3.2 - A Method to Enhance the Accuracy of Time of Flight Measurement Systems
- Event
- SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Nürnberg - Band
- Proceedings OPTO 2011
- Chapter
- O3 - Measuring technologies
- Author(s)
- J. Papadoudis, A. Georgiadis - Leuphana Universität Lüneburg (Germany), C. Koch - Fachhochschule Emden/Leer (Germany), S. Klein - Inosens GmbH, Dahlenburg (Germany)
- Pages
- 72 - 77
- DOI
- 10.5162/opto11/o3.2
- ISBN
- 978-3-9810993-9-3
- Price
- free
Abstract
A ToF-camera operates with modulated visible and near-infrared radiation, which is detected and demodulated simultaneously by a 2-D array of pixels. The phase shift between the sent signal and the received signal is an indicator for the object's distance. The amplitude of the radiation gives additional information for its reflectance.
Unfortunately, measurements of distances performed with almost all cameras available on the market show very high deviations. The main reason for them seems to be the influence of stray light, which is detected by the camera with the modulated radiation. Another source of uncertainties is the low sampling rate along the received signal. In the first part of this paper, experiments using different cameras and alternating environments have been performed in order to determine the deviations, which are up to 20 cm over a distance of 150 cm. A survey of existing cameras as well as their performance has taken place. In the second part of the present work two approaches for the eliminations of the deviations have been introduced. In the first approach spline functions have been calculated using reference measurements for distances between 5 cm and 200 cm. Those functions are used for the calculation of the uncertainties of each pixel. Alternatively the splines can be replaced by a gain matrix which is multiplied with the image matrix in order to equalize the error of the distance measurement of the pixel. Subsequently only one spline is needed in order to adjust the distances. In this case the calculation is faster but it cannot be applied in every case. The reflectance of the objects in the scene has not been taken into account yet. The opposite happens in the second approach in order to enhance the accuracy of the measurement. This is done by estimating the reflectance of the nearest object in dependency of the measured amplitude and distance. The reflectance of the background, respectively everything behind the nearest object, is estimated by using the measured amplitude of the background and the distance of the nearest object and the background. By using both reflectance factors and the distances as indicators for further smoothing functions the error of measurement due to the reflectance can be determined. After applying both methods the error of measurement decreased in our tests from about 20 cm to 3 cm.