All-sky imaging: a simple, versatile system for atmospheric research

Abstract

A simple and inexpensive fully automated all-sky imaging system based on a commercial digital camera with a fish-eye lens and a rotating polarizer is presented. The system is characterized and two examples of applications in atmospheric physics are given: polarization maps and cloud detection. All-sky polarization maps are obtained by acquiring images at different polarizer angles and computing Stokes vectors. The polarization in the principal plane, a vertical cut through the sky containing the Sun, is compared to measurements of a well-characterized spectroradiometer with polarized radiance optics to validate the method. The images are further used for automated cloud detection using a simple color- ratio algorithm. The resulting cloud cover is validated against synoptic cloud observations. A Sun coverage parameter is introduced that shows, in combination with the total cloud cover, useful correlation with UV irradiance.

Full text

All-sky imaging: a simple, versatile system
for atmospheric research
Axel Kreuter,* Matthias Zangerl, Michael Schwarzmann, and Mario Blumthaler
Division for Biomedical Physics, Department of Physiology and Medical Physics,
Innsbruck Medical University, Müllerstrasse 44, 6020 Innsbruck, Austria
*Corresponding author: axel.kreuter@i‑med.ac.at
Received 17 November 2008; revised 16 January 2009; accepted 17 January 2009;
posted 21 January 2009 (Doc. ID 104048); published 13 February 2009
A simple and inexpensive fully automated all-sky imaging system based on a commercial digital camera
with a fish-eye lens and a rotating polarizer is presented. The system is characterized and two examples
of applications in atmospheric physics are given: polarization maps and cloud detection. All-sky polar-
ization maps are obtained by acquiring images at different polarizer angles and computing Stokes
vectors. The polarization in the principal plane, a vertical cut through the sky containing the Sun, is
compared to measurements of a well-characterized spectroradiometer with polarized radiance optics
to validate the method. The images are further used for automated cloud detection using a simple color-
ratio algorithm. The resulting cloud cover is validated against synoptic cloud observations. A Sun cover-
age parameter is introduced that shows, in combination with the total cloud cover, useful correlation with
UV irradiance. © 2009 Optical Society of America
OCIS codes: 010.1615, 100.2960, 110.5405, 120.5410.
1. Introduction
Observations of the sky are one of the oldest methods
in planetary sciences such as meteorology and as-
tronomy. In many cases the human observer is still
indispensable, however autonomous digital technol-
ogy has grown in importance, e.g., in large observa-
tion networks. Digital all-sky imaging has been
utilized in the automated investigation of diverse
phenomena such as Auroras [1], urban light pollu-
tion [2], and photosynthetically active radiation un-
der forest canopies [3]. In astronomy, meteorology or
atmospheric physics, all-sky imaging in its most ele-
mentary form is a convenient way to record the gen-
eral atmospheric situation during observations [4].
More specifically, all-sky cameras with polarization
filters have been used for polarization mapping of
the sky hemisphere [5–10]. When calibrated against
a radiometric standard, such a system is a multiwa-
velength, multiangle radiometer measuring the radi-
ance of the whole sky in one exposure [5]. Compared
to sky-scanning grating spectroradiometers, acquisi-
tion speed is a trade-off against well-defined wave-
length bandwidth, dynamic range, and precision of
the detector. The sky’s polarization is sensitive to
the aerosol properties in the sky and is thus an ideal
complementary measurement device in combination
with aerosol optical depth measurements.
Systems for cloud observation and detection pur-
poses have been presented in [10–14]. Cloud type
identification has been attempted but remains a
challenging issue [15]. It seems that a single camera
picture on its own is not quite sufficient for a detailed
automated cloud type analysis since, e.g., cloud
height information is difficult to recover. However,
in combination with satellite images in different
wavelength regions and a ceilometer, an all-sky im-
age could add valuable information.
