Progress in field spectroscopy Edward J. Milton a, ⁎ , Michael E. Schaepman b , Karen Anderson c , Mathias Kneubühler d , Nigel Fox e a School of Geography, University of Southampton, SO17 1BJ, UK b Centre for Geo-Information, Wageningen University, Droevendaalsesteeg 3 6708 PB Wageningen, The Netherlands c Department of Geography, University of Exeter, EX4 4QJ, UK d Remote Sensing Laboratories, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland e National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK Received 7 November 2006; received in revised form 14 June 2007; accepted 13 August 2007 Abstract This paper reviews developments in the science of field spectroscopy, focusing on the last twenty years in particular. During this period field spectroscopy has become established as an important technique for characterising the reflectance of natural surfaces in situ, for supporting the vicarious calibration of airborne and satellite sensors, and for providing a means of scaling-up measurements from small areas (e.g. leaves, rocks) to composite scenes (e.g. vegetation canopies), and ultimately to pixels. This paper describes the physical basis of the subject and evaluates the different methods and instruments which have been employed across a range of studies. The development and use of field goniometers is described, and related to methods for estimating the bidirectional reflectance distribution function (BRDF) from directional reflectance measurements in the field. The paper also considers the practical aspects of field spectroscopy, and identifies a number of factors affecting the useability of field spectroradiometers, including the weight and cost of the instruments, limitations of some commonly used methodologies and practical issues such as the legibility of displays and limited battery life. The prospects for the future of field spectroscopy are considered in relation to the increasingly important contribution that field spectral data will make to EO-based global measurement and monitoring systems, specifically through their assimilation into numerical models. However, for this to be achieved it is essential that the data are of high quality, with stated levels of accuracy and uncertainty, and that common protocols are developed and maintained to ensure the long-term value of field spectroscopic data. The importance of employing a precise terminology for describing the geometric configuration of measurements is highlighted in relation to issues of repeatability and reproducibility. Through such refinements in methodology, field spectroscopy will establish its credentials as a reliable method of environmental measurement, underpinning quantitative Earth observation and its applications in the environmental and Earth sciences. © 2007 Elsevier Inc. All rights reserved. Keywords: Reflectance; Methodology; BRDF; Goniometer; Field portable spectrometers; Spectroscopy 1. Introduction Field spectroscopy pre-dates the development of imaging spectrometry by many years, but the two technologies have much in common, as they share the common goal of acquiring accurate data on the spectral reflectance of Earth surface materials from a remote location. Field spectroscopy is technically less challeng- ing, as the sensing instrument can remain fixed over the subject of interest for much longer, and the path length between the instrument and the object being measured is reduced. However, field spectroradiometers generally measure a much smaller area, therefore, how to sample the surface of interest becomes an additional consideration. Field spectroradiometers were first used to study human colour vision, and in particular the colour of the Earth's surface from the air (Penndorf, 1956). The development of airborne multispectral scanners in the 1960s spurred on the development of the first instruments capable of making accurate measure- ments of spectral reflectance in the field environment. One of the key challenges during the 1970s was to make accurate measurements in the short-wave infra-red region (1.1–2.4 μm) which was known from laboratory measurements to be a very important part of the electromagnetic spectrum for geological Remote Sensing of Environment xx (2007) xxx – xxx + MODEL RSE-06999; No of Pages 18 www.elsevier.com/locate/rse ⁎ Corresponding author. E-mail address: E.J.Milton@soton.ac.uk (E.J. Milton). 0034-4257/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2007.08.001 ARTICLE IN PRESS Please cite this article as: Milton, E. J. et al., Progress in field spectroscopy, Remote Sensing of Environment (2007), doi:10.1016/j.rse.2007.08.001