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This paper describes the methodology developed for characterizing a commercial charge-coupled device (CCD) camera as a luminance meter for analyzing lighting systems and especially for measurements in road light plants. Today, several luminance meters based on commercial CCD cameras are on the market. They are very attractive for the lighting engineer: The availability of a high number of closely spaced small detectors (pixels) on a single chip permits analyses almost impossible with a traditional luminance meter. These commercial-industrial CCD cameras are sold at prices lower than scientific grade ones. They are factory equipped with a dedicated filter to reach the correct photopic sensitivity V(λ), and they are factory calibrated in luminance SI units. The main counterparts in using these cameras are in the difficulties to define the measurement accuracy and the influence of the environment luminance on the measured values of the framed scene, in the low resolution of their A/D converter (usually 8 or 12 bit), and the higher noise level (usually the CCD chip is not cooled). To reach the measurement accuracy required by lighting norms, it is necessary to characterize metrologically a camera and quantify all the possible external influences which could degrade its performances, in real measurement situations, and which could affect the measurement results. A carefully controlled measurement set up and operating procedure could limit the causes of errors and improve the accuracy of measurements obtained in operating conditions. In this way, the measurement uncertainties might be evaluated completely, and considerations on the results could suggest particular operating practices to limit the causes of error due to measurement setup and environmental conditions.
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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 1, FEBRUARY 2005 171
Characterization and Calibration of a
CCD Detector for Light Engineering
Pietro Fiorentin, Paola Iacomussi, and Giuseppe Rossi
Abstract—This paper describes the methodology developed
for characterizing a commercial charge-coupled device (CCD)
camera as a luminance meter for analyzing lighting systems and
especially for measurements in road light plants. Today, several
luminance meters based on commercial CCD cameras are on the
market. They are very attractive for the lighting engineer: The
availability of a high number of closely spaced small detectors
(pixels) on a single chip permits analyses almost impossible with
a traditional luminance meter. These commercial-industrial CCD
cameras are sold at prices lower than scientific grade ones. They
are factory equipped with a dedicated filter to reach the correct
photopic sensitivity V , and they are factory calibrated in
luminance SI units. The main counterparts in using these cameras
are in the difficulties to define the measurement accuracy and the
influence of the environment luminance on the measured values
of the framed scene, in the low resolution of their A/D converter
(usually 8 or 12 bit), and the higher noise level (usually the CCD
chip is not cooled). To reach the measurement accuracy required
by lighting norms, it is necessary to characterize metrologically
a camera and quantify all the possible external influences which
could degrade its performances, in real measurement situations,
and which could affect the measurement results. A carefully con-
trolled measurement set up and operating procedure could limit
the causes of errors and improve the accuracy of measurements
obtained in operating conditions. In this way, the measurement
uncertainties might be evaluated completely, and considerations
on the results could suggest particular operating practices to limit
the causes of error due to measurement setup and environmental
conditions.
Index Terms—Blooming, calibration, charge-coupled device
(CCD), dark current, defocusing, linearity, luminance meter.
I. INTRODUCTION
THE STIMULUS received by retinal photoreceptors of the
human eye is related to a photometric quantity called lumi-
nance, i.e., the luminous intensity per unity area. Relative lumi-
nance differences on contiguous surfaces allow the perception
of object details. Therefore, luminance plays a leading role in
lighting engineering, determining the quality of lighting systems
and conditions both in indoor and in external environments.
Traditional luminance meters are only able to evaluate the
mean luminance of a surface viewed from a given direction [1].
The devices considered here are charge-coupled device
(CCD) luminance meters, where an optical system focuses the
framed surface on the CCD matrix. Any single pixel can be
Manuscript received June 15, 2003; revised June 15, 2004.
P. Fiorentin is with the Dipartimento di Ingegneria Elettrica, Università degli
Studi di Padova, 35131 Padova, Italy (e-mail: pietro.fiorentin@unipd.it).
P. Iacomussi and G. Rossi are with the Dipartimento di Fotometria, Isti-
tuto Naionale Elettrotecnio “Galileo Ferraris,” 10135 Torino, Italy (e-mail:
iaco@ien.it, rossig@ien.it).
Digital Object Identifier 10.1109/TIM.2004.834055
associated with an element of the surface under measurement
whose uniformity can be evaluated.
A CCD-based luminance meter could evaluate the luminance
of several small portions of a surface (“point” luminance) and
for several directions of observation: Large scenes can be cap-
tured and converted to digital photometric information [2].
CCD detectors are integrating devices: the number of charges
stored in each pixel is directly proportional to exposure time and
to array irradiance. To evaluate the luminance in SI units [can-
dela per square meter], the camera must be equipped with an ad
hoc filter to obtain a spectral sensitivity of the device equal to the
spectral luminous efficiency [3] V (i.e., the normalized spec-
tral sensitivity of the standard observer used as a weight factor
for photometric quantities) and calibrated against a luminance
standard source.
