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Diode-array UV solar spectroradiometer implementing a digital micromirror device
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2014 Metrologia 51 S289
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Bureau International des Poids et Mesures
Metrologia
Metrologia 51 (2014) S289–S292
doi:10.1088/0026-1394/51/6/S289
Diode-array UV solar spectroradiometer
implementing a digital micromirror device
A Feldman1,2 , T Burnitt3 , G Porrovecchio2 , M Smid2 , L Egli4 , J Gröbner4
and K M Nield5
1
National Institute of Standards and Technology, Boulder, Co, United States of America
Cesky Metrologicky Institut, Prague, Czech Republic
3
Principal Optics, Reading, UK
4
Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Dorf,
Switzerland
5
Measurement Standards Laboratory of New Zealand, Callaghan Innovation, Lower Hutt, New Zealand
2
Received 9 June 2014, revised 11 August 2014
Accepted for publication 21 August 2014
Published 20 November 2014
Abstract
The solar ultraviolet spectrum captured by commercially available diode-array
spectroradiometers is dominated by stray light from longer wavelengths with higher intensity.
The implementation of a digital micromirror device in an array spectroradiometer has the
potential to enable the precise selection of desired wavelengths as well as the ability to reduce
spectral intensity of some wavelengths via selective mirror modulation, both reducing long
wavelength stray light. A prototype consisting of off-the-shelf components has been assembled
to verify the validity of the base concept, and initial measurements have been performed to
confirm the throughput and image qualities such as spectral resolution and astigmatism.
Keywords: spectroradiometer, digital micromirror device, ultraviolet radiation
(Some figures may appear in colour only in the online journal)
Introduction
Diode-array spectroradiometers provide a low-cost and effective alternative to expensive scanning double monochromators,
yet their dynamic range is insufficient to accurately measure
ultraviolet (UV) radiation. While diode-array spectroradiometers can complete the acquisition of the entire UV solar spectrum in a few seconds, the portion of the spectrum below
320 nm is dominated by the stray light signal. The stray light
originates from high-intensity radiation at longer wavelengths,
which affects the signal at the detector pixels for wavelength
regions with low irradiance levels. The high dynamic range
of atmospheric solar UV radiation, approximately 6 orders of
magnitude [1], results in considerable bias of the low intensity
at around 290 nm to 320 nm measurements with conventional
array spectroradiometers.
An earlier theoretical study [2] of commercial spectroradiometers demonstrated a four order of magnitude stray light
contribution from wavelengths above the UV range of interest
(290 nm to 440 nm), which would interfere with an accurate
UV spectral intensity measurement. The same study proposed
0026-1394/14/060289+04$33.00
that implementing a digital micromirror device (DMD) could
significantly reduce the impact of stray light using two techniques. The first involves leveling the dynamic range of the
incoming radiation by selective wavelength modulation [2, 3]
where the undesired wavelengths are modulated at high frequency, creating an effective repetition rate, reducing their intensity and stray light contribution. The second method is to
select a precise range of wavelengths using the DMD as an
effective bandpass filter [2].
While the use of DMDs in spectroradiometer applications
has been previously explored for visible light [4–6] and
infrared applications [7], in this study a novel micromirror
diode-array UV spectroradiometer (µ-MUV) was assembled
from off-the-shelf components to demonstrate the validity
of the modelled stray light reduction concepts in the UV
range. Instrument specific attributes such as spectral resolution
and slit function were characterized.
In addition, the
impacts of the DMD modulation techniques, stray-light
spectral filters and physical intra-instrumental baffling were
quantified.
S289
© 2014 BIPM & IOP Publishing Ltd Printed in the UK
Metrologia 51 (2014) S289
A Feldman et al
Figure 1. Optical design schematic of the µ-MUV prototype composed of four spherical mirrors (M1–M4), a 600 G mm−1 diffraction
grating, DLP micromirror chip, and CCD detector array.
Experimental setup
Results
Initial modelling of the µ-MUV prototype using optical design
software was used as the basis for the experimental design
[8]. The optical design was carried out with the main aim to
preserve throughput of the optical system and to optimize its
spectral resolution. Figure 1 shows the key elements of the
model and subsequent prototype composed of a plain ruled
600 G mm−1 diffraction grating, a 1024 pixel × 720 pixel
extended graphics array (XGA) digital light processor (DLP)
micromirror chip [9], a back-lit 2048 pixel × 250 pixel charge
coupled device (CCD) detector array [10], and four spherical
mirrors (M1–M4).
Light is introduced into the system via a 100 µm core
optical fibre with subsequent collimation onto the grating by
the first spherical mirror (M1). The reflected wavelength
spectrum is focused via the second spherical mirror (M2) onto
the DLP, where the incident light is normal to chip face. The
DLP operates along a 16.7◦ angle where the ‘on’ position
projects the spectral image horizontally and vertically at the
same angle. Similarly, the ‘off’ position projects the image
at an identical opposing angle down and away. The ‘float’
position is unpowered and acts as an ordinary flat mirror. Due
to this angular functionality, utilization of the DLP requires
a second plane of operation that commercially available
spectrometers cannot currently accommodate. The projected
light is collimated and then focused onto the CCD array by
the last two spherical mirrors (M3 and M4). The astigmatism
generated by the use of off-axis spherics is exploited by binning
the CCD camera’s vertical pixels, effectively using it as a onedimensional array.