The combination of all-sky imaging with UV radia-
tion measurements has been shown in [16–18],
where cloud cover analysis was correlated with
enhanced UV irradiation. Under certain cloud
configurations the global UV radiation field can be
enhanced compared to the clear-sky value. For
radiation measurements in general, all-sky imaging
0003-6935/09/061091-07$15.00/0
© 2009 Optical Society of America
20 February 2009 / Vol. 48, No. 6 / APPLIED OPTICS 1091

adds an extra dimension to systematic data analysis.
Cloud cover and Sun coverage are key parameters for
estimating actual irradiances from clear-sky model
predictions.
In this study we present a particularly simple and
inexpensive all-sky imaging system for atmospheric
research and demonstrate two applications: polari-
zation maps and cloud detection. We show that rudi-
mentary characterization and relative calibration is
sufficient for these purposes.
2. General System Description
Images of the full-sky hemisphere are recorded with a
commercial compact digital camera (Canon A75) with
a fish-eye objective [field-of-view (FOV) ≈180°] and a
stepper motor controlled linear polarizer, situated be-
tween the objective and the camera. The system is
mounted in a weatherproof housing with a glass dome
and connected via ethernet cable to a personal compu-
ter for external automated control. A horizontal setup
is assured by a bubble level on the housing. Four
images at polarizer angles differing by 45° are
acquired at a fixed exposure time of 1=125 s and an
aperture of f=6. At one polarizer angle, a second under-
exposed image is acquired (a rudimentary high dy-
namic range (HDR) method, described, e.g., in [19])
to gain extra information close to the Sun where pixels
are often saturated. The images are transmitted in
JPEG format and have an intensity resolution of
8bit for three color channels (RGB) and a spatial re-
solution of 1536 ×2048 pixels. The whole process of ac-
quiring the set of five images, rotating the polarizer,
and transmitting the data takes less than 1min.
The system was installed on a building rooftop in
the city of Innsbruck, Austria, 47:26°N, 11:39°E,
620 m amsl. In routine operation, acquisitions are ta-
ken hourly at solar elevations >5°. The system was
also installed at two additional sites, featuring differ-
ent environments and horizons: A flat urban site in
Vienna (48:24°N, 16:33°E, 180 m amsl) and a mid-
altitude alpine site with an obscured horizon in
Kolm–Saigurn (47:07°N, 12:98°E, 1600 m amsl).
A shadow mechanism for obscuring the direct Sun
has been dismounted again since, for most of our in-
tentions here, the advantages (simplified system,
less moving parts, less obscured sky) outweigh the
disadvantages (area around the direct Sun difficult
for image processing, reflections in the lens system).
The hemisphere is projected onto the flat CCD
chip by an equiangular projection. Each point in
the sky, characterized by two angles (azimuth angle
ϕand zenith angle Θ) is mapped onto a circular
area in the x–yplane. For increased computation
speed, the original JPEG images are downscaled
by a factor of 4 by nearest neighbor interpolation
and cut so that the hemisphere of 168° FOV is a circle
in a 325 ×325 pixel square plane. The xand ycoor-
dinates are centered on the zenith pixel ðx0;y0Þand
converted to polar coordinates ϕand r. The radius ris
proportional to the zenith angle in the sky Θ¼
rðΘmax=rmax Þ, with Θmax ¼FOV=2and rmax ¼
325=2, while the azimuth angle is invariant in the
transformation:
r
ϕpolar
→r·Θmax
rmax
ϕ≡Θ
ϕsky
:ð1Þ
The zenith pixel is the geometric center of the
square plane, when the camera is perfectly level.
This is checked by marked points (mountain peaks)
near the horizon. The resulting angle that each pixel
subtends is Θmax=rmax ≃0:5°. Considering the scale
of the atmospheric structures in the sky, an adequate
spatial resolution is easily met by a 1Mpixel
CCD chip.
Since each pixel is illuminated by a radiant power
from a solid angle and integrated over the spectral
responsivity, the measured radiometric quantity is
radiance. In fact, an idealistic camera attempts to
imitate the human eye’s spectral responsivity, so the
quantity would then be luminosity.
Each pixel can be considered a set of three inde-
pendent broadband detectors with 8 bit resolution.
