TECHNICAL COMMUNICATIONS www:cerf -jcr:org Simple Techniques for Retrieval of Wind Wave Periods and Directions from Optical Images Sequences in Wave Tanks Henrique Rapizo †** , Victor d’Avila ‡ , Nelson Violante-Carvalho † *, Uggo Pinho § , Carlos Eduardo Parente † , and Fabio Nascimento † † Ocean Engineering Program Federal University of Rio de Janeiro Rio de Janeiro 21941-450, Brazil ‡ Faculty of Oceanography Rio de Janeiro State University Rio de Janeiro 21941-450, Brazil § Operational Oceanography Center (OCEANOP) Petrobras Rio de Janeiro 20031-912, Brazil ABSTRACT Rapizo, H.; d’Avila, V.; Violante-Carvalho, N.; Pinho, U.; Parente C.E., and Nascimento, F., 0000. Simple techniques for retrieval of wind wave periods and directions from optical images sequences in wave tanks. Journal of Coastal Research, 00(0), 000–000. Coconut Creek (Florida), ISSN 0749-0208. Optical techniques are potentially suitable for overcoming some of the limitations faced by single-point sensors, like buoys, in retrieving spatial properties of wind-generated waves. However, most of the approaches that have been addressed in the literature employ complex configurations that use a coupling system between cameras, making its implementation difficult. This article describes simple techniques for measuring the period and direction of multimodal, regular waves, employing a single, commonly available camera. Furthermore, there is no special requirement for light conditions, making its implementation feasible, in both indoor and outdoor environments. Several tests were conducted in a multidirectional tank, varying the steepness and direction of the waves. The optical measurements were in good agreement, especially for waves from moderate to greater steepness. The techniques described here could be employed as a preliminary step before tackling more-complex configurations, for instance, as a priori information on the main wave modes. ADDITIONAL INDEX WORDS: Wind waves, optical techniques, measurement of waves. INTRODUCTION Improvement in our understanding of wind-generated waves is directly linked to better measurements. For experimental, theoretical, or numerical approaches, it is crucial to employ effective data-recording methods, both in the ocean and in wave tanks. Conventional, single-point sensors, such as heave-pitch- roll buoys, considered to be the standard for wave measure- ments, are accurate enough to retrieve time series of surface elevations but have limitations in measuring the directional wave field and, therefore, in describing wave spatial properties (Wanek and Wu, 2006). In this context, optical techniques are particularly promising for imaging the sea surface with a low- cost, nonintrusive method and extensive spatial and temporal coverages of the wave field. Optical techniques have been employed for over a century to measure physical aspects of the ocean. In early works, before the 1970s, the photographs were processed manually, which involved a considerable amount of time, hindering its progress. With the advent of digital processing, image analysis has become a less-laborious and significantly faster task. Digital video cameras allow the acquisition of many images per second, which makes it possible to extend analysis to the temporal domain, with high resolution. Some important studies have used only one camera, based on models of either reflection of light (Cox and Munk, 1954a,b; Monaldo and Kasevich, 1981; Stilwell, 1969; Stilwell and Pilon, 1977) or refraction of light (Cox and Munk, 1954b; J ¨ ahne and Riemer, 1990) to obtain the spectrum of ocean surface waves. However, a significant part of published studies discuss the use of stereoscopic systems (Banner, Jones, and Trinder, 1989; Benetazzo et al., 2012; Holthuijsen, 1983a,b; Mironov et al., 2012; Santel et al., 2002; Santel, Linder, and Heipke, 2004; Shemdin, Tran, and Wu, 1988; Wanek and Wu, 2006), which is conceptually more complicated than those based on a single camera. Reflection models are based on water being a highly reflective, specular surface. Measured irradiance is, therefore, dependent only on the tilt of the water surface, which is related to wave steepness. Stilwell (1969) presented a linear model for the relationship between irradiance and surface slope, retriev- ing the wave-number spectrum from the distribution of brightness on the image plane. Monaldo and Kasevich (1981) proposed an extension to that model, including upwelled irradiance from lower layers of the water column. According to the authors, the appropriate choice for the geometry of the optical system would minimize nonlinearities in the transfer function between the optical terms and the surface slope. DOI: 10.2112/JCOASTRES-D-15-00072.1 received 28 April 2015; accepted in revision 11 June 2015; corrected proofs received 5 August 2015; published pre-print online 18 September 2015. *Corresponding author: violante_carvalho@yahoo.co.uk **Present address: Swinburne University of Technology, Hawthorn 3122, Australia; hrapizo@swin.edu.au Ó Coastal Education and Research Foundation, Inc. 2015 Journal of Coastal Research 00 0 000–000 Coconut Creek, Florida Month 0000