Radiation heat transfer between two surfaces can be reduced greatly by inserting a thin, high-reflectivity (low-emissivity) sheet of material between the two surfaces. Such highly reflective thin plates or shells are called radiationshields. Multilayer radiation shields constructed of about 20 sheets per cm thickness separated by evacuated space are commonly used in cryogenic and space applications. Radiation shields are also used in temperature measurements of fluids to reduce the error caused by the radiation effect when the temperature sensor is exposed to surfaces that are much hotter or colder than the fluid itself. The role of the radiation shield is to reduce the rate of radiation heat transfer by placing additional resistances in the path of radiation heat flow.The lower the emissivity of the shield, the higher the resistance.

Radiation heat transfer between two large parallel plates of emissivities$ε_1$ and $ε_2$ maintained at uniform temperatures T1 and T2 is given by

$Q_(12,no shield)$=$\frac{Aσ(T_1^4-T_2^4)}{(\frac{1}{ε_1} +\frac{1}{ε_2} -1)}$

Now consider a radiation shield placed between these two plates, as shown in Figure

Fig: The radiation shield placed between two parallel plates and the radiation network associated with it.

Let the emissivities of the shield facing plates 1 and 2 be ε_3,1and ε_3,2, respectively. Note that the emissivity of different surfaces of the shield may be different. The radiation network of this geometry is constructed, as usual, by drawing a surface resistance associated with each surface and connecting these surface resistances with space resistances, as shown in the figure. The resistances are connected in series, and thus the rate of radiation heat transfer is

$Q_(12,one shield)$=$\frac{(E_b1-E_b2)}{\frac{(1-ε_1)}{(A_1 ε_1 )}+\frac{1}{(A_1 F_12 )}+\frac{(1-ε_3,1)}{(A_3 ε_31 )}+\frac{(1-ε_3,2)}{(ε_(3,2) A_3 )}+\frac{1}{(A_3 F_32 )}+\frac{(1-ε_2)}{(A_2 ε_2 )}+\frac{1}{(A_3 F_32 )}+\frac{(1-ε_2)}{(A_2 ε_2 )}}$

Noting that F13 =F23 = 1 and A1 = A2 = A3 =A for infinite parallel plates,