Here is an estimation of the best spectrum for Chaetomorpha (based on an Action Spectrum of the green endolithic alga Ostreobium. An Action Spectrum determines the effect of monochromatic light on a parameter, say, oxygen production. There are some issues with this procedure, as it ignores the Emerson Enhancement Effect, but it should be pretty close.)
% Violet (400-430) 12.5%
% Blue (431-480) 22.2%
% Green-Blue (481-490) 4.5%
% Blue-Green (491-510) 5.4%
% Green (511-530) 4.2%
% Yellow-Green (531-570) 7.8%
% Yellow (571-580) 1.1%
% Orange (581-600) 5.2%
% Red (601-700) 37.0%
% Violet (400-430) 12.5%
% Blue (431-480) 22.2%
% Green-Blue (481-490) 4.5%
% Blue-Green (491-510) 5.4%
% Green (511-530) 4.2%
% Yellow-Green (531-570) 7.8%
% Yellow (571-580) 1.1%
% Orange (581-600) 5.2%
% Red (601-700) 37.0%