How does the vitamin d rapid self-test work? Why do I need an app?

The test system of the kiweno vitamin d rapid test consists of an immunological rapid test, which detects vitamin D (25(OH)-vitamin D) using specific antibodies, and a smartphone app, which quantitatively evaluates the result of the rapid test based on the colour intensity of the control and test band. The result is displayed and stored on the smartphone in use. The system requirements and supported devices can be found here.