In accordance with the hot issues of infrared detection and guidance, an IR transmission process from target to sensitive array was established to explore the IR signatures of an anisotropic-isothermal sphere model. The energy probability density distribution function was employed to describe the energy intensity levels of sensitive array units. The energy distributions of detector units were abtained with the aid of the Monte Carlo Method. The relationship between target directional IR radiation characteristics and energy distributions on the sensitive array was revealed. It indicates that in the case of the equal hemispherical emissivity of far-field small targets, increasing the emissivity in the near-normal direction can appreciably enhance the IR intensity of the image-center unit and improve the IR intensity contrast among the image units.