Back then, they were primarily fluorine-doped tin oxide coating at about 3000 nm thickness. No doubt now they are thicker to be more thermally efficinet, and sputtering has come back in the fold for really high-end applications.
Nonetheless, while the films do lower the percentage of transmitted light, they do so fairly uniformly accross the psectrum of light used by plants. In that old film, the reduction was about 25% at the blue end of the spectrum and about 20% at the red end. Think of it as built-in shade cloth. I reproduced a spectrum my guys generated in the lab here:
http://www.firstrays.com/plants_and_light.htm
Ray Barkalow - First Rays Orchids - www.firstrays.com Plants, Supplies, Artwork, Books and Lots of Free Info!
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