For the objective comparison of different cameras or image sensors, it is necessary to describe their image acquisition channel by suitable parameters. The EMVA 1288 standard, developed by camera manufacturers and research institutes, distinguishes itself from other standards by considering the camera as a linear model. The camera is treated as a black box of which only pixel size and exposure time must be known. The recording of standardized test images is also omitted, allowing the camera to be described without optics. The only input variable of the linear camera model of the EMVA 1288 standard is the number of photons that hit a pixel of the image sensor during the exposure time. Therefore, the correct determination of the photon count is essential to calculate important camera parameters from the linear camera model, such as quantum efficiency or signal-to-noise ratio. To determine the number of photons, the irradiance of the radiation incident on the image sensor must be measured. This is usually done using a radiometer instead of the camera or image sensor. The number of photons per pixel during the exposure time can then be calculated from the irradiance, considering constants like the wavelength of the incident radiation, the area of the pixel and the exposure time of the camera. Calibrated radiometers were procured which are sensitive in the UV-A, VISNIR and NIR wavelength ranges in order to be able to perform camera or image sensor characterizations according to the EMVA 1288 standard in these wavelength ranges in the future.