Conferences and supporting programme
Characterization of NIR light sources for 3D imaging using new Fourier optics viewing angle instrument
In the last years, different methods have been developed for 3D sensing: NIR cameras with structured lighting and time of flight sensors are the most common techniques. In all the cases, NIR illumination is a key component for such a system and need to be characterize carefully. Most of the current solutions use LED or laser diodes with wavelengths between 830nm and 950nm with power levels varying from a few milli-Watts to several Watts. In all the cases, the light sources need to comply with safety regulations . The IEC60825-1 specifies maximum permissible exposure limits that ensure eye safety which needs to be verified by specific measurements. It is particularly the case for laser imaging detection and ranging (LIDAR) applications for autonomous vehicles that are generally using direct time of flight detection. In this latter case to overcome sunlight and other parasitic sources the requirements on the illumination in terms of power becomes important. Generally vertical cavity surface emitting lasers (VCSEL) are used for that purpose because of their high brightness, narrow spectrum and few nanoseconds short pulse mode . For safety regulation their emission needs to be characterize in an absolute way. For large angle aperture sources as it is generally the case for 3D sensing (gesture recognition) and 3D image capture, these measurements need to be performed inside all their emissive cone. In case of structured lighting use for example for face recognition, the source shows generally a great number of spots within a large angular aperture , and high-resolution angular measurements are required. Angular characterization of the emission of such NIR sources is generally made with very simple setups. Projection on a screen and imaging with a CMOS camera can be easily made but it just gives and idea of the spatial extension of the emission and absolute quantities cannot be derived. More sophisticated methods for measuring the beam properties using a lens system and a CCD camera has also been developed  but the angular resolution is very limited for large apertures sources. In the proposed paper we present a new instrument to perform absolute angular measurements on such sources very rapidly and accurately. It is based on a Fourier optic concept that has been used for a long time to measure the viewing angle of displays in the visible range . Each light beam is collected by a set of Fourier lenses and collimated on a Fourier plane. Each point of this plane corresponding to one emission direction is reimaged on a high-resolution CMOS camera. The maximum angular aperture of the system is ±70° and its spot size is fixed to 2mm. The system includes neutral densities to manage very powerful sources and image acquisition can be automatically synchronized when working in multi pulse mode. The effective angular resolution is better than 0.05°. The paper will present the system in detail with examples of measurements on different types of NIR sources.
--- Date: 27.02.2019 Time: 2:15 PM - 2:35 PM Location: Conference Counter NCC Ost