The radiance Rat each pixel is a nonlinear function
of the stored pixel counts C:
R¼k·fðCÞ;ð2Þ
where kis a calibration constant for radiance in ab-
solute units (Wm−2sr−1), so fðCÞis a linearized pixel
intensity or relative radiance. This nonlinear conver-
sion is generally implemented in imaging systems to
increase contrast. Normally, this function is called
gamma correction and is of the form fðCÞ¼Cγwith
γ≃2:1[20]. For our camera this relationship has
been found to be too inaccurate over the full dynamic
range, and the function fhas been established ex-
perimentally by analyzing a series of images of an
illuminated reflection plate with increasing exposure
time τand fitting a 3rd order polynomial of the form
fðCÞ¼aC −bC2þcC3as an empirical function (see
Fig. 1). The result of the least squares fit for the
blue channel is ½abc¼½0:2410:0010:0000154.To
determine the constant k, a radiance standard like
an integrating sphere must be used. It is possible
Fig. 1. Linearization function fðCÞto convert pixel counts into
relative radiance for the blue channel. Data points are fitted with
a 3rd order polynomial.
1092 APPLIED OPTICS / Vol. 48, No. 6 / 20 February 2009

that kis a function of zenith angle Θ, an effect known
as vignetting in photography, in particular with fish-
eye objectives. It is a huge advantage of polarization
measurements and cloud detection presented in this
study that the issue of proper absolute radiance is
not relevant, which is a great experimental simplifi-
cation. Note that the dark noise is automatically sub-
tracted by the camera, so that our function fhas no
offset. As a test, a series of images was acquired at
exposure times of 0:0005–15 s with the camera being
completely in the dark. The mean counts of the JPEG
images ranged from 0.003 at 0:0005 s exposure time
to 0.25 at 15 s. It is concluded that for the exposure
times used here (around 0:01 s) the dark noise is suf-
ficiently subtracted.
Note also that, for the following image processing,
each image is rotated so that the line through the ze-
nith and the Sun is always vertical, the Sun being on
the upper half. This cut through the hemisphere is
called the principal plane (PP).
3. Polarization Maps
Polarization of the sky was first reported by Arago in
the early 19th century [21]. A first qualitative expla-
nation could be given by Lord Rayleigh using his
molecular scattering theory [22]. Indeed, the very
simplified consideration of single molecular scatter-
ing and geometric scattering angles reproduces the
polarization pattern for longer wavelengths in the
visible spectral range quite well. In the real atmo-
sphere, light is scattered multiple times and might
be backreflected by the ground, resulting in lower
than unity maximum polarization and points of zero
polarization, so-called neutral points. These neutral
points were observed quite early by Arago, Brewster
and Babinet and have more recently been imaged
in [23].
The generalized polarization state of light is con-
veniently described by the Stokes vector formalism
[21]. Decomposing the electric field vector into its
two orthogonal complex field amplitudes Erand El,
the 4-component Stokes vector is defined as
2
6
6
4
I
Q
U
V
3
7
7
5
¼2
6
6
4
ElE
lþErE
r
ElE
l−ErE
r
ElE
rþErE
l
iðElE
rþErE
lÞ
3
7
7
5
;ð3Þ
where Edenotes the complex conjugate amplitudes.