In comparison to the model, the technical implementation
of the µ-MUV prototype varied slightly due to real-world
conditions and available equipment. The modelling accounts
for the physical dimension of the CCD detector, but not
the accompanying housing and equipment. Therefore, slight
angular adjustments of M3, M4, and the CCD detector were
made to preserve the normal incidence and spectral image on
the detector. Additionally, due to availability, a 200 µm core
optical fibre was used in place of the modelled 100 µm core
optical fibre as the source. Lastly, an actively-cooled CCD
detector would be used, but owing to availability, an air-cooled
alternative was utilized.
The whole DMD operates as a plane mirror when all pixels
are configured in the ‘on’ position and determination of device
specific parameters such as spectral range, spectral resolution,
and bandwidth is possible. The spectral range is tunable using
the angular adjustment of the grating. The angle was then
adjusted to cover the range of 270 nm to 425 nm using the
characteristic peak spectra of a mercury pen lamp to calibrate
the wavelength range. The model intends on the image to
be 1 : 1 on the CCD detector from the DMD, yet this limits
the spectral range and usable space on the CCD because the
DMD device uses 1024 pixels to the 2048 pixels of the CCD
detector array. Due to this size mismatch, the spectral image
was centred on the detector and causes the spectrum truncation
effect seen in this study. As seen in figure 2(a), using a 407 nm
laser, the full width half maximum (FWHM) of the slit function
of the spectroradiometer was determined to be 2.5 nm. This
measurement was achieved by varying the integration time
of the CCD array until saturation, where the slit function is
determined based on the barely saturated spectrum. The model
predicts a 1 nm bandwidth using a 100 µm core optical fibre,
implying that employing the smaller specified core diameter
fibre could improve the bandwidth to 1.25 nm. As with most
array spectrometers, the diode-array is placed at the focal plane
of the incoming light and the spectral resolution capability is
determined by horizontal CCD pixel density. Based on the
incident spectral range and number of pixels illuminated, the
spectral resolution of the CCD array was determined to be
0.2 nm per pixel.
As the array detector bins all incoming light, any stray light
is incorrectly interpreted as spectral intensity. Stray light is
generated within commercial spectroradiometers by unwanted
reflection off optics, and can be introduced by both internal and
external elements [11]. For the µ-MUV prototype, the DMD
chip itself generates stray light due to the array-like nature of
the micro-mirrors with a 13.68 µm mirror pitch [9] in addition
to typical stray light sources. To identify the spectral range
of stray light incident on the CCD detector a tungsten halogen
white light source and Schott glass filters were placed in front of
the detector. According to their specifications, high-pass filter
UV360 allows wavelengths greater than 360 nm; bandpass
filter B38 allows the range from 320 nm to 700 nm; and
S290
Metrologia 51 (2014) S289
A Feldman et al
(a)
(b)
Figure 2. (a) Using a 407 nm laser, the slit function of the spectroradiometer was determined to be 2.5 nm using the FWHM of varying
integration periods. (b) Using various filters, the majority stray-light contribution was determined to be above the spectral range of
interest.
(a)
(b)
Figure 3. (a) Half of the DMD was modulated in the DMD ‘on’/‘off’ positions at specified duty cycles to demonstrate dynamic range
leveling and stray light reduction. (b) Modulation of longer wavelengths by DMD switching with B05 filter allowing only UV.
bandpass filter B05 permits the range from 340 nm to 540 nm.
Figure 2(b) demonstrates that majority source of stray light is
due to light greater than 550 nm as both bandpass filters data
sets have considerably lower stray-light contribution indicated
by lower noise floor. The stray light above 550 nm can best
be attributed to the spectral image incident on the DLP chip,
where the UV region of interest for this study is focused on
the center of the chip, and longer wavelengths extend past the
edge of the DMD.
To eliminate the scattering effects of the edges of the DLP
chip, physical baffling was implemented to reduce this source
of stray light. As noted earlier, the ‘float’ position of the DLP
acts as a flat mirror, scattering light that is not actively projected
along the desired path. Furthermore, the spectral range that
falls outside the DMD is scattered by the device housing.
Rudimentary baffles were installed throughout the system
including the edge of the DMD housing where the infrared
radiation was scattered and on the ‘inside’ edge of the CCD
detector. Such efforts were rewarded with an approximate 30%
broadband reduction in stray-light contribution, by comparing
measurements of intensity with and without baffles.
