With the development of LED illumination technology, LED illuminating devices are used more and more in various environments. In order to improve the optical efficiency and to realize predetermined light-distribution pattern, an LED illuminating device is usually equipped with a lens for light sources. In a commonly used light box, a lens having a strip profile covers light sources in a linear arrangement and is mounted together with the light sources in a box-shaped transparent housing, especially at the bottom of the housing. Such lens preferably is a total-internal reflection lens (TIR). Light emitted from the light sources can, for instance, strike on the top of the housing from the bottom, and part of light can strike on side walls of the housing. Such light box is particularly suited for realizing decorative illumination, for instance, used as advertisement light box.
However, since the lens installed at the bottom of the light box is usually configured as a total-internal reflection lens, light emitted from the light sources only can strike on the top of the light box through a lens surface used as a light emergent surface. For an area from the lens surface to the top of the light box, such conventional total-internal reflection lens leads to low luminance in the bottom region of the light box, and a shadowed region at the bottom of the light box can be seen obviously as viewed from outside of the light box. Based on such illumination effect, aesthetics and applicability of the light box are affected to different degrees.