acrylic and polycarbonate plastic greenhouse glazings allow liquid dyes
to be contained in hollow channels of the glazing as filtering materials.
In the 1970s and 1980s, liquid filters were widely investigated for filtering
out infrared radiation (heat) from sunlight as a mean to cool greenhouses.
Van Bavel noted liquid radiation filters reduce energy requirements by
20%-40% and virtually eliminate the need for forced ventilation in greenhouses.
The ability of various aqueous dye filters [red, green, yellow, blue, and
copper sulfate (CuSO4•5H2O)]
to selectively remove elongation-stimulating far-red light from the natural
spectrum and to reduce plant height was investigated in the late 1980s
in Norway and in the USA. Of the different liquid filters tested only liquid
CuSO4 filters were effective in removing elongation-stimulating
far-red wavelengths from the sunlight (Fig. 1). The CuSO4 liquid
filter reduced both red and far-red wavelengths, but the reduction of far-red
was greater than the reduction of red wavelengths thus, resulting in a
high R:FR ratio and high f.
Figure 1. Spectral distribution under liquid red dye (#259), blue dye #171 (CIBA-GEIGY, Greensboro, NC), and CuSO4 filters.