How to choose a UV lamp for photoisomerization of 4,4'-azobenzene dicarboxylic acid? I have prepared a thin film containing 4,4'-azobenzene dicarboxylic acid and I want to study the photoresponsive behaviour of them. For the trans-cis isomeration , I used a 365 nm UV LED with irradiation intensity varied in the range of 5-50 mW/cm². Not seeing any isomeration I have the doubt if I am using the right lamp.  Articles which have prepared thin films containing 4,4'-azobenzene dicarboxylic acid have used other types of lamps such as 200 W Xenon-doped mercury lamp(using a 355 nm cut off filter), 500 W high pressure mercury lamp or 400 W high pressure mercury lamp. I also took the UV-Visible spectroscopy of my films and noticed that the absorption of trans azobenzene-4,4'-dicarboxylic acid is centered at 330 nm.

Am I using the right lamp?
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.