A general formula for calculating radiative fluxes from point sources of radiation incident on spherical objects was derived using some fundamental laws of classical radiometry. This formula was derived in the Cartesian coordinate system, 0xyz, where the coordinates, x, y, and z, determine the position of the spherical object with respect to the point source. The obtained solution was dependent on the radius of the object, and on the function describing the intensity of the radiation. A specific solution for calculating fluxes of isotropic radiations was presented and selected calculations were illustrated graphically.
©2004 Optical Society of America
Many natural objects and industrial products of spherical shape are irradiated in nature by non-ionizing and ionizing radiation from a single point source or from many point sources. For example, non-ionizing and ionizing radiation from distant stars continuously irradiates planets, planetoids and moons, while a such radiation from non-natural point sources may be used for irradiation of fruits, vegetables and seeds, various microorganisms as well as molecules and atoms when they may be considered as spherical bodies [1–3]. In addition, a variety of spherical products may be irradiated by artificial point sources of radiation [4–7].
Treatment of some foods by non-ionizing and ionizing radiation have shown very promising results in destroying harmful pathogens and extending shelf life of some food products without any detectable physical and chemical changes. Such methods of irradiation have been successfully tested in laboratories and should find wide application in the future [1, 3–7]. Therefore, to study the effect of this radiation on irradiated objects quantitatively accurate analytical methods for estimating the radiative fluxes are necessary.
The main goal of this research was to derive an analytical formula for calculating radiative fluxes emitted by various point source models into a space subtended by a spherical object. As an example, selected results for an isotropic point source in various configurations with respect to the spherical object were calculated and illustrated graphically by three-dimensional surface plots.
2. Basic equations
An infinitesimal element of the radiative flux, dΦP(x, y, z), emitted by a point source, P, located at the point P(0, 0, 0), in the direction determined by the Cartesian coordinates x, y and z is defined by the expression 
where I(x, y, z) is the intensity of the radiation (radiant intensity) emitted by the source P and dωP(x, y, z) is the infinitesimal element of the solid angle subtended by the surface element dS(x, y, z) at the point P(0, 0, 0). The infinitesimal solid angle is defined as 
For the center 01 of the spherical object, at the distance z from the origin 0 of the coordinate system 0xyz, we can introduce an additional coordinate system 01 x 1 y 1 z as shown in Fig. 1. In these coordinates, Eq. (3) becomes
It is easy to see, that after integration of Eq. (4) with respect to the variables x 1 and y 1 it is possible to obtain expressions for calculating radiative flux as function of the x, y, and z coordinates determining the center 01 of the spherical object.
Transforming coordinates x 1 and x 2 as
where a and b denote half-lengths of the major and minor ellipse axes, respectively, and c 2=a 2-b 2.
The geometrical relationships in Fig. 2 lead us to the expression for the half-length of the minor ellipse axis
and from dependencies between some variables shown in Fig. 3 we can obtain the half-length of the major ellipse axis
and R is the radius of the spherical object.
and does not vanish for any angle φ. Therefore, Eq. (4) can be rewritten as
where rxy=(x 2+y 2)1/2.
It should be noted here that Eq. (13) determines the infinitesimal radiative flux incident on a spherical object when the center 01 of this object lies at the distance r>R from the point source P.
3. The total flux of radiation incident from a point source on a spherical object
The total flux, ΦP(x,y,z), of radiation (the total radiative flux) incident from a point source on a spherical object will be given as a double definite integral of dΦP(x,y,z;x 11,y 11) with respect to x 11 and y 11 with the limits for the variable y 11 given as the function of x 11 derived from Eq. (7). The limits for the variable x 11 are determined by the condition ρ-rxy-2a+b 2/a=0 and some geometrical dependencies between the parameters of the ellipse drawn in Fig. 2. Thus, the formula for ΦP(x,y,z) obtains the form
It can easily be seen that integrals in Eq. (14) may be further calculated if the radiant intensity function, I(x, y, z, x 11, y 11), is known. For some intensity functions, the inner integrals in Eq. (14) may be evaluated analytically. Then the obtained formula may be simply programmed and used directly in practice.
