12 There is currently a misconception among designers and users of free space laser communication (lasercom) equipment that 1550 nm light suffers from less atmospheric attenuation than 785 or 850 nm light in all weather conditions. This misconception is based upon a published equation for atmospheric attenuation as a function of wavelength, which is used frequently in the free-space lasercom literature. In hazy weather (visibility > 2 km), the prediction of less atmospheric attenuation at 1550 nm is most likely true. However, in foggy weather (visibility < 500 m), it appears that the attenuation of laser light is independent of wavelength, ie. 785 nm, 850 nm, and 1550 nm are all attenuated equally by fog. This same wavelength independence is also observed in snow and rain. This observation is based on an extensive literature search, and from full Mie scattering calculations. A modification to the published equation describing the atmospheric attenuation of laser power, which more accurately describes the effects of fog, is offered. This observation of wavelength-independent attenuation in fog is important, because fog, heavy snow, and extreme rain are the only types of weather that are likely to disrupt short (< 500 m) lasercom links. Short lasercom links will be necessary to meet the high availability requirements of the telecommunications industry.

/pdf/comparison-of-laser-beam-propagation-at-785-nm-and-1550-nm-14cpfndqfp.pdf

Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications

Knowledge of a laser scanner's spatial resolution is necessary in order to prevent aliasing and estimate the level of detail that can be resolved from a scanned point cloud. Spatial resolution can be decoupled into range and angular components. The latter is the focus of this paper and is governed primarily by sampling interval and laser beamwidth. However, emphasis is often placed on one of these—typically sampling interval—as an indicator of resolution. Since both affect the resolution of a scanned point cloud, consideration of only one factor independent of the other can lead to a misunderstanding of a system's capabilities. This will be demonstrated to be inappropriate except under very specific conditions. A new, more appropriate resolution measure for laser scanners, the effective instantaneous field of view (EIFOV), is proposed. It is derived by modelling the shift variance of the equal angular increment sampling process, laser beamwidth-induced positional uncertainty and observed angle quantisation with ensemble average modulation transfer functions (AMTFs). Several commercially available terrestrial laser scanner systems are modelled and analysed in terms of their angular resolution capabilities using the EIFOV.

Resume

Il est necessaire de connaitre la resolution spatiale des scanneurs a laser pour eviter des degradations de type aliasing et estimer le niveau de detail que l'on peut obtenir a partir d'un nuage de points scannes. On peut identifier dans la resolution spatiale une composante en distance et une angulaire. Cette derniere, qui fait l'objet de cet article, est essentiellement tributaire de l'intervalle d’echantillonnage et de la largeur du faisceau laser. Mais on considere generalement que c'est seulement l'intervalle d’echantillonnage qui constitue l'indicateur unique de resolution. Or les deux facteurs affectent la resolution de tout nuage de points scannes, de sorte que si l'on en examine un independamment de l'autre, on peut aboutir a une mauvaise estimation des possibilites du systeme. On a meme pu demontrer que c’etait tout a fait inadequat, sauf dans certains cas particuliers. C'est pourquoi on propose un nouveau moyen, mieux adapte, pour determiner la resolution des scanneurs a laser, en se basant sur le cone d'analyse instantane efficace (EIFOV). On derive ce dernier par une modelisation faisant intervenir la variance de glissement du processus incremental d’echantillonnage equi-angulaire, l'incertitude du positionnement provoque par la largeur du faisceau laser et la quantification angulaire observee et en melant les fonctions de transfert de modulation moyennes de l'ensemble (AMTFs). On a ainsi pu modeliser et analyser plusieurs systemes de scanneurs terrestres a laser du commerce en ce qui concerne leurs possibilites de resolution angulaire grâce a cette notion d'EIFOV.

