances are 25 and 30 times the

value of Ltypical, respectively.

7.3 Theoretical Band-to-Band Response

To allow for an analysis of the SeaWiFS bands over a

wide range of optical overdrives, the assumption was made

that the stray light responses of the eight bands are similar,

at least to a very good approximation. This assumption

allows the conclusion that the shorter wavelength bands,

tested at small optical overdrives, will behave in the same

manner as the longer wavelength bands when optical overdrives are large. Conversely, it allows the conclusion that

17
Stray Light in the SeaWiFS Radiometer

Fig. 12. SeaWiFS cloud and typical ocean radiances. Shown here are the: a) radiances for the eight SeaWiFS

bands; and b) cloud radiances given in reduced units. Here they are given as their ratios to the typical ocean

radiances.

18
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 13. Radiances from the SBRC test apparatus. These radiances are given in the same reduced units as

the maximum cloud radiances for each band. The radiances are given for images that are 3 and 10 pixels

wide. Shown here are the: a) radiances from the high output settings of the SBRC integrating sphere; and b)

radiances from the midrange output settings of the SBRC integrating sphere.

19
Stray Light in the SeaWiFS Radiometer

the longer wavelength bands, tested at large optical overdrives, will behave like the shorter wavelength bands when

overdrives are small. It assumes that none of the bands

have peculiarities in their bright target responses.

The assumption of band-to-band uniformity works better for the sets of four odd and four even SeaWiFS bands,

since there is evidence in the test results of odd-even band

differences due to optical cross talk. These odd-even differences are relatively slight, and their characteristics show up

in all of the pairs of odd-even bands. As discussed above,

this odd-even band difference in stray light response was

predicted before the post-modification tests. Indeed, the

character of the predicted odd-even differences was one of

the principal guides for the design of the post-modification

measurements.

Consistent and explanable effects, such as these, must

be demonstrated in the test results, if the assumption of

band-to-band uniformity in stray light response is to be

believed. The analysis presented below will refer to this

assumption; however, the proposed stray light corrections

are tailored individually to the eight SeaWiFS bands. For

these corrections, very small differences in the responses of

the bands become significant. The stray light corrections

can be a few tenths of a percent or less of the radiance

from the bright source. At levels above this, it will remain

important to check the assumption that the stray light

response is consistent from band to band.

7.4 Source Brightness Response

With the effective removal of the electronic recovery

tail (Section 5.0), there has been no sign of any significant

electronic component to the instrument’s bright target response at a distance of more than one or two pixels from

the edge of the bright source. There is a small electronic

overshoot in each band’s electronic response 1 pixel after

a bright source (see Fig. 1 of Barnes et al. 1994a). The

effect derives from the design of the Goldberg noise reduction circuit in each band’s intermediate electronics. That

response, in the along-scan direction, is present in the test

results, and it will be present in the measurements on orbit. That response will be located within 1–2 pixels from

the edge of bright sources. This overshoot will be discussed

in the analysis of the test results presented below.

Without an electronic effect, the remaining response

to a bright source is caused by scattered or reflected light

from the bright source. As the brightness of the source

varies, so does the amount of scattered or reflected light

within the radiometer and the amount of stray light in the

radiometer’s measurements. This is the assumption of a

stray light response linear with source brightness; again, it

will be important to check this assumption in the results.

7.5 Along-Scan Responses

The characterization of residual stray light in the instrument was performed using a 1 pixel wide image positioned at the nadir pixel for SeaWiFS, pixel 643. The

SeaWiFS response was measured with the standard detector configuration for the radiometer and with science gain

1. Each band’s response to that bright impulse was measured over a range of 20 pixels on either side of the nadir

pixel. Over these 41 pixels, the output for each band was

summed, and each summation was normalized to unity.

The calculations presented here assume that all energy

from the impulse is contained within the 41 pixels of the

instrument output.

The along-scan impulses and responses are listed in Table 8. The results are given for the range from 13 pixels

before the impulse (pixel offset −13) to 15 pixels after the

impulse (pixel offset 15). The responses are given for the

standard detector configuration for SeaWiFS, i.e., for measurements using all four detectors in each band. Stray light

measurements, using the same impulses, have also been

made individually for each detector in each band. They

are part of the SeaWiFS bio-optical archive and storage

system (Westphal et al. 1994). If changes to the detector

configurations are required on orbit, these test data can be

used to construct additional responses.

Table 8 gives the averages and standard deviations for

the odd and even band responses. Each of the eight responses summarized in Table 8 was normalized to unity

before the calculation of the table’s results. The standard

deviations in Table 8 range from about one-third to about

one-half of the corresponding mean values. These small

standard deviations are a measure of the band-to-band

similarity in the instrument response; however, there is

still the need for individually tailored responses for each

band in the stray light correction procedure presented below.

The impulse from the 1 pixel wide bright image is

shown in Fig. 14. The abscissa has been changed so that

the impulse is at pixel zero, rather than pixel 643. The

spread of this input into the odd band response is shown

in Fig. 15. This figure gives the average response for the

four odd SeaWiFS bands (bands 1, 3, 5, and 7). Figure 15a

shows that almost 99% of the response of the odd bands

can be found within two pixels of the source. Figure 15b

shows the remaining 1% of the odd band response, i.e., the

response that is three or more pixels from the source. In

these figures, the instrument scans left to right. Figure 15b

shows the effect of optical cross talk on the response of the

odd bands on the SeaWiFS focal planes. There is a significantly longer and larger response after the bright source.

For each odd band, this increased response results from

additional stray light from the even band (Section 6.5).

It is felt that the addition of a septum between the focal

planes would greatly reduce this additional stray light and

make the down-scan response in Fig. 15b nearly identical

to the up-scan response.

The spread of the impulse into the even SeaWiFS bands

is shown in Fig. 16. In the same manner as before, this figure gives the average response for the four even bands, and

20
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Table 8. Along-scan impulse and response. The impulse represents the illumination from a 1 pixel

wide slit. The responses come from the SCADP measurements performed at SBRC. The response values

give the means and standard deviations for the four odd and four even bands. All functions have been

normalized to give integrated values of unity.

Pixel Along-Scan Odd Band Response Even Band Response

Offset Impulse Mean σ Mean σ

−13 0.0 0.000000 0.000000 0.000000 0.000000

−12 0.0 0.000000 0.000000 0.000047 0.000011

−11 0.0 0.000000 0.000000 0.000150 0.000036

−10 0.0 0.000000 0.000000 0.000263 0.000066

−9 0.0 0.000000 0.000000 0.000435 0.000104

−8 0.0 0.000025 0.000016 0.000638 0.000147

−7 0.0 0.000069 0.000030 0.000773 0.000185

−6 0.0 0.000198 0.000079 0.000969 0.000238

−5 0.0 0.000573 0.000202 0.001136 0.000202

−4 0.0 0.001661 0.000553 0.001458 0.000187

−3 0.0 0.004368 0.001589 0.003672 0.001050

−2 0.0 0.012424 0.004133 0.009061 0.002671

−1 0.0 0.149717 0.057032 0.124966 0.067547

0 1.0 0.703367 0.029070 0.765813 0.082545

1 0.0 0.097853 0.086333 0.061887 0.081895

2 0.0 0.016655 0.005364 0.014468 0.012949

3 0.0 0.005788 0.002888 0.008829 0.002597

4 0.0 0.002021 0.000866 0.003241 0.000861

5 0.0 0.001459 0.000944 0.001259 0.000442

6 0.0 0.001127 0.000708 0.000493 0.000252

7 0.0 0.000931 0.000636 0.000214 0.000157

8 0.0 0.000675 0.000438 0.000106 0.000106

9 0.0 0.000541 0.000332 0.000049 0.000064

10 0.0 0.000310 0.000117 0.000025 0.000043

11 0.0 0.000154 0.000025 0.000025 0.000043

12 0.0 0.000059 0.000036 0.000015 0.000026

13 0.0 0.000017 0.000030 0.000007 0.000013

14 0.0 0.000007 0.000013 0.000000 0.000000

15 0.0 0.000000 0.000000 0.000000 0.000000

Sum 1.0 1.0 1.0

as with the odd bands, almost 99% of the even band response can be found within two pixels of the source. A

comparison of Figs. 15a and 16a shows a marked similarity in the pattern of the response. For the even bands,

the response in the central pixel is about 6% greater than

for the odd bands. As a result, the even band responses

for pixels −1 and +1 are smaller. The overall assessment

is that the relationships illustrated in Figs. 15a and 16a

between these two pixels are similar.

A comparison of the lower level responses in the outer

pixels (Figs. 15b and 16b) shows more of a mirror image

with respect to the central pixel. The extended down-scan

tail in the response of the odd bands is mirrored by the

extended up-scan tail in the even bands. In both cases, the

extended tails for the bands are caused by cross talk from

the bands that share the focal planes with them. For both

the odd and even bands, the tails with cross talk extend

about 11–12 pixels. For the responses without cross talk,

the tails extend about 7–8 pixels.

7.6 Along-Track Responses

The impulse in the along-track direction comes from a

3 pixel wide bright source. The measured along-track response is shown in Fig. 17. The vertical scales for Figs. 17a

and 17b duplicate those for the respective parts of Figs. 15

and 16, but the horizontal scales of Figs. 17a and 17b have

been expanded by a factor of three. The spacing of the

data points in Fig. 17 reflects the oversampling in the

measurements discussed in Section 7.1. Figure 17 shows

six along-track measurements per pixel. Again, for both

the along-scan and along-track stray light results presented

21
Stray Light in the SeaWiFS Radiometer

Fig. 14. Impulse for the stray light responses. The impulse has been set to unity, since it represents all of the

input light to the instrument.

here, a pixel is defined as a square with a side length of

1.6 mrad, which will be 1.13 km for nadir measurements

made from an altitude of 705 km (Barnes et al. 1994a).

