We present a method for improving a flash system for retroreflective beacon detection in CMOS cameras. Generally, flash systems are designed in such a manner that makes them suited for beacon detection in a small range interval. We strive to increase the flash system range interval by exploiting the directional properties of the retroreflector. Thus, light sources placed relatively far away from the optical axis of the camera will contribute only when the retroreflector is far away. This fact can be used to compensate for the 1/distance2 dependency of optical power. We present underlying theory and formulae, then describe a flash system consisting of several light-emitting diodes that was designed by considering the presented method. Simulations show that the usable flash range of the improved system can be almost doubled compared to a general flash system. Tests were performed indicating that the presented method works according to theory and simulations.
The lasing wavelength of a single-section distributed feedback (DFB) laser diode can be modulated by modulating the drive current. This makes it possible to utilize the DFB laser diode in a frequency-modulated continuous wave (FMCW) range and velocity measuring system. In FMCW, the frequency of the laser is ramped, and the frequency difference between the reflected wave and a local-oscillator wave is monitored. For maximum performance the frequency ramping should be linear. Due to thermal phenomena, a linear ramping of the current seldom results in a linear ramping of the optical frequency. We have derived a discrete thermal model, using resistors and capacitors, of our laser module. The thermal model was then used as a starting point to model the frequency behavior of the laser and to derive modulation currents that resulted in a linear frequency ramping at some different modulation frequencies.
An examination on how to divide the optical power between the local oscillator radiation and radiation sent towards the target for maximized signal-to-noise ratio, has been carried out. From the self-mixing equations, by substituting electrical fields with optical power and including relevant noise sources, an expression for the signal-to-noise ratio (SNR) for the photodiode has been obtained. From this equation the conditions for maximum SNR has been derived.
A laser beam scanner, used as an angle measuring device in a particular navigation system for mobile robots, has been developed. It measures heading angles to beacons made of vertical stripes of retroreflective tape. Expressions giving the received power and energy in the pulse, which is generated when a beacon is traversed by the laser beam, are given. Measurements support the derived expressions. The shape and amplitude of the pulse are functions of the range <i>R</i> to the beacon, but at long range the pulse shape and width become independent of range while the dependence of the received amplitude on range becomes <i>R</i><sup>-3</sup>. At long range the pulse shape is determined by the Gaussian irradiance distribution in the laser beam and the pulse width is governed by the laser beam divergence and the scanning speed. Design rules for the optimum field of view of the receiving optics and for an electrical filter, which maximizes the signal-to-noise ratio, are proposed. An expression giving a conservative estimate of the signal-to-noise ratio is derived.