Zusammenfassung

Mit der Kenntnis der raumlichen Auflosung eines Laserscanners lassen sich Aliasing Effekte vermeiden und der Detaillierungsgrad abschatzen, der aus einer gescannten Punktwolke abgeleitet werden kann. Die raumliche Auflosung lasst sich in eine Entfernungs- und eine Winkelkomponente aufteilen. Letztere steht hier im Vordergrund und wird vor allem durch das Abtastintervall und die Laserstrahlbreite bestimmt. Oftmals wird jedoch als Indikator fur die Auflosung nur eines dieser Kriterien—meist das Abtastintervall—herangezogen. Da jedoch beide Kriterien die Auflosung der gescannten Punktwolke beeinflussen, kann die von dem anderen Kriterium unabhangige Betrachtung zu Fehlinterpretationen der Systemeigenschaften fuhren. Es wird gezeigt, dass dies bis auf wenige Spezialfalle zutrifft. Ein besseres Mas fur die Auflosung eines Laserscanners ist das effektive aktuelle Gesichtsfeld (EIFOV). Es wird durch eine Modellierung der Verschiebungsvarianz des Abtastprozesses auf der Basis gleicher Winkelelemente, der durch die Laserstrahlbreite induzierten Lageunsicherheit und der beobachteten Winkelquantisierung mit Hilfe von gemittelten Modulationstransferfunktionen (AMTFs) abgeleitet. Mehrere kommerziell erhaltliche terrestrische Laserscanning Systeme werden hinsichtlich ihrer Winkelauflosungseigenschaften mit EIFOV modelliert und analysiert.

Resumen

Para evitar el pixelado de bordes y estimar el nivel de detalle que se puede obtener de una nube de puntos escaneada es necesario conocer la resolucion espacial del escaner laser. La resolucion espacial se puede descomponer en una componente de rango y una angular. El objeto de este articulo es esta ultima componente, que viene determinada principalmente por el intervalo de muestreo y la anchura del haz del laser. Sin embargo, con frecuencia se hace enfasis solo en uno de esos dos factores –habitualmente el intervalo de muestreo- como indicador de la resolucion. Como ambos afectan a la resolucion de una nube de puntos escaneada, considerar solo un factor independiente del otro puede conducir a una mala interpretacion de las capacidades de un sistema. Se demostrara que este enfoque no es el mas adecuado, excepto en circunstancias muy especificas. Se propone el campo efectivo instantaneo (EIFOV) como una nueva medida de la resolucion, mas adecuada para los instrumentos de barrido laser. Esta medida se obtiene mediante un modelo de la varianza del desplazamiento de los incrementos angulares constantes del proceso de muestreo, de la incertidumbre en la posicion inducida por la anchura del rayo laser y por la cuantizacion del angulo observado sobre la media poblacional de la funcion de transferencia de modulacion (AMTF). Se modelan varios sistemas comerciales de escaner laser terrestre y se analizan en funcion de su capacidad de resolucion angular utilizando el EIFOV.

Angular resolution of terrestrial laser scanners

This paper theoretically and experimentally investigate the spectrum attenuation of free space optical (FSO) communication systems operating at visible and near infrared (NIR) wavelengths $(0.6\ \mu{\rm m} under fog and smoke in a controlled laboratory condition. Fog and smoke are generated and controlled homogeneously along a dedicated atmospheric chamber of length 5.5 m. A new wavelength dependent empirical model is proposed to predict the fog and smoke attenuation operating at visible and NIR wavelengths. Comparison of the new proposed model with the measured continuous attenuation spectrum from visible—NIR in the fog and smoke channels shows a close relationship than the semi-empirical Kim and Kruse fog models. The experimental results also show the selection for the possible appropriate wavelengths from visible—NIR for FSO links to achieve the maximum link span in dense fog conditions.

Modeling of Fog and Smoke Attenuation in Free Space Optical Communications Link Under Controlled Laboratory Conditions

Cultural heritage recording and engineering surveying are prime applications for terrestrial laser scanners (TLSs) because of the high spatial resolution, high accuracy, and fast data capture rates this technology offers. To date, insufficient attention has been given to the many error sources contributing to the uncertainty of scanner datasets. A full error budget for directly georeferenced terrestrial laser scanner networks that considers both relevant error sources fundamental to surveying and those unique to sampled laser scanner systems is derived. In the case of the latter, new probabilistic models are proposed for angular positional uncertainty due to laser beamwidth and centroid-based target pointing. Analysis of a cultural heritage-recording project in Ayutthaya, Thailand, highlights the disparity between “expected” precision and the more realistic precision indicated by the error budget and demonstrates that the beamwidth error can be a significant factor. The causes and effects of several syste...

Error Models and Propagation in Directly Georeferenced Terrestrial Laser Scanner Networks

An important step in data processing from terrestrial laser scanning (TLS) is georeferencing, i.e. transformation of the scanner data (point clouds) into a real world coordinate system, which is im ...

Hugo Weichel

Papers

Laser Beam Propagation in the Atmosphere

Laser beam propagation in the atmosphere