Well that chamber, being the type pf plastic it is, might block out a lot of the needed spectrum of light. Do you have a different plastic maybe?
Also I'm just gonna throw this out there, at work I deal with UV lights almost all night long. We had to make these lights "shatter proof" and so we put the florescent tube covers over them. They are nothing more then a thin plastic nothing special just to cover the bulbs in case of breakage.
What I found out is even though the tubes were not listed as UV blocking DID block a lot of the UV light. Also with more study, trial and error, I found a lot of plastics do block a good portion of UV light, which could also block other usable light for plants. Now I know to much UVB can harm plants, UVA is still debatable about plant use but it gets close to the range at which plants start photosynthesis.

200 - 280 nm UVC ultraviolet range which is extremely harmful to plants because it is highly toxic.
280 - 315 nm Includes harmful UVB ultraviolet light which causes plants colors to fade.
315 - 380 nm Range of UVA ultraviolet light which is neither harmful nor beneficial to plant growth. Research being conducted and is debatable)
380 - 400 nm Start of visible light spectrum. Process of chlorophyll absorption begins. UV protected plastics ideally block out any light below this range.
400 - 520 nm This range includes violet, blue, and green bands. Peak absorption by chlorophyll occurs, and a strong influence on photosynthesis. (promotes vegetative growth)
520 - 610 nm This range includes the green, yellow, and orange bands and has less absorption by pigments.
610 - 720 nm This is the red band. Large amount of absorption by chlorophyll occurs, and most significant influence on photosynthesis. (promotes flowering and budding)
720 - 1000 nm There is little absorption by chlorophyll here. Flowering and germination is influenced. At the high end of the band is infrared, which is heat.
1000+ nm Totally infrared range. All energy absorbed at this point is converted to heat.
If anyone can provide more info on this help me out. I don't want to give bad info, this is just what I've researched myself. Thanks