The 2 pixel plateau at the top center of the response derives directly from the width of the illuminated slit. The

half-power points for the response are 3 pixels apart.

Figure 18 shows the along-track response in a format

identical to the along-scan figures. This was obtained by

removing just over 1.5 pixels from the central plateau in

Fig. 17a to create a narrower response. The responses in

Figs. 18a and 18b have been prepared for qualitative purposes only. They do show, however, that the side lobes

of the secondary reflections from the scrambler have been

moved from 5 pixels from the central image (see Fig. 7), to

1 pixel from the center. In addition, the along-track stray

light response exists for only 1 to 2 pixels from the central

image. This width is significantly less than that for either

the odd or even along-scan stray light results. The alongtrack results are also presented in Table 9. This table does

not separate the results into odd and even bands, since

there are no significant odd-even band differences. The

standard deviations in Table 9 are also reasonably small

when compared with the mean values. The narrower stray

light response in the along-track direction in SeaWiFS results from the basic design of the focal plane assemblies.

As shown in Fig. 10, each SeaWiFS band is 4 pixels long

(in the along-scan direction), but only 1 pixel wide.

Table 9. Along-track impulse and response. The

impulse represents the illumination from a 1 pixel

wide slit. The responses come from measurements

in the SCADP. The response values give the means

and standard deviations for the SeaWiFS bands.

All of the functions have been normalized to give

integrated values of unity.

Pixel Along-Track Response

Offset Impulse Mean σ

−6 0.000000 0.000000 0.000000

−5 0.000000 0.000003 0.000009

−4 0.000000 0.000100 0.000120

−3 0.000000 0.000238 0.000217

−2 0.000000 0.002165 0.001704

−1 0.000000 0.176247 0.045494

0 1.000000 0.645052 0.013227

1 0.000000 0.175668 0.041982

2 0.000000 0.000511 0.000183

3 0.000000 0.000016 0.000018

4 0.000000 0.000000 0.000000

Sum 1.0 1.0

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

The field stop at the inlet to the aft optics masks the

bands in the focal planes, leaving an open area that is

approximately 6 pixels long by 2 pixels wide per band.

The narrower the mask, the less stray light there is in

the measurement. Thus, the reduction of the along-scan

mask is one method for reducing stray light in that direction; such a suggestion was raised by W. Esaias (see Appendix A). For SeaWiFS, reducing the along-scan masks

over the band areas would eliminate using some of the detectors through vignetting and reduce the signal-to-noise

improvement from the use of those detectors. This suggestion was not implemented. Remodeling the instrument

to separate the individual detectors and place them under

individual filters was also considered impractical.

8. STRAY LIGHT ASSESSMENT

There are three primary contributors to the BTR in

SeaWiFS—MTF/optical blur, optical cross talk under the

interference filters, and mirror BRDF. Mirror BRDF, or

primary mirror scatter, was discussed in Section 6.1, and

included a calculation of the angular dependence of the

BRDF. Primary mirror scatter remains a stray light source

after all other sources have been removed. The ranges over

which primary mirror scatter dominates SeaWiFS BTR are

given in Table 10. These ranges can be compared with results from the laboratory measurements in Fig. 15 (alongscan odd band response), Fig. 16 (along-scan even band

response), and Fig. 18 (along-track response).

Optical cross talk has been described in Section 6.5.

It is the primary source of asymmetry in the along-track

BTR from SeaWiFS. For the stray light budget presented

here, optical cross talk covers all sources of reflections under the interference filters. For the odd bands, intraband

optical cross talk, i.e., reflections under the interference

filter within a band, dominates from 3–5 pixels before a

bright source. The so-called excess stray light from interband cross talk extends to 9 pixels after the bright source.

The same along-scan cross talk occurs in the even bands,

except that the excess interband cross talk comes before

the bright source.

The MTF has been discussed in terms of the line spread

function in Barnes et al. (1994a). The MTF is called optical blur in Fig. 6. In optical terms, this effect creates a

blur circle that extends 1 or 2 pixels around each pixel.

Other effects, such as intraband electronic cross talk and

reflections from the focusing lenses, are present in BTR;

however, they are considered to be secondary contributors

to stray light in SeaWiFS.

9. ALONG-SCAN CORRECTION

The individual responses for the eight SeaWiFS bands

are listed in Table 11. These values have been taken from

the SCADP, which also gives a brief description of their

derivation. The application of responses to remove residual stray light from an individual pixel uses the response of

that pixel plus the responses of pixels near it. Each pixel

is treated, in turn, as the central pixel in the response.

The correction is applied in two parts. First, the so-called

missing signal from the central pixel is restored. For the

odd bands, approximately 30% of the original impulse is

removed from the central pixel in the response (Table 8).

For the even bands, about 24% of the central pixel is removed and must be replaced. Second, the light added to

the central pixel must be removed from its neighbors.

The stray light correction illustrated in Table 12 includes only five pixels; the pixel to be corrected and the

two pixels on either side of it. This was done to produce

a simplified explanation. In practice, the SeaWiFS alongscan stray light correction will involve many more pixels,

and the correction is applied after the counts have been

converted into radiances. Using the notation from Table 12, the stray light correction (FSL) for an individual

pixel proceeds in the following sequence.

Step 1: Set the correction term to zero:

FSL = 0. (2)

Step 2: Replace the missing signal from pixel P0, as in

FSL = FSL + R0



1 − K0

K0



, (3)

where R0 designates the radiance of pixel P0, and K0

is the correction constant for pixel P0.

Step 3: Remove the contributions from the adjacent

pixels. This example includes only two pixels up scan

and down scan. In practice, there will be many more.

FSL = FSL − R−2



K+2

K0



, (4)

FSL = FSL − R−1



K+1

K0



, (5)

FSL = FSL − R+1

K−1

K0



, (6)

and

FSL = FSL − R+2

K−2

K0



. (7)

Step 4: Apply the correction to the radiance:

R0 = R0 + FSL. (8)

The correction in Step 2 is straightforward, since the radiance and the correction constant come from the same pixel.

For Step 3, the location of the pixel under correction, relative to the source of radiance, determines the appropriate

correction constant.

In the tests summarized below, the stray light corrections were accomplished using a commercial spreadsheet

23
Stray Light in the SeaWiFS Radiometer

Fig. 15. The response for the odd bands on the SeaWiFS focal plane assemblies. This is the instrument

response in the along-scan direction. The total response from the two portions of this figure is equal to unity,

since it represents all of the response to the impulse in Fig. 14. The figure shows: a) the response of the central

pixel and its immediate neighbors when the ordinate is set to unity; and b) the response of the outer pixels

when the ordinate is set to 1% of full scale. The excess stray light in the higher number pixels, relative to the

pixels on the lower number side, is the result of optical cross talk from the even bands.

24
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 16. The response for the even bands on the SeaWiFS focal plane assemblies. This is the instrument

response in the along-scan direction. The total response from the two portions of this figure is equal to unity,

since it represents all of the response to the impulse in Fig. 14. The figure shows: a) the response of the central

pixel and its immediate neighbors when the ordinate is set to unity; and b) the response of the outer pixels

when the ordinate is set to 1% of full scale. The excess stray light in the higher number pixels, relative to the

pixels on the lower number side, is the result of optical cross talk from the even bands.

25
Stray Light in the SeaWiFS Radiometer

Fig. 17. Measurements of the response in the along-track direction. These measurements were made by adjusting the height of the light source (Section 7.1). The measurements of the along-track response are oversampled

by a factor of 6.

26
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 18. The response in the along-track direction. This response represents all eight SeaWiFS bands. The

total response from the two portions of this figure is equal to unity, since it represents all of the response to the

impulse in Fig. 14. The figure shows: a) the response of the central pixel and its immediate neighbors when the

ordinate is set to unity; and b) the response of the outer pixels when the ordinate is set to 1% of full scale.

27
Stray Light in the SeaWiFS Radiometer

Table 10. Primary BTR components. This is a qualitative summary of the principal components in the

BTR/stray light response of SeaWiFS. The pixel ranges give estimates for the regions over which each component

dominates the BTR/stray light response. (The + sign indicates the pixel enumerated and beyond.)

Scan BTR Pixels Before Source Pixels After Source

Direction Component Odd Band Even Band Odd Band Even Band

Along-Scan MTF/Optical Blur 1–2 1–2 1–2 1–2

Optical Cross Talk 3–5 3–9 3–9 3–5

Mirror BRDF 6+ 10+ 10+ 6+

Along-Track MTF/Optical Blur 1–2 1–2 1–2 1–2

Mirror BRDF 3+ 3+ 3+ 3+

Table 11. Along-scan responses for SeaWiFS bands 1–8. These are the constants used in the correction

procedure.

