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Charge-coupled device spectrograph for direct solar irradiance and sky radiance measurements Natalia Kouremeti, 1, * Alkiviadis Bais, 1 Stelios Kazadzis, 1 Mario Blumthaler, 2 and Rainer Schmitt 3 1 Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, P.O. Box 149, Thessaloniki 54124, Greece 2 Division for Biomedical Physics, Innsbruck Medical University, Müllerstrasse 44, A-6020 Innsbruck, Austria 3 Meterologie Consult GmbH (METCON), Konigstein, Germany *Corresponding author: [email protected] Received 10 September 2007; revised 18 January 2008; accepted 18 January 2008; posted 24 January 2008 (Doc. ID 85906); published 31 March 2008 The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory of Atmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for direct irradiance and sky radiance measurements was determined, respectively, with an uncertainty of 4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelength ranges. The overall uncertainty associated with the direct irradiance and the sky radiance measure- ments is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g., at solar zenith angles (SZAs) larger than 70°], and 4% and 6% in the UV-A, visible, and NIR. Direct solar spectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM 300) were compared with the corresponding CCD measurements. Their agreement in the wavelength range of 310500 nm is within 0:5% 1:1% (for SZA between 20° and 70°). Aerosol optical depth (AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer [Aerosol Robotic network (AERONET)] agree to within 0:02 0:02 in the range of 315870 nm. Significant cor- relation coefficients with a maximum of 0.99 in the range of 340360 nm and a minimum of 0.90 at 870 nm were found between synchronous AOD measurements with the Bentham and the Cimel instru- ments. © 2008 Optical Society of America OCIS codes: 010.0280, 010.1100, 280.4788, 280.1100. 1. Introduction In the past decades great attention has been paid to the study of the ultraviolet and visible solar radia- tion and the factors influencing their propagation through the atmosphere. Spectral measurements of direct solar irradiance are used in various atmo- spheric science applications, such as the determina- tion of aerosol optical properties [14] and the columnar abundance of atmospheric species [59]. In addition, various modeling studies benefit from such measurements since their parameterization is much easier than that of global or diffuse radiation [10]. Lately, direct irradiance spectral measurements were used to determine the actinic flux in combina- tion with global irradiance measurements [1113]. Finally, they can be used to determine the spectrum of the extraterrestrial solar flux from ground-based measurements [14,15]. One of the most important global networks, the Aerosol Robotic Network (AERONET), provides data on the optical properties of aerosol through direct ir- radiance and sky radiance measurements [16,17] with filter radiometers. In recent years, UV and visi- ble measurements of sky radiance have been used to derive atmospheric columns and vertical profiles for a number of important trace species such as O 3 , NO 2 , 0003-6935/08/101594-14$15.00/0 © 2008 Optical Society of America 1594 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

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Charge-coupled device spectrograph for direct solarirradiance and sky radiance measurements

Natalia Kouremeti,1,* Alkiviadis Bais,1 Stelios Kazadzis,1

Mario Blumthaler,2 and Rainer Schmitt3

1Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, P.O. Box 149, Thessaloniki 54124, Greece2Division for Biomedical Physics, Innsbruck Medical University, Müllerstrasse 44, A-6020 Innsbruck, Austria

3Meterologie Consult GmbH (METCON), Konigstein, Germany

*Corresponding author: [email protected]

Received 10 September 2007; revised 18 January 2008; accepted 18 January 2008;posted 24 January 2008 (Doc. ID 85906); published 31 March 2008

The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory ofAtmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for directirradiance and sky radiance measurements was determined, respectively, with an uncertainty of4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelengthranges. The overall uncertainty associated with the direct irradiance and the sky radiance measure-ments is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g.,at solar zenith angles (SZAs) larger than 70°], and 4% and 6% in the UV-A, visible, and NIR. Direct solarspectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM300) were compared with the corresponding CCD measurements. Their agreement in the wavelengthrange of 310–500nm is within 0:5%� 1:1% (for SZA between 20° and 70°). Aerosol optical depth(AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer[AerosolRobotic network (AERONET)] agree to within 0:02� 0:02 in the range of 315–870nm. Significant cor-relation coefficients with a maximum of 0.99 in the range of 340–360nm and a minimum of 0.90 at870nm were found between synchronous AOD measurements with the Bentham and the Cimel instru-ments. © 2008 Optical Society of America

OCIS codes: 010.0280, 010.1100, 280.4788, 280.1100.

1. Introduction

In the past decades great attention has been paid tothe study of the ultraviolet and visible solar radia-tion and the factors influencing their propagationthrough the atmosphere. Spectral measurementsof direct solar irradiance are used in various atmo-spheric science applications, such as the determina-tion of aerosol optical properties [1–4] and thecolumnar abundance of atmospheric species [5–9].In addition, various modeling studies benefit fromsuch measurements since their parameterization is

much easier than that of global or diffuse radiation[10]. Lately, direct irradiance spectral measurementswere used to determine the actinic flux in combina-tion with global irradiance measurements [11–13].Finally, they can be used to determine the spectrumof the extraterrestrial solar flux from ground-basedmeasurements [14,15].

One of the most important global networks, theAerosol Robotic Network (AERONET), provides dataon the optical properties of aerosol through direct ir-radiance and sky radiance measurements [16,17]with filter radiometers. In recent years, UV and visi-ble measurements of sky radiance have been used toderive atmospheric columns and vertical profiles fora number of important trace species such as O3, NO2,

0003-6935/08/101594-14$15.00/0© 2008 Optical Society of America

1594 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

BrO, and OClO using differential optical absorptionspectroscopy (DOAS) or Multi Axis (MAX)-DOASmeasurement techniques [18–20].Ground-based spectrographs based on CCDs or

diode array detectors have been developed for globalirradiance, actinic flux, zenith sky [18,21], and skyradiance [19] measurements providing good qualityobservations. The advantage of these instrumentsis their ability to simultaneously measure the solarradiation spectrum over the entire operational wave-length range in high temporal frequency, thus beingable to capture short term irradiance variabilities inthe atmosphere.The CCD spectrograph presented here is designed

for direct solar irradiance and sky radiance measure-ments in the spectral range of 310–1000nm. We pre-sent a detailed characterization of its properties andperformance and a small scale validation against ascanning spectroradiometer that has been repeat-edly intercompared and validated in the past.

