In a previous work, we described the theoretical fundamentals for the design of radiance meters. There, we observed that the length between the first and the second aperture of the instrument plays an important role in the performance of the meter. Such a length is a parameter related with the instrument response to variations of the size of the source and of the distance between the instrument and the source. Such dependences are not predicted by the theory of geometrical optics, normally used in the instrument design. Even the approximation of wave optics gives a partial understanding of the propagation of optical radiation. A more general treatment, based in the theory of partial coherence, gives a better description of the propagation.
Photoelasticity is a stress measurement method extensively used in test laboratories of materials. This method can be a reference to validate numerical calculation of stress and strain distributions, however we need to evaluate errors and uncertainties to know the reproducibility of the method in order to compare with numerical calculation. Some transparent materials present birefringence when they are stressed. In a first approximation the birefringence depends of the stress in a linear way, the proportionality constant is known as stress-optic constant. When polychromatic light is used the wavelength becomes other important parameter for the method. Therefore the stress-optic constant is a source of error and uncertainty, also the resolution of the wavelength is a second source of error and uncertainty. In this work we present an evaluation of the sources of errors and uncertainties of the photoelasticity method for stress and strain measurements.
This work considers typical radiometers that measure the radiance of sources or of some surface (due to reflection). The
most important thing in a radiometer is its optical cavity, which defines the solid angle of measurement. In case of
instruments that use lenses, lens diameter and focal length define the solid angle. Radiometer design takes care that the
solid angle subtended by the optical cavity be minor that the solid angle subtended by the source to be measured,
however no considerations are taken on the length of the cavity. In this work it is discussed theoretical limitation in the
design of radiometers, particularly the influence of radiometer length (the cavity length). Theory of partial coherence is
used to obtain the instrument function and to show how the length of the instrument affects measurements.
Inhomogeneity measurement is an important test of reference materials. In case of hardness reference blocks it consists of measurements at five distributed points on the surface with a test indenter. Unfortunately the test is destructive and it is not possible to use the same point for a new measurement. In this work we propose an optical method to measure the inhomogeneity of hardness standards. As we reported in a recent work, variations of hardness in a steel produces variations in their optical properties, this fact can be used to detect variations in the hardness of blocks with a non-destructive method.
In this work, the observation of phase delay changes between parallel and perpendicular components of an optical beam
reflected on a metal surface is reported. Those changes have been induced by electrically charging a metal with static
charge. A quasimonochromatic lineally polarized beam is directed to a piece of steel where the beam is reflected, the
polarization of the reflected beam is in general elliptical. The module of each polarization component and their
difference of phase are measured with an ellipsometer. For the experiments we have started by making ellipsometry
measurements on a grounded steel sample, then a second measurement has been carried out on the same sample after
inducing electrostatic charge, results indicate a rotation of the elliptical polarization in the beam reflected on the
electrically charged sample.
The spectral responsivity of detectors is commonly measured through the comparison with a reference detector and an
optical system that provides monochromatic radiation. Such systems are designed to provide narrow bandwidth
monochromatic radiation whose optical flux is generally low. These levels of optical flux are not enough to excite
photopic instruments whose spectral response has to be measured. In this work we propose an optical arrangement with
enough optical flux to realize such measurements. The system consists of a color temperature calibrated lamp which is
the reference. The monochromatic separation is realized with a transmittance grating. The spectral distribution at the
plane of the instrument is calculated, it is practically the same that the lamp except for the level of irradiance. The
spectral response measured is corrected by the bandwidth of the system. Experimental results are presented and the
noise-to-signal level reached in the system is discussed.
Pyrometer calibration is a common task in most radiometry labs. When measurements are made in a wide range of
temperatures, it is necessary to use different blackbody radiators to cover the whole interval. A problem that arises with
this is the lack of concordance in the signals measured by the pyrometer when taken from different radiators.
In a recent publication, Fourier theory is applied to measure the temperature of inhomogeneous objects, particularly
periodic objects^{1}. Those results are used to measure the temperature of the coil filament of a reference lamp (in a range
from 300 K to 3300 K, approximately), considering the filament as a periodic object, which is modeled with a simple
functions arrangement. Measurement verification is also presented by comparing our calculations to the experimental
data of the lamp's temperature.
The instruments in temperature measurements with infrared radiation are the radiation pyrometers or radiometers, some
of them of double cavity and with or without imaging lenses. In the case of scale realizations is important to know the
function of the instrument. In a previous work we published such a function for the case of instruments without lens
considering the partial coherence theory. However the resulted expression consists of a four-fold integral which makes
difficult its physical interpretation. In this work we present such functions for instruments with lenses and use Fourier
techniques to obtain expressions easier to manipulate and to interpret. The instrument functions are evaluated and
discussed its physical interpretation for both cases; with lenses and without lenses, through two practical examples.
