ARM TR-043

> Whole-Sky Imager (WSI) Handbook
>
> January 2005
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 1](#general-overview)

[2. Contacts 1](#contacts)

[3. Deployment Locations and History
1](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 2](#near-real-time-data-plots)

[5. Data Description and Examples 2](#data-description-and-examples)

[6. Data Quality 8](#data-quality)

[7. Instrument Details 9](#instrument-details)

**Tables**

[Table 1. 5](#__RefHeading___Toc94675793)

# General Overview

The whole-sky imager (WSI) is an automated imager used for assessing and
documenting cloud fields and cloud field dynamics. The WSI is a
ground-based electronic imaging system that monitors the upper
hemisphere. It is a passive, i.e., non-emissive, system that acquires
images of the sky dome through three spectral filters (neutral, red, and
blue). From these sky images, we can assess the presence, distribution,
shape, and radiance of clouds over the entire sky using automated cloud
decision algorithms and related processing. The current WSI model (EO
System 6) is capable of image acquisition under daylight, moonlight, and
starlight conditions.

# Contacts

## Mentor

Ken Black\
Sandia National Laboratories\
P.O. Box 969\
Livermore, CA 94551\
Phone: 925-294-2889\
E-mail: [kblack@sandia.gov](mailto:klblack@sandia.gov)

Tim Tooman (data analysis)\
Sandia National Laboratories\
P.O. Box 969, MS# 9056\
Livermore, CA 94551\
Phone: (925-294-2752\
E-mail: [tim.tooman@arm.gov](mailto:tooman@sandia.gov)

Instrument Developer

Janet Shields\
Atmospheric Optics Group\
Marine Physical Lab, 0701\
Scripps Institution of Oceanography\
University of California San Diego\
9500 Gilman Drive\
La Jolla, CA 92093-0701\
Phone: 619-534-1769\
Fax: 619-553-0764\
E-mail: <jshields@mpl.ucsd.edu>

# Deployment Locations and History 

WSIs are currently located at the Southern Great Plains (SGP), North
Slope of Alaska (NSA), and Tropical Western Pacific (TWP) sites.

# Near-Real-Time Data Plots

[Barrow, Alaska Quicklooks](http://nanuna.gi.alaska.edu/nsadocs/)

[Atqasuk, Alaska Quicklooks](http://nanuna.gi.alaska.edu/atqdocs/)

# Data Description and Examples

Observed Data:

  ---------------------------------------------------- --------------------------------------------------------------------------------
  ![](media/image1.png){width="0.6659722222222222in"   Sample [Daylight
  height="0.5819444444444445in"}                       image](http://www.arm.gov/instruments/images/wsi/sep13.93.wsmr_daylight.gif).
                                                       Note that the occultor design on this imager is different from the design
                                                       employed at SGP and TWP.

  ![](media/image2.png){width="0.6659722222222222in"   Sample [Moonlight
  height="0.5729166666666666in"}                       image](http://www.arm.gov/instruments/images/wsi/sep30.93.wsmr.moonlight.gif).
                                                       Note that the occultor design on this imager is different from the design
                                                       employed at SGP and TWP.

  ![](media/image3.png){width="0.6659722222222222in"   Sample [Starlight
  height="0.5729166666666666in"}                       image](http://www.arm.gov/instruments/images/wsi/sep14.93.wsmr.starlight.gif).
  ---------------------------------------------------- --------------------------------------------------------------------------------

Calculated data:

  ---------------------------------------------------- -----------------------------------------------------------------------------------
  ![](media/image4.png){width="0.6659722222222222in"   Sample [cloud decision
  height="0.5819444444444445in"}\                      image](http://www.arm.gov/instruments/images/wsi/sep18.92.wsmr.clouddecision.gif)
  ![](media/image1.png){width="0.6659722222222222in"   determined from the red/blue ratio image and the ratio threshold calibration values
  height="0.5819444444444445in"}                       along with the corresponding red [daylight
                                                       image](http://www.arm.gov/instruments/images/wsi/sep18.92.wsmr.rawred.gif).

  ---------------------------------------------------- -----------------------------------------------------------------------------------

Time series (MPEG movies):

  ---------------------------------------------------- --------------------------------------------------------------------------------
  ![](media/image5.png){width="0.6659722222222222in"   This [time
  height="0.5729166666666666in"}                       series](http://www.arm.gov/instruments/images/wsi/jan31_96_snlnm_daylight.mpg)
                                                       (2MB) is from the alpha test for unit WSI02. The data were taken 31 Jan 1996
                                                       between 1748Z and 2358Z. Notice the modified occultor for ARCS1. This occultor
                                                       has only one degree of freedom because the trolley drive has been eliminated,
                                                       and it has been sized/positioned to allow continuous operation without
                                                       adjustment.

  ---------------------------------------------------- --------------------------------------------------------------------------------

Interesting/Unusual Data:

This section will highlight interesting/unusual data that are worthy of
special attention because they demonstrate some physical phenomenon or
they serve as illustrative examples for features of the WSI.

  ---------------------------------------------------- -------------------------------------------------------------------------------
  ![](media/image3.png){width="0.6659722222222222in"   A [Starlight](http://www.arm.gov/instruments/images/wsi/sep18.93.sor.laser.gif)
  height="0.5729166666666666in"}                       image taken near a high-power laser source used in atmospheric studies and
                                                       compensated imagery studies. (Note the laser beam in the upper half of the
                                                       image).

  ---------------------------------------------------- -------------------------------------------------------------------------------

## Data File Contents

### Primary Variables and Expected Uncertainty

The WSI measures the sky radiance in approximately 1/3 increments over
the entire sky dome. The measurements are made in two narrow spectral
regions centered at 650 nm and 450 nm. The sky radiances are acquired as
two 16-bit images and are used to determine the presence of opaque
clouds and thin clouds in the line of sight on a pixel-by-pixel basis.
These cloud data are used to calculate the following cloud cover and
statistical quantities:

#### Sector cloud cover values over the full sky and in the following nine sectors: 

-   a 10° circle at the zenith

-   four quadrants running from 0° to 45° zenith angle

-   four quadrants running from 45° to 90° zenith angle.

