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Measuring and improving Time-of-Flight camera light source electrical-to-optical power conversion efficiency

Time of Flight (ToF) cameras capture three-dimensional depth images of a scene. They do this by transmitting a modulated light signal from a light source, which scatters off objects in the scene and returns to the camera’s modulated image sensor. Distance values for each pixel of the image sensor are calculated from the measured difference in phase between the transmitted and received signal. There are two circuit topologies typically used for ToF camera light sources, shunt switching and series switching. Chronoptics Limited (Hamilton, New Zealand) is a company that develops ToF cameras. The light source of their Kea ToF camera drives four laser diodes using the iC-HG (iC-Haus GmbH, Bodenheim, Germany) Integrated Circuit (IC). The iC-HG IC, and therefore the Kea light source, has a series switching topology. Texas Instruments (Dallas, Texas) has designed a ToF camera with a light source that has the shunt switching topology. Texas Instruments claim that the shunt switching topology has a better electrical input power to optical output power than the series switching topology. The aim of this project is to investigate the shunt topology, in particular the Texas Instruments implementation, and to determine if the shunt topology could provide a substantial increase in efficiency for the Kea light source and future Chronoptics’ light source designs. In this thesis, we measure the electrical input to optical output power efficiency of the Texas Instruments TIDA 01173 light source and compare it to the Kea light source and find the TIDA1173 does indeed have a higher power efficiency than the Kea light source. However, it is not as efficient as we anticipated, so we also investigate options for improving the power efficiency of the TIDA 01173 light source to maximise the benefits of designing a new light source. To achieve the aim, an apparatus is designed and constructed to measure the electrical input power and optical output power of the TIDA 01173 and Kea light sources. The input and output power measurements are used to calculate the efficiency of the light sources over a range of modulation frequencies from 10 MHz to 100 MHz. The electrical to optical power efficiency results show that the shunt switching topology TIDA 01173 light source is more power efficient than the series switching topology Kea light source. The TIDA 01173 has a measured power efficiency of 22% at 10 MHz modulation frequency, and Kea has an efficiency of 11% at the same frequency. These hardware measurement results are corroborated by the results of SPICE simulations made for each of the ToF light sources. To make these simulations, the electrical and optical properties of the BOB850T1000 laser diode (Bob Laser Co, Guangdong, China) used in the ToF light sources are extracted to make a laser diode SPICE model. An account of the power consumption of each component in both light sources is conducted using the light source SPICE simulations. This account shows that the linear current limiter in the TIDA 01173 consumes the most power, so it is swapped for a buck current regulator. This modification increases the power efficiency of the TIDA 01173 light source to 28% at 10 MHz modulation frequency. The power efficiency of the Kea light source is increased to 13% at 10 MHz modulation frequency by decreasing the input voltage from 5 V to 4 V. Improving the efficiency of the ToF camera light sources allows higher optical output power for a given input power. This enables ToF cameras to be powered from low power sources such as, Universal Serial Bus (USB) ports or battery packs, therefore improving the versatility of ToF cameras.
Type of thesis
Corlett, D. O. (2020). Measuring and improving Time-of-Flight camera light source electrical-to-optical power conversion efficiency (Thesis, Master of Engineering (ME)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13796
The University of Waikato
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