They don't work if you put a hood on 'em as they gather the light and shine it back at you........ I keep a spare in that little spidertrap in the stock for emergencies and find that even with the rough treatment my rifle gets, I only have to change it about once every 2 or 3 years.

FWIW, when I'm doing a night time follow up, I tape a small surefire to the side of the barrel where I can flick the button with my thumb and also wear a headlamp that shines a bit of light onto the foresight with most of the light from that shining where I'm looking and it works a treat for me.

Also, FWIW, I also give my tracker my Surefire (blue light) Kroma which although helluva expensive, is worth it's weight in gold in these situations.

Have you swept the visioned valley with the green stream streaking though it?
Searched the vastness for a something you have lost?
Have you strung your soul to silence? Then for God's sake go and do it
Hear the challenge, learn the lesson, pay the cost