#### Kegelmeyer code:

-   Global (1) Statistics:

```{=html}
<!-- -->
```
-   cloud cover fraction

-   cloud path length mean and standard deviation

-   clear path length mean and standard deviation.

```{=html}
<!-- -->
```
-   Per-Cloud (2) Statistics:

    -   number of clouds

    -   cloud area mean and standard deviation

    -   cloud perimeter mean and standard deviation

    -   cloud circularity (3) mean and standard deviation.

> Notes:\
> (1) All pixels labeled cloud or clear are included in these
> statistics.\
> (2) Clouds that are not entirely visible (i.e., clouds that "touch"
> the edge of the field of view or the edge of the occultor/occultor
> arm) are not included in these statistics.\
> (3) Circularity is defined as the ratio of the square of the perimeter
> to the product of 4PI and the area.

The sector cloud cover values are the fraction of the sector containing
cloud pixels. The cloud statistical quantities have the units of pixels
if no cloud height is assumed; otherwise, they are in the same units as
the cloud height.

#### Definition of Uncertainty

There are three primary types of output generated by the WSI: 1)
calibrated radiance distributions, 2) quantities such as cloud cover
derived from the cloud algorithm results, and 3) visual imagery intended
for visual assessment. Each of these outputs are discussed below.

####  {#section .unnumbered}

**Radiometric Data**

The primary source of uncertainty in the radiometric data is the
uncertainty of the calibration lamps. For a visible system requiring
knowledge of the spectral lamp output and not just total lamp output,
lamps are typically accurate to about 3%. With these lamps, net
accuracies of 5% are normally achievable. Positioning accuracy is
generally 1 when fielded by experienced teams, with a precision of 1/3.
Positioning accuracy can be checked and corrected for, if necessary,
using the measured sun positions.

**Cloud Cover**

Quantities such as cloud cover depend on the accuracy of the cloud
algorithm. The algorithm works quite well for opaque clouds. bThin
clouds are more difficult to characterize because of the difficulty in
assessing a very thin cloud with respect to a fairly thick haze.
In-house processing has normally been quite accurate, but for the
near-real-time code being fielded with the instruments, only the opaque
results will be presented until the thin cloud algorithm can be verified
or improved for use in totally autonomous mode.

**Interpreting Images Visually**

When used for visual assessment, the red images are processed for easier
viewing. (See windowing in the Theory of Operations, Section 7.2.) While
this process introduces no errors, it is possible to misinterpret the
resulting images. Specifically, data which appear offscale bright or
dark are usually only outside the displayed range, and not actually
offscale. Similarly, if features appear to be too dim to see clearly,
they can usually be brought out by selection of a narrower display
range. It is important for the user to check the image time, and know
whether the image was taken under sunlight, moonlight, or starlight. It
will be a site responsibility to be sure the instrument is shadowed from
extraneous bright lights so that the night sky will be imaged properly.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

The following (Table 1) lists the flags applied to the data, the normal
operating ranges for each parameter, and the parameter values that
generate a flag.

[]{#__RefHeading___Toc94675793 .anchor}Table 1.