2. Instrument Technical Description

A new instrument has been developed at the Labora-tory of Atmospheric Physics (LAP), Aristotle Univer-sity of Thessaloniki, for measurements of the directsolar irradiance and the sky radiance in the spectralrange of 310–1000nm. It consists of componentsproduced by different manufacturers. The designand assemblage of the spectrograph was done bythe Meterorologie Consult GmbH, Konigstein,Germany (http://www.metcon.de) (Fig. 1).The detector of the spectrograph is a solid-state

CCD-type S7031 by Hamamatsu Photonics (Hama-matsu Photonics, K.K., Hamamatsu City, Japan,http://www.hamamatsu.com). The system comprisestwo telescopes that are for collecting the incomingradiation (Meterorologie Consult GmbH), an UV en-hanced optical fiber, a multichannel spectrometer(MCS-CCD) with a preamplifier (Zeiss GmbH, Jena,Germany, http://www.zeiss.de), and a housing withthe CCD cooling electronics and the front end electro-nics (FEE) (tec5, Oberursel, Germany, http://www.tec5.de). The FEE controls the signal genera-

tion of the CCD and digitizes the analog video signal.A personal computer with a PC interface card (tec5)is used for data acquisition, the electronic control ofthe FEE, and the generation of the integration time.The optical fiber is connected to the internal fiber ofthe spectrograph by a standard mechanical adapter(SMA). The radiation passes through an entranceslit (20 μm wide) and is dispersed by a flat field dif-fraction grating (248 lines=mm) blazed at 400nm. Inaddition to its dispersive function, the gratingimages the entrance slit on the CCD detector, whichis thermoelectrically cooled by a Peltier element toreduce the noise-to-signal ratio and to improve thetemporal stability of the offset. The average step ofthe measured spectrum is 0:77nm, ranging from0:8nm in the UV to 0:74nm in the near-infrared(NIR) region.

The optical telescopes consist of two parallel alu-minum tubes that are filled with cylindrical bafflesof different apertures. The field of view (FOV) isset to 1:2° for the direct irradiance and to 4:6° forthe sky radiance optics. Depending on the applica-tion, both FOVs can be adjusted by changing the baf-fles inside the tubes. The two telescopes areconnected to the spectrograph through a split fiberbundle. The distribution of single fibers in the splitpart of the fiber is 4 for the direct irradiance and 16for the sky radiance optics. The total number of sin-gle fibers is limited to 20 by the size of the entranceslit of the spectrograph. The direct irradiance opticsis equipped with an attenuation filter to reduce thelevel of the signal in the visible part of the spectrumand avoid saturation of the CCD’s signal. Withoutthis filter the saturation occurs even with the lowestintegration time of 8ms. The transmittance of the fil-ter has a slight wavelength dependence, which wasdetermined in the laboratory using a 1000W lamp.The maximum transmittance (12.2%) was found at350nm and the minimum (10.7%) at 600nm.

For positioning the telescopes toward the Sun orthe sky, they are mounted on a two axes tracker thathas been designed and produced at the LAP. Thetracker is equipped with two stepper motors of0:6° angular resolution. For improving the resolutiontwo timing pulleys (of 12 and 70 teeth each) and atiming belt have been used, resulting in a final angu-lar resolution of 0:103° for both the azimuth and thezenith. The maximum speed of both motors is ∼15 sper revolution. Both motors can be zeroed with theaid of a photodiode installed on each of the large pull-eys. For accurately positioning the telescopes, eitherto the Sun or to a specific direction on the sky, a sight-ing procedure is used to determine an offset for eachof the two motors. The tracker is positioned towardthe Sun using the solar azimuth and zenith anglesthat are calculated from astronomical formulas.Then the system searches for themaximum intensityby varying the zenith and azimuth in small steps andcalculates the two offsets.

Fig. 1. (Color online) CCD system: spectrograph and control unit(left), direct irradiance and radiance telescopes on tracker (right).

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1595

3. Instrument Characterization

The characterization of the system was performedthrough laboratory tests and measurements underreal atmospheric conditions. In Subsections 3.A wepresent the methodology followed for the determina-tion of different parameters and discuss the achievedresults.

A. Dark Signal and Stray Light

The dark signal and the stray light produce an offsetin the measured signal, which is removed before ap-plying the absolute calibration function on the rawsignal. The CCD detectors produce a small outputsignal even in the absence of radiation, which iscaused partly by thermal generation of electronsand partly by the electronics [22]. For the quantifica-tion of this dark signal (Sd) a set of 1000 measure-ments were taken with blocked input optics andfor 20 different integration times ranging from 8 to5000ms. During the measurements the temperatureof the instrument (warm side of the Peltier element)varied between 17 °C and 25 °C, covering its usualoperational range. It was found that, under constanttemperature, Sd is related linearly to the integrationtime (IT) and has a pixel dependence of ∼4%.Figure 2 shows the linear correlation between Sdand IT for four different wavelengths (300, 400,500, and 800nm) and for a temperature of 20 °C.Therefore, we consider that the dark signal Sdconsists of two pixel (and thus wavelength), depen-dent components: the electronic offset eðλÞ and thethermally produced signal Stðλ; TÞ:

Sd ¼ eðλÞ þ Stðλ;TÞ IT: ð1Þ

The electronic offset is the readout noise arisingfrom the electronics of the CCD and has very weaktemperature dependence. Equation (1) suggests that

eðλÞ represents the dark signal for IT ¼ 0, and there-fore eðλÞ can be determined at each wavelength byextrapolating the dark signal measured at differentintegration times to IT ¼ 0.

Figure 3 shows the wavelength dependence of theestimated electronic offset, ranging from ∼217counts at 300nm to 227 counts at 950nm. The varia-bility at each wavelength (represented by the 5th and95th percentiles) results from the temperaturechanges and it is ∼2%.

From the same set of measurements the average ofthe dark signal Stðλ;TÞ was derived from Eq. (1) foreach IT and converted to electrons per second bydividing with the corresponding integration time.Figure 4 shows the mean Stðλ;TÞ over the opera-tional wavelength range of 300–1000nm as a func-tion of temperature. Evidently, for an increase inthe instrument’s temperature of ∼6 °C the darksignal is almost doubled, in accordance with themanufacturer’s specifications [22]. It is clear that

0 1000 2000 3000 4000 50000

0.5

1

1.5

2

2.5

3x 10

4

Integration Time (ms)

Da

rk S

ign

al (

cou

nts

)

300400500800

wavelength (nm)

Fig. 2. (Color online) Dark signal versus integration time at fourselected wavelengths and for a constant temperature of 20 °C. Thestandard deviation for each integration time derived from a set ofeight measurements is also shown.

300 400 500 600 700 800 900 100021

215

220

225

230

235

wavelength (nm)

Ele

ctro

nic

offse

t (c

ounts

)

5th and 95th percentilesmedian

Fig. 3. Wavelength dependence of the electronic offset, eðλÞ, asderived from linear regressions between the dark signal and theintegration time at different temperatures.

17 18 19 20 21 22 23 243.5

4

4.5

5

5.5

6

6.5

7

7.5

Temperature(°C)

S t (

,T)

(e- /

sec

) (a

vera

ged

over

the

wav

elen

gth

rang

e 30

0 - 1

000

nm)

50004000300020001000 700 500 200 100 90 80 70 60 50 40 30 20 10 8

IT (ms)

Fig. 4. (Color online) Average dark signal as a function of theinstrument’s temperature for different integration times.

