In this work, we present a low-cost experimental setup to evaluate the image quality of digital-camera sensors, which can be implemented in undergraduate and postgraduate teaching. The method consists of evaluating the modulation transfer function (MTF) of digital-camera sensors by speckle patterns using a ping-pong ball as a diffuser, with two handmade circular apertures acting as input and output ports, respectively. To specify the spatial-frequency content of the speckle pattern, it is necessary to use an aperture; for this, we made a slit in a piece of black cardboard. First, the MTF of a digital-camera sensor was calculated using the ping-pong ball and the handmade slit, and then the MTF was calculated using an integrating sphere and a high-quality steel slit. Finally, the results achieved with both experimental setups were compared, showing a similar MTF in both cases.
KEYWORDS: Digital cameras, Image processing, Information science, Fourier transforms, Cameras, Digital imaging, Image quality, Modulation transfer functions, Image analysis, Calibration
In this work, we present a teaching methodology using active-learning techniques in the course “Devices and Instrumentation” of the Erasmus Mundus Master’s Degree in “Color in Informatics and Media Technology” (CIMET). A part of the course “Devices and Instrumentation” of this Master’s is dedicated to the study of image sensors and methods to evaluate their image quality. The teaching methodology that we present consists of incorporating practical activities during the traditional lectures. One of the innovative aspects of this teaching methodology is that students apply the concepts and methods studied in class to real devices. For this, students use their own digital cameras, webcams, or cellphone cameras in class. These activities provide students a better understanding of the theoretical subject given in class and encourage the active participation of students.
KEYWORDS: LCDs, Light emitting diodes, Visualization, LED displays, LED backlight, CIE 1931 color space, Transmittance, Liquid crystals, Human vision and color perception, Lamps
In this work we studied the color dependence with a horizontal-viewing angle and colorimetric characterization of two liquid-crystal displays (LCD) using two different backlighting: Cold Cathode Fluorescent Lamps (CCFLs) and light-emitting diodes (LEDs). The LCDs studied had identical resolution, size, and technology (TFT – thin film transistor). The colorimetric measurements were made with the spectroradiometer SpectraScan PR-650 following the procedure recommended in the European guideline EN 61747-6. For each display, we measured at the centre of the screen the chromaticity coordinates at horizontal viewing angles of 0, 20, 40, 60 and 80 degrees for the achromatic (A), red (R), green (G) and blue (B) channels. Results showed a greater color-gamut area for the display with LED backlight, compared with the CCFL backlight, showing a greater range of colors perceptible by human vision. This color-gamut area diminished with viewing angle for both displays. Higher differences between trends for viewing angles were observed in the LED-backlight, especially for the R- and G-channels, demonstrating a higher variability of the chromaticity coordinates with viewing angle. The best additivity was reached by the LED-backlight display (a lower error percentage). LED-backlight display provided better color performance of visualization.
KEYWORDS: Image filtering, Glasses, Autostereoscopic displays, LCDs, 3D image processing, Chromium, 3D metrology, 3D modeling, RGB color model, 3D displays
Spectroradiometric measurements have been made for the experimental characterization of the RGB channels of
autostereoscopic 3D displays, giving results for different measurement angles with respect to the normal direction of the
plane of the display. In the study, 2 different models of autostereoscopic 3D displays of different sizes and resolutions
were used, making measurements with a spectroradiometer (model PR-670 SpectraScan of PhotoResearch). From the
measurements made, goniometric results were recorded for luminance contrast, and the fundamental hypotheses have
been evaluated for the characterization of the displays: independence of the RGB channels and their constancy. The
results show that the display with the lower angle variability in the contrast-ratio value and constancy of the chromaticity
coordinates nevertheless presented the greatest additivity deviations with the measurement angle. For both displays,
when the parameters evaluated were taken into account, lower angle variability consistently resulted in the 2D mode than
in the 3D mode.