+--------------------------+-------+---------+--------+-------+-------+
| Function                 | N     | Yellow  | Yellow | Red   | Red   |
|                          | ormal | Flag    | Flag   | Flag  | Flag  |
|                          |       | Level   | Code   | Level | Code  |
+--------------------------+-------+---------+--------+-------+-------+
| CCD chip temperature     | -35   | \> -30  | 1      | \> 0  | 1     |
| (?C)                     |       |         |        |       |       |
+--------------------------+-------+---------+--------+-------+-------+
| Environmental housing    | 16    | \> 32   | 2      | \> 49 | 2     |
| temperature (?C)         |       |         |        |       |       |
+--------------------------+-------+---------+--------+-------+-------+
| Coolant flow through     | 0.25  | 0.125   | 3      | ![    | 3     |
| camera (gpm)             |       |         |        | ](med |       |
|                          |       |         |        | ia/im |       |
|                          |       |         |        | age6. |       |
|                          |       |         |        | png){ |       |
|                          |       |         |        | width |       |
|                          |       |         |        | ="9.2 |       |
|                          |       |         |        | 36111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 1112e |       |
|                          |       |         |        | -2in" |       |
|                          |       |         |        | heigh |       |
|                          |       |         |        | t="0. |       |
|                          |       |         |        | 10486 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 11in" |       |
|                          |       |         |        | }0.09 |       |
+--------------------------+-------+---------+--------+-------+-------+
| Camera housing           | 16    | \> 32   | 4      | \> 49 | 4     |
| temperature (?C)         |       |         |        |       |       |
+--------------------------+-------+---------+--------+-------+-------+
| No camera response       | \-\-- | \-\--   | \-\--  | ![    | 5     |
|                          |       |         |        | ](med |       |
| (If the camera does not  |       |         |        | ia/im |       |
| respond for any one      |       |         |        | age7. |       |
| grab, a red flag is set) |       |         |        | png){ |       |
|                          |       |         |        | width |       |
|                          |       |         |        | ="9.2 |       |
|                          |       |         |        | 36111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 1112e |       |
|                          |       |         |        | -2in" |       |
|                          |       |         |        | he    |       |
|                          |       |         |        | ight= |       |
|                          |       |         |        | "0.10 |       |
|                          |       |         |        | 48611 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | in"}1 |       |
+--------------------------+-------+---------+--------+-------+-------+
| \(1\) Arc/trolley        | \<    | ![]     | 5 -    | ![    | 6 -   |
| occultor errors: Type 2  | 10%   | (media/ | arc\   | ](med | arc\  |
|                          |       | image7. | 6 -    | ia/im | 7 -   |
| (A time out occurs       |       | png){wi | t      | age7. | tr    |
| before occultor reaches  |       | dth="9. | rolley | png){ | olley |
| specified position)      |       | 2361111 |        | width |       |
|                          |       | 1111111 |        | ="9.2 |       |
|                          |       | 2e-2in" |        | 36111 |       |
|                          |       | height  |        | 11111 |       |
|                          |       | ="0.104 |        | 1112e |       |
|                          |       | 8611111 |        | -2in" |       |
|                          |       | 1111111 |        | heig  |       |
|                          |       | in"}10% |        | ht="0 |       |
|                          |       |         |        | .1048 |       |
|                          |       |         |        | 61111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 111in |       |
|                          |       |         |        | "}90% |       |
+--------------------------+-------+---------+--------+-------+-------+
| \(1\) Arc/trolley        | \<    | ![]     | 7 -    | ![    | 8 -   |
| occultor errors: Type 1  | 10%   | (media/ | arc\   | ](med | arc\  |
| (Occultor is not in      |       | image7. | 8 -    | ia/im | 9 -   |
| specified position)      |       | png){wi | t      | age7. | tr    |
|                          |       | dth="9. | rolley | png){ | olley |
|                          |       | 2361111 |        | width |       |
|                          |       | 1111111 |        | ="9.2 |       |
|                          |       | 2e-2in" |        | 36111 |       |
|                          |       | height  |        | 11111 |       |
|                          |       | ="0.104 |        | 1112e |       |
|                          |       | 8611111 |        | -2in" |       |
|                          |       | 1111111 |        | heig  |       |
|                          |       | in"}10% |        | ht="0 |       |
|                          |       |         |        | .1048 |       |
|                          |       |         |        | 61111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 111in |       |
|                          |       |         |        | "}90% |       |
+--------------------------+-------+---------+--------+-------+-------+
| \(1\) Neutral density    | \<    | ![]     | 9      | ![    | 10    |
| filter errors            | 10%   | (media/ |        | ](med |       |
|                          |       | image7. |        | ia/im |       |
|                          |       | png){wi |        | age7. |       |
|                          |       | dth="9. |        | png){ |       |
|                          |       | 2361111 |        | width |       |
|                          |       | 1111111 |        | ="9.2 |       |
|                          |       | 2e-2in" |        | 36111 |       |
|                          |       | height  |        | 11111 |       |
|                          |       | ="0.104 |        | 1112e |       |
|                          |       | 8611111 |        | -2in" |       |
|                          |       | 1111111 |        | heig  |       |
|                          |       | in"}10% |        | ht="0 |       |
|                          |       |         |        | .1048 |       |
|                          |       |         |        | 61111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 111in |       |
|                          |       |         |        | "}90% |       |
+--------------------------+-------+---------+--------+-------+-------+
| \(1\) Spectral filter    | \<    | ![]     | 10     | ![    | 11    |
| errors                   | 10%   | (media/ |        | ](med |       |
|                          |       | image7. |        | ia/im |       |
| (Unable to put filter    |       | png){wi |        | age7. |       |
| into specified position) |       | dth="9. |        | png){ |       |
|                          |       | 2361111 |        | width |       |
|                          |       | 1111111 |        | ="9.2 |       |
|                          |       | 2e-2in" |        | 36111 |       |
|                          |       | height  |        | 11111 |       |
|                          |       | ="0.104 |        | 1112e |       |
|                          |       | 8611111 |        | -2in" |       |
|                          |       | 1111111 |        | heig  |       |
|                          |       | in"}10% |        | ht="0 |       |
|                          |       |         |        | .1048 |       |
|                          |       |         |        | 61111 |       |
|                          |       |         |        | 11111 |       |
|                          |       |         |        | 111in |       |
|                          |       |         |        | "}90% |       |
+--------------------------+-------+---------+--------+-------+-------+
| \(1\) Exabyte error\     | \-\-- | ![]     | 11     | \-\-- | \-\-- |
| (unable to write to      |       | (media/ |        |       |       |
| Exabyte)                 |       | image7. |        |       |       |
|                          |       | png){wi |        |       |       |
|                          |       | dth="9. |        |       |       |
|                          |       | 2361111 |        |       |       |
|                          |       | 1111111 |        |       |       |
|                          |       | 2e-2in" |        |       |       |
|                          |       | height  |        |       |       |
|                          |       | ="0.104 |        |       |       |
|                          |       | 8611111 |        |       |       |
|                          |       | 1111111 |        |       |       |
|                          |       | in"}10% |        |       |       |
+--------------------------+-------+---------+--------+-------+-------+
| Nitrogen pressure in     | 5     | ![]     | 12     | \-\-- | 12    |
| camera housing (psi)     |       | (media/ |        |       |       |
|                          |       | image6. |        |       |       |
|                          |       | png){wi |        |       |       |
|                          |       | dth="9. |        |       |       |
|                          |       | 2361111 |        |       |       |
|                          |       | 1111111 |        |       |       |
|                          |       | 2e-2in" |        |       |       |
|                          |       | heig    |        |       |       |
|                          |       | ht="0.1 |        |       |       |
|                          |       | 0486111 |        |       |       |
|                          |       | 1111111 |        |       |       |
|                          |       | 11in"}2 |        |       |       |
+--------------------------+-------+---------+--------+-------+-------+
| No WWV\                  | \-\-- | = \# of | 13     | \-\-- | \-\-- |
| (If the source of the    |       | images  |        |       |       |
| time stamp on all images |       | grabbed |        |       |       |
| grabbed is not WWV, a    |       |         |        |       |       |
| yellow flag is set)      |       |         |        |       |       |
+--------------------------+-------+---------+--------+-------+-------+

\(1\) Error figure = (# of occurrences) ? images grabbed).

