How to choose a UV lamp for photoisomerization of 4,4'-azobenzene dicarboxylic acid?
I'd probably try a more powerful lamp. Plain low-pressure mercury tubes have their emission maximum at 254 nm, which isn't what you want here. But these tubes also come in various coated variants with emission maxima around 300 or 365 nm.

A reasonable choice would be a Rayonet RPR-3000A or a Luzchem LZC-UVB lamp in a proper housing.