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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 amplifier. These operating conditions
should be recorded in the calibration certificate.
The knowledge of the calibration values is not enough to ob-
tain accurate measures because several parameters could influ-
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 influence.
In a CCD matrix, the luminance calibration coefficients 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 reflection 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 coefficient is called the
“calibration matrix”; it tries to correct the influence 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 influence 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 field diaphragm limits the field 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
first 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 influence 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 influence 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 flat 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 influence 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 simplified 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 reflection of the lens holder shall be
carefully avoided.
Alternatively, the camera with the lens frames the source. The
output image considers also the influence 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 define a grid of 35 equally
spaced groups of nine pixels, as in the original IEN method.
From these measurements, the influence 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 influence. 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, filter, and CCD) matches
the V , the less are the spectrum influences on the luminance
results.
Commercial CCD cameras are factory equipped with a cal-
culated filter approximating the V . How close the spectral
sensitivity fits 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 influence 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 first problem, the camera should be
calibrated in luminance units pacing the reference source at a
distance equal to 60 m. The influence 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 scientific 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 simplified 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 define 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
defines the depth of field, 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 coefficient normalized to 4 value versus
values.
Experimentally, the measurement is carried out with a lumi-
nance standard source at a fixed 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 field 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 verified 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 field 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 quantification 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 figure. 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
identified: 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 influences 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 dell’Universitàe
della Ricerca Scientifica 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 configurations 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, retroreflectors, 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 dell’Univer-
sitàe della Ricerca Scientifica 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 fields 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 field 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.
