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.
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<sup>1</sup>. 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.
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.
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.
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.
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.
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.