Surface properties are essential for a complete characterization of biomaterials. In restorative dentistry, the study of the surface properties of materials meant to replace dental tissues in an irreversibly diseased tooth is important to avoid harmful changes in future treatments. We have experimentally analyzed the surface characterization parameters of two different types of dental-resin composites and pre-sintered and sintered zirconia ceramics. We studied two shades of both composite types and two sintered zirconia ceramics: colored and uncolored. Moreover, a surface treatment was applied to one specimen of each dental-resin. All the samples were submitted to rugometric and microtopographic non-invasive inspection with the MICROTOP.06.MFC laser microtopographer in order to gather meaningful statistical parameters such as the average roughness (Ra), the root-mean-square deviation (Rq), the skewness (Rsk), and the kurtosis of the surface height distribution (Rku). For a comparison of the different biomaterials, the uncertainties associated to the surface parameters were also determined. With respect to Ra and Rq, significant differences between the composite shades were found. Among the dental resins, the nanocomposite presented the highest values and, for the zirconia ceramics, the pre-sintered sample registered the lowest ones. The composite performance may have been due to cluster-formation variations. Except for the composites with the surface treatment, the sample surfaces had approximately a normal distribution of heights. The surface treatment applied to the composites increased the average roughness and moved the height distribution farther away from the normal distribution. The zirconia-sintering process resulted in higher average roughness without affecting the height distribution.
Light propagation in biological media is characterized by the absorption coefficient, the scattering coefficient, the scattering phase function, the refractive index, and the surface conditions (roughness). By means of the inverse-adding-doubling (IAD) method, transmittance and reflectance measurements lead to the determination of the absorption coefficient and the reduced scattering coefficient. The additional measurement of the phase function performed by goniometry allows the separation of the reduced scattering coefficient into the scattering coefficient and the scattering anisotropy factor. The majority of techniques, such as the one utilized in this work, involve the use of integrating spheres to measure total transmission and reflection. We have employed an integrating sphere setup to measure the total transmittance and reflectance of dental biomaterials used in restorative dentistry. Dental biomaterials are meant to replace dental tissues, such as enamel and dentine, in irreversibly diseased teeth. In previous works we performed goniometric measurements in order to evaluate the scattering anisotropy factor for these kinds of materials. In the present work we have used the IAD method to combine the measurements performed using the integrating sphere setup with the results of the previous goniometric measurements. The aim was to optically characterize the dental biomaterials analyzed, since whole studies to assess the appropriate material properties are required in medical applications. In this context, complete optical characterizations play an important role in achieving the fulfillment of optimal quality and the final success of dental biomaterials used in restorative dentistry.
KEYWORDS: 3D displays, LCDs, CIE 1931 color space, 3D modeling, 3D image processing, Light emitting diodes, 3D imaging standards, Image quality, LED displays, LED backlight
An analysis has been made of the stability of the images generated by electronic autostereoscopic 3D displays, studying the time course of the photometric and colorimetric parameters. The measurements were made on the basis of the procedure recommended in the European guideline EN 61747-6 for the characterization of electronic liquid-crystal displays (LCD). The study uses 3 different models of autostereoscopic 3D displays of different sizes and numbers of pixels, taking the measurements with a spectroradiometer (model PR-670 SpectraScan of PhotoResearch). For each of the displays, the time course is shown for the tristimulus values and the chromaticity coordinates in the XYZ CIE 1931 system and values from the time periods required to reach stable values of these parameters are presented. For the analysis of how the procedure recommended in the guideline EN 61747-6 for 2D displays influenced the results, and for the adaption of the procedure to the characterization of 3D displays, the experimental conditions of the standard procedure were varied, making the stability analysis in the two ocular channels (RE and LE) of the 3D mode and comparing the results with those corresponding to the 2D. The results of our study show that the stabilization time of a autostereoscopic 3D display with parallax barrier technology depends on the tristimulus value analysed (X, Y, Z) as well as on the presentation mode (2D, 3D); furthermore, it was found that whether the 3D mode is used depends on the ocular channel evaluated (RE, LE).
