It's a 60s or 70s SF novel. Maybe by a French or Japanese author.
The plot is that some prominent scientists suddenly dies in strange occasions. It turns out they finds a kind of eye drops enabling them to see previously invisible light globes floating in the sky. These light globes suck on human's emotions and thoughts for food. The light globes would kill anyone who detects their existence. The victims can only escape the fate if they can keep their minds occupied by other things when they see the globes around. At the end, some scientists develop a sort of reflective antenna that can burn the light globes and won the war with them.