We use consult brand flu tests at work. Sometimes the positive line is hard to see by the naked eye. But if you shine a light behind the test cassette, you can clearly SEE the lines.

I made two versions. One is a small box you can stuff an LED light strip into and the other one you can use your cell phone to move the light around. You don't need this invention but it keeps you from blinding yourself. the box/led design so you don't have to use your cell phone in the lab.