1596 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

the temperature dependence of eðλÞ is negligible com-pared to that of Stðλ;TÞ).Since the temperature dependence of the dark sig-

nal has been established, during regular operationthe dark signal is measured only once per day asthe average of ten consecutive measurements forthe entire range of integration times. The dark-sig-nal correction for each direct Sun (or radiance) spec-trum is derived by adjusting this measured darksignal for the actual temperature at the time ofthe particular spectral measurement. The adjust-ment is done in proportion to the ratio of the darksignal and of the average measured irradiance inthe wavelength range of 255–262nm. In this rangeno photons are available from atmospheric or solarradiation, and the stray-light contribution from high-er wavelengths is very weak; thus the measured sig-nal can be safely assumed as a dark signal. Note thatany wavelength interval below ∼270nm gives simi-lar results.Stray light is defined as the spurious signal origi-

nating from photons of different wavelengths thanthe measured one, which succeed to pass throughthe exit slit of a spectrometer and are recordedtogether with the true signal. These photons are scat-tered on particles or reflected on various optical com-ponents and move almost randomly inside thespectrometer. Unlike spectrometers equipped withan exit slit, the entire CCD is exposed to the radia-tion dispersed from the grating and consequently tostray-light photons. Stray light becomes more impor-tant in the UV-B region, where the true signal is veryweak and therefore the many photons of longerwavelengths can significantly influence the mea-sured signal [23–25].The stray-light contribution to the atmospheric

radiation measurements of the CCD spectrographwas investigated by comparison to spectral measure-ments conducted by a collocated double-monochro-mator Brewer MKIII spectroradiometer, which hassuperior stray-light rejection [26]. A set of 9500direct Sun spectra recorded under cloud-free condi-tions and for solar zenith angles (SZRs) ranging be-tween 20° and 85° were used. The correction of straylight in the spectral measurements of thissystem is based on the assumption that stray lightis a wavelength independent offset. A similar as-sumption is used for the stray-light correction ofthe irradiance spectra measured with the single-monochromator Brewer spectroradiometers [23].An improvement to the Brewer methodology is toconsider stray light as intensity dependent. Thestray-light correction is calculated as the average sig-nal in a variable wavelength band, which depends onintensity, according to the following methodology.First, synchronous spectra were acquired with

the CCD spectrometer and a collocated double-monochromator Brewer spectroradiometer during3 days. Then, taking into account the detectionthreshold for the Brewer spectral measurements(∼10−6 Wm−2 nm−1) the first measurable wavelength

for each Brewer spectrum was determined. Assum-ing that below this wavelength the incident radiationon both instruments is zero, the signal of the CCD atthis wavelength is considered to originate only fromstray light. Both the wavelength and the level of thesignal depend on the irradiance level at the time ofthe measurement. Figure 4 shows the fraction of thestray-light signal in the irradiance at 340nm as afunction of the irradiance at 340nm. This wave-length was chosen because of its negligible stray-light effect, and because the CCD signal at thiswavelength is never saturated. From this figure itappears that the stray-light contribution is generallyless than 1% of the signal at 340nm and increasesonly under very low irradiance levels (e.g., for highSZAs and/or thick cloud conditions). The spread inthe data is a result of small differences in exactlymatching the Brewer starting wavelength with thecorresponding pixel of the CCD due to differencesin the spectral resolution and wavelength step ofthe two instruments and also due to atmosphericnoise. A third degree polynomial fit on the data isused to determine the signal-dependent stray-lightcorrection factor, which is used to calculate the straylight at any pixel in the entire operational wave-length range of the CCD system.

Using ∼9000 of direct irradiance spectra recordedduring a period of 3 days, the stray light has beendetermined using the polynomial fit of Fig. 5 andthe signal at 340nm of each scan and used to correctthe irradiance at the entire spectral range. The ratioof the stray light to the measured signal provides ameasure of the contribution of stray light in eachmeasurement. Averages of this ratio over subsetsof cloudless sky spectra, recorded within a smallrange of SZAs are calculated and presented as a func-tion of wavelength in the upper panel of Fig. 6. Intotal, 13 bins of 5° SZAs were used. For wavelengthslonger than 350nm the stray-light contribution is

0 2000 4000 6000 8000 10000 12000 14000 16000 18000

100

101

Irradiance at 340nm (counts)

Str

ay

light c

orr

ect

ion

as

perc

enta

ge o

f Ir

radia

nce

at 340nm

datapolynomial fit

Fig. 5. (Color online) Fraction of the stray-light signal withrespect to the measured irradiance at 340nm as a function of irra-diance. The symbols represent the actual measurements and thesmooth line a third degree polynomial fit.

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1597

less than 1% for SZAs smaller than 80° and becomesimportant only for SZAs larger than 75° in the UV-Band at the shorter UV-A wavelengths. Finally, thestray-light contribution is less than 1% for wave-lengths longer than 400nm irrespective of SZAs.Assuming that the uncertainties introduced by the

stray-light correction are less important when thecorrection is smaller than 10% of the measured sig-nal, the minimum wavelength with reliable mea-surement of irradiance as a function of SZAs couldbe determined (see lower panel of Fig. 6). In thistypical example, the threshold wavelength is302nm for SZAs smaller than 35°, increasing to320nm for SZAs of 80°, and to 340nm for SZAs of85°. This panel also provides an indication of how re-liable the spectral measurements of this system atvarious SZAs can be, suggesting that measurementstaken under strong irradiance conditions are of ac-ceptable quality down to ∼310nm.Although in the above analysis the stray-light de-

pendence on the SZA has been determined from datataken under cloud-free conditions, the correction fac-tors are applicable to measurements recorded underall-sky conditions, since they are determined as a

fraction of the level of the measured irradianceat 340nm.

B. Linearity

According to manufacturer’s specifications, the non-linearity of the CCD detector is less than 1% [22].This was verified using a set of direct Sun measure-ments taken on a clear day with the SZA varyingfrom 20° to 78°, resulting in a variation of irradianceof up to 80% of the maximum depending on the wa-velength. Each time the measurements were per-formed using four integration times (10, 30, 50,and 100ms) and were corrected for the dark signaland stray light. Thus each set comprises measure-ments with the CCD illuminated with radiation ofdifferent levels (between three and ten times stron-ger). Ideally, the ratio of the measurements for eachintegration time Sðλ; ITÞ to the measurementSðλ; 10msÞ (for IT ¼ 10ms) should be equal to theratio IT=10ms. Our measurements show small de-viations that depend on wavelength, IT, and irradi-ance, and are summarized in Tables 1 and 2. Theaverage deviations are small, even at 320nm, wherethe variability of the deviations is larger, owing to

300 310 320 330 340 350 360 370 380 390 400

1

10

100

wavelength(nm)

Ratio

of

stra

y lig

ht

corr

ect

ion to Irr

adia

nce

(%

)

20 25 30 35 40 45 50 55 60 65 70 75 80 8585300

310

320

330

SZA (deg)

Th

resh

old

(nm

)

20-2525-3030-3535-4040-4545-5050-5555-6060-6565-7070-7575-8080-85

SZA (deg)

Fig. 6. Percentage of stray-light correction with respect to spectral direct irradiance for different SZAs (upper panel). Wavelength thresh-old for stray-light correction as a function of SZAs (lower panel).