Lastly, to demonstrate the intended DMD concepts, basic
light-modulation techniques were implemented for initial
stray-light reduction attempts, where half of the DMD was
modulated at various duty cycles. For this test, the same
tungsten halogen white light source was used and the portion
of the DMD chip with longer wavelengths was chosen to
flatten the dynamic range and reduce stray light in the shorter
wavelength region. Duty cycles ranging from 5% to 100%
are shown in figure 3(a), and as expected the intensity of
the modulated region decreases proportionally with the duty
cycle. As the duty cycle decreases however, the range in the
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Metrologia 51 (2014) S289
A Feldman et al
‘on’ portion from 290 nm to 350 nm subsequently decreases
by a fraction of a percent. It is interesting to note that while
the contribution in this range does demonstrate some stray
light reduction, the reduction in amplitude of the background
signal (430 nm to 450 nm) is more pronounced. As noted with
the brief filter study, the majority of the stray light comes
from longer-wavelength radiation, so a large improvement
in signal to noise ratio was not expected, yet the concept of
DMD modulation to level the dynamic range still holds true.
Another interesting feature is the nominal 10 nm roll-off, which
further enforces the bandwidth study. Furthermore, utilizing
the B05 filter and the modulation techniques demonstrates that
indeed there is still a large stray light contribution from longer
wavelength light, yet the modulation does indeed reduce the
scattered light as seen on the edge of the DMD at wavelengths
430 nm and above.
Conclusion
A prototype solar UV spectroradiometer implementing a DMD
was constructed from off-the-shelf components. Basic device
parameters such as bandwidth, spectral range, and spectral
resolution were measured. In comparison to commercially
available spectroradiometers, the µ-MUV prototype has a
wider slit function and poorer spectral resolution, but is capable
of reducing stray light via modulation techniques as predicted.
Initial results indicate that dynamic range leveling can suppress
stray-light contribution in the UV range of interest, yet further
testing is needed.
There is a considerable amount of future work to attain
the specifications outlined in the in the Global Atmosphere
Watch (GAW) report no.
191 [1] and the European
Metrology Research Program (EMRP) project goals. The
second stray-light reduction technique of using the DMD
to select and raster the spectral range selection needs to be
demonstrated. Additionally, improvements on the prototype
such as; replacing the detector with an actively cooled model
will reduce dark currents and improve the dynamic range
detection capabilities; removal of the thin window on the
DLP enclosure will eliminate unnecessary UV absorption and
improve the UV signal; and a new light-tight enclosure has
also been fabricated, which will reduce external contributions
to stray light.
Acknowledgments
Part of this work has been supported by the European
Metrology Research Programme (EMRP) within the joint
research project Traceability for Terrestrial Solar UV
Irradiance Measurement (Solar UV). The EMRP is jointly
funded by the EMRP participating countries within
EURAMET and the European Union.
The authors
acknowledge support from NIST and CMI.
Contribution of an agency of the US government; not
subject to copyright.
References
[1] Seckmeyer G, Bais A, Bernhard G, Blumthaler M, Drüke S,
Kiedron P, Lantz K, McKenzie R and Riechelmann S 2010
GAW Report No . 191, Instruments to measure solar
ultraviolet radiation 4. Array spectroradiometers Technical
Report 191 World Meteorological Organization—Global
Atmosphere Watch
[2] Egli L, Grobner J, Smid M, Porrovecchio G, Burnitt T,
Nield K M, Gibson S, Dubard J, Nevas S and Tormen M
2013 New technologies to reduce stray light for measuring
solar UV with array spectroradiometers AIP Conf. Proc.
1531 825–28
[3] Rice J P, Neira J E, Kehoe M and Swanson R 2009 DMD
diffraction measurements to support design of projectors for
test and evaluation of multispectral and hyperspectral
imaging sensors Proc. SPIE Emerging Digital Micromirror
Device Based Systems and Applications 7210 72100D
[4] Meyer R D, Kearney K J, Ninkov Z, Cotton C T, Hammond P
and Statt B D 2004 Proc. SPIE 5492 200–19
[5] Spudich T M, Utz C K, Kuntz J M, Deverse R a,
Hammaker R M and McCurdy D L 2003 Appl. Spectrosc.
57 733–6
[6] Wagner E and Smith B 1995 Appl. Spectrosc. 49 1715–9
[7] Beasley D and Bender M 2001 Proc. SPIE 4366 96–102
[8] Burnitt T 2012 Optical layout for prototype DLP spectrometer
Technical Report Principle optics
[9] Texas Instruments Inc 2013 DLP 0.7 XGA 2xLVDS type A
DMD technical data sheet Technical Report June Texas
Instruments Inc (www.ti.com/lit/ds/symlink/dlp7000.pdf)
[10] Hamamatsu Inc 2012 CCD area image sensor technical data
sheet Technical Report Hamamatsu Photonics K.K., Solid
State Division Hamamatsu City, Japan
[11] Zong Y, Brown S W, Johnson B C, Lykke K R and Ohno Y
2006 Appl. Opt. 45 1111–9
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