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A compact and versatile snapshot-type multi-channel spectrometer was constructed by mounting a patterned spectral filter array onto a CCD area sensor. The individual filter element is consisted of a wavy Nb2O5/SiO2 multilayer (2-D photonic crystal) fabricated by the rf-bias sputtering process. By utilizing the distributed passband characteristics of the filter and the spectral estimation algorithm (Wiener estimation method), we confirmed that the incoming spectra at red to near infrared band could be estimated with good accuracy. We investigated the measurement performance for fruits, which is one of the most important targets of the near infrared spectroscopy, by simulating the realistic condition with optical shot noise.

We propose and demonstrate a compact snapshot-type multispectral image sensor. The sensor consists of a CCD area sensor and a photonic crystal (PhC) type patterned spectral filter array. The perpendicular refractive index profile of the PhC roughly determines the spectral response, while the in-plane periodic structure slightly modifies it. As a result, a multi-patterned filter array with each spatial region exhibiting different transmission characteristic. The filter was directly mounted onto a CCD to construct an imaging spectrometer. Application to near infrared inspection of agricultural products (fruits) was investigated. Spectral reflectivity images at wavelengths between 610nm to 810nm were predicted with the aid of Wiener estimation algorithm.

We fabricated 16-, 25-, 36-, and 64-channel distributed passband-type multi-spectral filter arrays by utilizing a multilayer-type photonic crystal and integrated them onto a CCD to form a snapshot-type spectroscopic sensor. Reflection spectra from target objects (fruits) under broadband light illumination were estimated directly using the Wiener estimation method. A root mean square error of the reflectivity on the order of 2 ∼ 5 % was obtained under optical shot noise with 6 × 6 pixel binning. A number of constituent filters of 36 was sufficient for this type of fruit spectral measurement. We also visualized reflection images at specified wavelengths by applying the estimation method to a multiple filter region on the sensor.