All the components are real, physical quantities,
namely, irradiances EE¼jEj2¼Iαmeasured at a
different polarizer angle α:
2
6
6
4
I
Q
U
V
3
7
7
5
¼2
6
6
4
I0þI90
I0−I90
I45 −I135
Iþ−I−
3
7
7
5
;ð4Þ
where Iþand I−denote circular polarized irra-
diances. Equation (4) can be simplified for the spe-
cific case here by noting that the irradiance is propor-
tional to the radiance for each pixel and neglecting
the circular polarization of the sky:
2
4
I
Q
U3
5¼2
4
R0þR90
R0−R90
R45 −R135 #:ð5Þ
Rαdenotes the measured radiances at relative polar-
izer angles α. The resulting degree of (linear) polar-
ization Πand its angle χis given by [21]
Π¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q2þU2
pI;χ¼0:5arctanU
Q:ð6Þ
Geometrically, we can visualize Q=Iand U=Ias the
two orthogonal components in a unit circle (horizon-
tal cut through the Poincaré sphere), whose vector
sum is the degree of polarization. It is clear that Π
is invariant under rotation of the coordinate system,
so the zero offset of the analyzer is irrelevant. Note
also that, in the expressions for Πand χ, any calibra-
tion constant cancels so that it suffices to compute
sums of relative radiances fðCÞ. The polarization of
the sky’s hemisphere is computed by combining four
images at relative polarizer angles of 0°, 45°, 90°, and
135° [Fig. 2(a)]. The images are linearized by the con-
version function fto obtain four relative radiance
maps. Applying Eqs. (5) and (6) at each pixel yields
the polarization map [Fig. 2(b)]. The maps are
smoothed by a 20 ×20 pixel 2D median filter for
spatial noise reduction without obscuring real atmo-
spheric structures. In this study we restrict ourselves
to the blue channel of the camera, since it has the
strongest signal and can be compared best to the
UV spectroradiometer. The center wavelength is
about 450 mm with a full width at half-maximum
(FWHM) of 50 nm.
Finally, the polarizing property of the composite
optical system (dome/objective/polarizer/CCD-chip)
is tested. Errors introduced by optical components
can be described by the Müller matrix, operating
(a) (b)
Fig. 2. (a) Blue pixel counts Cfrom JPEG images at four polarizer
angles. Counts are converted to relative radiance and inserted into
Eqs. (5) and (6) to yield the polarization map. (b) Corresponding
polarization map of the cloud-free sky for a wavelength of 450 
50 nm (blue channel). The degree of polarization is coded in gray
shades, undetermined areas are rendered white on 26 February
2008, 10:30 UTC.
20 February 2009 / Vol. 48, No. 6 / APPLIED OPTICS 1093

on the input Stokes vector [21]. The Müller matrix
describes a nonunitary polarization transformation,
i.e., it can rotate and change the length of the Stokes
vector. Here we confirm that the length of the Stokes
vector is conserved. A large, completely polarized
area (polarizer in front of a white reflectance plate)
is used as the test field and analyzed from different
angles of incidence (0°, 45°, and 80°) and polarizer
angle offsets (0°, 15°, and 30°) to confirm the invar-
iance of Πunder these angles. The measured polar-
ization is averaged over the test area and shown
along with the standard deviation as an error bar
in Fig. 3. No dependence is found for these angles of
incidence and polarizer offset angles. So we assume
the Müller matrix to be the identity for all the angles
of incidence.
However, small bright sources like the Sun, may
cause internal reflections at certain angles in the ob-
jective, which are superimposed in the sky’s image.
These reflections may locally introduce large errors
in the polarization but are clearly visible in the raw
images and can be identified as artifacts.
Since the Stokes vector is independent of a polar-
izer offset angle, it is clear that first Stokes para-
meter Iis overdetermined: I¼R0þR90 ¼R45þ
R135, i.e., three polarizer angles would suffice for a
full Stokes-vector recovery. This is expected because
the left-hand side of Eq. (5) contains only three un-
knowns. Now we use the additional information for
better statistics, averaging the first Stokes para-
meter Ito

I¼1
2P4
αRα. Furthermore, the difference,
d¼ðR0þR90Þ−ðR45 þR135 Þshould be zero and is
used as a quality check of the images.
To estimate the statistical error in our polarization
analysis, we measured consecutive maps of the sky
every 2min under stable atmospheric conditions.
From the variation of these series, we estimate a 1σ
standard deviation of 3% for Π.
For validation of this method with respect to sys-
tematic errors, we compare the polarization in the
principal plane to the values measured with a well-
characterized spectroradiometer with polarized
radiance optics with a small FOV of 1:5°[24]. The
wavelength of the spectroradiometer has been set
to 495 nm. As displayed in Fig. 4, the general shape
and value of polarization agree well within 3%.
