Device (10; 50; 100; 150; 200; 250; 300; 350) for determining the magnetic flux density with a first material (18; 358) which generates a luminescence and/or a photocurrent on physical excitation, the luminescence or the photocurrent being dependent on the magnetic flux density, means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation and means (24; 54; 104; 160; 212; 262; 312; 362) for measuring the luminescence or the photocurrent with a detection signal which represents the strength of the detected luminescence signal or photocurrent signal, wherein the first material (18; 358) and the means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation are formed in one piece and/or the first material (18; 358) and the means (24; 54; 104; 160; 212; 262; 312; 362) for measuring the luminescence or the photocurrent are formed in one piece, characterized in that the means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation are formed in one piece. of the photocurrent are formed in one piece, characterized in that the means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation comprise a source (14; 60; 108; 156; 204; 254; 304; 354) and conversion means (20; 308; 356), the source (14; 60; 108; 156; 204; 254; 304; 354) generating a first physical excitation in the light spectrum 200 nm to 500 nm and the conversion means (20; 308; 356) converting the first physical excitation into a second physical excitation in the light spectrum 500 nm to 600 nm. a first material (18; 358) which generates a luminescence and/or a photocurrent on physical excitation, the luminescence or the photocurrent being dependent on the magnetic flux density, means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation and means (24; 54; 104; 160; 212; 262; 312; 362) for measuring the luminescence or the photocurrent with a detection signal which represents the strength of the detected luminescence signal or photocurrent signal, wherein the first material (18; 358) and the means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation are formed in one piece and/or the first material (18; 358) and the means (24; 54; 104; 160; 212; 262; 312; 362) for measuring the luminescence or the photocurrent are formed in one piece, characterized in that the means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation are formed in one piece. of the photocurrent are formed in one piece, characterized in that the means (14, 20; 60, 20; 108, 20; 156, 20; 204, 20; 254, 20; 304, 308; 354; 358) for generating the physical excitation comprise a source (14; 60; 108; 156; 204; 254; 304; 354) and conversion means (20; 308; 356), the source (14; 60; 108; 156; 204; 254; 304; 354) generating a first physical excitation in the light spectrum 200 nm to 500 nm and the conversion means (20; 308; 356) converting the first physical excitation into a second physical excitation in the light spectrum 500 nm to 600 nm.