Recently, i added some protective acrylic glass in front of the LEDs. Before, i was afraid that some plants could grow into my lamp, if i don't pay enough attention - well, that's unlikely, but you never know. The glass doesn't seem to impact efficiency too much. 

Acrylic glass does reflect about 8% of incident light (at 90° to surface), and almost 92% are transmitted. Of those 8% that are reflected, most will be reflected again by the white PCB, making a second attempt to pass the glass plane. This again will be 92% successful. In the end, the efficiency will most likely be reduced by 1-2%.
On the plus side, the light distribution will be a bit more uniform, so shadows are not that harsh. 

Also, i finally put some near UV LEDs onto the board. I don't think they will help too much in irradiating the plants, since the are really low-powered, but they're nice to have. 

To have any effect on my plants, however, the UV LEDs probably should have some shorter wavelengths as well as more power. It has been shown that UVB radiation can alter the content of carotenoids in some plants: https://www.sciencedirect.com/science/article/pii/S1319562X10001063
And those seem to work well as antioxidants in the human body: https://www.researchgate.net/publication/9035299_Antioxidant_activity_of_carotenoids

So, UV light might be good to have, but i doubt that my weak UVA lights will have any chance of doing anything useful. But at least i can try to observe some differences.