Table 1. Linearity Uncertainty Budget for Different Wavelengths and Integration Times

Deviation from Ratio IT=10ms (%)

30ms 50ms 100ms

Wavelength (nm) Mean �1σ Median

Percentiles

Mean �1σ Median

Percentiles

Mean �1 σ Median

Percentiles

5th 95th 5th 95th 5th 95th

320 −1:1 � 1:77 −1:00 −4:18 1.51 −1:0 � 1:75 −0:92 −4:21 1.48 −1:10 � 1:82 −0:91 −4:68 1.33400 0:45 � 0:44 0.46 −0:27 1.17 0:76 � 0:42 0.77 0.08 1.44 1:01� 0:46 0.98 0.32 1.76500 0:59 � 0:30 0.59 0.10 1.11 0:70� 0:27 0.71 0.24 1.14 —

a—

a—

a—

a

900 0:16 � 0:62 0.16 −0:85 1.21 0:38 � 0:60 0.39 −0:59 1.38 0:87 � 0:57 0.88 −0:05 1.82

aNo measurements at 100ms because of saturation of the detector.

1598 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

the weaker signal of the radiation and the increasinguncertainties in the correction of stray light. Gener-ally the linearity assumption results in a meanerror of 0:50%� 0:19% for the spectral range of320–1000nm.

C. Absolute Calibration

The absolute sensitivity Rðλ; ITÞ of the CCD spectro-graph can be determined from

Rðλ; ITÞ ¼ Sðλ; ITÞE ðλÞ ; ð2Þ

where Sðλ; ITÞ is the measured signal, corrected forthe dark signal and stray light, at a distance wherethe lamp can be considered as a point source [27] andEðλÞ is the irradiance of the calibration lamp at thisdistance.The first step of the procedure is to determine the

reference point from where the distance of the cali-bration lamp is measured, also used to calculatethe corresponding lamp’s irradiance by the inversedistance square law. The spectrum of a 1000Wtungsten halogen lamp (type DXW) traceable tothe Physikalisch-Technische Bundesanstalt (PTB)standards of spectral irradiance wasmeasured at dif-ferent distances (in steps of 20 cm) using both thedirect irradiance and the sky radiance optics, andfor a range of integration times. The closest positionfrom the edge of the fiber to the center of the lamp’sfilament was 109:1 cm and the farthest 300:1 cm. Atthe same positions, measurements were also per-formed by inserting the attenuation filter, which isused in the direct Sun measurements, in front ofthe sky radiance optics, to investigate possible effectsfrom this filter.For the determination of the reference point only

the measurements made with the sky radianceoptics, which are more sensitive compared to the di-rect irradiance optics, were used in order to improvethe signal-to-dark-signal ratio. The unknown dis-tance (x) between the reference point and the edgeof the fiber is calculated from Eq. (3) for all possiblecombinations of spectral irradiance measurementsSðλ; IT;dÞ performed at different distances d:

Eðλ; IT;diÞEðλ; IT;djÞ

¼�dj þ x

di þ x

�2: ð3Þ

Although this distance, x, should be independent ofwavelength, the application of Eq. (3) for differentwavelengths yields different values for x, which areshown in Fig. 7. The blue area represents the 10thand 90th percentiles of x. The shaded area denotesthe wavelength range where the ratio of the signalto the dark signal is greater than 10 for all distances.This ensures that small uncertainties in calculatingthe dark signal at the temperature of the measure-ment would not have a significant effect on the mea-surement. In the same figure a spline fit on theestimated values of x for the entire wavelength rangeis shown, as well as the mean value of x over theshaded wavelength range, denoted as “trusted wave-length area.” Evidently, there is a wavelength depen-dence resulting in x to deviate between 0 cm (at920nm) and þ2:4 cm (at 300nm). This wavelengthdependence is yet unexplained, probably caused bysmall nonlinearities, variation in the FOVof the fiberbundle with wavelength, and small errors in thedark-signal correction. The mean distance x overthe trusted wavelength range of 500–930nm isþ0:4 cm without the attenuation filter and þ1:8 cmwith the filter. This difference of 1:4 cm can be mostlyattributed to small errors in the dark-signal correc-tion, as the filter appreciably reduces the ratio of thesignal to the dark signal, to between 3.5 and 16. Theabove tests indicate that the reference point islocated a few millimeters behind the edge of the fiberbundle, which is located 16:4 cm from the edge of thetelescope. Although we have not fully understood thevariation of x with wavelength, we have taken thisvariation into account in the absolute calibration,even outside the “trusted” wavelength region wherethe uncertainty is much higher. The importanceof the variation of x can be assessed by calculatingthe change in the irradiance resulting from the var-iation of x about its mean value. For a distance of182 cm, this variation would introduce an error inthe irradiance of the lamp ranging from −0:5% to2% (see lower panel of Fig. 7).

Table 2. Linearity Uncertainty Budget for Average Spectrumand Integration Times

Deviation from Ratio IT=10ms in the SpectralRange 320 to 1000nm (%)

IT (ms) Median

Percentiles

Mean� 1 σ5th 95th

30 0.39 0.11 0.68 0:39 � 0:1850 0.50 0.22 0.77 0:49 � 0:18

100 0.65 0.29 0.94 0:63� 0:20

Dis

tanc

e x

(cm

)

-2

-1

0

1

2

3

300 400 500 600 700 800 900 1000

-0.5 0.0 0.5 1.0 1.5 2.0

wavelength (nm)

Error in Irradiance (%

)

Trusted wavelength range10th and 90th percentilesfitmean

Fig. 7. (Color online) Wavelength variation of the calculated dis-tance x using the sky radiance optics without the attenuation filter(upper panel). Error in lamp irradiance at a distance of 182 cm re-sulting from the variation of x with wavelength (lower panel).