Optical detectors for illumination measurements have the known photopic response V(λ), which is implemented with the
help of colored glass filters attached to silicon detectors. In the case of high precision photometry the filter of each
detector is hand prepared and the spectral response measured until it is matched to the function V(λ). This method takes
a long time and the precision is limited. In this work we propose an optical method to implement holographic lens with
predefined spectral selection, this method permits to make the optical design of the hologram when the spectral response
of the silicon detector is known.
In a recent work we reported dependence between the hardness of steels and its refraction complex index, showing that
this optical property can be taken as a measure of the electronic interaction inside the molecular structure of metals. If
the molecular structure changes then the electronic interaction changes and it is observed as a modification of its
refraction index. In this work we present experimental results on steel pieces thermally treated and maintained in rest in
the laboratory for material stabilization. The refraction complex index showed variations through a several days period.
Variations are attributed to released stresses of the material. The steel sample is thermally treated with a tempering
process and tested with an optical setup. The refractive index of the sample is measured through several days, showing
variations. The ratio of changes is grater in the first days, showing an exponential decaying in subsequent periods of
time.
The mechanical properties of the materials are related with the atomic arrangement of their constituent elements. Particularly, the electronic cloud at the surface of steels shows spatial properties of the charge distribution of the metallic crystals. In this work we report the conductivity properties of the electronic cloud in the directions normal and parallel to the surface. These conductivity features are studied through the interaction with an optical field. The reflectance components of the materials are measured and related with the conductivity in the respective directions. We show that for the different Steel samples that were measured, a relation between the hardness and the reflectance components is observed.
In this work is shown that hardness properties of metals can be related with its refraction index. Given a metal, its surface hardness is a function of the molecular structure which can be studied with the refraction of optical radiation. In general refraction index of conductors are complex, it produces reflected light with elliptical polarization. It is observed that the ellipse rotates for different hardness of a given metal. Different hardness of two types of steel are measured as well as their respective rotation. The measurements show that there exist a direct relation between the hardness and the refraction index.
KEYWORDS: Sensors, Near field diffraction, Diffraction, Point spread functions, Radiometry, Optical engineering, Signal detection, Radio optics, Stray light, Computer simulations
We present numerical calculations of the Fresnel diffraction for periodic structures in an optical system with two apertures. In such a system, measurements are affected by the relation between the spatial frequency of the sample and the geometrical parameters involved (i.e., aperture diameters, radiometer-sample distance, in-plane rotation, and translation of the sample). This numerical calculation of the Fresnel diffraction enables us to establish criteria to choose the right geometrical parameters of the system to ensure invariance of the measurements when the sample is rotated or shifted. We use the theory of partial coherence to calculate the Fresnel diffraction through two successive apertures. By using the point spread function of the system, as in the theory of partial coherence, we avoid complicated statistical processes that are commonly used in this theory. We show some numerical results that verify our proposal.
In this work we design and build an optoelectronic demultiplexer to distribute audio and video signals transmitted through an optical carrier toward different users using a liquid crystal display (LCD) as a router. The output plane of our proposed device is located within the Fresnel Diffraction regime. With this feature we obtain a simultaneous distribution, high-speed of processing, easiness of path reconfigurations and a high density of interconnections. The optical carrier modulation includes a Frequency Shift Keying signal (FSK) to indicate the interconnection path. This last control signal is manipulated with a personal computer. We present some preliminary results of the feasibility of our proposal.
A common and known problem in textile industry is the measurement of color and of their fabrics for quality control. Commercial equipment is limited to color measurements of objects with homogeneous surfaces. In the case of samples with no-homogeneous surfaces it is recommended the use of accessories such as an integrating sphere that averages the in-homogeneities along with the averaging of several measurements taken for different orientations of the sample. However, single measurements with these devices are still not precise enough. In order to solve this problem we proposed a novel system for color measurement of periodic objects. In a first stage we have presented novel illumination geometry capable to produce an homogeneous illumination over periodic objects. The measurements were made with a two-aperture radiometer which is coupled to the illumination setup. This system was used to measure the reflectance produced by a textile sample. In this work, we present the second stage, where the system is improved in order to measure color directly from textile samples. We present the results from a comparison between our system and a commercial one.
One of the main sources of error when making precise measurements of radiance is the one associated to the variation in the output signal of the radiometer due to changes in the size of the source. This effect is known as the size-of-source effect (SSE). It is observed experimentally that as the size of the source increases, the output signal of the radiometer increases as well. No standard method for measuring the SSE exists. The SSE is estimated as the ratio of the output signal at a given diameter of the source to the signal at a reference diameter. One method considers this reference diameter as the diameter of an infinite source. A second method sets the reference diameter to the largest diameter experimentally possible. Commonly, the second method is the one used since it is more practical. However, the first one is a better model, even though the limit to infinite is not available experimentally. In this work, we discuss a formal method to calculate this limit. The limit can be used in the first method for a better quantification of the effect in practical measurements.
