Is there a material suitable for use as RF shielding that would allow at least 50% light transmission in the visible range? Thinking about a sensitive light receiver.
Yes, this is actually a pretty common product in these security conscious days: RF shielded glass or add-on plastic sheeting. Many firms market this for the construction of office buildings where you want to reduce EM emissions to prevent eavesdropping.
Picking one of these at random, Holland Shielding Systems BV markets a glass that transmits 84% of the light and attenuates RF between 24 dB and 98 dB depending on frequency. I imagine this is a spendy product, but the company website I looked at offered free samples; considering your project, that might be enough.
Another company, LessEMF, sells a variety of plastics that shield RF and can be added to existing windows. This firm lists prices that seem to start around 29 USD per linear foot with the following characteristics: