Red Light On Helmet with Rucksack — Invisible

I have seen this several times now.  Please don’t ride with only a red light on your helmet. As you tip your head forward, if you are wearing a rucksack, your red light is invisible from the rear. 

I came up on a cyclist today and told him he was invisible. He pointed to the red light and I pointed to his rucksack and said it was hiding it.  His other red light wasnt working