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

ARM-TR-011.1, [Whole Sky Imager Retrieval
Guide](http://www.arm.gov/publications/tech_reports/arm-tr-011-1.pdf).
T.P. Tooman, November 2003.

## Frequently Asked Questions

Hardware

Why does the WSI need to use an imager?

Originally, for acquiring radiance distributions, we used scanning
photometers. Whereas they provided excellent radiance distributions, the
more demanding task of high-quality cloud detection requires more
simultaneity in the data. The imager acquires measurements in all
directions at once, so cloud motion presents little problem. By using a
512 by 512 imager, we essentially have nearly 250,000 calibrated
radiometers staring at the sky simultaneously.

How did you decide to use the 16-bit charge-coupled device (CCD), as
opposed to using film, a vidicon, video camera, or intensified CCD?

We used film fisheye systems in the 60s and 70s; however, they cannot be
processed quickly, and the radiometric content of the data is severely
degraded or lost. For measuring daylight skies only, we moved to a CEPEX
integrated data system (CID)-based video camera in the 80s. This had the
advantage over vidicons in that calibration and geometric positioning
accuracy are maintained. (This is not true of all video cameras;
however; it is very important that they have fixed gain, and electronic
characteristics such that they can be well calibrated.) In moving to the
day/night application, we determined that the video cameras we evaluated
did not have the sensitivity we required. Likewise, intensified CID
systems did not have sufficient stability and noise control for our
application. We have been very happy with the performance of the
slow-scan, low-noise 16-bit CCDs we are using.

What is the purpose of the occultor?

The occultor is used to shade the lens and dome, so we don\'t get stray
light. Most photographers have at one time taken pictures looking toward
the sun and seen the resulting artifacts in the image; the occultor
prevents these artifacts by shading the lens.

Why is the occultor so big?

It is not sufficient to cover the ½ ° solar disk. It is necessary to
shade the full physical extent of the lens and the dome. Because the
solar rays are parallel, the shade must be at least as big as the dome,
which is 7\". To avoid obscuring too large a solid angle, we place it
24\" from the dome. The actual solid angle obscured by the shade is less
than 1% of the sky dome. At the TWP site, we will be using a larger
shade, which is fixed in the N-S direction and moves only in the E-W
direction; it will obscure 6% of the sky, but should be more reliable
for a 6-month unattended operation.

Why do you need the occultor under moonlight?

The flux control is set to bring the radiance of the moonlit sky and
clouds on scale. Under moonlight, the moon is as bright relative to the
sky (in the absence of urban lights) as the sun is relative to the
daylight sky.

Observed Images

The image appears round. What is the user looking at?

The center of the image is the zenith (overhead), and the edge of the
circular image is the horizon. The view cannot be the same as looking
down at a map, because the WSI is looking up. If you imagine lying on
the ground looking up, with your toes to the north, you can imagine the
view of the WSI: E is right, W is left, S is top, and N is bottom.

Why do you use 16 bits, if you can only display 8 bits?

An 8-bit image has only 256 grey levels, which means it has a limited
radiometric resolution and/or range. By saving and using the original
16-bit data, data over a larger range and with a finer resolution are
available to the numerical processing.

Can you see the whole sky?

Yes, because we underfill (rather than overfill) the chip so that the
full optical image falls on the chip.

What can the WSI see under starlight? Is it really visible and not
infrared (IR)?

We do use visible at night, although currently we open up the passband
to roughly 400 - 900 nm. We can see the milky way, stars such as the
stars in Orion's belt, clouds that are not visible to the human at
night, and the light of distant cities. Often when the night visually
appears clear, there are cirrus layers easily detectable by the sensor.

How does the sensor adjust to the changing light conditions?

The flux control algorithm embedded in the control software computes the
solar zenith angle and the lunar zenith angle, phase angle,
earth-to-moon distance, and resulting relative brightness. Using these
values, the desired combination of filter and exposure selection is
chosen. The imager has sufficient dynamic range within each image so
that the system need not correct for variations in lighting conditions
because of cloud cover to acquire onscale data.

Cloud Decision Images

How is cloud cover computed?

Please see cloud cover determination in Theory of Operations, Section
7.2. In general, an opaque cloud is defined as a cloud of a given
whiteness (or measured red/blue ratio). A thin cloud is defined as a
region that is whiter than the clear sky would be. In this process, an
adjustment is made so that haze is normally not identified as thin
cloud. The algorithm does a very nice job of detecting clouds, whether
they are large or small (the resolution is about 17 m for a cloud at
3000 m, and 6 m for a cloud at 1000 m). It is not fooled by dark clouds,
and it has no problem seeing high clouds. Sometimes thin clouds near the
sun will be incorrectly labeled opaque clouds, and the choice of when to
define an increasingly thick haze as a thin cloud is something that we
hope to better define in the future.

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

> [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)
>
> [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
> plotting using.

The tables and graphs shown contain the techniques used by the
Atmospheric Radiation Measurement (ARM) Program's data quality analysts,
instrument mentors, and site scientists to monitor and diagnose data
quality.

## Data Reviews by Instrument Mentor

-   QC frequency: Limited

-   QC delay: N/A

-   QC type: N/A

-   Inputs: WSI images

-   Outputs: Not specified

-   Reference: N/A.

At the present time, data quality control beyond inspection of images is
very limited. A considerable effort might be required to compare cloud
retrievals on a regular basis with the Belfort laser ceilometer (BLC),
the Marine Physical Laboratory (MPL), or VCEIL data. A contract is being
set up with Mission Research Corporation to produce night and thin cloud
retrieval algorithms. Additionally, instrument mentor Tim Tooman is
trying to develop a calibrated radiance retrieval.

## Data Assessments by Site Scientist/Data Quality Office

All Data Quality (DQ) Office and most Site Scientist techniques for
checking have been incorporated within [DQ HandS](http://dq.arm.gov/)
and can be viewed there.

