Block diagram of instrument setup. DDS: direct digital synthesis. moḏf1 and moḏf2: diode modulation frequencies. LD: laser diode. TEC: thermoelectric cooler. and : wavelengths 1 and 2. DSP: digital signal processor.
Communication interface between administering microcontroller and direct digital synthesizers.
Progression of signal conditioning required before synthesized waveform may drive laser diode. Steps include an initial gain of the output DAC signal followed by a bandwidth narrowing filter. The final stage consists of a variable gain and offset adjustment.
Block diagram of general detection layout. Detector channels contain analog conditioning circuitry and are then sampled by the ADC. Digitized data are transferred to the DSP under precise control of the CPLD. DSP performs digital phase sensitive detection and carries out system management tasks.
Signal flow and computational sequence of extracting amplitude and phase using the quadrature digital lock-in detection method.
(Color online) Frequency response of 150-point averaging filter sampled at . The frequency axis runs till , and magnitude is calculated in decibels.
Flow chart of DSP operations during an imaging cycle.
State machine flow of imaging progression carried out by the CPLD.
Illustration of sample read process. Two ADC’s are accessed simultaneously, each direct two detector channels worth of data over separate data port lines.
The timing diagram during the data acquisition sequence for each source position. Most of the time constraints are caused by the settling time of the optical switch and analog electronics. The lock in and data transfer for source occurs in parallel with the settling time of source .
(Color online) Interconnection between digital DAQ board and DSP timing board. The upper region shows the signaling for gain-bit transfers while the lower region displays the imaging protocol.
(Color online) Gain control panel. This interface is used to set, confirm, and test the gain settings for all source-detector pairs. An automatic calibration is available to determine the optimal gain settings in less than .
(Color online) Image acquisition control panel. This screen is used to manage the imaging process and monitor incoming data. Numerous display options are available and configurable on the fly.
Mean rms dark-measurement readings for each gain setting.
Linearity curve of instrument detection over measureable dynamic range.
Mean CV of detector channels as a function of gain setting.
Experimental setup for phantom experiments shown in cross-sectional view. (a) Single inclusion rotated clockwise around cylindrical vessel, (b) single inclusion translated across vessel, and (c) double inclusion apart rotated 90°.
Time traces of all detectors for a sample source position (src 1) wavelength of during the first dynamic phantom experiment. Responsivity of each channel changes as the absorbing target moves.
(Color online) Cross-sectional reconstruction results from the second phantom experiment. Images taken from the time of the inclusion is immersed and every thereafter until the translation across the probe is complete. The recovered object also seems to follow the object pathway.
(Color online) Reconstruction results from the first phantom experiment. Images taken from the time of the inclusion is immersed and every thereafter until a complete rotation is formed. The recovered object is observed rotating in a counterclockwise direction.
Reconstruction results from the third and final phantom experiment shown in the three dimensional (3D) volumetric mesh. The left side represents the initial positions of the inclusions and the right side depicts the final positions.
Performance characteristics of digital instrument.
Comparison chart between first generation analog and second generation digital optical tomography systems.
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