In dental applications, optimizing appearance is desirable and increasingly demanded by patients. The specular gloss is among the major appearance properties of dental biomaterials, and its relationship with surface roughness has been reported. Roughness and gloss are key surface aspects that complement each other. We have experimentally analyzed the specular gloss and surface roughness of two different types of dental-resin composites and pre-sintered and sintered zirconia ceramics. We have studied two shades of both composite types and two sintered zirconia ceramics: colored and uncolored. Moreover, a surface treatment was applied to one specimen of each dental resin. Gloss measurements were performed with a standardized reflectometer and the corresponding gloss percentages were calculated. All the samples were submitted to rugometric non-invasive inspection with the MICROTOP.06.MFC laser microtopographer in order to determine meaningful statistical parameters such as the average roughness (Ra) and the root-mean-square deviation (Rq). For a comparison of the different biomaterials, the uncertainties associated to the measure of the surface gloss and roughness were also determined. The differences between the two shades of both kinds of composites proved significant in the case of the roughness parameters but not for the specular gloss. The surface treatment applied to the dental-resin composites increased the average roughness but the changes in the specular gloss were significant only for the A2 enamel nano-composite. For the zirconia ceramic the sintered process resulted in an increase in the surface roughness with a decrease of the specular gloss, corroborating that the relationship between the gloss and the roughness shows the expected behavior.
In recent years, the flat-panel display (FPD) technology has undergone great development. Currently, FPDs are present
in many devices. A significant element in FPD manufacturing is the display front surface. Manufacturers sell FPDs with
different types of front surface which can be matte (also called anti-glare) or glossy screens. Users who prefer glossy
screens consider images shown in these types of displays to have more vivid colours compared with matte-screen
displays. However, external light sources may cause unpleasant reflections on the glossy screens. These reflections can
be reduced by a matte treatment in the front surface of FPDs. In this work, we present a method to characterize the front
surface of FPDs using laser speckle patterns. We characterized three FPDs: a Samsung XL2370 LCD monitor of 23"
with matte screen, a Toshiba Satellite A100 laptop of 15.4" with glossy screen, and a Papyre electronic book reader. The
results show great differences in speckle contrast values for the three screens characterized and, therefore, this work
shows the feasibility of this method for characterizing and comparing FPDs which have different types of front surfaces.
KEYWORDS: Voltage controlled current source, Modulation transfer functions, Video, Cameras, Video surveillance, Speckle pattern, Charge-coupled devices, Sensors, Spatial frequencies, Control systems
Today, video cameras connected to frame grabbers or video capture cards (VCCs) are used in many applications such as
traffic control, surveillance, medical systems or machine vision. In this work, we present a method for determining the
spatial-frequency response of VCCs. This method is based on the modulation transfer function (MTF) determination
from speckle patterns using a low-cost experimental setup. We have evaluated and compared three different VCCs. The
three VCCs produce an amplification (boost) in the horizontal MTF in a different spatial-frequency range, also differing
in the maximum amplification value.
Charge-coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) matrices offer excellent features
in imaging systems. A suitability evaluation of either technology according to a specific application requires a complete
characterization of the different detector types. A system is optically characterized by the modulation transfer function
(MTF), which represents its response in spatial frequency of this system. One of the methods to measure the MTF uses a
laser speckle pattern as the object.
Here, we comparatively examine the results provided by the speckle method to determine the MTF for detectors
of two types: CCD and CMOS. We generate the speckle pattern using a laser and an integrating sphere with an aperture
at its exit port. The aperture determined the spatial-frequency content of the pattern registered in the detector. The
precision in determining the MTF of the CCD was studied using two different apertures: a single-slit and a double-slit.
For the single-slit, we propose a new procedure of fitting the experimental data which resolves the drawbacks of the
conventional procedure. To study the CMOS detector, we used the single-slit because it offered lower uncertainty and
better reproducibility. The differences between the MTF values of the CCD and the CMOS detectors proved significant
for the spatial frequencies higher than 50 cycles/mm, which is half of the interval studied with both arrays. For these
spatial frequencies, our results demonstrate that the CCD detector presented MTF values higher than those of the CMOS
array.
Knowledge of the optical properties of biological structures is useful for clinical applications, especially when dealing
with incoming biomaterials engineered to improve the benefits for the patient. One ceramic material currently used in
restorative dentistry is yttrium cation-doped tetragonal zirconia polycrystal (3Y-TZP) because of its good mechanical
properties. However, its optical properties have not been thoroughly studied. Many methods for the determination of
optical parameters from biological media make the assumption that scattered light is isotropically distributed over all
angles. Nevertheless, real biological materials may have an angular dependence on light scattering, which may affect the
optical behaviour of the materials. Therefore, the recovery of the degree of anisotropy in the scattering angular
distribution is important. The phase function that represents the scattering angular distribution is usually characterized by
the anisotropy coefficient g, which equals the average cosine of the scattering angle. In this work, we measured angularscattering
distributions for two zirconia ceramic samples, pre-sintered and sintered, with similar thicknesses (0.48 mm
and 0.50 mm, respectively) and also for a human dentine sample (0.41 mm in thickness). The samples were irradiated
with a He-Ne laser beam (λ = 632.8 nm) and the angular-scattering distributions were measured using a rotating
goniometer. The g values yielded were: -0.7970 ± 0.0016 for pre-sintered zirconia, -0.2074 ± 0.0024 for sintered zirconia
and 0.0620 ± 0.0010 for dentine. The results show that zirconia sintering results in optical behaviour more similar to
those of dentine tissue, in terms of scattering anisotropy.
