Dark Adaptation

One of the sensory adaptations of the retina to the intensity of light. If you suddenly go from a bright place to a dark place, you cannot see anything at first, but after a while you can see things. This phenomenon of your eyes getting used to the darkness is called dark adaptation.

Dark adaptation can be measured with a dark adaptometer. A constant light for adaptation is applied in advance, and the threshold for a constant test light in a dark place is determined. The dark adaptation curve obtained shows a bend about 7 minutes after entering the dark place. This is called the Kohlrausch inflection point. The degree of dark adaptation differs depending on the location of the retina, with the center having poor adaptability and the ability to adapt increasing toward the periphery. At first, the cones of the retinal photoreceptor cells are mainly active, and the adaptation of the cones increases their sensitivity by about 10 times. As dark adaptation progresses, the sensitivity of the rods increases and they take over from the cones. The adaptation of the rods increases their sensitivity by about 10,000 times. This causes the dark adaptation curve to show a bend.

It takes about an hour for dark adaptation to be complete, but on the other hand, it only takes a minute or two to go from a dark place to a bright place and no longer feel glare (photoadaptation). Dark adaptation corresponds to the time it takes for the visual red (rhodopsin) in the rods to be gradually synthesized in the dark. Visual red is synthesized in the dark and accumulates in the rods, making them more sensitive to light. A condition in which dark adaptation is impaired is called night blindness (commonly known as night blindness), and includes conditions such as retinitis pigmentosa, Oguchi's disease, retinitis albeit punctata, and idiopathic night blindness (vitamin A deficiency). Vitamin A is an important component in the synthesis of visual red, so a deficiency results in night blindness.

[Takashi Oshima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

光の強さに対する網膜の感覚順応の一つ。明るい所からいきなり暗い所に入ると、最初は何も見えないが、しばらくたってから物が見えてくる。このように暗さに目が慣れてくる現象を暗順応とよぶ。

　暗順応は暗順応計で測ることができる。あらかじめ一定の明順応光を与えておき、暗所で一定のテスト光に対する閾値(いきち)を求める。得られた暗順応曲線は、暗所に入ってから7分間ぐらいで折れ曲がりが生じる。これをコールラウシュKohlrauschの屈曲点という。暗順応の程度は網膜の場所により差が生じ、中心部は順応能力に乏しく、周辺部になるほど増大する。初めは網膜視細胞の錐体(すいたい)（錐状体）がおもに働き、錐体の順応で約10倍の感度を増す。暗順応が進むにつれて桿体(かんたい)（桿状体）の感度が高まり、錐体にとってかわる。桿体の順応によって感度は約1万倍に増す。このために暗順応曲線には折れ曲がりが生じる。

　暗順応が完了するのに約1時間を要するが、逆に暗い所から明るい所に出てまぶしさがなくなるまで（明順応）には1、2分しかかからない。暗順応は桿体内にある視紅（ロドプシン）が暗所で徐々に合成される時間に一致する。暗所で視紅が合成されて桿体に集まり、その働きを敏感にさせるわけである。暗順応が障害された状態を夜盲症（俗に鳥目）というが、網膜色素変性症、小口(おぐち)病、白点状網膜炎、特発性夜盲症（ビタミンA欠乏症）などが含まれる。ビタミンAは視紅の合成にたいせつな成分であるため欠乏症は夜盲になる。

［大島　崇］

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