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1599

1. Direct Optics

Ideally, the calibration function, Rðλ; ITÞ, for the di-rect irradiance optics is determined by measuringthe irradiance emitted from a standard lamp of spec-tral irradiance placed at a distance where the appar-ent diameter of the lamp equals that of the Sun. Thisdistance for our system is 340 cm. Since the sensitiv-ity of the direct irradiance optics is low [owing to thesmall number of single fibers (four) and the attenua-tion filter] a shorter distance (∼180 cm) from thelamp is required to achieve measurable signal, whileat the same time the lamp is “seen” as a point source,and falls well within the FOV of the optics (1:2°).However, even at this distance the signal at certainwavelength bands is still too low, introducing largeuncertainty in the calibration function. Therefore atwo stage procedure is followed: (a) a measurementSfarðλ; ITÞ at a distance of 182:2 cm to determine theabsolute level of the calibration at a wavelength withsufficiently strong signal (588nm) and a measure-ment Scloseðλ; ITÞ at a closer distance (122:2 cm) todetermine the spectral shape of the calibrationfunction.For the Scloseðλ; ITÞ measurement an integration

time of 500ms (ITcRef ) was used, being the maximum

possible without saturating the signal, while2000ms (ITref ) was used for the Sfarðλ; ITÞ measure-ment to derive Rð588nm; ITref Þ using Eq. (2). Bothmeasurements were first corrected for the dark sig-nal and for stray light using a long-pass filter (SchottWG 320) that cuts off all photons below 320nm.Through a series of alternating measurements withand without the filter, the stray-light contributionand level of the dark signal were determined, andthe corrected measurement Scðλ; ITÞ) is derived from

Scðλ; ITÞ ¼ Sðλ; ITÞ

− Sdarkðλ; ITÞ

2666664

X240nm230nm

SWG 320ðλ; ITÞ

X240nm230nm

Sdarkðλ; ITÞ

3777775

− ⟨SWG 320ðλ; ITÞ⟩; ð4Þ

where Sdarkðλ; ITÞ is the dark current measurementperformed at the end of the calibration, SWG 320ðλ; ITÞis the lamp measurement through the long-pass fil-ter, corrected for the dark signal and smoothed with aspline fit in the range of 250–310nm. The transmit-tance of the filter is 10−6 at 295nm and reaches 10−12at 287nm and below. Assuming that the near-fieldstray-light contribution from a specific wavelengthis significant only within �25nm from this wave-length (right panel of Fig. 10), the signal in the spec-tral band of 230–240nm could be considered asstray-light free and therefore can be used for the ad-justment of dark current at the temperature of themeasurement Sðλ; ITÞ. The far-field stray light may

also contribute, but it is believed to be less important.The term ⟨SWG 320ðλ; ITÞ⟩, representing the stray-light offset, is the mean over the wavelength range290–292nm. The remaining noise in Scðλ; ITÞ dueto the pixel to pixel variation is removed throughspline fitting, and it is on average 0:46%� 1:32%for the entire operational range.

Finally the irradiance of the lamp EðλÞ at the dis-tance of 182:2 cm is calculated through the inversesquare law and the calibration function of the instru-ment for any IT is calculated from

Rðλ; ITÞ ¼ ITITRef

EðλÞSfarðλ; ITref Þ

; ð5Þ

where

Sfarðλ; ITref Þ

¼ Scfarð588nm; ITref Þ

�Sccloseðλ; ITc

ref ÞSccloseð588nm; ITc

ref Þ�:

In Fig. 8 the direct irradiance absolute calibrationfunction of the CCD spectrograph is presented for theintegration time of 100ms. The highest responsivityappears in the wavelength range of 450–550nm. Theetalonlike behavior, characteristic for the thinnedback-illuminated CCDs, is evident for wavelengthsgreater than 800nm [28,29].

2. Sky Radiance Optics

The calibration of the sky radiance is performed withthe use of the 1000W DXW calibration lamp and astandard reflectance plate (Spectralon) calibratedby Gigahertz-Optic GmbH. The reflectance platewas set at a distance of 50 cm from the lamp andthe telescope of the radiance optics was alignedto view the plate at an angle of less than 5°from the normal on the center of the plate. The tele-scope was positioned behind the lamp, in such a waythat the lamp was completely out of its FOV. Thereflectivity of a reflectance plate can be described

Fig. 8. (Color online) Direct irradiance (left axis, blue line) andradiance (right axis, green line) calibration function of the CCD.

1600 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

through the following quantities [30]: the bidirec-tional reflectance distribution function (BRDF), f r;the bidirectional reflectance factor (BRF), R; andthe hemispherical reflection factor, ρ.For a perfectlyLambertian surface, f r equals 1=π sr−1, while Rand ρ are equal to unity. The radiance from thereflectance plate equals the incident irradiance mul-tiplied by the BRF. The calibration certificateof the reflectance plate used in this study providesthe hemispherical reflection factor, ρ:

ρðθi;ϕi; λÞ ¼1π

Z ϕr¼2π

ϕr¼0

Z θr¼π=2

θr¼0Rðθi;ϕi; θr;ϕr; λÞ · cosðθrÞ

· sinðθrÞdθrdϕr;

ð6Þ

where θi ϕi are the zenith and azimuth angles of in-cidence irradiance, respectively, and θr ϕr the corre-sponding zenith and azimuth viewing angles of thereflectance surface, while R is unknown. Assumingthat the reflectance plate behaves very closely to aLambertian surface, the radiance emitted by the re-flectance plate is given by

LðλÞ ¼ EðλÞρðλÞπ ; ð7Þ

where EðλÞ is the lamp’s irradiance at the distance of50 cm. The error imposed by this assumption can beup to 5% [30]. Finally, the distance of the telescopefrom the reflectance plate is not essential, as longas the plate overfills the FOV of the entrance optics.Similar to the procedure for the calibration of di-

rect optics, the maximum possible integration timewas used in the measurements. For the radiancecalibration a second measurement at a closer dis-tance was not necessary as the signal to dark ratiowas sufficiently large at all wavelengths. The func-tion was calculated in a similar way as for the directSun calibration from the following equation:

Rðλ; ITÞ ¼�

ITITref

�LðλÞ

Scðλ; ITref Þ; ð8Þ

where Scðλ; ITref Þ is the dark current and stray-lightcorrected measurement. In Fig. 8 (right axis) the ra-diance absolute calibration function of the CCD spec-trograph is presented for the integration time of1000ms. The spectral characteristics are similar tothe direct response function.

D. Signal-to-Noise Ratio

The noise characterization is the determination ofthe noise associated with the measured signal for re-presentative irradiance levels. There are numeroussources that generate noise in CCDs and can be ca-tegorized to photon shot, dark shot, and readoutnoise. A quantitative measure of the noise is the

signal-to-noise ratio (SNR), which is defined by theequation

SNR ¼ S − Sdarkffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiσ2 Sþ σ2 Sdark

p ; ð9Þ

where S and Sdark are the mean signal and darkcurrent, respectively, and σðSÞ and σðSdarkÞ are thecorresponding standard deviations. For the determi-nation of the SNR a set of at least ten measurementsof a stable light source is required [27].

