Conformity Magazine - August 2008 - (Page 56)

are two main possibilities to reduce the domain as shown in Figure 3: 1) the reduced computational domain Ω is a volume containing the eyeball; 2) the reduced computational domain Ω coincides with the only eyeball. For both possibilities it is crucial to define the adequate boundary conditions to be applied on the boundaries ∂Ωi internal to the human head which are necessary to truncate the computational domain. The boundary conditions of (2) on the surfaces ∂Ωi can be of Dirichlet type as: Equation 6 Figure 3: Reduced computational domains and boundary conditions: cubic reduced volume containing the eyeball (a) and the eyeball reduced volume (b). Tissues Skin Muscle Bone Fat Nerve Gray Matter White Matter Sclera Choroid Retina Vitreous Humor Aqueous Humor Post Chamber Lens Lens Muscle Cornea C K B [J/(Kg °C)] W/(m °C)] [J/(s °C m3)] 3500 3600 1300 2500 3600 3700 3600 4200 3900 3700 4178 3997 3997 3000 3430 4200 0 .42 0 .50 0 .40 0 .25 0 .503 0 .57 0 .50 0 .58 0 .51 0 .56 0 .58 0 .58 0 .58 0 .40 0 .498 0 .58 9100 2700 1000 520 35000 35000 35000 0 85000 9500 0 0 0 0 2700 0 A [W/m3] 1000 690 0 180 10000 10000 10000 0 20000 2500 0 0 0 0 690 0 or of the generalized Neumann boundary conditions as: Equation 7 where H is the convection coefficient that takes into account mainly the convective effect and Te is now the fluid temperature of the tissues internal to the human head, which in many cases can be assumed to be equal to the body-core temperature (Te = 37 °C). It should be noted that T∂Ω and F∂Ω are not easy to known because in the real human head the convection coefficient H and the fluid temperature Te are function of the position, time and exposure. In our study we perform different simulations to find the reduced domain with adequate boundary conditions which leads to a good approximation of the thermal elevation in the eye zone, which is obtained by subtracting the normothermal distribution from the calculated temperature T [6]. In particular we perform a numerical thermal simulation of the whole head by using a coarse grid with cell size of 1.0 mm in order to identify the appropriate boundary conditions that can be applied in the reduced domains. Thermal Parameters The thermal parameters of tissues are shown in Table 1. Note that the values shown in [6] are used, with the addition of the thermal properties of choroid, retina, post chamber and lens muscles. Due to the lack of experimental data, the same material properties of muscle and aqueous humor have been used respectively for the lens muscles and post chamber, while new values of blood perfusion and metabolic rate are here adopted for the retina and choroid layers. The term related with blood perfusion (B) is obtained by means of the same weighted procedure applied in [6], thanks to recent researches on subjects with systematic vascular diseases. Instead, the metabolic rate of these tissues are not so easy to be found and so we have assumed for the retina the same of the grain matter and a value proportional to the blood flow for the choroid (see Table 1). It should be noted that the blood perfusion of choroid layers are higher than those of any other tissue, reflecting the fame of highly vascularized tissue cited in the literature. Table 1: Thermal properties of tissues Figure 4: Calculated temperature T map for the vertical dipole antenna exposure (P = 1 W, f = 450 MHz, d = 3 cm) in the whole head (Δ = 1.0 mm) where the cubic reduced volume (solid black line) and eyeball volume (dotted black line) are highlighted. 5 Conformity AUGUSt 2008

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Conformity Magazine - August 2008 Contents Editor’s Note FCC Seeks Comment on Reauction of 700 MHz D Block Commission Cracks Down (Again!) on Junk Fax Marketer Canada Levies Fees for Public Safety Radio Spectrum FDA Issues Guidance on Medical Device Tracking Greenpeace Challenges Video Game Manufacturers The IEEE EMC Society: A Proud History of Accomplishments ESD Open Forum An Update on Changes to the Automotive ESD Standard, ISO 10605 Modeling of Radiated Electromagnetic Disturbances in Automotive Applications Design Issues in Automotive Radio Frequency Systems IEEE EMC Symposium IEEE 2008 Show Preview Exhibitors Buyer’s Guide Product News Most Eco-Friendly Companies Updated Standards List For the EU’s Pressure Equipment Directive Commission Issues Updated Standards List For Directive on Gas-Fired Appliances HHS Releases Fact Sheet on U.S. Imports from China Customs Agents Seize Holiday Lights IEEE PSES Symposium Set for October CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Product Reviews Advertisers

Conformity Magazine - August 2008

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