Around the Sun at a polar angle of ≈60°, the camera’s
pixels are close to overexposure resulting in a large
deviation of the polarization from the spectroradi-
ometer measurement. Also, around the maximum po-
larization, at −30° zenith angle (90° behind the Sun), a
polarized reflection within the fish-eye objective
causes a distortion of the polarization curve. To con-
firm the effect of the reflections from the unobscured
Sun, a direct comparison between the polarization in
the principal plane with and without a mounted sha-
dow band is shown in Fig. 5. So omitting the shadow
band, the measured polarization is perturbed only at
the locations of the reflections (around −45°) and
around the Sun (62°). Taking care of these limitations,
all-sky maps of the polarization can be investigated.
Two interesting examples of polarization maps are
given. Figure 6(a) shows clear-sky polarization after
sunset, when two neutral points are distinctly visi-
ble. The minima around þand −70° zenith angle in
the principal plane are called the Babinet and Arago
neutral points, respectively. Their positions are de-
pendent on aerosol parameters in the atmosphere
and ground reflectivity [21] and will be investigated
more closely in the future.
In contrast, Fig. 6(b) shows the polarization map of
a partly covered sky around noon (solar elevation is
41°). High, thin cirrus clouds reduce the maximum
polarization below 50%, while lower, optically thick
cumulus clouds are barely polarized. It has also been
noted that scattered cumulus clouds reduce the
polarization also in the clear sky in between, due
to reflected light, corresponding to a higher ground
albedo. So although clouds have a dramatic effect
on the sky’s polarization, cloud detection is based
on a different method, described in Section 4.
Fig. 3. Analysis of a fully polarized test field (white reflectance
place) at different angles of incidence and polarizer angle offsets
0°, 15°, and 30°. Within the experimental uncertainty, no influence
of these parameters on the measured polarization can be found.
Fig. 4. Comparison of the all-sky camera blue polarization and
spectroradiometer radiance measurements in the principal plane
on a clear-sky day, 26 February 2008, 10:30 UTC. Solar zenith
angle is 58°.
1094 APPLIED OPTICS / Vol. 48, No. 6 / 20 February 2009

4. Cloud Detection
Color is the primary property that allows visual dis-
tinction of clouds in the sky. Because of different
wavelength dependence of scattering, the color of
the clear-sky region is blue rather than the whitish
or gray color of clouds. So in a digital color image,
clear-sky pixels have a higher ratio of blue/red radi-
ance than cloud pixels. The cloud detection method
is based on setting a threshold on this ratio [11–14].
Considering a cloudless, aerosol-free sky, the blue/
red ratio is a function of both zenith and azimuth an-
gles in the sky and the solar elevation. Toward the hor-
izon, the sky appears more palish than at the zenith.
In addition, scattering by aerosols, which in general
is less wavelength dependent and much stronger in
the forward direction than molecular scattering
[25], also diminishes the blueness of the sky, most pro-
minently in the region around the Sun. However, aero-
sol content and scattering properties may vary so
much in time that this temporal variation masks
the spatial variation of the pristine cloudless sky color.
Hence, without aerosol information, a constant blue/
red ratio threshold is taken for the entire hemisphere
to account for universal atmospheric conditions.
Using a diverse set of images with typical cloud
situations (low and high clouds, illuminated and
dark clouds, different solar zenith angles), a suitable
threshold of 1.3 on this ratio for marking cloud areas
was found, that best discriminated cloud and clear
sky. The threshold was confirmed by investigating
the number of cloud-marked pixels as a function of
threshold. The number first increases before reach-
ing a plateau for the optimal value of the threshold,
after which it increases again. It should be noted that
the threshold is unique to each camera system since
it depends on the color response of the CCD chip as
well as any gamma correction. Also, the location has
an influence on the threshold as altitude and typical
aerosol background will result in a different clear-
sky color.