A complete calibration procedure to verify and, if possible,
improve the performance of the CCD luminance meter, when
applied in lighting engineering, was developed and described
here. The main causes of error are described, together with their
analysis, and possible solutions in the measurement procedure,
are presented.
The methodology is a general one and was arranged at the
Istituto Elettrotecnico Nazionale (IEN) for the characterization
of scientific CCD cameras for metrological applications [4].
This paper shows some simplifications in the calibration pro-
cedure in order to reduce its cost and time when we have the
following:
commercial or industrial grade CCDs are considered;
the CCD luminance meter is mainly used for environment
characterization, i.e., road lighting plant evaluation.
The measurement examples described in this paper are ob-
tained using a commercial camera “LMK96.” The original lens
had a fixed diaphragm, the right choice to simplify the work
in the field, but a great restraint in research work. It was re-
placed with a Zeiss 50 mm photographic lens. The sci-
entific grade camera used in the development of the charac-
terization procedures was a “Photometrics 350” camera with a
1024 1024 CCD and a Nikon 50 mm lens.
II. CALIBRATION AND CHARACTERIZATION PROCEDURE
A camera calibration procedure gives for each pixel of the
CCD array the value that links the A/D converter output with the
luminance of the pixel-framed surface (luminance calibration
coefficient).
These values depend essentially on the pixel sensitivity, on
the optical system characteristic (i.e., the diaphragm) and on the
0018-9456/$20.00 © 2005 IEEE
172 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 1, FEBRUARY 2005
gain of the CCD output amplier. These operating conditions
should be recorded in the calibration certicate.
The knowledge of the calibration values is not enough to ob-
tain accurate measures because several parameters could inu-
ence the pixel reading in real measurement conditions (linearity,
spectral sensitivity, dark frame, presence of saturated pixels,
ghost images, etc.). Furthermore, the same calibration proce-
dure could introduce some discrepancies; it usually happens as
it is carried out with a source lighting the entire chip [5][7].
For these reasons, the above quoted parameters should be
evaluated to understand their inuence.
In a CCD matrix, the luminance calibration coefcients can
be different for each pixel and must be evaluated [8]. Discrep-
ancies between pixels depend mainly on three factors:
manufacturing tolerances on the pixel sensitivity over the
chip (sensitivity matrix);
optical system setup;
lens shutter position.
Due to optical reasons, for a given luminance, the optical
system produces on the CCD surface an illuminance distribu-
tion according to the fourth power of the cosine of the angle
subtended by the pixel, approximately. The real distribution de-
pends on the correction applied in the system design. Further-
more, some ghost images appear due to internal reection in the
optical system and in the CCD case [9].
A well-mounted central radial mechanical shutter does not
introduce any nonuniformity (due to its opening and closing
times) on the CCD surface illuminance distribution. Otherwise,
its contribution shall be considered [8].
The table of the luminance calibration coefcient is called the
calibration matrix; it tries to correct the inuence of all the
factors considered above. It is recorded in the CCD computer
control memory and applied to every measured frame.
Following the procedure developed at IEN for calibration of
the CCD detector used for luminance measurements in metro-
logical applications, both the calibration matrix and the pres-
ence and inuence of ghost images can be evaluated. A uniform
diffuser was lighted by a calibrated intensity lamp, the funda-
mental property of which is its stability [4].
Referring to Fig. 1, a standard illuminant A lights the cali-
brated white diffuser: Its luminance and uniformity are there-
fore known. A eld diaphragm limits the eld of view of the
camera: only groups of at least 16 pixels are lighted, but only
the four inner pixels are considered for calibration. The groups
of pixels are selected by rotating the CCD camera around the
rst nodal point of the lens system in front of the CCD matrix.
In this way, each group of four pixels can be calibrated for lumi-
nance measurements. Contemporarily, ghost images can be dis-
covered and their inuence correctly evaluated (if other pixels
appear lighted, this is due to ghost images).
The calibration could be applied for different camera condi-
tions, and the inuence of each parameter on the device perfor-
mance could be easily obtained. Usually, the procedure is re-
peated for different exposure times and lens diaphragms. For
road applications, the distance between the detector and white
standard target is 60 m, to simulate the real working conditions.
Fig. 1. Calibration set up.
This is a very time-consuming procedure: It is possible to
reduce the characterization time by considering bigger groups
of pixels, but, obviously, this brings lower accuracy.
A. Commercial CCD Camera Calibration Procedure
Commercial CCD cameras do not have such technical char-
acteristics to justify the use of such a heavy procedure.
The usual approach [6], [10] requires a at luminance source,
but three points should be considered:
the uniformity of a state-of-the-art luminance source is not
better than 1%;
the calibration is done at low distances because the source
light shall shoot all the chip pixels;
ghost images are not highlighted and their inuence is er-
roneously recorded in the calibration matrix.