SNR depends on the intensity of the measuredradiance flux therefore for the determination ofSNR the inverse square law measurements (Subsec-tion 3.C) with and without the attenuation filterwere used. At each distance ten measurements usinga number of ITs (14) were performed, and the spec-tral SNRwas calculated. Because this quantity is notrepresentative as the lamp’s irradiance increaseswith wavelength we limit the characterization ofnoise level in the dependence of SNR from IT. Thelogarithm of the SNR and the dark and stray-lightcorrected signal expressed in counts per second(Scps) at each IT are linked through a second-orderlinear fit. Based on these polynomials the SNR ateach IT was calculated for the measured range ofScps (Fig. 9). As expected, SNR increases as IT andmeasured signal (Scps) increases. SNR ranges be-tween 2.5 and 770 when Scps takes values from 1to 1200. The signal’s range from the lamp measure-ments covers more of the corresponding range duringdirect Sun measurements (up to 1500 counts=s),which are performed with ITs up to 100ms. Forradiance measurements, excluding an area of 10°around the Sun, the signal Scps may reach the valueof 80 while the ITs used range from 200 to 5000ms.The calculated values of SNR cover the variability ofthe signal during atmospheric measurements andalthough not all ITs have been characterized, it couldbe concluded that the SNR exceeds the value of 2 forall ITs and Scps.

100

101

102

103

100

101

102

Signal (counts/sec)

SN

R

10 20 30 40 50 60 70 80 90100200300400500

IT (ms)

Fig. 9. (Color online) SNR as a function of the measured signalexpressed in counts per second for IT ranging from 10 to 500ms.Colored lines correspond to polynomial fits on the data for differentintegration times (actual data are shown only for IT ¼10ms).

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1601

E. Noise Equivalent Irradiance and Radiance

The noise equivalent irradiance (or radiance), Ne, ofthe instrument corresponds to the lowest detectablevalue of irradiance (or radiance) at a wavelength λand is represented by the ratio of the standard devia-tion (Σ) of the dark current to the responsivity of theinstrument Rðλ; ITÞ) [25]. The Ne preserves the spec-tral characteristics of the calibration function as thewavelength dependence of Σ is negligible (Subsec-tion 3.A). Moreover, Ne preserves the dependencyof Σ, as far as the IT and temperature are concerned,as the response function is considered independent oftemperature and has linear correlation with the IT.In Figure 10 shows the detection limit for the max-imum IT used for atmospheric direct irradiance andsky radiance measurements.

F. Slit Function

The slit function of the CCD spectrograph has a tri-angular shape throughout the operational spectralrange, but the FWHM is wavelength dependent. Ameasurement of the slit function throughout theoperational wavelength range of the CCD spectro-graph was performed using quasi-monochromaticlight produced by a 1000WXe lamp and a Jobin Yvondouble monochromator. The monochromator isequipped with two 1200 lines=mm holographic grat-ings blazed at 330nm. The width of the entrance andexit slits of the monochromator was set to 0:25mm,resulting in a triangular slit function with FWHM of0:45nm. The effect from the finite bandwidth of theoutput beam was taken into account through a de-convolution process. The FWHM of the CCD spectro-graph was found to vary between approximately 1.2and 3:7nm and is shown in the left panel of Fig. 11 asa function of wavelength. On average, the FWHMwas found to be 1:8� 0:09nm in the UV-B, 1:93�0:22nm in the UV-A, and 2:65� 0:17nm in the visi-ble. Above 780nm the FWHM increases rapidly. Inaddition to the monochromator, measurements ofthe slit function were performed with Hg and Cdspectral lamps and a He–Cd laser source with cen-

tral wavelength at 325:15nm. The slit function mea-surements using the laser at 325:15nm and theoutput from the double monochromator at two wave-lengths corresponding to the two pixels either side of325:15nm are shown in the right panel of Fig. 11.The results from both methods are generally in goodagreement, as far as it concerns the shape of the slitfunction near the maximum. The weak intensity ofthe radiation from the monochromator increasesthe noise at the wings of the slit function. The disad-vantages of using spectral lamps for the wavelengthcalibration of CCDs is the limited availability of thespectral lines, their uneven distribution over theinstrument’s wavelength range, and the mismatchof the central wavelength of the spectral lines withthe central wavelength of the CCD pixels [31].

G. Uncertainty Analysis

The overall uncertainty in direct Sun and radiancemeasurements is the square root of the sum ofsquares of the different error components arisingfrom the calibration procedure, the dark-signal andstray-light correction, and for the direct measure-ments, the discrepancies in tracking the Sun. Inthe following more detailed presentation of the errorcomponents, the uncertainties correspond to the con-fidence level of one sigma (68.3%), unless statedotherwise.

The uncertainty of the absolute calibration proce-dure followed at the LAP is estimated to ∼1% (E1),which is the combined uncertainty of the stability ofthe lamp current, the stray light in the dark room,and the effects from the variations in the dark roomambient temperature. The uncertainty of the lamp’sspectral irradiance quoted by the supplier is 2.25%,2%, and 3%, respectively, for the wavelength bands of300–400, 400–800, and 800–1000nm (E2). The corre-sponding mean uncertainty in the spectral reflectiv-ity of the reflectance plate is 2:25%� 0:1% for thewhole operational range (E2.1). As discussed in Sub-section 3.C, the uncertainty in the spectral determi-nation of the reference distance x would induce anuncertainty in the calibration function of the instru-ment of 2.04% between 300 and 320nm, and 0.90%for λ>320nm, for a distance of 182 cm. In addition, apossible positive offset in x (þ1:4 cm) due to the pre-sence of the attenuation filter would result in an ad-ditional error of 1.5%. Thus the maximum combineduncertainty arising from the determination of x (E3)is 2.53% for the UV-B, and 1.75% in UV-A to NIR. Thecalibration function of the instrument has a statisti-cal noise caused by the weak intensity of the stan-dard of spectral standard that is used (1000WDXW lamps). The average noise level is 2.4% be-tween 300 and 400nm and 0.15% in the visibleand NIR regions (E4). The noise is removed bysmoothing the calibration function with a polynomialspline fitting, but the uncertainty is inherited in thecalibration function. For the radiance measurementsa 5% uncertainty arising from the calibration setuphas to be considered (E3.1). Finally, if a different

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

wavelength (nm)

De

tect

ion

Lim

it (m

W.m

-2.n

m-1

)

300 400 500 600 700 800 900 10000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

De

tectio

n L

imit (m

W.m

-2.nm

-1.sr -1)

IT = 100ms

IT = 5000 ms

Fig. 10. (Color online) Detection limit for direct irradiance mea-surements (blue axis–line) and sky radiance measurements (lightgreen axis–line).

1602 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

reference wavelength is selected for the measure-ment Sc

close, the resulting differences in the responsefunctions would be of the order of�0:2% (E5), as longas this wavelength lays in the region of 550–750nm,where the SNR is high and the wavy feature ofthinned back-illuminated CCD is absent.The wavelength dependent uncertainty arising

from the stray-light correction (E6) could be quanti-fied through the residuals of the polynomial fit (seeSubsection 3.A) that introduces a 15% mean uncer-tainty on the calculated stray light. Combining theabove percentage with the results shown in Fig. 6,we calculated an uncertainty up to 1.5% and 7.5%for the UV-B range and for SZAs lower and higherthan 70°, respectively, and a negligible (0.03%) uncer-tainty for higher wavelengths. The errors associatedwith the assumption of linearity (E7) of the detector

were found to be 0.69% (averaged over the threeintegration times used) for the spectral range of320–1000nm (Subsection 3.B).

