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This refers to the slight photographic density that appears in the unexposed parts of photographic materials when they are developed. Generally, it is more prevalent in high-sensitivity materials and less prevalent in low-sensitivity materials. In a broader sense, it can also refer to stains or blackening caused by the following:
(1) Photosensitivity due to the entry of unnecessary light,
(2) Unnecessary development reactions proceed.
(3) Contamination caused by unnecessary chemical reactions during processing
To prevent (2), conditions must be adjusted to ensure an appropriate development speed, and antifoggants such as potassium bromide or benzotriazole are added to the developer to inhibit the reaction of unexposed silver halide. An example of (3) is the deposition of colored matter caused by the air oxidation of components in the developer, such as hydroquinone. To prevent this, a large amount of a weak reducing agent such as sodium sulfite is added to the developer to inhibit coloration caused by oxidation.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

写真感光材料の現像時に，未露光部分にわずかながら現れる写真濃度をいう．一般に，高感度の材料に多く，低感度の材料では少ない，広義には，次の原因による汚れ，または黒化などをいうこともある．
(1)不必要な光が入ったための感光，
(2)不必要な現像反応の進行，
(3)処理中の不必要な化学反応による汚れ，
などである．(2)を防ぐには現像速度を適当にするように条件を整えるほか，臭化カリウムやベンゾトリアゾールのようなかぶり防止剤を現像液中に添加して，未露光のハロゲン化銀の反応を抑制する．(3)の例としては，現像液中の成分，たとえば，ヒドロキノンの空気酸化による着色物の付着があるが，これを防ぐには，亜硫酸ナトリウムのような弱い還元剤を多量，現像液に加えて酸化による着色を抑制する．

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