The last point could be very important if the energy in ghost
images, produced by scattering in the lens system, is greater
than the device noise or resolution (the A/D converter has 8 or
10 bits); in fact, an overestimation of the sensitivity matrix is
introduced.
For the above reasons, a simplied procedure was developed.
A commercial calibrated luminance source (illuminant standard
A equipped with a diffusing device like a sphere) is used. At
a short distance, the source lights the whole CCD chip of the
camera without lens. In this way, the illuminance on the de-
tector surface is extremely uniform, and the sensitivity matrix
is obtained. Of course, the reection of the lens holder shall be
carefully avoided.
Alternatively, the camera with the lens frames the source. The
output image considers also the inuence of the lens, but the
illuminance among adjacent pixels could be considered uniform
and the sensitivity matrix is obtained.
To improve accuracy several measurements can be done by
rotating the source or the camera.
Then, the luminance source is placed 60 m from the camera
(the typical working distance in road plant measurements). In
this condition, the luminance source lights uniformly a group of
nine pixels. The camera is rotated to dene a grid of 35 equally
spaced groups of nine pixels, as in the original IEN method.
From these measurements, the inuence of the lens is obtained.
This function represents a very regular surface, without spikes,
FIORENTIN et al.: CHARACTERIZATION AND CALIBRATION OF A CCD DETECTOR FOR LIGHT ENGINEERING 173
which can be interpolated for intermediate pixels with adequate
accuracy.
The calibration matrix is then obtained by extrapolating the
results of the two sets of data and minimizing discrepancies.
In Fig. 2, a tridimensional interpolation shows these results.
The standard source used in the calibration procedure had a lu-
minance of 1035 cd m , and the camera lens aperture was .
Usually, commercial cameras, like the model here considered,
have an electronic shutter and automatic exposure time. Foreach
acquisition, the automatic exposure feature was used to reduce
noise inuence. With the same aim, the calibration results were
the mean values of three records.
III. CCD CAMERA CHARACTERIZATION:ROAD LIGHTING
MEASUREMENT CONSTRAINTS
To improve the accuracies in road lighting measurements,
the camera performances should be characterized following an
ad hoc procedure. Two constraints are very important in photo-
metric road measurements:
the luminaries use discharge lamps;
international or national norms [11] and technical docu-
ments [12] require the luminance of road surface at an ob-
servation angle of , i.e., the camera must be
at about 1.5 m over the road surface and 60 m from the
observed point.
Discharge lamps have an emitted spectrum strongly different
from the illuminant A spectrum of the source used for lumi-
nance calibration (Black Body at 2856 K). The more the total
spectral sensitivity of the system (lens, lter, and CCD) matches
the V , the less are the spectrum inuences on the luminance
results.
Commercial CCD cameras are factory equipped with a cal-
culated lter approximating the V . How close the spectral
sensitivity ts the V is stated in the data sheet: The CIE
parameter [1] must be considered in the measurement uncer-
tainty budget.
Theoretically, the output emission of road lighting lamps has
a period half of the power supply one. In Europe, the line fre-
quency is 50 Hz: Therefore, the camera integration time (ex-
posure) should be an integer multiple of 10 ms. Practically, for
some types of lamps, the light output during the negative part of
the electric energy wave could differ by several percent from the
output during the positive semiperiod. Therefore, if high-level
accuracy measurements are required, the integration time shall
be an integer multiple of 20 ms.
Luminance meter calibrations are usually carried out at short
distances, with only the calibration source in the environment.
In road lighting measurements, the lighted surfaces are at great
distances (60 m), and several lighting sources are present in the
environment. Therefore, it is necessary to consider both the ef-
fect of the difference in lens position (focus) between the cali-
bration and measurement setup and to evaluate the inuence of
ghost images (i.e., light sources in the environment, even if not
framed, can generate stray-light in the lens and on the detector
surface). To overcome the rst problem, the camera should be
calibrated in luminance units pacing the reference source at a
distance equal to 60 m. The inuence of off-axis light sources as
Fig. 2. Normalized calibration matrix.
ghost images is described in detail in the following, where some
parameters of the optical system performances are analyzed too.
The camera should also be tested for the most important quality
aspects of a CCD: linearity, blooming effect, and dark current
(dark frame). The performances are good indicators of the dif-
ferences between commercial and scientic CCDs.
IV. INFLUENCES OF THE OPTICAL SYSTEM
A. Lens Aperture
In our example, the calibration matrix was determined for
an aperture value of ; different calibration matrices should
be used for different aperture values. This means a calibration
process for every aperture value. To avoid so many calibrations,
it is possible to introduce a multiplicative factor to scale the cal-
ibration matrix at different apertures. The values of this correc-
tion factor are not constant for every pixel, but in our camera, the
differences are less than 4%. Obviously, this simplied method
lowers the global measurement accuracies of the system; in any
case, it is acceptable for the particular application we are in-
terested in, especially if the measured surface is framed in the
center of the CCD matrix.
