A radiation shield that has the same emissivity ε_{3} on both sides is placed between two large parallel plates at T_{1} = 650 K and T_{2} = 400 K with emissivities ε_{1} = 0.6 and ε_{2} = 0.9. Determine the emissivity of the shield if the radiation heat transfer between the plates is to be reduced to 15 % of that without the shield (answer as a decimal fraction, dimensionless).