Finally the error arising from the tracking discre-pancy of the direct Sun optics was quantified byinvestigating the variability of direct irradiancemeasurements between two updates of the pointingposition during clear sky conditions. The set of mea-surements used for the analysis covers the wholediurnal course (SZA varied from 20° to 78°) and re-vealed an error of 0.36%. It has to be mentioned thatthe error is rather small and most probably reflectsthe statistical error of the CCD measurements, asthe same percentage of uncertainty appears in thecalibration measurements.

Based on the above discussion, which is summar-ized in Table 3, the overall uncertainty of direct and

Table 3. Summary of Uncertainty Analysis (Level of Confidence 68.3%)

Percentage Uncertainty

Source of UncertaintyDirect Irradiance Radiance

UV-B UV-A, Vis, NIRCombined

Uncertainties UV-B UV-A,Vis, NIRCombinedUncertainties

U1 Calibration facilities 2.69 2.69 E1, E2 3.52 3.52 E1, E2, E2.1U2 Calibration procedure 3.49 1.77 E3, E4, E5 5.55 5.00 E3.1, E4U3 Dark and stray light correction 7.5a, 1.5 0.03 E6 7.5a, 1.5 0.03 E6U4 Linearity assumption 1.41 0.69 E7 1.41 0.69 E7U5 Tracking accuracy and/or

measurement repeatability0.36 0.36 E8 0.36 0.36 E8

Overall uncertainty 8.8a, 4.9 3.8 10.0a, 6.9 6.2

aFor solar zenith angle >70° where Ui ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiX

iE2

i

qand overall uncertainty ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiX5i¼1

U2i

vuut .

300 400 500 600 700 800 900 10001

1.5

2

2.5

3

3.5

4

wavelength (nm)

FWH

M (n

m)

MonochromatorSpectral LampsZeiss (manufacturer)

-25 -20 -15 -10 -5 0 5 10 15 20 2510-4

10-3

10-2

10-1

100

Wavelength Offset (nm)

Nor

mal

ized

Sig

nal

Monochromator - Pixel#145Monochromator - Pixel#155

LASER - Pixel#149

Fig. 11. (Color online) FWHM of the CCD slit function as a function of wavelength, measured by three different methods (left). The solidcurve corresponds to a spline fit on the data. The slit function shape, which is assumed representative for the entire spectral range of theinstrument, was measured with a He–Cd laser at 325:15nm center wavelength, and with a Xe lamp and a monochromator at 322.06 and330:01nm (right).

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1603

radiance measurements of the CCD is 5% and 7% inthe UV-B region and 4% and 6% in UV-A, visible, andNIR, respectively. The error in the UV-B region,especially for a SZA greater than 70° (10%), is ratherlarge for irradiance measurements, but one shouldconsider that in some applications, e.g., the aerosoloptical depth (AOD) retrieval; the uncertainty inthe product is much smaller, of the order of 3%.

4. Atmospheric Measurements: Validation

A. Atmospheric Measurements

The atmospheric measurements, direct Sun irradi-ance, and sky radiance, with the CCD spectrographare performed and processed according to the follow-ing procedure. Each spectrum is constructed by com-bining two or three spectra measured with differentITs. This procedure results in higher SNRs in spec-tral regions where either the instrument’s sensitivityor the radiation levels are small. In addition, it elim-inates the saturated signals in the spectral range of∼400–600nm, where the instrument’s sensitivity ismaximum. As the CCD signal saturates at ∼64,000counts, any measurements exceeding 55,000 countsare discarded, together with those at the 15 neigh-boring pixels, which could have been influencedby the high signals. A typical set of integration timesfor direct Sun measurements is 30, 50, and 100ms,while for radiance measurements it is 2 and 4 s forSZAs smaller than 75° and 5 s for larger ones. Thescans are first corrected for dark current and straylight and then are combined to form a single scanby selecting for each pixel the highest signal. Finally,the appropriate response function, Rðλ; ITÞ, is ap-plied to derive the absolutely calibrated spectrum.

B. Validation

The CCD measurements in the wavelength range of300–500nm were validated by comparison to spec-tral direct irradiance measurements with the Ben-tham DTM300 scanning spectroradiometer of theMedical University of Innsbruck during the Strato-spheric-Climate Links with Emphasis on the UpperTroposphere and Lower Stratosphere (SCOUT–O3)campaign held at Thessaloniki from 12 to 24 July2006. This instrument has participated in many ex-perimental campaigns during the past 15 years andproved to be one of the most stable and reliable UVspectroradiometers operating in Europe (e.g.,[32,33]). Details about this instrument and on the as-sociated calibration and quality control procedurescan be found in Huber et al. [5].Due to differences in the slit functions of the two

instruments, all spectral measurements were pro-cessed through the SHICRIVM algorithm [34], whichcorrects possible wavelength shifts and transformsthe measured spectra to standardized ones, with aspectral resolution of 2nm FWHM (triangular slitfunction) and a wavelength step of 0:5nm.In total 68 direct irradiance spectra recorded un-

der cloudless skies and covering a range of SZAs from

20° to 70° were compared. A complete spectral mea-surement with the Bentham spectroradiometertakes a few minutes, while the CCD records a largenumber of spectra during this time interval. To makethe spectral measurements from the two instru-ments directly comparable and synchronous, a com-posite spectrum is created from all the CCD spectrathat were recorded during the period required to ac-quire a scan with the Bentham spectroradiometer.The irradiance at each wavelength of the compositespectrum is the average of the CCD-derived spectralirradiances that fall within the time period requiredfor the specific wavelength to be measured by theBentham spectroradiometer. In Figs. 12 and 13 theratio at seven wavelengths and the spectral ratioof the two instruments, respectively, are presented.The average spectral agreement is at the level of0.5% for wavelengths higher than 320nm (1.7% in300–320nm, 0.1% in the UV-A, and 0.9% in the visi-ble range of the spectrum). In addition there is aspread of �3%, which can be attributed to synchro-nization failure under the presence of thin clouds infront of the Sun. Moreover, there is no evident pat-tern in the diurnal course of the ratio especiallyfor wavelengths greater than 310nm. For wave-lengths smaller than 310nm and SZA > 60° thereare differences of the order of 10% to 15%, whichwas expected as the stray-light contribution is inmost cases higher than 10% (Subsection 3.A).

Differences in the calibration standards used onthe two instruments were also considered. Measure-ments of both calibration lamps in the laboratory re-vealed a difference of 2%� 0:8% in the wavelengthrange of 300–500nm, which result in overestimationof the CCD measurements. No correction factor hasbeen applied to the data as these differences arewithin the uncertainty of the calibration standards.The validation of the CCD measurements for wave-lengths higher than 500nm and for the same timeperiod was performed by comparing the aerosol opti-cal depth values with a collocated Cimel instrument,which is part of AERONET (Subsection 4.C).