Experimentally, a uniform luminance source is framed at
different lens apertures. For every aperture, the average of the
reading (corrected using the sensitivity matrix of Section II) of
seven pixels along the diagonal of the CCD matrix is consid-
ered. Then, each mean value is normalized referring to the value
obtained with the luminance source in the same conditions at
aperture (condition used to dene the calibration matrix).
An example of these correction values is plotted in Fig. 3.
B. Defocusing Effect
If an object is not perfectly focused on the CCD matrix, an
error arrives in measuring its luminance. As the lens aperture
denes the depth of eld, the focusing effect should be consid-
ered for all the lens apertures available.
174 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 1, FEBRUARY 2005
Fig. 3. Aperture correction coefcient normalized to 4 value versus
values.
Experimentally, the measurement is carried out with a lumi-
nance standard source at a xed distance, changing the lens fo-
cusing. In our example, to highlight the problem, a uniform lu-
minance source ( cd m ) was placed at a distance of
2.5 m from the CCD camera (Table I) and at 15 m to simulate ex-
perimental condition more convenient for lighting engineering
applications (Table II).
Toevaluate the errors due to defocusing, the mean value of the
nine central pixels is considered and compared to the value ob-
tained in the same condition but with the lens perfectly focused.
The error increases rapidly if a shorter focus distance is used
and reaches unacceptable values. If the focus distance is very
large, the luminance error remains small. Comparing different
apertures, obviously, the closer presents the smallest error.
C. Influences of Off-Axis Sources
To quantify the effect of sources not directly framed by the
camera, a light source of uniform luminance (1035 cd m)is
positioned behind a dark target on a lateral movable support.
The experimental setup is shown in Fig. 4. The luminance of
the target versus the angle between the normal to the target and
the line connecting the CCD sensor and the luminance source
(in Fig. 4) was considered. This situation is very common
during street light measurements.
The result of the test is shown in Fig. 5, where luminance
values of the target (normalized to the luminance of the source)
versus the angular displacement of the source are shown. It
is also apparent that a light source, not directly framed by
the camera, produces an illuminance on the CCD sensor. The
farther the source is from the eld of view of the detector, the
less is the effect on the CCD. This is a typical behavior in
road lighting measurements: The luminance of a target on the
road surface is measured at night in the presence of several not
framed luminaries.
V. L INEARITY CHECK
Linearity has been checked for about 37 000 pixels of a cen-
tral zone of the CCD matrix. They were excited by a source of
TABLE I
LUMINANCE ERROR CAUSED BY AN INCORRECT FOCUSING:
FOCUSING DISTANCE 2.5 M,DIAPHRAGM APERTURE
TABLE II
LUMINANCE ERROR CAUSED BY AN INCORRECT FOCUSING:
FOCUSING DISTANCE 15 M,D
IAPHRAGM APERTURE
a uniform luminance, variable from 1.2 to 40 000 cd m with a
relative uncertainty of 0.1%. Even if the optical system has sev-
eral diaphragm apertures, the linearity was veried under the
condition of . The results obtained can also be considered
partially valid for the other diaphragm values, provided that the
gain value is opportunely scaled.
To estimate the repeatability of the instrument, three frames
of the luminance source were correlated with the luminance
values of the standard source. A good response was found, as
shown in Fig. 6. To emphasize the discrepancy from linearity,
the mean square linear interpolation of the data was considered:
The maximum error (considered as the dispersion of the three
measurements) relative to the maximum measured luminance is
about 0.5%.
The errors relative to the read luminance are less than 5% if
the average linearity factor is used to interpolate the data, instead
of the mean square method. For the two approximations, Fig. 7
presents the absolute errors normalized to the maximum read
luminance versus the standard luminance.
FIORENTIN et al.: CHARACTERIZATION AND CALIBRATION OF A CCD DETECTOR FOR LIGHT ENGINEERING 175
Fig. 4. Road lighting measurements set up.
Fig. 5. Effect of light sources not included in the framing eld on the
luminance of the target.
Fig. 6. Linearity test.
VI. PIXEL SATURATION AND BLOOMING
When light shoots the CCD for a too long time, the charge ac-
cumulated into a pixel can exceed its ability of storing it (called
Fig. 7. Errors with respect to the minimum square error interpolation ( ) and
to the interpolation with the mean gain ( ).
full-well capacity). The exceeding charges move to the adjacent
pixels: This effect is called blooming. If blooming is present,
an error arrives in the evaluation of luminance: Fig. 8 shows the
same source shot at different exposition times.
