7.1 Color-Separating and Color-Combining Prisms

7.1.1 Three-channel Philips RGB separating prism

One of the first commercial applications of trichroic separation prisms was developed by Philips Corporation for use in early color television cameras. Widely known as the Philips prism, it consists of three prism sections and is used today in three-channel charge-coupled device (CCD) video camcorders. Figure 7.1 shows one configuration.¹ Converging light from a camera objective lens is split into red, green, and blue components that are directed to separate photocathode or CCD receivers. The geometry must be such that the optical path lengths for each color are equal and there are two reflections for each split ray to preserve image orientation. Face 2 of prism A has a first dichroic layer that reflects green light and transmits red and blue light. There is a small air-gap separation between face 2 and face 3 of prism B. Reflected green light from face 2 is directed to the green sensor by TIR from entrance face 1. A second dichroic layer on face 4 of prism B reflects the red light to face 3, where it is directed to the red sensor by TIR. Prism C is optically bonded to prism B, and the transmitted blue light is directed to the blue sensor. The varying angle of incidence on each dichroic layer causes the transmission and reflection curves to be shifted toward shorter wavelengths. However, transmitted rays that are incident on the first dichroic layer at larger angles of incidence have smaller angles of incidence on the second dichroic layer, and vice versa. Hence, the wavelength shifts that occur in each dichroic compensate each other. This application of the Philips prism uses a single-pass mode with unpolarized light.