## Value-Added Procedures and Quality Measurement Experiments

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into "value-added"
products or VAPs. Despite extensive instrumentation deployed at the ARM
sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not to fill unmet measurement needs,
but to improve the quality of existing measurements. In addition, when
more than one measurement is available, ARM also produces "best
estimate" VAPs. A special class of VAP, called a Quality Measurement
Experiment (QME), does not output geophysical parameters of scientific
interest. Rather, a QME adds value to the input datastreams by providing
for continuous assessment of the quality of the input data based on
internal consistency checks, comparisons between independent similar
measurements, or comparisons between measurement with modeled results,
and so forth. For more information, see the [VAPs and QMEs web
page](http://www.arm.gov/data/vaps.stm).

# Instrument Details

## Detailed Description

### List of Components

The WSI is designed and built by Marine Physical Lab. The primary
components of the system are listed below:

[![](media/image8.png){width="0.9166666666666666in"
height="1.1458333333333333in"}](http://www.arm.gov/instruments/images/wsi/sensor_subassembly.gif)

#### [**Camera System:**](http://www.arm.gov/instruments/images/wsi/sensor_subassembly.gif)

-   Fisheye Lens - 180? field of view

-   Filter Changer with spectral and neutral density filter wheels

-   Shutter - electrically driven mechanical

-   Digital CCD Camera - Photometrics 16 bit with fiber-optic taper

-   ![](media/image9.png){width="0.9166666666666666in"
    height="1.1458333333333333in"}Purged Camera Housing to enclose above
    items.

#### [**Automatic Solar/Lunar Occultor:**](http://www.arm.gov/instruments/images/wsi/wsi_schematic.gif)

-   Arc with Arc Drive (along East-West axis)

-   Trolley and Trolley Drive (along North-South axis)

-   Sunshade and 4-log neutral density filter.

#### [**Environmental Housing:**](http://www.arm.gov/docs/instruments/images/wsi/images/wsi_schematic.gif)

-   Weatherproof housing to enclose camera system and the following
    items:

-   Thermo-electric Air Cooler

-   Temperature Controller

-   Liquid Coolant system with Flow Meter and Flow Switch

-   Relative Humidity Sensor.

#### [![](media/image11.png){width="0.9166666666666666in" height="1.15625in"}](http://www.arm.gov/instruments/images/wsi/bluebox_med_labeled.gif)**Controller:**

-   Sensor Accessory Control Panel

-   Occultor Accessory Control Panel

-   Field Hardened PC Computer (486) and boards

-   Monitor

-   8 mm Tape Drives

-   External accurate clock

-   Field Hardened Computer Rack to enclose the above items.

### System Configuration and Measurement Methods

The WSI consists of the sensor unit and the controller. The sensor unit
includes the optical system with a solid-state CCD camera and a filter
changer, the solar/lunar occultor, and environmental protection

The radiance images are acquired through a Nikkor 8mm, f/2.8 fisheye
lens. The lens has a full 180-degree field of view for viewing the
complete sky dome simultaneously. The lens has equi-distant projection,
i.e., the zenith angle in object space is nearly linear with respect to
the position of the corresponding pixel in image space.

For the WSI to acquire images under all possible lighting conditions
(sunlight, moonlight, and starlight), it must be able to handle an
approximately 9-decade range of lighting. To help accomplish this, a
neutral density (ND) filter wheel containing 2- and 3-log ND filters is
positioned between the lens and CCD imaging chip. The spectral filter
wheel containing the red (650-nm) and blue (450-nm) filters is also
positioned between the lens and CCD chip.

The imaging camera is a Photometrics Slow Scan CCD. This camera
generates very low noise, has outstanding sensitivity characteristics,
and produces a high quality image. Its 16-bit digitization
(approximately 4-log range) allows for fine radiometric resolution.

To prevent direct sunlight or moonlight from hitting the protective dome
covering the lens and producing artifacts in the image from scattering,
a solar/lunar occultor is actively positioned between the sun/moon and
dome. The occultor has drive controls for 2 degrees of freedom and is
automatically positioned in the correct place by the controlling
computer. The central portion of the occultor contains a 4-log ND filter
that allows detection of the sun and moon.

A modified solar/lunar occultor has been designed for deployment to some
locations. The modified design has 1 degree of freedom; that is, the
trolley is replaced with a fixed shade. The width of the fixed shade is
the same as the diameter of the shade on the trolley design, and the
length of the shade is determined by site requirements. For the Surface
Heat Budget of the Arctic Ocean (SHEBA) and NSA sites, the shade will
cover the full 180 degrees to allow blockage of the sun as it moves
around the horizon. For the TWP sites, the shade will extend from about
+23° to -23°. The arc drive for the modified occultor design is the same
as for the original design. The ND filter is not present in the modified
design.

Special Data Collection Processing (IDCP Information)

The attached
[file](http://www.arm.gov/instruments/publications/wsi_idcp.html) lists
all the file names for the WSI instrument data products.

### Specifications

This section lists physical parameters describing the WSI. Some measured
characteristics may vary slightly.

Physical Characteristics:

-   Sensor Dimensions: 28\" W x 36\" D x 36\" H

-   Sensor Weight: 410 lb

-   Sensor Power: 614 watts (79 must be on UPS)

-   Controller Dimensions: 26\" W x 26\" D x 52\" H

-   Controller Weight: 450 lb

-   Controller Power: 150 watts (must be on UPS).

Optical Characteristics:

-   Spectral Filters at 450 nm and 650 nm and open hole

-   Spectral passband width: 70 nm

-   Neutral Density Filters at 0, 2, and 3 logs (decades) neutral
    density

-   Fisheye-Nikkor 8mm f/2.8 lens

-   Angular resolution: 1/3°

-   Field of view 180 degrees (typically 181?)