This effect is particularly important when the luminance of a
dark target must be evaluated and there is a very bright surface
or source in the frame. The object is dark and a long integration
time is necessary, but because of the source, some pixels can
be saturated. This situation is quite common in street lighting
measurements. To evaluate the luminance of the scene correctly,
the quantication of the blooming effect is necessary.
A uniform luminance source of 1035 cd m was placed on a
black background and framed with the CCD camera under test.
The aperture was used to simulate a situation analogous to
street measurements (Fig. 8). Three acquisitions were recorded
for twelve different integration times. The results, as the number
of counts along a central column of the CCD matrix, are shown
in Fig. 9. The limits equal to 10%, 15%, and 25% of the satu-
ration value are also shown in the same gure. For integration
times longer than 1 ms, the effect of charge dispersion from the
central pixels toward the adjacent pixels is clearly shown and
176 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 1, FEBRUARY 2005
Fig. 8. Blooming effect depending on the integration time for the analyzed
device. Integration time 40 ms (top). Integration time 400 ms (middle).
Integration time 15 s. (bottom).
is stronger as the integration time increases. Four areas can be
identied: 1) the source circle (in the center); 2) the area where
the effect goes down to 25% of the saturation value; 3) the area
where it decreases to 15%; and 4) the area where it decreases
to 10%. The dimensions of these areas can be used to evaluate
the errors in measuring the luminance of objects close to an in-
tense source. When the integration time of the electronic shutter
is quite equal to the reading time of the CCD, other effects could
appear (see 0.1 ms in Fig. 9). This situation is not analyzed here,
the application of interest always requires an integration time
greater than 20 ms (see Section III).
VII. DARK FRAME
Even if a CCD is not exposed to a light source, the output
signal of each pixel is not zero, due to noise of both the CCD
sensor and of the acquisition electronics. The CCD image ob-
tained when the sensor is not exposed to light is called the dark
frame. The counts of each pixel can be considered only due to
the noise. Fig. 10 shows the average values of the dark frame
Fig. 9. Blooming effect.
Fig. 10. Dark current versus the integration times.
counts versus the integration time. The uniform values at inte-
gration times shorter than 1 s are due to an internal algorithm of
the control software of the CCD camera; it is able to compen-
sate partially the effects of the thermal noise. The values at the
shortest integration times are mainly due to the noise of reading
electronics: It is about 3% of the maximum measurable lumi-
nance (the camera has an 8-bit A/D converter).
VIII. CONCLUSION
The paper shows the experimental procedure used to charac-
terize a commercial CCD camera for luminance measurement
in a road lighting plant. In order to calibrate it and to evaluate
its measurement uncertainties, several tests have been carried
out. The camera has been checked for several aspects closely
connected with its measurement application: road lighting
measurement. As commercial CCD cameras could show low
metrological performances, the best solution to improve the
measurement accuracy is to study how the camera behaves in
its typical measurement conditions. Ad hoc corrections and
attentions can be used so as to reduce the inuences of the dark
noise, the blooming and the optical system nonidealities (espe-
cially ghost images). In the presented case, the above practices
allow obtaining an overall uncertainty of 5%. A nonoptimized
measuring procedure could cause the uncertainty to grow up
to about 10%. The improvements in the global measurement
uncertainty, obtained when the corrections are applied, can be
considered proportionate to the technical characteristic of the
device and suitable for lighting engineering applications.
FIORENTIN et al.: CHARACTERIZATION AND CALIBRATION OF A CCD DETECTOR FOR LIGHT ENGINEERING 177
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Pietro Fiorentin was born in Chioggia (Venice),
Italy, on February 12, 1964. He received the Laurea
degree in electronic engineering from the University
of Padova, Padova, Italy, in 1988. In 1991, he
received the Dottorato di Ricerca degree in electrical
engineering from the Ministero dellUniversitàe
della Ricerca Scientica e Tecnologica.
From 1988 to 1991, he was with the Ionized
Gas Institute of the Research National Council
in Padova, doing thermonuclear experiments.
He worked with the control of the equilibrium
magnetic congurations in thermonuclear plasmas and magnetic sensors in
thermonuclear experiments. In 1995, he joined the Department of Electric
Engineering, University of Padova, where he is Associate Professor on Electric
and Electronic Measurements. His research interests include the character-
ization of voltage recorders, analysis of power line waveform in distorted
conditions, and photometric and radiometric characterization of light sources
and surfaces like road surface paving, retroreectors, and fresco surfaces.
Paola Iacomussi was born in 1969 in Italy. She received the degree in physics
from the University of Turin in 1993. In 1994, she had a fellowship at the Fiat
Research Center in the Optoelectronic Division, Photometry and Man-Machine
Interface Group.
Since 1995, she has been a Researcher at the Lighting Engineering Research
Group at the Photometry Department of the IEN Galileo Ferraris. The IEN
Galileo Ferraris is the Italian metrological institute for the electromagnetic and
perceptive units. Her current research interests includes relative and absolute
photometric measurements techniques for characterization of materials and
lighting systems, realization and conservation of photometric units, photo-
metric applications of digital detectors, and cultural heritage colorimetric
measurements.