Proc. SPIE. 5526, Optical Systems Degradation, Contamination, and Stray Light: Effects, Measurements, and Control
KEYWORDS: Diffraction, Point spread functions, Radio optics, Spatial frequencies, Sensors, Numerical simulations, Near field diffraction, Radiometry, Stray light, Signal detection
In this work, we present a study of Fresnel diffraction of periodic structures in an optical system of two apertures. This system of two apertures was used successfully for measuring color in textile samples solving the problems of illumination and directionality that present current commercial equipments. However, the system is sensible to the spatial frequency of the periodic sample’s area enclosed in its optical field of view. The study of Fresnel diffraction allows us to establish criteria for geometrical parameters of measurements in order to assure invariance in angular rotations and spatial positions. In this work, we use the theory of partial coherence to calculate the diffraction through two continuous apertures. In the calculation process, we use the concept of point-spread function of the system for partial coherence, in this way we avoid complicated statistical processes commonly used in the partial coherence theory.
Radiation pyrometers are widely used in industries and laboratories for non-contact temperature measurement of objects. In the case of very accurate pyrometry, the measurements are affected by two effects, namely, the size-of-source effect (SSE) and the distance to the source effect (DE). The lack of accuracy in the measurements due to the SSE is associated to variations in the size of the object for a fixed measuring distance, whereas for the DE is associated to variations of the measuring distance for a fixed size of the object. In this work we present a numerical method that can be used for the calculation of corrections for both effects. In this case the method is applied to a lensless double aperture pyrometer. The method is based on the theory of partial coherence for the calculation of the energy transport through the pyrometer. The corrections can be made for sources of any size and shape and for any distance. In this case we consider sources of circular shape given our black body radiators. We present experimental results that confirm our numerical calculations.
Recently it was reported a method to calculate the instrument function of a two-aperture radiometer which describes the energy transport through two apertures using the theory of partial coherence. The result of that work was expressed as a multiplication of a two-fold integral and its complex conjugate. In this work we solve partially this two-fold integral, particularly we introduce a semi-cicle angular integral that reduces the integration. This new representation allows a faster numerical evaluation as well as an easier interpretation of energy transport for radiometric considerations.
Color measurements in textile samples is a very well known problem, current measurement methods are repositioning-of-sample dependent. In particular, the orientation of the sample is the first parameter of discrepancies in the reproducibility of measurements, even when we use the same instrument and the same sample. In this work we propose a new optical arrangement which is insensible to rotations. Preliminary experimental results show the invariance under rotations of two-dimensional periodic samples.
We obtain in this work the instrument function of a two-aperture radiometer by applying the general case of diffraction with quasi-monochromatic partially coherent sources. Such instrument function describes the energy transport through the two apertures and allows the absolute calculation of the source emittance.
The radiance and the law of conservation of radiance are two concepts created in an effort to explain experimental observations with extended incoherent sources. In this work we show that such observations are an effect of the partial coherence of the optical fields and that they can be explained by calculating the irradiance of the diffraction pattern of apertures with partial coherent illumination.
The holmium oxide solution is a common standard reference material used in the calibration of wavelength scales of spectrophotometers. The wavelength values of minimum transmittance of several bands are reported by national laboratories, these values depend on the bandwidth of the instrument (FWHM) with which the transmittance is measured. In high accurate spectrophotometry the FWHM is smaller than 3nm. In this work we simulate the holmium oxide spectral transmittance and obtain extrapolated values to be used as reference values in the calibration of spectrophotometers with 10nm and 20nm of FWHM.
Proc. SPIE. 4087, Applications of Photonic Technology 4
KEYWORDS: Diffraction, Metrology, Sensors, Copper, Electroluminescence, Near field diffraction, Radiometry, Standards development, Radio propagation, Near field optics
In this work we present a solution to the problem of the cross spectral density propagated through a circular aperture in the Fresnel approximation. Our proposal is a generalization to partially coherent illumination of the classical solution of the problem of near field diffraction due to a circular aperture. Our result can be used to improve the evaluation of the diffraction errors in Radiometry. We show that our generalization contains as particular cases the ones already reported in the literature.
In this work we calculate, in the Fresnel approximation, the spectral radiant flux density of the diffracted field of a circular aperture which is illuminated by an extended incoherent source. Our formulation allows us to obtain easily numerical simulations for different parameters of the optical configuration. To verify our proposal, we present numerical and experimental results.
We analyze the possibilities of white light interconnections by controlling the mixture of diffractive colors. In our experiments we used conventional holographic materials to reproduce the spatial frequency codification to obtain the desired color mixture. Experimental verifications are included.
We describe a holographic technique suitable to manipulate the chromaticism and brightness of the diffractive colors. This technique allows us to reproduce a wide range of color sensations as true color holographic images. Experimental results that verify our proposal are included.