-   CCD: Thomson TH7895B, Grade 1

-   512 x 512 pixels

-   16 bit A/D readout at 40,000 pixels/second

-   25 mm to 11 mm fiber optic reducer.

Thermal Characteristics:

-   CCD cooled to -35?C via 3-stage Peltier and liquid water/alcohol
    coolant

-   Environmental Housing held at 60°F via thermo-electric air cooler

-   Camera electronics (including A/D and pre-amp) help to 60°F.

Calibration Characteristics:

-   Measures all sky conditions, including full sun to starlight

-   Dynamic Range (overall): 10.6 log or 40000000000:1

-   Dynamic Range (single image): 45800:1

-   Readout Noise: 1.6 counts out of 65,536 grey levels

-   Uniformity (Spatial Variation): 1.6% (correctable)

-   Precision (Temporal Variation): 0.2%

-   Nonlinearity: \<= 1% up to signal 10k, \<= 3% up to signal 50k
    (correctable)

-   Pixel Resolution: 0.36?

-   Avg. Deviation from Equi-Distant: 1.2? (correctable).

Control Characteristics:

-   Image Exposure Time: Variable, 100 msec day, 1 min starlight

-   Automated Exposure Adjustment based on solar/lunar conditions

-   Solar/Lunar Occultor to shade lens and optical dome

-   Automated Occultor Position based on solar/lunar position

-   Self-checks for proper readouts and data acquisition

-   Automated self-shutdown under pre-defined conditions

-   Automated recovery from power down when power returns.

## Theory of Operation

The WSI is designed to run either autonomously or networked to a site
data system. It runs 24 hours a day, acquiring images at user-specified
intervals. For the ARM Program, this interval is normally 10 minutes.
For other applications, acquisition of image sets as often as once a
minute is possible.

The system performs its own housekeeping, which includes positioning the
solar/lunar occultor, acquiring the image set, saving data to tape drive
for archival and to hard disk for network access, processing the data to
yield cloud cover, and performing self-checks. To complete these steps,
the system is able to determine the solar or lunar position (given the
correct input location). Also, the WSI includes a flux control
algorithm, which checks sun location, moon location, phase, and
earth-moon distance; from these parameters the ND filter and exposure to
use are determined.

When data are acquired, the first step is to display an image for the
user. The raw data are 16 bit, (i.e., have a grey scale range of 0 -
65,535). To view the data, it is necessary to determine the portion of
the range occupied by the data (such as grey levels 1000 - 8000) and map
this into the 8-bit image (256 grey levels) used to display an image.
The WSI includes an automated windowing algorithm, which determines a
reasonable range to display, and derives the 8-bit image and displays
it. (The 16-bit raw data are also always retained in unmodified format
for further use.)

Once the raw data have been saved, and the windowed image displayed, the
WSI proceeds with the cloud decision algorithm (unless this option is
turned off). This algorithm first applies calibration factors to the
16-bit data, then ratios the red and blue images to provide a ratio
image. As a first step, the ratios are thresholded to identify the
opaque clouds. Thus, opaque clouds are identified by their spectral
character.

In addition, a library of clear-sky ratio images for a full range of
solar zenith angles must be stored in advance. This library is
site-dependent, because it depends on such factors as site altitude
(thus it can only be determined after the imager has been at the site
for a reasonable period). This library is used to determine the
background clear-sky ratio for a given solar zenith angle, which is then
adjusted to compensate for variations in the aerosol load. The ratio
image being analyzed is then compared with this background ratio on a
pixel-by-pixel basis. The algorithm identifies a pixel as thin cloud if
the test image ratio exceeds the background ratio by 20%. Thus, a pixel
is identified as thin if its spectral ratio is not as high as that of an
opaque cloud, but is significantly higher than that of the clear sky at
the same look angle and solar/lunar angle.

If the aerosol load is so high (i.e., the sky is so hazy) that the
clear-sky ratio exceeds the opaque ratio in any pixels, then these
pixels may be labeled indeterminate. For example, on a very hazy day,
the aureole may be labeled indeterminate. Once the full image has been
processed, the cloud cover may be determined by evaluating the number of
pixels inside the image, which are labeled opaque or thin cloud.
(Normally no correction for the solid angle per pixel is required, as
this introduces very small changes in the total result.)

The algorithm also labels pixels that are offscale bright or dark, and
in the future it will label pixels that are blocked by the occultor. The
current interactive cloud decision algorithm identifies both opaque and
thin clouds; however, the near-real-time version of the algorithm
fielded with the instruments only presents the opaque cloud results at
this time. The extension of the thin cloud algorithm for near-real time
use will be under development in the near future.

From the cloud decision images, in which each pixel is identified as
opaque cloud or no cloud, the cloud cover over the full sky is computed,
as well as the cloud cover in nine sectors: a 10-degree circle at the
zenith, four quadrants running from 0 to 45° zenith angle, and four
quadrants running from 45 to 90° zenith angle. In addition, other cloud
products, such as cloud path length, area, and perimeters, will be
computed at the Experiment Center. (See Value-Added Procedures and
Quality Measurement Experiments, Section 6.4, for details and Data
Descriptions and Examples, Section 5, for sample plots).

These cloud product results may be used for evaluation of the nature of
the cloud heterogeneities, their impact on radiative transfer models,
and the impact of the cloud cover on the surface fluxes. The zenith
measurements may be used to evaluate zenith -ooking instruments.

An additional product is the sky radiance distribution. When each image
is calibrated for absolute radiance, the image consists of more than
200,000 measurements of sky radiance acquired simultaneously at 1/3°
spatial resolution. This product can be used in several ways: evaluation
of the clear-sky radiance distributions, potentially including
extraction of aerosol characteristics, evaluation of the cloud radiances
and their relations to heterogeneities, evaluation of the variations in
the diffuse irradiance (which can be computed from the radiances),
evaluations of the solar disk for potential determination of optical
depth and direct irradiance, and evaluation of the fine-scale temporal
and spatial variations in these quantities.

