The light source must therefore be immensely brighter （主系表）
这是主句,the light source为主语,must be 为谓语动词,brighter为表语
therefore 和immensely为状语
if it is to be used as a headlight to illuminate the path
if it is to be used as a signal to others
两个条件状语从句为比较对象.
整个句子的意思：因此这种光源,如果被用作头灯来照亮路,比把它用来作为给别人的信号一定要明亮得多.