Pixel Band 1 Band 2 Band 3Band 4 Band 5 Band 6 Band 7 Band 8

Offset (412 nm) (443 nm) (490 nm) (510 nm) (555 nm) (670 nm) (765 nm) (865 nm)

−13 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

−12 0.00000 0.00005 0.00000 0.00006 0.00000 0.00005 0.00000 0.00003

−11 0.00000 0.00015 0.00000 0.00020 0.00000 0.00016 0.00000 0.00010

−10 0.00000 0.00030 0.00000 0.00034 0.00000 0.00027 0.00000 0.00016

−9 0.00000 0.00050 0.00000 0.00052 0.00000 0.00049 0.00000 0.00026

−8 0.00004 0.00074 0.00004 0.00070 0.00002 0.00077 0.00000 0.00039

−7 0.00010 0.00090 0.00010 0.00078 0.00005 0.00099 0.00003 0.00048

−6 0.00030 0.00110 0.00025 0.00090 0.00015 0.00131 0.00010 0.00064

−5 0.00080 0.00130 0.00075 0.00104 0.00045 0.00142 0.00031 0.00087

−4 0.00240 0.00148 0.00199 0.00156 0.00134 0.00170 0.00097 0.00119

−3 0.00639 0.00519 0.00546 0.00442 0.00342 0.00296 0.00235 0.00241

−2 0.01758 0.01335 0.01565 0.01040 0.00921 0.00607 0.00769 0.00716

−1 0.13183 0.10236 0.21913 0.10948 0.06523 0.05654 0.19012 0.23871

0 0.73314 0.87317 0.74859 0.85724 0.66876 0.71775 0.68751 0.66864

1 0.06392 −0.00445 −0.00273 −0.00104 0.23318 0.20043 0.09758 0.05559

2 0.02000 0.03400 0.02000 −0.00416 0.00743 0.01400 0.02000 0.01600

3 0.01039 0.01335 0.00621 0.00910 0.00297 0.00558 0.00377 0.00803

4 0.00350 0.00440 0.00180 0.00300 0.00149 0.00197 0.00135 0.00385

5 0.00300 0.00148 0.00149 0.00090 0.00059 0.00082 0.00080 0.00193

6 0.00230 0.00055 0.00110 0.00026 0.00052 0.00030 0.00062 0.00090

7 0.00200 0.00020 0.00085 0.00008 0.00045 0.00011 0.00045 0.00048

8 0.00140 0.00007 0.00066 0.00003 0.00035 0.00004 0.00031 0.00029

9 0.00110 0.00002 0.00050 0.00000 0.00030 0.00002 0.00028 0.00016

10 0.00050 0.00000 0.00032 0.00000 0.00022 0.00000 0.00021 0.00010

11 0.00013 0.00000 0.00020 0.00000 0.00015 0.00000 0.00014 0.00010

12 0.00000 0.00000 0.00007 0.00000 0.00007 0.00000 0.00010 0.00006

13 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00007 0.00003

14 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00003 0.00000

15 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Table 12. Application of the along-scan stray light correction. The correction in this table is demonstrated for

a reduced set of five pixels. The correction is shown for the central pixel, P, only.

Correction Pixel

Variable P−2 P−1 P0 P+1 P+2

Radiance R−2 R−1 R0 R+1 R+2

Correction Constant K−2 K−1 K0 K+1 K+2

Complete Correction R0(1 − K0) − R−2K+2 − R−1K+1 − R+1K−1 − R+2K−2

program. In that program, all calculations were made with

radiances that were not corrected for stray light. This

spreadsheet technique gives results that are different from

calculations which correct pixels in series down a scan line.

In that type of calculation, called a moving window calculation, some of the radiances used in the stray light correction have been adjusted for stray light, and some have not.

The actual correction routine for flight data will not duplicate the spreadsheet process that derived these corrections;

however, there are other algorithms that can provide essentially identical results.

10. ALONG-SCAN TEST RESULTS

The 32 stray light tests presented here correspond to

the combination of radiances and slit widths in Figs. 13a

and 13b. There are two sets of radiances, called high and

midrange, and there are two slit widths—3 pixels and 10

pixels. A sample calculation from the test results is presented in Table 13.

10.1 Sample Calculations

The results in Table 13 are for SeaWiFS band 7, with

high radiance, and a 10 pixel slit. For the calculations,

a subset of pixels ranging from pixels 601–680, is taken

from the 1,285 pixel scan line. The laboratory measurements were made in a darkened room with an illuminated

slit (Section 7.1). From those measurements, the instrument output (in counts) for pixels 601–620 was averaged

to provide the zero (or dark) count; this zero count was

subtracted from all of the pixels in the subset. The darkcorrected counts have been converted into radiances using

the prelaunch calibration constants in Barnes et al. (1994b)

for bilinear gains (Section 4).

These measured radiances form the input for the stray

light correction. For band 7, with high radiance and a

10 pixel slit (Table 13), the input radiance shows a peak

value of about 41.8 mW cm−2 sr−1 μm−1 at pixel 640. The

output radiance from the stray light correction for that

pixel is nearly the same. The radiance at pixel 640, in

normalized units, is very close to Lcloud and is about 26

times greater than Ltypical.

The 10 pixel bright source extends from pixel 634 to

pixel 643. In all of the test set measurements, the rightmost edge of the bright source has been placed at pixel 643.

The zero values in this case (pixels 615–620 and 657–660)

range from 0.001–0.004 mW cm−2 sr−1 μm−1; i.e., from approximately 0.33–1.33 counts above the zero offset calculated from pixels 601–620. In other terms, the zero values

range from 0.06–0.25% of Ltypical. On the surface, this

is a small offset, but the SeaWiFS performance specifications call for the output of the band to fall within 0.5% of

Ltypical within 10 pixels of the bright source. In these results, where 2 or 3 counts are equivalent to 0.5% of Ltypical,

these zero values can be significant.

The decision was made not to fine tune the zero values in these calculations. In addition, the decision was

made not to fine tune the responses in these calculations.

A hypothetically improved set of response constants could

remove more of the residual stray light tails in Table 13.

Individual tailoring of these parts of the calculations could

produce improved results. Those improvements would be

of laboratory measurements of bright slits, and will not be

made on orbit. There can, and will be, multiple bright

sources in each scan of the Earth. Most of those bright

sources, clouds in particular, will not have the sharp edges

characteristic of laboratory slits.

Extrapolation of these controlled laboratory measurements of stray light to the conditions to be found on orbit

is not completely straightforward. As a result, it was decided to define the procedure for stray light reduction before analysis of the actual measurements and to use those

results without empirical fitting. The calculation of the

zero values and the constants in the along-scan responses

were not adjusted. Based on the test results presented

here, improving the laboratory results in this manner does

not necessarily lead to better results from orbit. In essence,

scatter, and possibly some bias, has been retained in these

test results.

10.2 Results

The laboratory stray light correction results are shown

in Figs. 19–50. The data from which these results were calculated have been stored by the SeaWiFS Project (Westphal et al. 1994), and are available to the interested user.

Each of these figures has four subpanels, a–d, respectively,

and the format for each part is the same from figure to figure. In all of the figures, the abscissa runs from pixel 615 to

pixel 665. Figure 19 will be used for illustrative purposes.

Subpanels 19a and 19b show the input (uncorrected) radiances for the measurement, with 19a scaled to show the

29
Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

Table 13. Results from the stray light correction for band 7 with a 10 pixel slit, and high radiance output from

the source. This table gives the input values to, and the output values from, the stray light correction. The

radiance values have units of mW cm−2 sr−1 μm−1.

Pixel Radiance Units Lcloud Normalized Ltypical Normalized

Number Input Output Input Output Input Output

615 0.00101 0.00101 0.00002 0.00002 0.00063 0.00063

616 0.00101 0.00101 0.00002 0.00002 0.00063 0.00063

617 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

618 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

619 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

620 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

621 0.00707 0.00707 0.00016 0.00016 0.00439 0.00439

622 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

623 0.00707 0.00707 0.00016 0.00016 0.00439 0.00439

624 0.01009 0.01009 0.00023 0.00023 0.00627 0.00627

625 0.01009 0.01009 0.00023 0.00023 0.00627 0.00627

626 0.01312 0.01283 0.00031 0.00030 0.00815 0.00797

627 0.01615 0.01402 0.00038 0.00033 0.01003 0.00871

628 0.01615 0.00811 0.00038 0.00019 0.01003 0.00504

629 0.03129 0.00485 0.00073 0.00011 0.01943 0.00301

630 0.07671 −0.00081 0.00178 −0.00002 0.04765 −0.00050

631 0.19177 −0.02908 0.00446 −0.00068 0.11911 −0.01806

632 0.57633 −1.68687 0.01340 −0.03923 0.35797 −1.04775

633 9.69040 5.06424 0.22536 0.11777 6.01888 3.14549

634 38.88820 42.32283 0.90438 0.98425 24.15416 26.28747

635 41.13418 42.22813 0.95661 0.98205 25.54918 26.22865

636 41.47972 41.82420 0.96464 0.97266 25.76380 25.97776

637 41.65248 41.82302 0.96866 0.97263 25.87111 25.97703

638 41.82525 41.97839 0.97268 0.97624 25.97842 26.07353

639 41.82525 41.94110 0.97268 0.97537 25.97842 26.05037

640 41.82525 41.95810 0.97268 0.97577 25.97842 26.06093

641 41.82525 42.06381 0.97268 0.97823 25.97842 26.12659

642 41.82525 43.51957 0.97268 1.01208 25.97842 27.03079

643 35.26007 40.46626 0.82000 0.94108 21.90066 25.13432

644 5.37120 2.56723 0.12491 0.05970 3.33615 1.59455

645 0.25536 −1.28311 0.00594 −0.02984 0.15861 −0.79696

646 0.38254 −0.03002 0.00890 −0.00070 0.23760 −0.01865

647 0.17663 −0.01295 0.00411 −0.00030 0.10971 −0.00804

648 0.12213 −0.00370 0.00284 −0.00009 0.07586 −0.00230

649 0.08579 −0.00546 0.00200 −0.00013 0.05329 −0.00339

650 0.06157 −0.00432 0.00143 −0.00010 0.03824 −0.00268

651 0.04643 −0.00095 0.00108 −0.00002 0.02884 −0.00059

652 0.03129 −0.00279 0.00073 −0.00006 0.01943 −0.00173

653 0.01918 −0.00366 0.00045 −0.00009 0.01191 −0.00227

654 0.01009 −0.00417 0.00023 −0.00010 0.00627 −0.00259

655 0.01009 0.00172 0.00023 0.00004 0.00627 0.00107

656 0.00707 0.00285 0.00016 0.00007 0.00439 0.00177

657 0.00404 0.00261 0.00009 0.00006 0.00251 0.00162

658 0.00404 0.00388 0.00009 0.00009 0.00251 0.00241

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Table 13 (cont.) Results from the stray light correction for band 7 with a 10 pixel slit, and high

radiance output from the source. This table gives the input values to, and the output values from, the

stray light correction. The radiance values have units of mW cm−2 sr−1 μm−1.