Understanding the behaviour of light propagation in biological materials is essential for biomedical engineering and its
applications. Among the key optical properties of biological media is the angular distribution of the scattered light,
characterized by the average cosine of the scattering angle, called the scattering anisotropy coefficient (g). The value of g
can be determined by experimentally irradiating the material with a laser beam and making angular-scattering
measurements in a goniometer. In this work, an experimental technique was used to determine g by means of
goniometric measurements of the laser light scattered off two different dental-resin composites (classified as nano and
hybrid). To assess the accuracy of the experimental method, a Mie theory-based computational model was used.
Independent measurements were used to determine some of the required input parameters for computation of the
theoretical model. The g values estimated with the computational method (nano-filled: 0.9399; hybrid: 0.8975) and the
values calculated with the experimental method presented (nano-filled: 0.98297 ± 0.00021; hybrid: 0.95429 ± 0.00014)
agreed well for both dental resins, with slightly higher experimental values. The higher experimental values may indicate
that the scattering particle causes more narrow-angle scattering than does a perfect sphere of equal volume, assuming
that with more spherical scattering particles the scattering anisotropy coefficient increases. Since g represents the angular
distribution of the scattered light, values provided by both the experimental and the computational methods show a
strongly forward-directed scattering in the dental resins studied, more pronounced in the nano-filled composite than in
the hybrid composite.
Since biological tissues can have the intrinsic property of altering the polarization of incident light, optical polarization
studies are important for a complete characterization. We have measured the polarized light scattered off of different
dental tissues and biomaterials for a comparative study of their optical polarization property.
The experimental setup was composed by a He-Ne laser, two linear polarizers and a detection system based on a
photodiode. The laser beam was passed through one linear polarizer placed in front of the sample, beyond which the
second linear polarizer (analyzer) and the photodiode detector were placed. First, the maximum laser-light intensity
(reference condition) was attained without the sample in the laser path. Then, the sample was placed between the two
polarizers and the polarization shift of the scattered laser light was determined by rotating the analyzer until the reference
condition was reached.
Two dental-resin composites (nanocomposite and hybrid) and two human dental tissues (enamel and dentine)
were analyzed under repeatability conditions at three different locations on the sample: 20 measurements of the shift
were taken and the average value and the uncertainty associated were calculated. For the human dentine the average
value of the polarization shift found was 7 degrees, with an associated uncertainty of 2 degrees. For the human enamel
and both dental-resin composites the average shift values were found to be similar to their corresponding uncertainties (2
degrees). The results suggest that although human dentine has notable polarization properties, dental-resin composites
and human enamel do not show significant polarization shifts.
We present a method for comparing the optical quality of spectacle lenses based on determining the modulation transfer function (MTF) using random-dot patterns. Furthermore, we determine the precision of the method under repeatability conditions of measurement. The experimental device is composed of a laptop computer in which the LCD monitor presents the random-dot pattern, a liquid-crystal tunable filter, the lens under test, and a charge-coupled device detector connected to its control card installed in another computer. The method proposed has major advantages: the lenses can be characterized at different wavelengths; no additional sources to illuminate the pattern are required, the monitor's lighting source itself is used; and the characteristics of the pattern can be quickly and easily modified, as we install the control software in the laptop computer. We analyze three spherical spectacle lenses (+5 D) from different manufacturers and, in terms of MTF, the greatest difference found between them is 13.7%. The uncertainty associated with this method falls within the range of 0.001 and 0.06. Given the low uncertainty values, differences found between the lenses are significant. Therefore, the method proposed is a versatile and quick technique to distinguish the optical quality of spectacle lenses from different manufacturers.
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