Several design engineers have come to us recently requesting lenses designed for characterizing light sources. Two hot topics are measuring light sources for LIDAR and for facial recognition. Both involve characterizing the angular distribution of light from an NIR (850 – 980nm) laser using a conoscope and camera, but that’s where the similarities end. Both these applications are intriguing, so we thought we’d take a moment and discuss both LIDAR and facial recognition systems.
LIDAR System Measurement
Measuring the light distribution from a laser for LIDAR is simple because the beam is simple. LIDAR is typically based on the time of flight concept – the distance from the laser to the object and back is determined by measuring how long it takes for a pulse of light to travel from the laser to the object and back. The laser beam must have low divergence so it can resolve relatively small objects that are up to a few hundred meters away from the laser. The only real challenge is that the lasers for this application are also powerful, over 10W, so it is necessary to dispose of a lot of light before it reaches the camera. Because of the small divergence angle, it is easy to resolve a few arc minutes.
Measuring Facial Recognition Systems
Facial recognition presents a much more difficult challenge. For this application, the laser light passes through a diffractive optical element to project a pattern on the user’s face. The distance from the laser to the face is small, so the beam must be spread over a large angle. This means that the measurement system must be accurate over a wide range of angles, both in terms of distortion (mapping of angles to positions on the image sensor) and how uniformly the sensor is illuminated.
Both of these applications are well suited for measurement with conoscopes. These lenses create a map on the image sensor in which each pixel within a circle captures light emitted by the source in to a small range of angles. Because conoscopes capture all of the emitted angles in a single frame, the data can be collected in a few milliseconds. This makes it possible to measure numerous light sources in a short time.
Other types of light sources, such as optical fibers and LED’s can be measured with conoscopes. The main limitations are that the lens must be able to get close to the source and the source must be no more than a few millimeters across. How close is close? It depends, of course. If you want to collect light within a 160° cone (80° half-angle), close is about 2 mm unless you want to pay for some very large lenses. On the other hand, if the full angle (side to side) of the cone is only 60° (30° half-angle), 10 mm is reasonable.
In recent months, facial recognition has been implemented with increasing frequency (and with mixed success). LIDAR, too, is undergoing scrutiny for its potential role in the development of self-driving cars. We hope that these explanations give you a better understanding of the methods these technologies rely on.