科学計測用カメラについて興味がある方
CMOS、CCD、EM-CCDそれぞれのセンサの構造や特性について説明します。
図1 : CCDイメージセンサーの模式図
CCDイメージセンサ (図1) の各画素は、フォトダイオードとポテンシャルウェル (=光電子を溜めるためのバケツ) で構成されています。
センサに入射した光は、電荷 (光電子) に変換されます。この波長に依存した光から光電子への変換効率が、量子効率 (QE) として表されます。
図2 : 光電子は画素の各列をリレーされ、リレー先の容器に画素単位で順次電荷が集められる
光電子は各バケツに蓄積され、読み出し時間になると、すべての光電子がバケツから次のバケツへと、画素の各列をリレーしていきます (図2)。電荷はピクセル単位で、リレー終了時に読み出し用のコンテナに集められます。コンテナに入った光電子は電圧に変換され、カメラの回路基板上で画像として処理されます。
光電子は共通のポートで信号 (電圧) に変換されるため、画像取得の速度に制限があります。
図3 : EM-CCDセンサの模式図
EM-CCDセンサ (図3) には、センサの読み出し前に光電子を増倍するための増倍レジスタという部品が追加されています (増倍レジスタについて詳しくはこちら)。しかし、この工程でエクセスノイズと呼ばれる別のノイズが発生し、このノイズの影響でカメラの実効QEは元のQEの約半分にまで低下します。
光電子は共通のポートで信号 (電圧) に変換されるため、画像取得の速度に制限があります。
図4 : CMOSセンサの模式図
CCDセンサやEM-CCDセンサとは異なり、CMOSイメージセンサ (図4) の各画素は、フォトダイオードとアンプの対で構成されています。
CCDセンサと同様に、CMOSセンサに入射した光は、量子効率 (QE) に応じて光電子に変換されます。しかし、CCDセンサとは異なり、光電子は各画素のフォトダイオードアンプペアで電圧に変換されます。電圧への変換はCCDセンサのように直列ではなく並列に行われるため、CMOSセンサでは画像取得をより高速化することができます。
科学計測用CMOSセンサの最適化されたアーキテクチャは、EM-CCDの増倍レジスタによってもたらされるエクセスノイズを排除し、高いQE、速いフレームレート、低い読み出しノイズを兼ね備えています。
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