Pixel Radiance Units Lcloud Normalized Ltypical Normalized

Number Input Output Input Output Input Output

659 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

660 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

661 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

662 0.00404 0.00404 0.00009 0.00009 0.00251 0.00251

663 0.00101 0.00101 0.00002 0.00002 0.00063 0.00063

664 0.00101 0.00101 0.00002 0.00002 0.00063 0.00063

665 0.00101 0.00101 0.00002 0.00002 0.00063 0.00063

full illumination from the bright source, and 19b scaled

to ±0.03, i.e., ±3% of Ltypical. The horizontal scales for

19c and 19d are identical to those for 19a and 19b, respectively; however, 19c and 19d give the output (corrected)

radiances from the stray light algorithm. For 19b and 19d,

horizontal lines are added at ±0.005 (±0.5% ) of Ltypical.

These limits are part of the stray light specification.

In all of the measurements, the down scan (rightmost)

part of the bright slit was placed at pixel 643. The edges of

the bright slits were set so that they divided the endmost

pixels. Thus, for a 10 pixel slit, the slit was adjusted to

give 9 full pixels in the center of the bright source, plus

0.5 pixel on each end. For a 3 pixel measurement, there

were two central pixels plus 0.5 pixel on each end. The slit

edge positions were repeatable to about ±0.5 pixel from

measurement to measurement. Both the measurement setup and the presentation of Figs. 19–50 were designed to

provide results that are as uniform as possible. This is

particularly true for the design of subpanels b and d of the

figures, which show the details of the instrument’s response

to bright sources. This was done to allow quick visual

comparisons of the results from each band, radiance level,

and slit width.

Figure 25 shows the results for band 7, listed in Table 13. Both the tabulated results for this measurement

and the figure (subpanel 25d) show the stray light correction to overshoot the zero level both before and after the

bright source. Figure 25 also shows that the algorithm undercorrects for the stray light before the slit. The impulse

provides essentially no correction for pixel 627 and below.

There is a small electronic overshoot in each band’s

response one pixel after a bright source. This characteristic lies ingrained within the instrument. As shown in

Fig. 1 of Barnes et al. (1994a), each SeaWiFS band has

a nearly identical one-time overshoot in its along-scan response. This effect derives from the design of the Goldberg

noise reduction filter in each band’s intermediate electronics. This design helps to ensure that the instrument meets

the MTF portion of the Ocean Color Data Mission performance specifications in the along-scan direction (see Section 14 of Barnes et al. 1994a).

Since the instrument’s radiometric calibration used an

extended source with nearly uniform radiance, the electronic effect has been, in effect, calibrated into ocean scenes

that have roughly constant radiances. For scenes with

sharp transitions in radiance, particularly in the down-scan

transitions from bright sources shown here, the electronic

overshoot is present one pixel from the radiant source.

There is no sign of the overshoot before bright targets.

The effect of the Goldberg filter can be seen in Figs. 19–

50: subpanels a and b. For bands 1–7 in these figures, the

overshoot is present down scan of the bright source. For

band 8 in these figures, the electronic overshoot cannot

be seen to the right of the slit. For this band, the stray

light down scan of the slit may be so great as to mask the

electronic effect.

It is obvious from the results in Figs. 19–50 that the

stray light algorithm overcompensates close to the bright

source. In nearly all of the test cases, the corrected results

run negative for the pixel before the bright source, returning to zero as distance from the bright source increases.

For the down scan portion of the measurements, the correction algorithm also exaggerates the electronic overshoot

one pixel from the bright source. This overcorrection by

the stray light algorithm need not be considered a problem,

as it may form the foundation for a method for determining

the edges of clouds or other bright targets on orbit.

Clouds observed on orbit will not have the sharp edges

characteristic of brightly lit slits in the laboratory. There

is every possibility that the corrected instrument radiances

from orbit will show the laboratory pattern of overcorrections, however, before and after cloud pixels. Such effects

should have distinct fingerprints if the magnitude of the

change in radiance of a pixel from its predecessor, along

with the first- and second-derivatives of that change, is examined. Recognition of these patterns is not beyond the

capacity of current computer programs. The location of

the overshoot occurs one pixel from the edge of the source.

Figs. 35–36 also show the over-ranging to be present for

bright targets that are brighter, by 0.5–0.75 of Ltypical,

than the ocean radiances.

47
Stray Light in the SeaWiFS Radiometer

As noted above, the positions of the edges of the slits

were repeatable to about ±0.5 pixel. On orbit, the locations of the bright sources will not be known a priori. It

will be interesting to see if this proposal for cloud edge location can be implemented in the on-orbit data reduction

procedures. It is also important to note that these discussions of stray light corrections apply only to along-scan

measurements.

10.3 Post-Correction Stray Light Effects

The results from Figs. 19–26, for high input radiances

and 10 pixel slits, are summarized in Fig. 51a. This figure gives the average of the residual responses from the

eight bands, plus their standard deviations (1σ). The abscissa and ordinate of Fig. 51a duplicate those found in

subpanel d in each of the figures for the individual bands.

In the same manner, Fig. 51b summarizes the results from

Figs. 27–34, with high input radiances and 3 pixel slits.

Both subpanels of the figure show the residual stray light

responses located 4–10 pixels from the slit edge; however,

this statement is not entirely correct. The edges of the

slits, as explained above, were set so that they divided the

endmost pixels in the bright sources. The positions of the

edges of the slits, also explained above, were repeatable

to about ±0.5 pixel. Thus, the responses in Fig. 51 are

found from 3.5–9.5 pixels from the slit edges, give or take

one-half pixel. With respect to the locations of the points

where the correction algorithm overshoots, the responses

are located 2–8 pixels away. This observation demonstrates

an advantage of using the stray light algorithm results,

which contain the overcorrection. The overshoot occurs at

a pixel that can be determined with confidence, although

the edges of clouds are less well defined.

For all of the results in Fig. 51, except those closest

to the bright source, the 1σ error bars lie at, or within,

the requirements of the instrument specifications (0.5% of

Ltypical). For on-orbit measurements, radiances more than

four pixels from clouds (or more than three pixels from the

overshoot in the correction algorithm) should be usable in

the SeaWiFS data reduction.

Uniformity exists in the pattern to the residual stray

light responses in Fig. 51, which can be seen in the tabulated values in Table 14. The mean values and standard

deviations from this table were used to create Fig. 51. The

similarity between the 3 and 10 pixel results reveal no significant dependence on the width of the bright source. Earlier examinations (unpublished) of the uncorrected stray

light in SeaWiFS indicated a small but noticeable increase

in stray light with slit width, for slits up to 6 pixels wide.

Those preliminary results showed no such dependence for

slit widths greater than 6 pixels. Since the masks over

the detector arrays for the SeaWiFS bands are 6 pixels

long, this effect appears to have a basis in the instrument

design. This source-size dependence was the reason for incorporating the two different bright source widths in the

measurements presented here. The post-correction results

in Fig. 51, however, show no significant differences between

the stray light residuals of 3 and 10 pixels. Since the correction algorithm is based on the instrument’s measured

response to a 1 pixel laboratory slit, it was concluded that

no significant dependence exists in the corrected results on

the width of the bright source, i.e., there is no cloud size

dependence.

The sizes of the standard deviations in Fig. 51 are also

uniform. For all of the residuals, except for those closest to the bright source, the 1σ error bars are 0.5% or

less of Ltypical. As explained above, the closest pixels will

be masked in the reduction of the data from orbit. The

brightness of the sources for the measurements ranged from

1–30 times the value of Ltypical, but the scatter in the results has remained uniformly small. As a result, there is

no dependence on source brightness in the stray light correction algorithm. It can be applied to very bright clouds,

as well as the clouds producing less radiance.

The results from Figs. 35–50, i.e., with midrange input

radiances, are summarized in Figs. 52a and 52b. These

figures give the average of the residual responses from the

eight bands, plus their standard deviations (1σ). The format for these figures duplicates that for Figs. 51a and 51b.

In fact, the results in Figs. 52a and 52b also duplicate the

corresponding results in Figs. 51a and 51b. At the 0.1–

0.2% level, there are differences between the high range

and midrange residuals. For measurements whose light intensities vary by a factor of 2, however, the residuals are

essentially identical.

The results shown in Figs. 51 and 52 provide the validation for these along-scan stray light corrections. There are

no peculiarities in the results, so the band-to-band stray

light response in the instrument is uniform. There are no

differences in the results for radiances that vary by a factor

of 2, so the stray light response of the instrument is linear

with source brightness. These results, however, have been

shown only for laboratory measurements. Flight results

will provide the ultimate test of these corrections.

11. PROPOSED LAC CORRECTION

The following sections discuss the corrections for LAC

(1 km resolution) data in both the along-scan and alongtrack directions.

11.1 Along-Scan LAC Correction

The calculation of the along-scan correction was described in Section 9. In that technique, a second 1,285

pixel array is created to hold the corrected radiances. For

along-scan LAC data, the correction procedure in Section 9

will be applied to all scan lines.

There are significant stray light residuals, however, in

the corrected radiances near bright sources (Section 10.3).

Pixels with these large residuals lie within five pixels of

48
R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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Stray Light in the SeaWiFS Radiometer

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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Stray Light in the SeaWiFS Radiometer

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Stray Light in the SeaWiFS Radiometer

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Stray Light in the SeaWiFS Radiometer

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Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Table 14. Residual along-scan stray light responses. These values are also shown in Figs. 51 and 52. The mean

values and the standard deviations are presented here as a percentage of Ltypical.

