| 摘要 |
A filter comprises a solid or liquid photochromic material which becomes opaque to visible light when it absorbs a determined quantity of ultra-violet light, and an infra-red filter fixed on one side of it. In the goggles shown, the eye-pieces each comprise an infra-red filter 22, a photo-chromic filter 23 and an ultra-violet filter 24. The photo-chromic filter may comprise derivatives of spiro indolo benzopyran or may comprise known photo-chromic liquids arranged in a sealed flat container 23a. A dried solution of a mixture of several photo-chromic compounds, Acryloid B 72 (polymethylmethacrylate) and toluene is referred to. The <PICT:0973405/C4-C5/1> <PICT:0973405/C4-C5/2> photochromic material can be 0.001 of an inch thick. Several layers of photo-chromic material can be used. The goggles are reversible to allow constantly clear vision to be obtained.
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