Giuseppe Rossi received the laurea degree in elec-
tronic engeneering from the Politecnico di Torino,
Italy, in 1979. In 1988, he received the Dottorato di
ricerca in Metrology from the Ministero dellUniver-
sitàe della Ricerca Scientica e Tecnologica.
He has been with the Istituto Elettrotecnico
Nazionale (IEN) since 1984. His main activities
are in photometry, metrological aspects of lighting
engineering, and the photoradiometric characteri-
zation of materials. He has participated in several
European research projects in the photometric and
road visibility elds and for glass unit photometric characterization. He is
active in international (CEN) and national (UNI) standardization bodies. At
IEN, he developed instruments and measuring systems for the characterization
of lighting systems in the eld and a gonio-photometric systems for accurate
and detailed characterization of diffusing specimens.
Mr. Rossi is a member of several CIE Technical Committees and chairman
of CIE TC4-26.
... According to the requirements of EN-13201:2015 standard, the image luminance measuring device (ILMD) Techno Team LMK 98-3 Color was placed in the center of each (left/right) lane at a height of 1.5 m, 60.0 m from the measuring module ( Figure 11). It has been demonstrated in earlier works of the authors of this article [63,65,[91][92][93][94] and other research papers [95][96][97][98] that using this type of the meter to measure the luminance distribution of road lighting is well-justified. [34]. ...
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This paper presents the research of optimization of road lighting energy consumption by utilizing the fact of human twilight and night vision (mesopic vision) dependency on luminance level and lamp’s light spectral composition. The research was conducted for a suburban street illuminated by smart LED road luminaires with a luminous flux control system with which different luminance levels can be achieved on the road. This road is an access road leading to a town located on the outskirts of Warsaw which is the capital of Poland and a large metropolitan area. Therefore, the traffic here is quite heavy on this road in the morning and in the evening and it is very light at other times of the day. In accordance with EN 13201 standard, lighting control can be applied to illuminate this road. This paper compares energy consumption for different lighting scenarios of the road in question. In the first scenario, the road luminance is compliant with M4, M5, and M6 lighting class requirements depending on the time of the day. In the second scenario, for each M lighting class, the values of luminance levels provided by EN 13201 standard have been reduced to the values resulting from their conversion to the corresponding mesopic luminance values. The conducted research has shown that a 15% saving per year in electricity consumption on the road is possible with such a conversion. Therefore, energy efficiency of a lighting installation can be improved by matching the lighting levels provided by the standard to the mesopic vision.
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The directional polarimetric camera (DPC) is a space-borne polarimetric sensor which has the characteristics of ultra-wide angle and low-distortion imaging. The charge coupled device (CCD) detector is the core component of the photoelectric conversation for DPC which is used to obtain multi-angle, multi-spectrum polarization information, so it is necessary to measure its electric-optical performance comprehensively before installed in the DPC. Instrumentation is proposed and developed with the aim to evaluate the electro-optical performance of array CCD. The instrument mainly consists of light source subsystem, detector imaging subsystem and detector refrigeration subsystem with the structure of optical-mechanical-electric integration. The photoelectric response performance and the spectral characteristics of the detector are comprehensively and quantitatively tested and analyzed. The key parameters, such as quantum efficiency, dark current, full well capacity and system gain are measured; the spectrum response of the detector is measured from 400nm to 940nm with the spectral resolution around 1nm. The experimental results show that the CCD used for DPC has no defective pixels, the system gain is 0.0245DN/e⁻, the full well charge is 138.8ke⁻/pixel, the dark current is 990.5e⁻/pixel/s when the detector works at 6℃. The photo response non-uniformity under different wavelengths is better than 3% and the linearity error under different wavelengths is less than 1%. The quantum efficiency has a strong dependence on temperature in the near-infrared band, the maximum fluctuation of QE in 865nm and 910nm is 5.84% and 6.65%, respectively, in the temperature range from 0℃ to 20℃. The Instrumentation can be used in the screening and testing the electro-optical performance of scientific grade CCD.
... Aiming at using the smartphone camera as an instrument, a correction of that error is required. A complete procedure would require to scan all lighting directions observing a reference source with a known stable luminance and build a correction matrix, it is presented in [21] . A fastest approach is here considered and an analytical function obtained by trial and error is used to compensate the radial decrease in luminance. ...