C. Aerosol Optical Depth

The retrieval of the total column of the AOD wasbased on the methodology described by Marenco

3 6 9 12 15 180.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

TIME (hours,UT)

RA

TIO

CC

D / B

enth

am

310nm320nm340nm380nm420nm450nm480nm

centerwavelength (nm)

Fig. 12. (Color online) Diurnal variation of direct irradiance ra-tios at seven selected 5nm wide wavelength bands, as measuredby the CCD and the Bentham instruments at Thessaloniki be-tween 14 and 24 July 2006.

1604 APPLIED OPTICS / Vol. 47, No. 10 / 1 April 2008

et al. [1]. The calibrated direct irradiance spectralmeasurements, standardized to a 2nm FWHM trian-gular slit function with the SHICRIVM algorithm[34] in the range of 300–500nm in steps of 0.5 andthe ATLAS 3 extraterrestrial solar spectrum [35]convolved with the same triangular slit functionwere used to derive the total extinction optical depth.From this, the Rayleigh optical depth, calculatedaccording to Hansen and Travis [36], and the ozoneand the SO2 optical depths were subtracted to inferthe AOD. Both the ozone and SO2 columns were mea-sured by the collocated Brewer spectroradiometer.For the ozone absorption cross sections we used thoseby Bass and Paur [37] and for the SO2 those byVandaele et al. [38]. For the calculations of AODfor the comparison with the Cimel sunphotometerthe synthetic extraterrestrial spectrum by Guey-mard [39] was convolved with the slit function ofthe CCD to account for wavelengths longer than500nm.Figure 14 shows the comparison of AODs as de-

rived from the CCD system, a Bentham DTM300spectroradiometer and a Cimel sunphotometer. Morespecifically, the CCD-derived AOD was comparedwith 68 synchronous measurements with the Ben-tham and 110 synchronous measurements withthe Cimel. The correlation coefficient between theCCD and the Bentham AOD ranges between 0.99and 0.94, while for the CCD and the Cimel between0.99 and 0.90, suggesting good agreement among thethree instruments. The mean absolute differencesrange between 0.06 in the UV-B and 0.01 in the visi-ble wavelengths.The good agreement found between the CCD and

the other two instruments for both the spectral directirradiance measurements and the retrieved spectralAOD, verifies the capability of the CCD instrumentto perform reliable measurements down to a wave-length of at least 310nm.

5. Conclusions

In this study we have presented a detailed character-ization of the new CCD spectroradiometer that wasestablished at the Laboratory of Atmospheric Phy-sics, University of Thessaloniki, which is necessaryfor assessing the quality of its measurements, aswell as preliminary comparisons with measurementsfrom other types of instrument. The absolute sensi-tivity of the instrument for direct Sun and skyradiance measurements was calculated with anuncertainty of 4.4% and 6.6% in the UV-B and 3%and 6% in the UV-A, visible, and NIR wavelengthranges, respectively. The noise equivalent irradianceand radiance were determined expressed by thedetection thresholds of the instrument for themaximum integration time that is used for measure-ments in the UV and NIR. For the direct irradiancemeasurements the detection threshold was foundto be 10−4 Wm−2 nm−1 at 300nm and 2:7 ×10−4 Wm−2 nm−1 at 1000nm; while for the radianceit was found to be, respectively, 1:5 × 10−4 and2:7 × 10−4 Wm−2 nm−1 sr−1. The noise associatedwith the measured signal was quantified throughthe signal-to-noise ratio (SNR), which was found toincrease with integration time and radiation level.It ranges between 2.5 and 770 for signals rangingbetween 1 and 1200 counts=s.

The signal offset introduced by the dark currentand the stray light was investigated as a functionof the temperature of the CCD, the integration time,and the level of the measured signal. The dark cur-rent increases with temperature. The stray-light con-tribution was found to be significant for wavelengthsshorter than 340nm.

The overall uncertainty associated with the directSun and sky radiance measurements is of the orderof 5% and 7%, increasing to 10% for weak signals(SZA > 70°) and 4% and 6% in the UV-A, visibleand NIR, respectively.

Measurements of direct solar spectral irradiancefrom an independently calibrated spectroradiometer(Bentham DTM300) were compared with those ofthe CCD instrument. For solar zenith angles (SZAs)

0.2 0.4 0.6 0.8 1 1.2

0.2

0.4

0.6

0.8

1

1.2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

0.2

0.4

0.6

0.8

1

1.2

Aerosol Optical Depth – CCD

Aer

osol

Opt

ical

Dep

th 310320340380420450480

wavelengths (nm)Bentham

675870

Cimel

Fig. 14. (Color online) Comparison of aerosol optical depth atnine wavelengths (310, 320, 340, 380, 420, 450, 480, 675, and870nm) retrieved from direct irradiance measurements withthe Bentham, the Cimel, and the CCD instruments during theSCOUT-O3 campaign (12–24 July 2006) at Thessaloniki, Greece.

300 320 340 360 380 400 420 440 460 480 5000.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

wavelength(nm)

RA

TIO

CC

D / B

enth

am

NBSpectra = 68

Range of values5/95th percentileMedian

Fig. 13. (Color online) Median of the spectral ratio of 68 directirradiance measurements by the CCD and the Bentham instru-ments at Thessaloniki between 14 and 24 July 2006. The greenenvelope represents the extreme values and the red lines the5th and 95th percentiles.

1 April 2008 / Vol. 47, No. 10 / APPLIED OPTICS 1605

ranging between 20° and 70°, they were found toagree within 0:5%� 1:1% and 4:2%� 2%, respec-tively, in the wavelength ranges of 310–500 and305–363nm. The agreement of the two instrumentsin aerosol optical depth (AOD) measurements is∼0:02� 0:02 in the range of 315–500nm. Significantcorrelation coefficients were found, ranging between0.99 in the range of 340–360nm and 0.94 at 500nm.Results of similar quality were also found by compar-ing the spectral AOD derived by a collocated Cimelsunphotometer. A more thorough validation of theCCD system is ongoing within the framework of acollaborative project with four other EuropeanInstitutes operating similar instruments, the resultsof which will be published separately. The accuratecharacterization and a detailed uncertainty analysisof the instrument will facilitate its use for applica-tions other than retrieval of the AOD. Spectral directSun and radiance measurements could also be usedfor the retrieval of trace-gas columns, which isplanned for the near future.

This work has been conducted partly within theframework of the project SCOUT-O3 funded by theEuropean Commission. The authors thank JulianGröbner for his substantial help in the developmentof the CCD instrument and the two anonymousreferees for their work on the manuscript. Finally,we thank S. Rapsomanikis, PI of ThessalonikiAERONET site.

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