【教えて下さい】「No salt zone ends」の意味

The image recording medium has on a substrate an image recording layer which contains a compound producing the change in the density in an absorption zone of 360-900 nm under the action of heat or an acid and contains no silver salt and a protective layer which has a coating amount of binder of 3 g or more per 1 m2.
支持体上に、熱または酸の作用により３６０〜９００ｎｍの吸収域に濃度変化を生じる化合物を含有し、かつ銀塩を含まない画像記録層と、バインダー塗布量が１m^2あたり３ｇ以上保護層とを有する画像記録媒体

http://ejje.weblio.jp/content/No+salt+zone+