Radiance Distance from Slit 10 Pixel Slit 3Pixel Slit

Range [pixels] Mean σ Mean σ

High −10 0.177 0.214 −0.013 0.085

−9 0.160 0.247 −0.070 0.180

−8 0.165 0.315 0.092 0.266

−7 0.126 0.389 −0.061 0.172

−6 0.075 0.376 −0.105 0.243

−5 −0.039 0.412 −0.198 0.325

−4 −0.295 0.573 −0.410 0.548

4 −0.519 0.717 −0.633 0.527

5 −0.011 0.358 −0.154 0.193

6 −0.014 0.230 −0.043 0.075

7 0.090 0.189 0.028 0.136

8 0.115 0.214 0.052 0.146

9 0.142 0.232 −0.006 0.138

10 0.105 0.166 0.027 0.143

Middle −10 0.067 0.113 0.005 0.088

−9 0.069 0.109 −0.052 0.160

−8 0.036 0.179 0.113 0.221

−7 0.034 0.170 0.008 0.263

−6 0.002 0.185 −0.031 0.300

−5 −0.054 0.201 −0.108 0.316

−4 −0.145 0.366 −0.272 0.631

4 −0.429 0.307 −0.316 0.133

5 0.004 0.155 −0.018 0.074

6 −0.034 0.129 −0.029 0.079

7 0.011 0.145 0.027 0.101

8 0.003 0.078 0.032 0.121

9 0.068 0.148 −0.007 0.129

10 0.029 0.134 0.008 0.099

the edges of those bright targets. Thus, pixels located four

pixels, or closer, to the edge of a cloud or another bright

source must be masked. Such masking procedures exist

for clouds, sun glint, and land surfaces (McClain et al.

1995 and Arrigo and McClain 1995). Those masks will be

extended in size to account for stray light in the instrument. In addition, those masks will be applied to the LAC

data after the stray light correction. The corrections and

the masking procedures developed here will also supersede

those masks for bright sources.

The masking procedure for bright targets presented

here is far from an accomplished fact. Indeed, there is

no guarantee that the actual computer code for flight data

will resemble this procedure at all, as a final (i.e., based on

observational data) masking procedure will be developed

post-launch. A proposed procedure for mask creation is

discussed here in terms of a single SeaWiFS band. The

masks for the flight data must be uniform for all bands,

so a mask for a pixel in any band requires a mask for that

pixel in all eight bands. Also, the correction and masking

procedures are presented here in terms of Ltypical radiances. This presentation puts the radiances and the radiance

changes, for the eight SeaWiFS bands, on equal footings.

Two methods of bright target detection (BTD) are presented here—detection based on differences from Rayleigh

scattering radiances, and detection based on changes in

radiance along the scan line.

11.1.1 Rayleigh Scattering Radiance BTD

It is possible to consider SeaWiFS as a sensor of atmospheric radiances having small perturbations due to color

changes in the ocean. This result is shown in Fig. 3 of

McClain et al. (1992). It is also possible to calculate the

radiances viewed by the sensor from the scattering of solar

flux by air molecules, i.e., from Rayleigh scattering. Radiances from the ocean surface increase those from molecular

scattering by only 5–10%, while radiances from clouds give

increases of an order of magnitude or more. In theory, the

use of Rayleigh scattering for BTD is reasonably simple.

A bright target contains a pixel (or pixels) with radiances greater than, for example, 1.3 times the Rayleigh radiance. This fractional difference, or bright target threshold,

65
Stray Light in the SeaWiFS Radiometer

Fig. 51. The residual SeaWiFS along-scan stray light responses after correction. These results give the means

and standard deviations (1σ) for the post-correction residuals. Shown here in this figure are the: a) results from

the 10 pixel high radiance measurements—the averages for all eight bands, i.e., for the results from subpanel d

of Figs. 19–26; and b) results from the 3 pixel high radiance measurements—the averages for all eight bands,

i.e., for the results from subpanel d of Figs. 27–34.

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

Fig. 52. The residual SeaWiFS along-scan stray light responses after correction. These results give the means

and standard deviations (1σ) for the post-correction residuals. Shown here in this figure are the: a) results from

the 10 pixel high radiance measurements—the averages for all eight bands, i.e., for the results from subpanel d

of Figs. 35–42; and b) results from the 3 pixel high radiance measurements—the averages for all eight bands,

i.e., for the results from subpanel d of Figs. 43–50.

67
Stray Light in the SeaWiFS Radiometer

is arbitrary and based on judgement and experience. The

threshold, in effect, defines what is or what is not a bright

target. The value for this threshold can also be changed.

The bright targets in the laboratory measurements with

the smallest optical overdrives (Figs. 35 and 43) had radiances that correspond to 1.6–1.7 of those from Rayleigh

scattering on orbit. In the laboratory, the bright targets

were displayed relative to a black background. On orbit,

the bright targets are located in the midst of typical radiances. An initial estimate for the bright target threshold is

1.3 times the Rayleigh radiance. This is the fractional multiplier for a bright source with a radiance of approximately

half of the radiance found in Fig. 35.

Each pixel’s radiance from Rayleigh scattering can be

calculated, based on values transmitted in the data stream

from the satellite. The actual data products, calculated in

the initial step of SeaWiFS data reduction, are:

The view angle of the instrument relative to

nadir (also called the scan angle);

The zenith angle of the sun; and

The azimuth angle between the view angle and

the solar zenith angle of the instrument.

Rayleigh scattering accounts for two effects in the radiances measured by SeaWiFS on orbit. First, the radiances become brighter as the instrument scans away from

nadir, since the pathlength through the atmosphere (and

the number of molecular scatterers in that pathlength) increases. Second, the radiances become dimmer with increasing solar zenith angle, since the solar flux becomes

attenuated by passing through more atmosphere before it

reaches the molecular scatterers in the path between the

instrument and the Earth. As a result, Rayleigh scattering

gives a relatively flat background radiance over a scan line

for BTD, i.e., it gives a background that changes smoothly

with view angle and with solar zenith angle. The exact calculation of the Rayleigh radiance is somewhat lengthy and

computationally cumbersome. Calculation of the Rayleigh

radiances for each pixel in each scan line would take a

significant amount of computer time. The direct calculation of individual Rayleigh radiances is not recommended

here. It seems more reasonable to determine the radiances

from a set of simplified equations based on approximations

or, more simply, from a set of look-up tables. These tables could be somewhat complex, since the azimuth angle

changes as the satellite moves from north to south in an

orbit, and also changes with the seasons.

For the Rayleigh radiance procedure, a bright target

exists where there are radiances above the threshold value,

and the edge of the target exists at the transition through

the threshold. The stray light mask in this procedure includes the pixel next to the transition and the three pixels

beyond it. For the benefit of the along-track correction

procedure presented in Section 11.2, it is important to distinguish between the pixels masked for the bright source

and those pixels in the mask’s extension for stray light.

11.1.2 Scan Line Radiance Change BTD

This technique examines the first and second derivatives of radiance changes across a scan line. The transitions

between ocean and bright target radiances are expected to

be relatively sharp, i.e., occurring over intervals from one

to a few pixels. The transitions are expected to be relatively large, since bright sources are, by definition, bright

compared to ocean measurements.

This technique also examines slopes and slope changes,

without a fixed reference, such as the Rayleigh radiance. It

uses these values to determine the edges of bright sources

with greater precision than the Rayleigh radiance technique. In portions of the scan line where radiances are

relatively constant, the radiance change technique cannot

discriminate between a bright source and a background

ocean measurement.

The first derivative is examined in terms of the difference of a given pixel’s radiance from that of its predecessor

in the scan line. As the scan reaches a bright source, it is

expected that the change will be 0.25 times the value of

Ltypical (0.25Ltypical), or greater, i.e., a positive slope with

a relatively large change in radiance. As the scan leaves a

bright source, another relatively large change is expected,

but with a negative slope. As with the Rayleigh radiance

procedure, the size of the changes required to determine a

bright target is arbitrary. The value of 0.25Ltypical used

here represents a smaller transition than in the Rayleigh

technique, since transitions between the ocean and bright

targets are not necessarily completed over the span of one

pixel.

The second derivative is also examined in terms of

changes in radiance. It is used to find the overshoot in the

correction algorithm discussed in Section 10.3. The pixel

at the point of this overshoot shows less radiance than

either its predecessor or its successor. The slope before

the overshoot pixel is negative; the slope after is positive.

In this procedure, the magnitude of the changes both before and after the overshoot pixel must be 1% of Ltypical

or greater. Insofar as is currently known, when the correction algorithm has been applied, in all cases, the overshoot

occurs after the bright source, and in the large majority of

cases, it also occurs before the bright source.

Similar methods are used to determine the left and

right sides of bright sources. Approaching the bright source

from the left side, i.e., from the start of the scan line,

a search is made for a positive change of 0.25Ltypical or

greater. The pixel after this change is considered to be the

leftmost pixel in the bright target. A search is then made

backwards (towards the beginning of the scan line) for the

pixel at the overshoot. When this pixel has been located,

the range of pixels starting three pixels before the overshoot and ending immediately before the leftmost pixel

of the bright target is then masked for stray light effects.

Note that the overshoot from the correction algorithm may

not be present in all bands; however, its presence in at least

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

four bands should be sufficient to define the location of the

overshoot for all eight bands.

Leaving the bright source, a search is made for a negative change of 0.25Ltypical or greater. The pixel before

this change is considered to be the rightmost pixel in the

bright target. A search is then made forward (toward the

end of the scan line) for the pixel at the overshoot. When

this pixel has been located, the range of pixels starting

immediately after the rightmost pixel of the bright target

and ending three pixels after the overshoot is then masked

for stray light effects.

