Fog is produced by the suspension of very fine moisture droplets in the air. When light hits these droplets, it scatters and results in a loss of contrast and a dense white background. As these droplets get smaller, fog gets thicker and makes roadways more blanketed, reducing visibility, limiting contrast, and distorting the perception of speed. Reports from the Federal Highway Administration cite an annual average of 31,385 fog-related car accidents resulting in more than 500 deaths.
To help drivers achieve improved visibility through fog, researchers from Purdue University and the University of Science and Technology of China developed an experimental off-axis spatiotemporally gated multimode laser scanning system. Extensive testing has shown the system yields high-quality images at seven scattering path lengths, which far exceeds the capability of conventional imaging solutions, such as LIDAR that typically lacks the spatial resolution and contrast of optical measurement.
During testing, image capture was performed using a Photonfocus 2-megapixel CMOS camera with full-well capacity recording at 128 x 118 pixel resolution to simulate pupil plane detection. The camera was configured for external exposure control mode so that the external trigger signal controlled both the exposure start and duration. Using a region of interest containing the 128 × 118 pixels, researchers achieved 4 kHz frame rate with 50% duty cycle. These images were continuously transferred to a computer memory through a BitFlow Neon CLB Base/PoCL Camera Link frame grabber. Featuring PoCL, this board can acquire from all Base CL cameras up to 24 bits at 85 MHz and has enough industrial I/O to handle even the most complicated synchronization tasks.
Image quality was evaluated by placing a flat wood deer shape figurine inside a rectangle glass tank filled with water and subject to different levels of scattering. Milk gradually was added into the water tank while scattering path length was measured. Researchers utilized a 592 nm diode laser source of 7 mm coherence length and employed hologram recording to achieve temporal gating.
To adapt the system for practical implementation on motor vehicles, researchers plan to abandon laser interferometry and directly employ a nanosecond pulsed light source and electronic gating on the detected signal as in LIDAR imaging. Also, they will locate an illumination module and detection module on each side of a vehicle using two separated synchronized beam scanners that will scan a common focus.