Finally, a third product that should be of use to the science team is a
visual presentation of the images. Because the data can be presented as
visual images (like pictures), they present tremendous information to
the trained observer. One can qualitatively evaluate cloud type and
opacity, and the general character of the cloud field. Cloud time lapse
loops are often particularly interesting and beautiful, and useful for
qualitative evaluations of cloud motion and cloud cover persistence.
Cloud loops are an easy tool for evaluating features such as orographic
clouds that may affect the net flux, and yet not be detected by the
zenith-looking sensors.

Potential future developments include use of the instruments for
sub-visual cirrus detection, 3-D retrievals, and cloud typing. At this
time, two instruments are fielded, and more emphasis is being placed on
transfer of the analysis techniques from the vendor to the experiment
center to enable fast data transfer to the science team.

## Calibration

### Theory

Presently, calibration occurs at Marine Physical Lab prior to fielding
the instruments. These calibrations are done quite rigorously, so that
the limiting accuracy should be the accuracy of the calibration sources.
Some of the calibrations perform the primary function of testing whether
system performance is up to the quality standard demanded by the nature
of the sensing task.

Long-term maintenance of the calibrations will probably depend on the
extent to which the absolute radiance distributions are utilized. It is
envisioned that a field calibration device will be important for this
application. For the application of determining cloud cover, the
calibration stability is less critical, because the system uses image
ratios, and only relative calibration must be stable or corrected for.
The most critical calibration, the dark calibration, is measured in the
field as part of the automated data acquisition procedure. The
development of a field calibration device is recommended to ensure
long-term results.

The following sections give a brief overview of the types of
calibrations normally acquired for a WSI. Most of these calibrations are
acquired in a calibration facility at MPL with a 2-meter precision
calibration bar. The source is a 1000W FEL lamp traceable to the
National Institute of Standards and Technology (NIST). A lambertian
reflectance plaque is placed at the 0 point on the bar, and the imager
views this plaque. A high-accuracy resistance shunt is used to monitor
the lamp current, and the power supply automatically adjusts the voltage
to maintain fixed current.

**Dark Level vs. Exposure**

A dark image is an image acquired with the shutter closed, and provides
a measure of the dark current, electronic bias, and readout noise. The
dark current is pixel-dependent, and is normally subtracted from the
measured images in the field. In the lab, the dark image is acquired as
a function of shutter exposure to characterize camera performance. In
the field, a dark image is acquired whenever the exposure changes, so
that a current dark image may be subtracted in the near-real-time
processing in the field.

**Dark Level Repeats**

Ten to 20 repeat dark images are acquired at each of two exposures to
characterize the spatial vs. temporal variance in the dark images.

**Exposure Calibration - Short Exposures**

Images of the uniform lambertian reflectance plaque are acquired at
short exposure times ranging from 15 msec to 1000 msec to measure the
effective shutter opening time.

**Exposure Calibration - Long Exposures**

Images are acquired at longer exposure times ranging from 15 msec to 3
minutes to verify system linearity as a function of input exposure time.

**Radiometric Linearity**

In this calibration, the distance between the calibration lamp and the
reflectance plaque are varied by known amounts on the precision bar, so
that the radiance presented to the sensor may be varied by known
amounts. This calibration is used to determine whether the sensor
response is linearly related to the radiance, and measure the
non-linearities, if any, so that the data may be corrected for
linearity.

**Precision and Uniformity**

Twenty images of the reflectance plaque are gathered in quick
succession. From these images, a variety of spatial and temporal
statistics are determined. These are used to characterize the precision
(temporal variance) and the uniformity (spatial, i.e. pixel-to-pixel)
variance.

**Filter Passband Calibrations**

To provide absolute radiance, it is necessary to know the effective lamp
irradiance over the passband of the instrument. To determine this, it is
necessary to know the lamp output as a function of wavelength, the CCD
sensitivity as a function of wavelength, and the transmittance of each
filter as a function of wavelength. Lamp and CCD calibrations are
provided by the vendors, and the filter passband calibrations are
measured separately.

**Absolute Radiance Calibrations**

Absolute calibrations are acquired at a fixed ND filter setting, in each
spectral filter. In each spectral filter, measurements are acquired at
each of five lamp positions, and then repeated. The 5 lamp positions
yield five signals for five different radiances. If non-linearities have
been corrected for properly, these five measurements should yield the
same calibration constant, in the absence of stray light or other
measurement error. The redundancy in this procedure yields a more
accurate determination of the calibration constant, and also enables an
evaluation of calibration uncertainty (this uncertainty is less than
0.5%). This calibration is performed for each combination of spectral
and neutral density filter.

**Aperture Calibration**

This calibration characterizes the variation in the overall sensitivity
as a function of aperture setting.

**Rolloff Calibration**

The rolloff calibration characterizes the variance in overall
sensitivity as a function of look angle or angle with respect to the
lens normal. This calibration is acquired using a rotary table to
accurately vary the look angle of the lens, while holding the position
of the lens with respect to the plaque fixed. Measurements are acquired
for each spectral band.

**Flat Field Calibration**

The flat field calibration is a measure of the variation in pixel
sensitivity at each pixel over the whole image. This correction is not
required for cloud decision results or image viewing, but is required
for the absolute radiance distribution extraction. Together, the dark
image and the flat field image may be used to adjust the pixel-to-pixel
variations in bias and gain. The WSI presents special difficulty in flat
fielding, as a result of some of the characteristics of the fiber-optic
taper. A unique technique was developed using a 1-meter integrating
sphere, and using numerical corrections to remove certain artifacts of
the sphere.

**Geometric Calibration**

The spatial relationship between zenith angle in object space and pixel
position in image space is measured in this calibration. Whereas this
relationship is nearly linear, it is MPL\'s practice to characterize the
relationship, and correct for the non-linearity.