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This work presents the possibility of using the extremely popular compact digital cameras of smartphones or action cameras to perform sky photometry. The newest generation of these devices allows to save raw images. They are not as good as digital single-lens reflex camera, in particular in terms of sensitivity, noise and pixel depth (10 bit versus 12 bit or more), but they have the advantage of being extremely widespread on the population and relatively cheap. These economical digital compact cameras work with an electronic shutter, it overcomes the consumption of mechanics and allows to gather images for long time. The work uses a simple calibration method to transfer raw data from the proprietary RGB color space to the standard CIE 1931 color space. It allows the measurement of sky luminance in cd m⁻² with an expected uncertainty of about 20%. Furthermore, the colorimetric calibration allows to know the correlated color temperature of a portion of the sky, it can help the identification of the kind of polluting sources. Aiming at better clarifying the performances of calibrated digital compact cameras, a comparison with a calibrated DSLR camera is presented in outdoor situations showing a good agreement both for luminance and color temperature measurements.
... Due to the wide view angle of the lens system, a vignetting effect [35] was expected for all cameras, i.e., a reduction of the image's luminance, with respect to luminance of the framed surfaces, going towards the periphery of the sensor from the image center. The effect is usually corrected by using a dedicated calibration procedure, which requires placing each camera with its optics on a goniometer [36], to expose the system to a reference source with a known stable luminance and to revolve the camera around both axes orthogonal to its optical axis, scanning any lighting direction, according to the procedure described in [37,38]. A sketch illustrating the revolving of the camera is presented in Figure 6. ...
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The paper presents the calibration activity on the imaging system of the MINLU instrument, an autonomous sensor suite designed for monitoring light pollution using commercial off-the-shelf components. The system is extremely compact and with an overall mass below 3 kg can be easily installed as a payload for drones or sounding balloons. Drones and air balloons can in fact play an important role in completing upward light emission measurement from satellites allowing an increased spatial and time resolution from convenient altitudes and positions. The proposed system can efficiently measure the luminous intensity and the spectral power density of on-ground emissions providing a useful tool to identify polluting sources and to quantify upward light flux. The metrological performance of the imaging system has been verified through an extensive laboratory test activity using referenced light sources: the overall uncertainty of the multi-luminance meter has been calculated to be 7% of the reading, while the multi-spectrometer has shown a full width at half maximum (FWHM) equal to 10 nm within the measuring range between 400 nm and 700 nm. When operating at an altitude of 200 m, the system can achieve a horizontal resolution at a ground level of 0.12 m with a wavelength resolution able to identify the different lamp technology of outdoor light sources, including light-emitting diode (LED) lights that are undetected by satellites.
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Bioluminescence, that is the emission of light in living organisms, has been extensively explored and applied for diverse bioanalytical applications, spanning from molecular imaging to biosensing. The unprecedented technological evolution of portable light detectors opened new possibilities to implement bioluminescence detection into miniaturized devices. We are witnessing a number of applications, including DNA sequencing, reporter gene assays, DNA amplification for point-of care and point-of need analyses relying on BL. Several photon detectors are currently available for measuring low light emission, such as photomultiplier tubes (PMT), charge-coupled devices (CCD), complementary metal oxide semiconductors (CMOS), single photon avalanche diodes (SPADs), silicon photomultipliers (SiPMs) and smartphone-integrated CMOS. Each technology has pros and cons and several issues, such as temperature dependence of the instrumental specific noise, the power supply, imaging capability and ease of integration, should be considered in the selection of the most appropriate detector for the selected BL application. These issues will be critically discussed from the perspective of the analytical chemist together with relevant examples from the literature with the goal of helping the reader in the selection and use of the most suitable detector for the selected application and to introduce non familiar readers into this exciting field.
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Rapidly developing thin-film solar cells prompts the demand for artificial narrow-band spectral illumination. Here, an LED-based compact spherical cap solar simulator is proposed to provide multiple independent color channels, while compatible with the traditional broadband IEC60904-9 standard in terms of illumination non-uniformity, stability and solar-like spectrum. Firstly, a one-dimensional model is established and then validated to analyze locations of maximum power point which may degenerate the spatial homogeneity. Secondly, the LED layout is optimized by Monte Carlo ray tracing, and it is found that a good illumination uniformity can be achieved if they are rotational symmetrically distributed. Thirdly, another two structural parameters, including output slot size and cavity height, are simulated to balance the output uniformity and output power. Finally, an instance simulator based on the structure is developed for performance verification. In the implementation, heat dissipation strategies are introduced to stabilize the system, including the use of heat sinks and flip-connected printed circuit boards to keep heat sources away from the optical chamber, thereby ensuring 0.052% short-term and 1.13% long-term stability. In addition, seven types of LEDs are adopted to simulate the solar spectrum, and its maximum deviation between the AM 1.5G is −8.82%. A control system consisting of a graphical user interface, STM32 microprocessor and constant current sources is developed to configure the current of different color channels. As a result, in predominant area of the entire output slot, six narrow-band and broadband illuminations can achieve 2% non-uniform, which is of interest for testing thin film solar cells.