In this procedure, the left and rightmost pixels in the

bright targets are determined, and the signatures of the

overshoots from the correction algorithm are found. From

the locations of these overshoot pixels, the stray light masks

are added to the bright target masks. By itself, however,

this procedure is flawed. There is no certainty that the

transitions in the procedure, that is, the first derivatives,

will always be between ocean measurements and bright targets. Transitions from land to (or from) clouds, or transitions from ocean glint to (or from) clouds, can be confused

with transitions to and from the oceans. A combination

of the procedure presented here and the Rayleigh radiance procedure from Section 11.1.1 is recommended. The

Rayleigh radiances should be used first to determine the

location of the bright sources and mask those bright targets. Then, there should be a search for the overshoots in

the correction algorithm, in order to determine the locations of the pixels for the stray light additions to the bright

target masks.

11.1.3 Arbitrary Radiance Threshold BTD

Bright target corrections will be applied early in the

set of computer programs that reduce SeaWiFS data. The

SeaWiFS data volume is large, and as a result, the detection schemes presented in Sections 11.1.1 and 11.1.2 may

be too cumbersome and time consuming to incorporate

into these programs. An alternative detection scheme may

be to declare all radiances above an arbitrary value, such

as the knee radiance in the bilinear gain, to come from a

bright source. A second alternative may be to apply the

existing cloud mask, which is provided in McClain et al.

(1995). Both alternatives can give the edges of bright targets and the reference pixels for the start of the stray light

masks.

The BTD schemes in Sections 11.1.1 and 11.1.2 may

prove to be too computationally complex for the routine

reduction of flight data, and therefore, a less complicated

scheme may be needed. Sections 11.1.1 and 11.1.2, however, provide insights into the characteristics of bright targets that can be used to test and improve the algorithms

for the on-orbit data. Since the stray light analysis in

this document is prelaunch and zeroeth order, in the same

manner, so are the masking algorithms that will be used

at launch.

11.2 Along-Track LAC Correction

There is no correction algorithm in the along-track direction, and thus, there are no calculations to be made.

The along-track LAC correction only includes expansion

of the bright target masks. The stray light correction procedure masks two additional pixels on each along-track

side of bright targets. In principle, this masking procedure

is very simple. Using the along-scan procedures in Section 11.1, the locations of the bright targets have already

been determined for each scan line in the LAC scene. Thus,

the scene need only be searched in the along-track direction for bright target masks, and stray light masks can be

added before and after the bright targets. In this alongtrack search, it is important to discriminate between bright

target masks and the stray light masks that were added to

them in the along-scan direction.

The addition of along-track masks requires a new appreciation of the relationship between scan lines in LAC

(and GAC) scenes. Before the study of stray light in SeaWiFS, individual Earth scans by the instrument were considered independent of each other. As a result, the analysis

of on-orbit measurements was scheduled to be performed

on a line-by-line basis only. The two-dimensional ocean

scenes were considered to be composites of independent

one-dimensional scan lines. The inclusion of along-track

stray light requires an additional, albeit simple, analysis

in a second dimension. The LAC (and GAC) data arrays

must be manipulated in a different fashion. This requirement adds possible complications to the SeaWiFS on-orbit

data analyses.

12. PROPOSED GAC CORRECTION

As discussed in Section 1.1, there is a 3 pixel spacing

between GAC pixels, both in the along-scan and alongtrack directions. Knowledge of the edges of bright targets,

in the manner of LAC measurements, is much coarser.

It is possible, however, to determine which of the GAC

pixels are bright targets, and which are ocean measurements. This can be done through the Rayleigh scattering

technique from Section 11.1.1. In that technique, the background Rayleigh radiance is determined from geometric

factors in the SeaWiFS measurements. Using the procedure in Section 11.1.1, it is possible to mask the bright

targets in each scan line.

A prelaunch method for determining the edges of bright

targets must be developed if GAC stray light corrections

are to be applied at the start of on-orbit sampling. As the

set of LAC and GAC measurements from SeaWiFS accumulates, an improved method for determining bright target edges will be designed and implemented (Section 1.2).

From those on-orbit samples, a statistically derived data

set should improve the two assumptions presented here.

First, in GAC scenes, it is assumed that the edge of

each bright target, i.e., each cloud, is located midway between the endmost GAC pixel that is masked as a bright

69
Stray Light in the SeaWiFS Radiometer

source and the next GAC pixel. This places the bright

target edge 2 LAC pixels (±1 LAC pixel) from the nearest unmasked GAC pixel. Thus, it is assumed that the

unmasked GAC pixels (i.e., unmasked as bright targets)

lie in the progression of 2, 6, 10 LAC pixels, and so forth,

from the bright source.

Second, it is assumed that the edge pixels for all of

the bright sources have the same radiance as the closest

pixel masked as a bright target. In other words, it is assumed that the endmost few pixels of bright sources have

the same radiances. This particular assumption can be

improved using on-orbit measurements. It is known that

clouds and cloud banks can taper off at their edges, and

many clouds do not end abruptly. A cloud radiance located one or two pixels from the edge of a cloud may be

significantly different from those closer to the edge. Thus,

the use of the nearest bright pixel for stray light corrections

may cause a miscorrection in the nearby GAC pixels. In

this case, a statistically derived data set based on on-orbit

measurements should determine the magnitude of any such

miscorrections.

With these assumptions, and with knowledge of the

locations of the bright sources in the GAC scene, it is possible to apply stray light corrections and masks to the adjoining pixels. The procedure is simple in the along-track

direction—mask the GAC pixels that neighbor the bright

targets for stray light. These GAC pixels are a distance of

1–3 LAC pixels from the bright source, and the along-track

LAC mask is applied at distances up to 2 LAC pixels from

bright targets. The GAC pixels that are one place further

removed from the bright sources should not be masked,

as they are from 5–7 LAC pixels distant from the bright

source. As with the along-track LAC masking procedure in

Section 11.2, masking for along-track GAC stray light will

require the manipulation of the GAC scene in a second dimension. This can also add significant complications to the

analysis of on-orbit GAC measurements from SeaWiFS.

In the along-scan direction, the GAC stray light masks

are determined using the same procedure as that for the

GAC along-scan masks. The GAC pixel closest to the

bright source is masked for stray light. For along-scan

GAC pixels that are two and three GAC positions away

from the bright target, however, a stray light correction

must be applied. In terms of LAC distances, these GAC

positions are 6 ±1 and 10 ±1 LAC pixels removed from the

bright source. The stray light corrections for these GAC

positions are based on the along-scan impulses discussed

in Section 7.5. However, the application of the corrections

must reflect the uncertainties in position and brightness of

the full set of pixels in the bright target.

As shown in Table 12, the stray light correction algorithm is cumulative. It includes contributions from all of

the adjacent pixels along the scan line. When the adjacent pixels on both sides are counted, the total for the

contributing pixels reaches about twenty. As a result, the

LAC along-scan algorithm, with its impulses, cannot be

used in GAC, since there are gaps in the GAC scenes. Instead, the GAC along-scan correction presented here uses

the test results shown in Figs. 27–34. The radiances in

those results include the stray light from the full set of

adjacent pixels.

The calculation of the GAC along-scan correction factors can be demonstrated using the test results in Table 13.

These results are for band 7, with high radiance output,

and a 10 pixel slit. For the purpose of GAC correction

factor calculation, the bright source in Table 13 extends

from pixel 634 to pixel 643. The data point that is 6 LAC

(or 2 GAC) pixels before the bright source is located at

pixel 628. The correction factor is the corrected residual,

minus the uncorrected residual, divided by the radiance at

the source. For pixel 628, the relative correction from the

stray light algorithm (using data from Table 13) is

FGAC = 20.00504 − 0.01003

24.15416 + 26.28747

, (8)

where FGAC is the GAC correction factor (dimensionless),

0.01003 and 0.00504 are the input and output radiances

from the stray light correction at pixel 628, and 24.15416

and 26.28747 are the radiances at the edge of the bright

source that correspond with those input and output radiances. The average of the two edge radiances (corresponding to the corrected and uncorrected residuals) is used in

the correction factor to minimize the bias that might arise

from selecting either one. These radiances are given in

terms of Ltypical, but the actual radiances or the radiances

normalized to Lcloud can also be used, since the results are

given as a relative correction.

The calculated correction factor, −0.00020, compares

favorably with the correction factor of −0.00027 in Table 15 for band 7 (2 GAC pixels before the bright source).

The correction factor is applied to the measured radiances

as follows:

R(Pedge±P∆) = R(Pedge±P∆)

+ FGAC(Pedge±P∆)R(Pedge), (9)

where Pedge represents a pixel located exactly on the edge

of the bright source in the GAC scene; ±P∆ is the location of the pixel to be corrected in GAC pixels relative

to the (bright target) edge pixel; and FGAC is the correction factor from Table 15. For a pixel from band 1

that is 2 pixels before a bright source, the correction factor, FGAC(Pedge − 2), is −0.0079 (Table 15). This value is

multiplied by the radiance of the edge pixel in the bright

source. The product is then added to the on-orbit radiance

for the pixel from band 1 that is 2 pixels before the bright

source.

Since these correction factors are ultimately based upon

the along-scan impulses, they should show the odd-band

even-band asymmetry from Figs. 15 and 16. Band 8 shows

a special case. For this band, there is a correction after the

70
R.A. Barnes, A.W. Holmes, and W.E. Esaias

Table 15. Along-scan GAC correction factors (FGAC). These factors are multiplied by the radiance from the

endmost pixel of the bright target, and that product is subtracted from the radiance in the given GAC pixel. The

correction factors are dimensionless, and the correction calculations are performed, using the standard SeaWiFS

radiance units. There are no corrections for the GAC pixels immediately before and after bright sources.