4. Radiative flux from an isotropic point source incident on a spherical object
In this section, we will present a particular solution to Eq. (14) obtained for radiation emitted by an isotropic point source. The intensity of isotropic radiation from a point source fulfils the relation
where I(x=0, y=0, z)=Iz=constant, and denotes the intensity of radiation emitted in the direction of the z-axis.
Applying Eq. (15) to the space subtended by the spherical object, we get
and Eq. (14) becomes
Equation (17) can be integrated analytically with respect to y 11 yielding
Evaluating the integrals in Eq. (18) with respect to x 11, we do not obtain any closed form expressions. However, an analytical solution to Eq. (18) exists and may be given by superposition of some elementary functions with elliptic integrals of all three kinds. This solution due to its very long and complicated form is not presented here. For our purposes, Eq. (18) was programmed for numerical simulation of some sample results.
5. Selected results of computer simulation
Selected results of numerical simulation from Eq. (18) obtained using Mathematica 2.2.3 software  are presented in Fig. 4(a) and (b). The data illustrate the total radiative flux, ΦP(x,y,z), normalized with respect to the total flux ΦP(x=0,y=0,z) incident on the object at the point 0′(0,0,z), as a dependency on x and y at given z, R, and ρ, Fig. 4(a), and as the function of y and z at given x, R, and ρ, Fig. 4(b). Figure 4(a) shows that the flux ΦP(x,y,z)/ΦP(x=0,y=0,z) depends on x and on y, analogously. However, Fig. 4(b) illustrates that the flux ΦP(x,y,z)/ΦP(x=0,y=0,z) is more sharply dependent on the variation of z than on the variation of y.
We have shown that the total radiative flux, ΦP(x,y,z), incident on spherical objects from any arbitrary point source models may be evaluated from Eq. (14) determined by two double integral expressions dependent on the radiant intensity function I(x, y, z, x 11, y 11). However, no general analytical solution to Eq. (14) can be given until the function I(x, y, z, x 11, y 11) is known. Usually, even then the radiant intensity function is known it is still difficult to obtain an analytical equation in closed form for ΦP(x,y,z) calculation. Therefore, in most cases, numerical procedures for double integral evaluation must be used to estimate the total flux ΦP(x,y,z) of radiation.
For some radiant intensity functions I(x, y, z, x 11, y 11) the inner integrals in Eq. (14) may be expressed in closed form. In these situations it is easy to calculate the flux ΦP(x,y,z) by applying one of many simple numerical procedures for single integral estimation. Sometimes the outer integrals in Eq. (14) may also be expressed simply. In most cases, however, the expressions obtained may be very complicated, so that the numerical procedures of single integral evaluation may be more practical than extended closed analytical solutions.
Equation (14) was then applied to evaluate the radiative flux emitted into the space subtended by the spherical object from an isotropic point source model. For this source model, the solution to Eq. (14) is represented by Eq. (18) described by two single integral expressions. These single integral expressions are calculated as a complex superposition of elementary functions with elliptic integrals of all three kinds. However, this complex solution was not presented in the paper and the flux ΦP(x,y,z) was calculated numerically.
Selected results of numerical simulations were presented in Figs. 4(a) and (b). The three-dimensional relationships of the radiative flux on the spherical object position with respect to the x, y, and z coordinates indicate that the flux is more sensitive to the variation of z than to x or y. These results indicate that the radiative flux changes considerably when the spherical objects moves in the radiation field from a point source. Such variation must be taken into account when systems for spherical object irradiation are designed. The calculation of the radiative flux may also be helpful in estimating the dose of radiation in the processes of spherical food product irradiation if these products are moving in the radiation field from a single point source or from several point sources.
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