## 2 Materials and Methods
### Processing of RGB Color Sensors
#### 2.2.1 Physical Structure and Working Principle of RGB Color Sensors  
An RGB color sensor basically comprises three main components including the underlying semiconductor structure of the photodiodes, the array of optical filters defining its spectral sensitivity, and the electronic circuit used for signal readout [110]. Its physical principle is illustrated in Figure 1. As incident light hits the surface of the RGB color sensor, it is filtered by the red, green, and blue optical bandpass filters attached to the photodiode layer, which combines at least three single photodiodes, i.e., one for each color channel. Without the bandpass filters, a typical silicon photodiode would be sensitive to wavelengths from the ultra-violet up to the infrared region with an absorption maximum located in the range from 800 nm to 950 nm. The different transmissive color filters are therefore intended to reshape and optimize the spectral response of the color sensor to mimic the trichromatic vision capabilities of the human eye. The correspondingly transmitted photons are thus converted into respective photocurrents, whose amplitudes depend on the spectral composition and irradiance of the incident light. These typically low currents can subsequently be converted into amplified proportional voltage signals using dedicated electronic circuits, such as transimpedance amplifiers or other suitable types of current-to-voltage converters. An analog-to-digital converter (ADC) is eventually applied to convert the amplified voltage outputs into digital signals and, thus, prepare them for further digital processing. Alternatively, a light-to-frequency integrated circuit architecture can be used to directly convert the photocurrents into pulse trains or square waves, whose frequencies are proportional to the irradiance of the correspondingly filtered light [111], which facilitates multiplexing and increases immunity against noise on signal transmittance.  
For a schematic overview, Figure 2 summarizes the different structural layers of a color sensor. The first layer comprises the optical elements, such as the housing and aperture, optical lenses, or diffusor plates, that are indented to guarantee a proper, conceivably cosine-corrected irradiation of the sensing area or to adjust the color sensor's angle of view according to the application requirements. The second layer is composed of the optical bandpass filters \(R^{*}(\lambda)\), \(G^{*}(\lambda)\), and \(B^{*}(\lambda)\) that determine the sensor's spectral sensitivities \(R(\lambda)\), \(G(\lambda)\), and \(B(\lambda)\). The third layer represents the array of underlying photodiodes, i.e., the actual semiconductor layer, which is responsible for converting the filtered light into corresponding electrical currents. The fourth layer summarizes the electronic circuitry required to convert these photocurrents into proportional, usable signals. The fifth or ADC layer is optional depending on whether a standard current-to-voltage or an integrated circuit architecture is used. In the case of the former, the ADC layer is required to convert the amplified voltage outputs into digital signals. The sixth layer comprises the digital and computational processing of the digital raw sensor signals including the implementation of  
Figure 1: Physical and electronic principle of an RGB color sensor.  
potential compensation and correction strategies (e.g., to correct for changes in temperature and humidity [112]), which eventually gives the final \(RGB\) readouts.