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A photodiode is a type of photodetector capable of converting incident light into either current or voltage, depending upon the mode of operation. The substrate used to make the photodiode sensing part is critical to defining its properties. In particular, only photons with sufficient energy to excite electrons across the material's bandgap will induce photocurrents. Materials commonly used to produce photodiodes include: Silicon (for the wavelength range 190-1100 nm), Germanium (for 400-1700 nm), Indium-Gallium arsenide (for 800-2600 nm) and Lead (II) sulphide (1000-3500 nm), among others. In many applications the semiconductor diode is placed within an opaque housing with different possible sizes (such as TO18, TO5, TO3..) and hermetically sealed with a cover of a material that allows transmission of the desired wavelength range of observation and permits radiation to reach the sensitive part of the device. Some applications require measurement of one specific wavelength range and it is essential that other radiation wavelengths of the range where the substrate shows responsivity do not contribute to the photodiode's current. For this purpose a filter is mounted above the detector to select the wavelength range of responsivity. Common configurations of filtered photodiodes include a metallic platform to hold the filter at an intermediate height between the cover window and the detector, see Figure 1. This mechanical configuration allows the implementation of different filters preserving the external packing dimensions and physical properties of the photodiode sealing with independence of the filter type. The geometry of arrangement of these elements (cover, housing, and sensing device) defines the nominal field of view (FOV) or geometrical FOV of the sensor. A careful look at this configuration shows that, because of secondary internal reflections against the inner walls of the sensor housing, the sensor may also be excited by photons from trajectories with incident angles greater than this nominal geometrical FOV. Notice that the filter crystal is thick, flat and cut in square shape. Its implementation on top of the sensing dice covers the nominal FOV but leaves a fraction of the photodiode horizontal cross-section uncovered, see Figure 2. As we will explain later, this will have implications for non-normal, noncollimated and diffusive irradiance sources. |
