Development of THz technology has been an interest research topic over the past decade due to its unique radiation characteristic. Recent development in THz technology enables THz radiation for various imaging applications such as remote sensing, biomedical and security detection because it has decent advantages over the old techniques of X-Ray and microwave imaging. For imaging applications, a system must meet the requirement of a high gain or narrow beamwidth radiation to produce a high-resolution spectra output. A high radiation efficiency is also the considered parameter because it will affect the amount of the radiated energy to the target. The matching layer (ML) technique is a common technique to minimize the internal reflections and improve the radiation efficiency performance of the optical based antenna such as lens. In the previous research, we have studied a bow-tie planar antenna combined with Silicon dielectric substrate and extended hemispherical lens for 1 THz resonant frequency. The ML method has also been applied in both single and cascaded technique to increase the radiation efficiency. However, the complexity of the design is one of the main concern because high number of stacked MLs used to achieve maximum radiation efficiency. In this paper, we study the effect of an optimum ML to the lens antenna. An optimum ML has different layer thickness among the lens surface to covers the different angle of incidence. From the simulation results, radiation efficiency can be increased up to 92.06% while the high gain performance can be kept consistent around 32 dB with a narrow beamwidth of below 3°. It shows that our proposed design is suitable for future THz imaging applications.