A radiance measuring device comprises an imaging device, a light measuring device (2) and a connection seat (3) for connecting the imaging device with the light measuring device (2). The emission port of the imaging device corresponds to the incidence port of the light measuring device (2). The imaging device comprises an imaging lens (1-1) and an aperture diaphragm (1-2), and the aperture diaphragm (1-2) is located on the front focal plane of the imaging lens (1-1).