出力が飽和する光量は、素子サイズ、入射スポット光サイズ、逆バイアスによりますが、約10 mW程度です。また、素子にダメージを与えないために、1 W/mm2以下で使用してください。
フォトダイオードのノイズは、ジョンソンノイズとショットノイズのそれぞれの2乗の和の平方根で表されます。ジョンソンノ イズは暗電流に依存します。暗電流は温度依存性をもっており、センサ温度が下がると暗電流も下がります。一方、ショットノイズは特殊な場合 (カットオフ波長付近)を除いて、温度に依存しません。温度が下がるとフォトダイオードのノイズも下がる傾向にあります。ノイズの計算式については、お近くの営業所までお問い合わせください。
光起電力素子は、光入射により素子に電流が流れます。これに対して光導電素子は、光入射により素子の抵抗 (電導度)が変化するもので、使用に際してはバイアス電圧または電流を加える必要があります。
必ずしもチョッピングする必要はありません。しかし、周囲環境などからの外来光を信号光と区別してS/Nを向上させるためにチョッピングする場合が一般的です。
以下に目安となるチョッピング周波数を示します。低周波側では素子自体の1/fノイズが支配的に、高速側では応答速度により制限されます。特に制約がなければフラットな領域を推奨します。なお、当社の検査時のチョッピング周波数は、InSb光導電素子の場合は1200 Hzです。
製品名 | チョッピング周波数 |
---|---|
InSb光導電素子 | 数kHz |
信号値は受光面積に依存しませんが、ノイズは受光面積の平方根に反比例します。S/Nは受光面積の平方根に比例します。
専用の再排気治具(メタルデュワ用バルブオペレータ A3515)を利用して排気装置に接続し、再排気を行ってください。なお、当社でも再排気作業を承っています (有償)。
液体窒素注入時に発生する白煙が見えなくなってから、10分程度経過したら使用することができます。
メタルデュワの窒素保持時間は購入直後は20時間程度ですが、徐々に低下し、1年後には10時間程度になります。そのため数年に1度は再排気が必要です。
物体の温度が高いほど放射エネルギーは大きく、その波長分布は短波長側にシフトします。反対に温度が低いと、エネルギーは小さく、その波長分布は長波長側にシフトし短波長側の強度は急激に小さくなります。そのため対象物の温度が低くなると長波長帯のセンサの利用が必要になります。おおよその目安として、以下に対象物の温度に適した検出器を示します。
対象物の温度 | 検出器 |
---|---|
500 ℃~ | Si |
200 ℃~ | InGaAs |
100 ℃~ | InAs |
50 ℃~ | InSb, InAsSb (8 μm帯) |
0 ℃~ | InAsSb (10 μm帯) |
-50 ℃~ | サーモパイル, TypeⅡ超格子赤外線検出素子 |
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