For cloud-marked pixels, the underexposed image
is used as a second criterion, applying another
threshold for the blue/red ratio. This step is neces-
sary for the region near the unobscured Sun, where
pixels close to saturation would always be cloud-
marked. The total cloud cover (TCC) is then com-
puted as the ratio of cloud-marked pixels to total
pixel number in the hemisphere, in which the hori-
zon (up to 20° for the Innsbruck site) is masked
out. It is noted that the fish-eye projection is not
area-conserving and the projected solid angle per
pixel strictly is a function of zenith angle (see, e.g.,
[11] for a detailed mathematical formulation). So
the simple ratio is an approximation with the rela-
tive error growing with increasing zenith angle.
However, the absolute error in the resulting TCC
is below 0.01 for most situations and can safely be
ignored, considering the accuracy of the TCC value,
which is normally rounded to one decimal place or
given in octas.
Furthermore, the area around the Sun is investi-
gated in more detail. When the Sun is unobscured,
diffraction around the blades of the camera’s aper-
ture produces a star flare pattern around the Sun
with a sixfold symmetry. The number of flares allows
a quantitative definition of a Sun coverage para-
meter (SCP) as a measure of how much the Sun is
obscured by clouds. Three discrete cases are being
distinguished: when no pixels in the underexposed
image are saturated, the Sun is completely obscured,
and the SCP is assigned unity. In all cases, when pix-
els are saturated but the number of detected flares is
less than five, the Sun is assumed to be partially cov-
ered with an associated SCP of 0.5. When five or
more Sun flares are detected, the Sun is considered
unobscured and has an SCP of 0.
Here we use the JPEG image with the polarizer set
parallel to the principal plane, which approximates
an unpolarized relative radiance image. In principle,
the degree of polarization could also be applied for
cloud discrimination, but it was found to be less
selective than the color-ratio threshold and requires
more image processing.
Two representative examples of the performance
of the cloud detection method are shown in Fig. 7.
Cumulus-type clouds [Figs. 7(a) and 7(b)] have a par-
ticularly sharp boundary and good contrast in the
Fig. 6. (a) Polarization map at dawn, solar elevation is 6° and the
Sun has set behind the mountains. Note the neutral points at
about 70° zenith angle (9 September 2008, 17:00 UTC). (b) Polar-
ization map around noon with high and low clouds. (24 September
2008, 12:00 UTC).
Fig. 5. Comparison of the polarization in the principal plane,
with and without obscuring the Sun using a shadow arm. The mea-
sured polarization is only perturbed at the locations of the reflec-
tions around −45° and around the Sun (Solar zenith angle ¼62°).
24 October 2008, 09:46 and 09:48 UTC.
20 February 2009 / Vol. 48, No. 6 / APPLIED OPTICS 1095

blue/red ratio against the clear sky. In this case, the
detection works very well, even close to the unobs-
cured Sun. More problematic are thin clouds in front
of the Sun [Figs. 7(c) and 7(d)], where blue/red ratios
are very similar. Nevertheless, total cloud cover re-
sults are not severely affected. Note that the contrail
is nicely detected, as well as cumulus-type clouds
near the horizon.
For a quantitative statistical validation, more than
1 yr of hourly data in the period from 2 August 2007
to 27 October 2008 has been compared to the synop-
tic (SYNOP) observation at Innsbruck airport, lo-
cated 3km to the west of the camera site; see
Fig. 8. 73% of a total of 3903 analyzed camera images
agree to within 1octa with the SYNOP observations.
The distribution of the differences shows a slight
asymmetry, i.e., our cloud cover results tend to un-
derestimate those of the SYNOP observers. However,
cloud observations always bear certain interpreta-
tional variances, some differences will even exist be-
tween human observers. Specifically, the transition
from haze to cloud is continuous. For example, on
some hazy days (aerosol optical depth at 500 nm of
>0:4) our analysis results in zero cloud cover
whereas observers often interpret such a situation
as totally overcast. Furthermore, clouds in front of
mountains are considered only by the SYNOP obser-
vers, and add to the bias.
