InGaAs/InP‐photodiodes from different manufacturers have got rather low noise level, a good response uniformity over the sensitive surface and a wide dynamic range. Therefore they are good devices to built radiometers in the NIR spectral region. As in any photodiode, the spectral short‐circuit responsivity is determined by the wavelength and the photodiode’s reflectance and internal quantum efficiency. Then if these quantities were known, the photodiode’s responsivity would be known without being compared to another standard radiometer; i. e. the photodiode would be an absolute radiometer for optical radiation measurements. This idea was firstly developed for silicon photodiodes in the eighties, once the technology was able to produce low defects photodiodes[1]. Following this reference, the reflectance could be approached from a superimposed thin layers model. By knowing the thicknesses of the layers and the optical constants of the materials, it is possible to determine the device reflectance. However, this information is not completely available for InGaAs/InP photodiodes: the actual thickness of the layers is not known and optical constants of materials are only approximately known for bulk. Nevertheless it’s possible to measure reflectance at some wavelengths and to fit the thicknesses of a layer model that would reproduce those experimental values. The internal quantum efficiency cannot be determined as in [2], since InGaAs photodiodes are hetero‐junctions rather than homo‐junctions as silicon photodiodes are. In the other hand, since the internal structure is not accurately known, it is not possible to model the internal quantum efficiency without having experimental values for it. Therefore the attainable scope at present is just to obtain a model to be able to calculate spectral responsivity values at any wavelength. To get this, a model has been developed to calculate reflectance values from experimental ones at some wavelengths and another model has been developed to interpolate spectral internal quantum efficiency values from some values got from reflectance and responsivity measurements at some wavelengths. Both models will be presented in this communication

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