does putting a glass lense on your hood decrease lumens or anything?

got a aircooled hood and i was wondering if i put a glass lense on my hood ( to keep the heat in hood to be exhausted quicker) does this block out some of the lumens or anything? also, do i have to use tempered glass or can i use a piece of plexiglass

I am pretty sure plexi will just melt.

Anything you put between the light and the plant will decrease the lumens. Tempered glass is MUCH better than plexiglass; acrylic will deform, as bigv said. It might be fine during the winter but during the summer, you'll be really glad you have the tempered glass. You can probably get a piece locally from a glass shop. I've found that the pieces that come with the reflectors are barely big enough to work; in fact, I used silicone to permanently seal mine into place after my last grow. I can change the bulb through the 6" holes and now I don't have to worry about the glass falling out.

Regular tempered glass will only block the shorter light waves that are not neccessary to grow pot.

a good glass lens only blocks the the high ultraviolet rays. The amount of light that is blocked that is beneficial to your plants is practically nothing.