Band GAC Pixels Before Bright Source GAC Pixels After Bright Source

Number 3 21 12 3

1 −0.00000 −0.00079 Mask Mask −0.00733 −0.00090

2 −0.00060 −0.00363 Mask Mask −0.00110 −0.00001

3 −0.00000 −0.00069 Mask Mask −0.00303 −0.00055

4 −0.00074 −0.00322 Mask Mask −0.00043 −0.00000

5 −0.00000 −0.00028 Mask Mask −0.00194 −0.00053

6 −0.00055 −0.00359 Mask Mask −0.00061 −0.00000

7 −0.00000 −0.00027 Mask Mask −0.00199 −0.00055

8 −0.00037 −0.00201 Mask Mask −0.00233 −0.00033

bright source that is the same magnitude as that for band

7. At the wavelength range for band 8, the silicon in the

detectors starts to become transparent to long wavelength

photons. This effect causes some photons to penetrate

within the silicon substrate before the photon is absorbed

by the detector. This increased distance causes the photoncreated electrons to travel further before leaving the detector. These delays cause the small additional tail in band

8 after the bright source. The transparency of the silicon

increases with longer wavelength until the response deteriorates badly at about 1,000 nm. The correction factors for

radiances located 2 GAC pixels from bright sources seem

very small. For example, the factor for band 8 for 2 GAC

pixels before a cloud is −0.00201. Clouds in band 8 can

be over 30 times brighter than the typical ocean radiance,

however. If the correction factor is incorrect by 0.00020,

i.e., by 10% of its value, then the error in the corrected radiance is 0.006Ltypical. The limit for residual stray light in

the SeaWiFS performance specifications is 0.005Ltypical.

According to this error analysis, the correction factors

for radiances located two GAC pixels from bright sources

may be inadequate to provide the required proper stray

light removal. This is also true of the knowledge of the

brightness of clouds near their edges in GAC scenes. It

is suggested that the along-scan mask for stray light be

extended to 2 GAC pixels from bright targets. This is a

determination made before launch. It may be possible that

the statistical analysis of on-orbit LAC measurements, as

described in Section 1.2, will show this suggestion to be

overly cautious.

Appendix A

SUMMARY OF SeaWiFS STRAY LIGHT

REDUCTION SCENARIOS

(Note:This appendix is a reprint of an internal SeaWiFS document written by Wayne Esaias. It is presented here to describe

some of the items taken under consideration by the SeaWiFS

Project to find alternative modifications to reduce stray light.

Some minor modifications of the original text has been made

in order to conform to the format of the SeaWiFS Technical

Report Series.)

From:Wayne E. Esaias

Date:May 11, 1993

This is a partial compilation of possible alterations being

studied to alleviate stray light/bright target recovery. Studies

are under way, and this document is expected to change considerably. Short of scrapping the whole thing or launching as

is, changes in the following areas may make some difference.

There is no priority indicated, and some of the scenarios are

mutually exclusive. Magnitudes of improvements are not well

quantified, and usually are not additive. Additional approaches

might arise from the studies, or impacts refined. Risks, cost,

and schedule impacts are crude estimates at the present time,

but are expected to be refined considerably as studies are completed.

1. Re-coat optics.

2. Tilt interference filters.

3. Change TDI.

4. Completely mask one or two detectors.

5. Implement bilinear gain for band 1.

6. Implement bilinear response for more bands, intermediate focus mask.

7. Reverse the band positions on focal planes.

8. Software corrections.

9. Collect one full LAC band in GAC data.

10. Assemble the HRPT LAC data.

11. Redesign the focal planes.

CHANGE 1

Re-coat optics with better anti-reflection coatings.

Intent: Decrease reflections from lenses. Improve response near

all bright targets. Decrease distance affected.

Magnitude ofimprovement: Awaiting results. Could decrease

stray light by 20–200%. Lenses already show very good

anti-reflection properties.

Negative impact: None to science data. Affected data will still

show asymmetry (from interband cross talk).

Risk: Medium.

Cost/Schedule impact: Major. Requires new lenses, reassembly, recalibration.

71
Stray Light in the SeaWiFS Radiometer

CHANGE 2

Tilt interference filters.

Intent: Redirect reflections from primary source and to eliminate them by tilting the interference filters or the focal

planes along the axis of the detectors.

Magnitude ofimprovement: This is uncertain pending modeling. Could virtually eliminate the stray light problem.

Possible 120% increase in uncontaminated data (2.2 times

as much) if 75% effective. This could mean 3 pixel cloud

masks versus 12 pixel masks. Could reduce degree of asymmetry (from interband cross talk).

Negative impact: Broadening of bandpass or some loss of transmission if new filters are required.

Risk: High

Cost/Schedule: Major. May require buying new filters unless

specifications are relaxed. May require making new focal

plane filter mount.

CHANGE 3

Change TDI to two detectors (Operational change).

Intent: Select only the central two detectors for TDI. Change

the distance affected by two IFOVS.

Magnitude ofimprovement: Increase ocean data by 8% relative to 12 pixel mask. No change in amount of stray light

present or asymmetry (correction still difficult).

Negative impact: Decrease SNR by 30% (still within spec).

Decrease redundancy.

Risk: None.

Cost/Schedule: None. TDI under command control already.

CHANGE 4

Mask one or more detectors on focal plane

(paint filters partially black).

Intent: Permanently blacken or mask a portion of the filter to

both decrease the amount of stray light and decrease the

distance affected.

Magnitude ofimprovement: Uncertain until model and test

are done, but could cause significant improvement in intensity at edges. Increases ocean data by 8% relative to

12 pixel mask (potentially more depending on stray light

reduction), plus aids correction near land.

Difficulties: Masking the f/2 system in front of the filter will

lead to vignetting.

Negative impact: Decrease SNR (by 30% if two channels are

blackened). Permanent change compared to #3 above.

Possible MTF change. Loss in redundancy.

Risk: Medium. Focal plane 1 is hard to access.

Cost/Schedule: Significant.

CHANGE 5

Bilinear response for band 1.

Intent: Increase dynamic range of band 1 (412 nm) so it can

quantify bright target radiances. Reduces gain of one of

four detectors in band 1 to 20% of present value, so that

the sum of 4 detectors shows a bilinear response with a

knee at about 800 counts and overall saturation at cloud

radiance.

Magnitude ofimprovement: Necessary for any correction that

requires knowledge of intensity of bright targets. Solves

no problem by itself. May provide sufficient information

to infer cloud radiances for all bands. Not sufficient for

overall land radiance.

Negative impact: Decreases resolution of ocean level radiance

by 20% , decreases the range of quantization 20%. Both

may be acceptable, but will impact DOM corrections to

chlorophyll derivations. Range limit would primarily affect

scan edges and low latitude bright waters.

Risk: Low.

Cost/Schedule: Significant, but changes only the electronics

module, not the instrument.

CHANGE 6

Bilinear response for bands other

than band 1. Partially mask one detector per

band at intermediate focus.

Intent: Increase dynamic range of all bands to quantify bright

target radiance. Reduce stray light by partially masking

the outer detectors on each band at the intermediate focus

mask (pinhole).

Magnitude ofimprovement: Awaits model results. Provides

full Earth albedo dynamic range on all channels. Aids corrections at all wavelengths (land). Solves no problem by

itself. Occlusion decreases distance of effect by about a

pixel, and reduces stray light by 20%. Provides redundant

bright target sensors. Increases ocean data by less than 5%

relative to 12 pixel mask, without correction.

Negative impact: Decreases radiometric resolution by 20–30%

in the ocean range. Limits range of high resolution data

(primarily affects scan edges and low latitudes, but probably acceptable). Requires both instrument and electronics

module work. Pinhole would have to reduce light. level

by about 1:16 (all bands) to accommodate bands 7 and 8.

Potential impacts on diffuser and lunar accuracy.

Risk: Medium-high.

Cost/Schedule: Major. Requires instrument work, calibration,

tests, and electronics work.

Added benefit: Mission would collect good (250 levels) global

land and cloud data as well (making an 8-bit MODIS simulator).

CHANGE 7

Reverse odd and even channel positions

on one or more focal planes.

Intent: To provide a useful quartet of spectral bands which

do not anticipate bright targets, by switching positions on

various focal planes. Ratios of the counts in odd bands

show very similar responses to the anticipation stray light

and have minimal bright target anticipation. Present odd

channels are 412, 490, 555, and 765 nm.

Magnitude ofimprovement: Partial. Potentially enables CZCSlike results up to 5 pixels from the up-scan edge of land and

clouds (by reversing the 412 and 443 bands), or reliable

atmospheric correction for the same region (by reversing

three focal planes). Provides 22% increase with CZCS-like

data relative to 12 pixel masks.

Negative impact: None. Contributes little to solution of trailing

edge or electronic recovery.

Risk: Medium-high. Filters have been replaced on one focal

plane before. Focal plane 1 is hard to access.

Cost/Schedule: Major. Requires disassembly and removal of

filters, with risk of damage. Only one spare filter exists for

band 5. Would require recalibration, realignment.

72
R.A. Barnes, A.W. Holmes, and W.E. Esaias

CHANGE 8

Software Corrections.

Intent: Solve the problem in software. Need the focal plane

response function and methodology.

Magnitude ofimprovement: Uncertain. Asymmetry of odd and

even band effects places a major constraint on the errors

allowed. If effects are corrected to 50% (of 12 pixels total),

this would provide a 44% increase in ocean data, or 75%

increase if 75% correction is possible (relative to 12 pixels).

Negative impact: Major software and coding task. Requires

knowledge of bright target spectral radiance and focal plane

response. Amounts to passing a variable response, 25 element filter through the data. Any change in focal plane

response over time will be difficult to resolve. May necessitate production and archive of a level-1b data set.

Corrections to GAC data are more difficult.

Risk: None to flight segment, high risk of guaranteed success.

Cost/Schedule: Requires some combination of changes 1 or 2,

5 or 6,and 7, plus a full focal plane BRDF, significant computer upgrades, intensive validation measurements, and 6

months to a year post-launch for a significant improvement.

