P. Donelli

Monte Carlo calculations of the modulation transfer function of an optical system operating in a turbid medium

Using a Monte Carlo method, we investigate the effect of a turbid medium on image transmission by means of the modulation transfer function approach. We present results that refer to a medium that consists of a random distribution of water spherical particles in air. We analyze the effect of geometric conditions (medium width and position) and source characteristics (Lambertian, beam emission). We present results for small spheres (Rayleigh scattering) and spheres (1.0-microm diameter) that are not small in comparison with the wavelength lambda = 0.6328 microm. Numerical data show a large modulation transfer function dependence on the source emission aperture and a substantial independence of the medium width for a fixed value of the optical depth. In accordance with reciprocity principles, we test an inverse scheme of Monte Carlo calculation, the advantage of this scheme being a substantial reduction in calculation time.

Experimental validation of a Monte Carlo procedure for the evaluation of the effect of a turbid medium on the point spread function of an optical system

Abstract The presence of particulate matter interposed between the object and the receiver affects the quality of the image produced by an optical system. This paper presents the results of measurements pertaining to the effect of a turbid medium on the point spread function of an optical system. The results refer to transmitted received power measurements obtained in controlled laboratory experiments. A random distribution of polystyrene microspheres suspended in water constituted the investigated turbid medium. Measurements were carried out for particulate with diameters of 0·33 μm, 0·995 μm, 15·7 μm at a wavelength of 0·6328 μm and for different values of sphere concentration in water. The measured data are favourably compared with results obtained by means of a Monte Carlo based numerical method. This numerical procedure allows us to obtain the point spread function and the modulation transfer function (MTF) of an optical system when a turbid medium is present. Examples of calculated MTFs that refer t...

Attenuation of energy in time-gated transillumination imaging: numerical results.

Numerical simulations based on a semianalytic Monte Carlo procedure have been developed to investigate the feasibility of a time-gated transillumination imaging system that could be useful for breast cancer screening. Numerical results showed that attenuation of earlier received photons strongly increases when the gating time decreases and strongly depends on single-scattering properties of the medium. For gating times shorter than ~ 5 ps, attenuation of scattered received energy approaches the exponential attenuation expected for unscattered photons. For typical optical properties of a healthy breast the use of gating times shorter than 100 ps seems to be questionable because of the low level of received energy. The image resolution expected with gating times longer than lOOps is of the order of 10mm. A comparison with predictions or the diffusion equation showed the inadequacy of this theory to describe the dependence of earlier photons on the single-scattering properties of the medium. However, thepredictions of the diffusion equation are within the range of values obtained from numerical simulations for the different scattering properties investigated.

Cloud, fog and aerosol effect on the MTF of optical systems

A numerical code is used to examine the features of the effect of atmospheric turbidity on the modulation transfer function of an optical system operating on ground, on an airplane or a satellite. Models of size distributions and optical properties of particulate suspended in the atmosphere are considered. The relevant scattering phase functions are calculated by Mie theory and are later used by a code using both Monte Carlo and geometrical optics procedures to evaluate the contribution of atmospheric turbidity to the augmentation of the point spread function. Comparison of ours with other researchers procedures is shown. The effect of atmospheric turbidity is evaluated as due to the presence of scatterers (the secondary sources) whose defocused images are distributed on the plane of the image of the primary source. The positions of the scatterers are determined by a Monte Carlo procedure, while the contribution of each secondary source to the irradiance on the image plane is evaluated by means of geometrical optics. Cases of different aerosols types, geometry aspects of viewing through the atmosphere and atmospheric absorption effects on the MTF are shown.

Numerical codes for multiple scattering in the atmosphere

The features of a code for calculation of lidar returns from clouds are recalled. Some laboratory measurements on models showed the validity of the code with regard to polarization of received power. Reference is also made to a code for calculations of turbidity effects on optical systems.

Atmospheric multiple scattering effect on spatial resolution of imaging systems

In this paper a physical model that describes the relationship between the optical properties of the atmosphere and the characteristics of an imaging system is suggested. The model describes how different components of the light reaching the imaging system, after passing through the atmosphere, are detected by it. The model includes the effects of the final size of the detector elements of the imaging system and the dynamic range and the final field of view limits of the imager. It is found that for common imaging systems (with resolution of 8 bit or 12 bit) working in general atmosphere conditions (VIS >= 5 km), the processes of atmospheric scattering and absorption hardly contribute to spatial blurring of the recorded images. A field experiment was carried out in order to verify the predictions of the suggested model. The measurements were performed using a scanning point radiometer, while a local meteorological station and a visibility meter measured the properties of the atmosphere. Theoretical predictions, which were accomplished by using a Monte-Carlo simulation of atmospheric scattering effects, are compared with the experimental data acquired in the field tests. A good agreement was obtained between the measured data and the theoretical predictions.

Atmospheric scattering effect on spatial resolution of imaging systems: theory

The atmosphere is the optical medium between the imaging system and the observed object. The effect of this optical turbulent, absorbing and scattering element on the quality of an image is difficult to predict. Since the beginning of the sixties there were several attempts to build a model that will describe the degrading effect of the atmosphere on spatial resolution of imaging systems. In the field of atmospheric turbulence there is a common agreement on its relative contribution to the degradation of the spatial resolution of an image. On the other hand in the field of atmospheric scattering there is a disagreement on its degrading effects and an international scientific discussion has been developed in the past five years in this scientific field. A model, that was suggested several years ago by Sadot and Kopeika, claimed that the effect of the atmospheric scattering on the spatial resolution of imaging systems is a function of the properties of the imaging system, apart from the inherent properties of the atmosphere. The results of their model were in contrast with the results of the work of other scientists and therefore, caused a scientific debate. The purpose is to propose an alternative theoretical model which describes the effects of atmospheric scattering by focusing on the point spread function of the imaging system.

Monte Carlo procedure to deal with inhomogeneity of medium turbidity and multiple scattering effects

A Monte Carlo procedure to deal with propagation of radiation in a turbid medium presenting some simple kinds of inhomogeneity is briefly outlined. Cases of lidar returns from inhomogeneous clouds are shown.

Monte Carlo calculations of point spread function and MTF of an optical system operating in a turbid medium

A numerical procedure is applied to studying the effect of some geometrical parameters on point spread function and modulation transfer function of an optical system operating in a turbid medium. The possibilities offered by an inverse scheme of calculation are also examined.

Numerical procedure for calculating the effect of a turbid medium on the modulation transfer function of an optical system

A Monte Carlo based method of calculating the effect of a turbid medium on the MTF of an optical system is presented. Results are compared with experimental data.