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The directional polarimetric camera (DPC) is a polarization sensor with the characteristics of ultra-wide-angle and low-distortion imaging. The multi-angle polarization information is helpful to obtain the spatial distribution of target radiation, and multiple data fusion relies on the non-uniformity calibration of image plane. The non-uniformity consists of many factors such as lens, detector assembly, spatial stray light, etc. The single correction method can not distinguish the error source effectively. In consideration of the in-flight operation mode of DPC based on the adjustment of exposure time, the non-uniformity correction method of the detector based on multi parameters is proposed. Through the electro-optical performance measurement system of the CCD detector, the sensitive factors such as temperature, dark current, exposure time and spectral response are obtained. After a series of preprocessing of the image including removal of dark signal, removal of smearing effect and temperature compensation, the non-uniformity calibration based on multi-parameters is imposed on the detector. The low-frequency unbalanced response difference of the image surface is eliminated, and the high-frequency difference is effectively suppressed. The experimental results show that the photo response non-uniformity of 95% full well single frame data is reduced from 2.86% to 0.36%. After correction, the data noise is shown as shot noise, and the detector has good ability of dynamic range adjustment. The non-uniformity calibration by the proposed method can offer data support for the instrumental calibration and in-flight fast calculation, and provide effective reference for the subsequent polarization remote sensing instruments.
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This article proposes a new technique to reduce the readout noise in charge-coupled devices (CCDs). The objective is to minimize the common noise between the channels. This noise is either induced by the readout electronics or by the environment, and it could be present in small (single CCD and a few channels) or large systems (dozens of CCDs and hundreds of channels). By interleaving the video signals, we show that it is possible to reduce the readout noise by around 50% without sacrificing output channels at the expense of a moderate increase of the readout time, which is less compared with other techniques. Up until now, similar approaches have used half of the output stages only for noise measurement doubling the total readout time of the sensor. Simulations and experimental results are presented to validate the proposed approach. Moreover, the technique is measured using a Skipper CCD, and its operation is validated together with the ability of the Skipper sensor for nondestructively reading, several times, the same pixel charge. With the combination of both techniques, we achieve deep subelectron readout noise levels (0.1 e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−</sup> ).
Conference Paper
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Present technology allows to produce high resolution and low cost CCD (Charge Coupled Device) cameras. Videography uses this kind of camera to make up innovative measurement instruments, above all with reference to luminance and lighting level on a surface. The main advantage of CCD photometer is the possibility to collect, in a much shorter time and at a higher resolution with respect to the traditional luminancemeters, large quantity of measurement data referred to extended surfaces. This in turn leads to a simpler and faster measurement procedure when the evaluation of luminance distribution on an extended surface is required rather than the knowledge of luminance values in few single points. The knowledge of videographic technique and of the related calibration problems is essential to set up a CCD photometer. Photometric measurements based on videography require calibration procedures concerning several aspects, depending on CCD sensor, optical system and coupling camera-optical interface. This paper refers to the main problems in the set up of a CCD photometer: zenith (or "on axis") calibration, vignetting characteristic of the instrument, background influence in the photometric measurements and spectral response. The interesting obtained results have allowed to complete the set up of this innovative luminancemeter for lighting research and application, particularly for indoor built environment.
Article
One of the most significant advances in light detection methods in recent years is the development of CCD technology. There is a great difference, however, in merely detecting light versus providing accurate photometric measurements of a field of view. Many factors must be taken into account in controlling CCD and camera characteristics, data acquisition, and subsequent data processing. Calibration techniques are particularly significant if we are to ensure proper photometric analysis.
CCD matrix detector for photometry
  • G Rossi
  • P Iacomussi
  • M Sarotto
  • P Soardo
G. Rossi, P. Iacomussi, M. Sarotto, and P. Soardo, "CCD matrix detector for photometry," in Proc. VIII Int. Metrology Congress, Besançon, France, 1997.
Melusine: Luminance measurement in digitized image, design and calibration of the system
  • C Brusque
  • R Hubert
C. Brusque and R. Hubert, "Melusine: Luminance measurement in digitized image, design and calibration of the system," in Proc. 2nd CIE Workshops Photometric Measurement Syst. for Road Lighting Installations (Liège 1994, Poitiers 1996), Wien, Austria.
Luminance calibration of the Nikon coolpix 950 digital camera
  • D Dumortier
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D. Dumortier and P. Vetro, "Luminance calibration of the Nikon coolpix 950 digital camera," in Proc. 9th Lux Europa Conference, Reykjavìk, Iceland, 2001.
The calibration of CCD matrix detectors for the measurement on photometric materials and lighting installations
  • G Rossi
  • G Fusco
  • P Soardo
G. Rossi, G. Fusco, and P. Soardo, "The calibration of CCD matrix detectors for the measurement on photometric materials and lighting installations," in Proc. 23rd Session CIE, 1995, pp. 132-134.