CHANGE 9

Collect full scan of one band in GAC data.

Intent: Provide high resolution data along-scan for one band

on GAC scan lines, to enable position of bright targets to

be determined in GAC data. Provides unambiguous flag

for corrupt data when small clouds are missed in GAC

subsamples.

Magnitude ofimprovement: Impact being assessed.

Negative impact: Major impact on spacecraft data system, requires data compression.

Risk: High.

Cost/Schedule: Increased data storage by 11/8, and changing 2

Gbits unpacked data per day to 2.75 Gbits to keep the same

potential coverage frequency. Major rework of spacecraft

data system, data formats.

CHANGE 10

Collate global LAC HRPT data.

Intent: Since the location of bright targets is not known in GAC

data to better than 3 pixels, correction of the stray light

effect is only possible with LAC data.

Magnitude ofimprovement: Dependent on change 8. Most of

North Atlantic and coastal waters are covered.

Negative impact: None, benefit regional science by having all

data available.

Cost/Schedule: Major, since it implies doing a correction, plus

processing 16 times the GAC volume to arrive at a global

data set.

CHANGE 11

Rearrange bands on focal planes (parallel bands).

Intent: Put bands in parallel instead of in line, to reduce-along

scan distance effect, and odd-even responses.

Magnitude ofimprovement: Could be significant from about 12

to about 5 pixels anticipation. Total area of effect remains

about the same.

Negative impact: Increases distance along-track in which a

bright target would affect a given pixel. Total area affected

would remain the same.

Risk: High.

Cost/Schedule: Major, major impact.

Glossary

A/D Analog-to-Digital

BRDF Bidirectional Reflectance Distribution Function

BTR Bright Target Recovery

BTD Bright Target Detection

CZCS Coastal Zone Color Scanner

FOV Field of View

GAC Global Area Coverage

GSFC Goddard Space Flight Center

IFOV Instantaneous Field-of-View

LAC Local Area Coverage

MTF Modulation Transfer Function

NASA National Aeronautics and Space Administration

NIMBUS Not an acronym, but a series of NASA experimental

satellites containing a wide variety of atmosphere,

ice, and ocean sensors.

OSC Orbital Sciences Corporation

RC Resistor-Capacitor (circuit)

SBRC Santa Barbara Research Center

SCADP SeaWiFS Calibration and Acceptance Data Package

SCDR SeaWiFS Critical Design Review

SeaWiFS Sea-viewing Wide Field-of-view Sensor

SNR Signal-to-Noise Ratio

SPR SeaWiFS Preship Review

SPMPR SeaWiFS Post-Modification Preship Review

SSLSP SeaWiFS Stray Light Signal Paths

TDI Time Delay and Integration

Symbols

FGAC GAC correction factor.

FSL Correction factor for stray light.

Ki Correction constant at the ith pixel.

Lcloud Maximum radiance from reflected light off of clouds.

Ltypical Expected radiance from the ocean measured on orbit.

Pedge A pixel located on the exact edge of a bright source

in a GAC scene.

Pi The ith pixel under correction.

Pslit Designates the number of pixels after the slit for the

instrument to return to the residual counts allowed

in the specification.

Pzero Designates the number of pixels required for the instrument to settle to a level of zero residual counts.

P∆ The location of the pixel to be corrected in GAC

pixels relative to the (bright target) edge pixel.

Ri Radiance of the ith pixel.

RB Bidirectional reflectance distribution function.

α Off-axis angle.

σ Standard deviation of a set of data values.

73
Stray Light in the SeaWiFS Radiometer

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R.A. Barnes, A.W. Holmes, and W.E. Esaias

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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.

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, 1995: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, 47 pp.

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.

Vol. 24

Firestone, E.R., and S.B. Hooker, 1995:SeaWiFS Technical Report Series Summary Index:Volumes 1–23. NASA Tech.

Memo. 104566, Vol. 24, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, 36 pp.

Vol. 25

Mueller, J.L., and R.W. Austin, 1995:Ocean Optics Protocols

for SeaWiFS Validation, Revision 1. NASA Tech. Memo.

104566, Vol. 25, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 66 pp.

Vol. 26

Siegel, D.A., M.C. O’Brien, J.C. Sorensen, D.A. Konnoff, E.A.

Brody, J.L. Mueller, C.O. Davis, W.J. Rhea, and S.B.

Hooker, 1995:Results of the SeaWiFS Data Analysis

Round-Robin (DARR), July 1994. NASA Tech. Memo.

104566, Vol. 26, S.B. Hooker and E.R. Firestone, Eds.,

NASA Goddard Space Flight Center, Greenbelt, Maryland, 58 pp.

Vol. 27

Mueller, J.L., R.S. Fraser, S.F. Biggar, K.J. Thome, P.N. Slater,

A.W. Holmes, R.A. Barnes, C.T. Weir, D.A. Siegel, D.W.

Menzies, A.F. Michaels, and G. Podesta, 1995:Case Studies for SeaWiFS Calibration and Validation, Part 3. NASA

Tech. Memo. 104566, Vol. 27, S.B. Hooker, E.R. Firestone,

and J.G. Acker, Eds., NASA Goddard Space Flight Center,

Greenbelt, Maryland, 46 pp.

75
Stray Light in the SeaWiFS Radiometer

Vol. 28

McClain, C.R., K.R. Arrigo, W.E. Esaias, M. Darzi, F.S. Patt,

R.H. Evans, J.W. Brown, C.W. Brown, R.A. Barnes, and

L. Kumar, 1995:SeaWiFS Algorithms, Part 1. NASA

Tech. Memo. 104566, Vol. 28, S.B. Hooker, E.R. Firestone,

and J.G. Acker, Eds., NASA Goddard Space Flight Center,

Greenbelt, Maryland, 38 pp., plus color plates.

Vol. 29

Aiken, J., G.F. Moore, C.C. Trees, S.B. Hooker, and D.K.

Clark, 1995:The SeaWiFS CZCS-Type Pigment Algorithm. NASA Tech. Memo. 104566, Vol. 29, S.B. Hooker

and E.R. Firestone, Eds., NASA Goddard Space Flight

Center, Greenbelt, Maryland, 34 pp.

Vol. 30

Firestone, E.R., and S.B. Hooker, 1995:SeaWiFS Technical Report Series Summary Index:Volumes 1–29. NASA Tech.

Memo. 104566, Vol. 30, S.B. Hooker and E.R. Firestone,

Eds., NASA Goddard Space Flight Center, Greenbelt,

Maryland, (in production).

Vol. 31

Barnes, R.A., A.W. Holmes, and W.E. Esaias, 1995:Stray

Light in the SeaWiFS Radiometer. NASA Tech. Memo.

104566, Vol. 31, S.B. Hooker, E.R. Firestone, and J.G.

Acker, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 76 pp.

76
 Form Approved

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 information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite

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 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Laboratory for Hydrospheric Processes

Goddard Space Flight Center 95B00108

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. 31

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words) Some of the measurements from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) will not be

useful as ocean measurements. For the ocean data set, there are procedures in place to mask the SeaWiFS measurements of clouds and ice.

Land measurements will also be masked using a geographic technique based on each measurement's latitude and longitude. Each of these

masks involves a source of light much brighter than the ocean. Because of stray light in the SeaWiFS radiometer, light from these bright

sources can contaminate ocean measurements located a variable number of pixels away from a bright source. In this document, the sources

of stray light in the sensor are examined, and a method is developed for masking measurements near bright targets for stray light effects. In

addition, a procedure is proposed for reducing the effects of stray light in the flight data from SeaWiFS. This correction can also reduce the

number of pixels masked for stray light. Without these corrections, local area scenes must be masked 10 pixels before and after bright

targets in the along-scan direction. The addition of these corrections reduces the along-scan masks to four pixels before and after bright

sources. In the along-track direction, the flight data are not corrected, and are masked two pixels before and after. Laboratory measurements have shown that stray light within the instrument changes in a direct ratio to the intensity of the bright source. The measurements

have also shown that none of the bands show peculiarities in their stray light response. In other words, the instrument's response is uniform

from band to band. The along-scan correction is based on each band's response to a bright source that is 1 pixel wide. Since these results

are based solely on preflight laboratory measurements, their successful implementation requires compliance with two additional criteria.

First, since SeaWiFS has a large data volume, the correction and masking procedures must be such that they can be converted into

computationally fast algorithms. Second, they must be shown to operate properly on flight data. The laboratory results, and the corrections

and masking procedures that derive from them, should be considered as zeroeth order estimates of the effects that will be found on orbit.

14. SUBJECT TERMS 15. NUMBER OF PAGES

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17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

 OF REPORT OF THIS PAGE OF ABSTRACT

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

 REPORT NUMBER

Unclassified–Unlimited

Subject Category 48

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SeaWiFS, Oceanography, Stray Light, Bright Target Recovery, GAC Correction, 76

LAC Correction

Report is available from the Center for AeroSpace Information (CASI),

7121 Standard Drive, Hanover, MD 21076–1320; (301)621-0390

 Unclassified Unclassified Unclassified Unlimited

 REPORT DOCUMENTATION PAGE OMB No. 0704-0188

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI Std. 239-18, 298-102

 SeaWiFS Technical Report Series

Volume 31–Stray Light in the SeaWiFS Radiometer Code 970.2

 July 1995 Technical Memorandum

6. AUTHOR(S)

 Robert A. Barnes, Alan W. Holmes, and Wayne E. Esaias

 Series Editors: Stanford B. Hooker and Elaine R. Firestone

 Technical Editor: James G. Acker

Elaine R. Firestone: General Sciences Corporation, Laurel, Maryland; Robert A. Barnes: ManTech, Inc., Wallops Island, VA;

and Alan W. Holmes: Santa Barbara Research Center, Goleta, CA