A day and night MOS imager spectrally adjusted for a wide range of color temperatures
Shinzo Koyama, Keisuke Tanaka, Mitsuru Muguruma, Manabu Usuda, Kazuo Fujiwara, Toshinobu Matsuno, Yutaka Hirose, Yasuhiro Shimada · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2009
We present a day and night MOS imager based on a single plate on-chip interference color filter. The filter comprises periodic multiple layers of TiO2 and SiO2, with an intermediate color selection layer (SiO2) to disturb the period of the layers, analogous to a "defect" layer in the one-dimensional photonic crystal. A particular advantage of this filter is flexibility of designing a spectral profile of each color. Thus, one unit cell of the present MOS imager is designed to have three multi-spectral, i.e. R+IR, G+IR, B+IR, pixels and one IR dedicated pixel, which would never be realized by ordinary pigment materials. Daytime color image signals are obtained by subtracting the IR pixel signal, as a reference, from each signal of R+IR, G+IR and B+IR pixels. Nighttime black and white imaging is simply realized by using the IR components of all the pixels as brightness signals. This enables seamless switching between the day and night operations of a camera. Although the subtraction operation usually reduces the dynamic range (DR) and signal-to-ratio (SNR), in particular at low color temperatures, we overcome the issues by employing a new design scheme of the color filter comprising double defect layers for each visible pass band and narrow IR pass bands for common IR components. As a result, signal degradations in SNR and DR are suppressed even at low color temperatures enabling daytime imaging in a wide range of color temperatures from 2300 K to 6500 K.