In addition to what fennecfoxen already stated, using their own visual sensor gives them control over the exact format of the data coming off the sensor, thus p

In addition to what fennecfoxen already stated, using their own visual sensor gives them control over the exact format of the data coming off the sensor, thus permitting them to use FPGA or ASIC based processing that would not be an option if they had to deal with image files coming off the iPhone's camera. It also allows them to collect extra data to help register the visual and IR images to each other.