Finally, the significance of the SPC is validated by
correlating it with the erythemally weighted global
UV irradiance (UV index or UVI) and the TCC
[Figs. 9(a)–9(c)]. The method for measuring the UVI
and determining the clear-sky model value is de-
scribed in detail in [26]. For the cases when the Sun
was labeled obscured (SCP of 1), the sky is mostly
found totally covered and the measured UVI is much
lower than the predicted clear-sky value, with the ra-
tio peaking around 0.3. For cases of a partially cov-
ered Sun, the whole range of TCC covers are found
and, as expected, the ratio of measured-to-predicted
clear-sky UVI decreases with increasing TCC. For a
SCP of 0, predominantly cloud-free sky occurs with
small TCC and the measured UVI is close to the pre-
dicted clear-sky value. These observations compare
well with those in [16]. The classification of three
Sun coverage scenarios poses a refinement to
Fig. 7. (a) Image of a sky with convective cloud type (cumulus) of
low and medium height. This cloud type has a sharp contrast and
is relatively easy to discriminate from the clear sky. (19 September
2008,12:00 UTC). (b) Processed image after cloud detection, clouds
are rendered white, while clear sky is gray and mask is black. The
underexposed image (not shown here) allows good discrimination
close to the Sun. Total cloud cover here is 0.63. (c) Image of a sky
with both low and high clouds, including altostratus and a contrail.
(24 September 2008, 12:00 UTC). (d) Total cloud cover is 0.29. Note
the problematic area near the Sun when thin clouds are present.
Fig. 8. Comparison of total cloud cover (TCC) obtained from the
camera images and SYNOP cloud observations (TCCcamera-
TCCSYNOP). 73% of a total of 3903 analyzed camera images agree
within 1octa with the SYNOP-observations.
Fig. 9. (a) For SCP of 1, the Sun is totally occluded, which coincides with a TCC near 1. The ratio of measured UVI and clear-sky pre-
diction is peaked around 0.3. In the range 0:98 <TCC <1, more than 620 data points are accumulated. (b) SCP of 0 implies a partially
covered Sun. In this case increasing TCC is correlated with a decreasing ratio of measured-to-clear-sky prediction. (c) For SCP of 0, the Sun
is assumed totally unobscured, which correlates with small TCC and the measured UVI converging toward the clear-sky prediction.
1096 APPLIED OPTICS / Vol. 48, No. 6 / 20 February 2009

previous work where only two cases were distin-
guished (Sun obscured or not obscured). It is further
noted that the extreme cases SCP of 0 and 1 repre-
sent a well-defined class of scenarios, while SCP of
0.5 spans a larger scope, which may suggest a further
improvement in the future by refined partitioning of
this case.
5. Conclusion and Outlook
A simple all-sky imaging system with a polarizing
filter has been described and characterized with
respect to measuring the sky´s degree of polarization
and detecting total cloud cover. Both types of analysis
have been validated against independent measuring
methods and found to agree well within their respec-
tive uncertainties. A Sun coverage parameter has
been obtained from image processing around the
Sun. It has been shown that the combination of total
cloud cover and Sun coverage parameter form a solid
basis for UV radiation estimation under cloudy
conditions. A cloud type analysis could be attempted
using pattern-recognition techniques in combination
with satellite images. Polarization maps of the sky
contain valuable information of aerosol content in
the atmosphere. The correlation of aerosol optical
depth and degree of polarization is one of the para-
mount topics in our ongoing work. Furthermore,
in contemporary climate research, the interaction
of aerosol and clouds are a key uncertainty in
the Earth’s energy budget. As a complementary de-
vice together with an aerosol optical depth measure-
ment like a sunphotometer all-sky imaging could be
a powerful tool for investigation of aerosol–cloud
interaction.
This work was supported by the Austrian Science
Fund (FWF) under Project P18780. We gratefully
acknowledge Lanzinger at Austrocontrol, Innsbruck
Airport, for supplying the SYNOP cloud observa-
tions. The camera system was developed in coopera-
tion with Schreder (CMS-Ing.Dr.Schreder GmbH).
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