### Procedures

### History

Calibration for WSI01 occurred in September through November 94, with
additional calibrations in December 94 and March 95. Calibrations for
WSI02 occurred in August 95. As of March 96, not all of these have yet
been evaluated, but results to date are excellent. For example, the
self-consistency of the absolute calibrations was found to be within
0.5% or better. Some of the calibration results are listed in the WSI
characteristics discussion.

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

This section is not applicable to this instrument.

### Software Documentation

ARM-TR-011.1, [Whole Sky Imager Retrieval
Guide](../%20http://www.arm.gov/publications/tech_reports/arm-tr-011-1.pdf).
T.P. Tooman, November 2003.

### Additional Documentation

See the [SGP Preventative Maintenance
Procedures](http://www.db.arm.gov/emp-bin/MDS/ListPM.pl?instrument=WSI).

## Glossary

See the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

AdaM ARCS Data Management

ARCS Atmospheric Radiation and Cloud Stations

[ARM](http://www.arm.gov/index.html) Atmospheric Radiation Measurement
(Program)

CCD charge-coupled device

CID CEPEX integrated data system

DQ Data Quality

IR infrared

MPL Marine Physical Laboratory

ND neutral density

NIST National Institute of Standards and Technology

[NSA](http://www.arm.gov/sites/nsa.stm) North Slope of Alaska

QC quality control

QME Quality Measurement Experiment

SDS site data system

SGP Southern Great Plains

[SHEBA](http://sheba.apl.washington.edu/) Surface Heat Budget of the
Arctic Ocean

TWP Tropical Western Pacific

VAP value-added product

WSI Whole-Sky Imager

Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Johnson, R.W., W.S. Hering, and J.E. Shields. 1989. Automated Visibility
and Cloud Cover Measurements with a Solid-State Imaging System.
University of California, San Diego, Scripps Institution of
Oceanography, Marine Physical Laboratory, SIO 89-7, GL-TR-89-0061, NTIS
No. ADA216906.

Johnson, R.W., J.E. Shields, and T.L. Koehler. 1991. Analysis and
Interpretation of Simultaneous Multi-Station Whole Sky Imagery.
University of California, San Diego, Scripps Institution of
Oceanography, Marine Physical Laboratory, SIO 91-33, PL-TR-91-2214.

Shields, J.E., R.W. Johnson, and M.E. Karr. 1992. An Automated Observing
System for Passive Evaluation of Cloud Cover and Visibility. University
of California, San Diego, Scripps Institution of Oceanography, Marine
Physical Laboratory, SIO 92-22, PL-TR-92-2202, NTIS No. ADA216906.

Sun, C.-H., and L.R.Thorne. Inferring Spatial Cloud Statistics from
Limited Field-of-View Zenith Observations. To be submitted to J. Appl.
Meteor.

Other References

Buch, K.A., and C.-H. Sun. 1995. Cloud Classification Using Whole-Sky
Imager Data. Ninth Symposium on Meteorological Observations and
Instrumentation, Paper 7.5, Charlotte, North Carolina.

Johnson, R.W., W.S. Hering, and J.E. Shields. 1986. Imagery Assessment
for the Determination of Cloud Free Intervals. University of California,
San Diego, Scripps Institution of Oceanography, Visibility Laboratory,
Atmospheric Visibility Technical Note No. 200.

Shields, J.E., T.L. Koehler, M.E. Karr, and R.W. Johnson 1990. Automated
Cloud Cover and Visibility Systems for Real Time Applications.
University of California, San Diego, Scripps Institution of
Oceanography, Marine Physical Laboratory, Optical Systems Group
Technical Note No. 217.

Shields, J.E., R.W. Johnson, and T.L. Koehler. 1991. Imaging Systems for
Automated 24-Hour Whole Sky Cloud Assessment and Visibility
Determination. Proceedings of the Cloud Impacts on DoD Operations and
Systems.

Shields, J.E., R.W. Johnson, and R.L. Koehler. 1993. Automated Whole Sky
Imaging Systems for Cloud Field Assessment. Fourth Symposium on Global
Change Studies, January 17-22, 1993, Anaheim, California. Published by
the American Meteorological Society, Boston, Massachusetts.

Shields, J.E., R.W. Johnson, M.E. Karr, B.J. Kroeger, D.R. Sauer, and
J.R. Varah. 1994. Operations Manual: Day/Night Whole Sky Imager (E/O
Camera System 6). University of California, San Diego, Scripps
Institution of Oceanography, Marine Physical Laboratory, Optical Systems
Group Technical Note No. 236.

Shields, J.E. 1995. WSI Acceptance Test Plan. University of California,
San Diego, Scripps Institution of Oceanography, Marine Physical
Laboratory, Optical Systems Group Technical Note No. 238.

Shields, J.E. 1995. WSI Functional Test Report D/N WSI Field Unit 3.
University of California, San Diego, Scripps Institution of
Oceanography, Marine Physical Laboratory, Optical Systems Group
Technical Note No. 239.

Shields, J.E., R.W. Johnson, M.E. Karr, D.R. Sauer, J.R. Varah, and R.A.
Weymouth. 1996. Maintenance and Trouble Shooting Manual: Day/Night Whole
Sky Imager (E/O Camera System 6). University of California, San Diego,
Scripps Institution of Oceanography, Marine Physical Laboratory, Optical
Systems Group Technical Note No. 241.

Sun, C.-H., and L.R. Thorne. 1995. Inferring Spatial Cloud Statistics
from Limited Field-of-View Zenith Observations. ARM Science Team
Meeting, San Diego, California.

Tooman T.P. 2003. [Whole Sky Imager Retrieval
Guide](../%20http://www.arm.gov/publications/tech_reports/arm-tr-011-1.pdf),
ARM-TR-011.1.