这个网友显然不熟悉比较结构。另外was是谓语动词，不是修饰语。这个句子含有more...than...比较结构，than从句有省略。

Mann was determined not to use more artificial light than was absolutely necessary.

= Mann was determined not to use more artificial light than it was absolutely necessary to use.

than从句省略了形式主语it 和外置主语to use。use的宾语为比较对应项，必须省略。