International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
FULL WAVEFORM ACTIVE HYPERSPECTRAL LIDAR
T. Hakala**, J. Suomalainen?, S. Kaasalainen*
^ Department of Photogrammetry and Remote Sensing, Finnish Geodetic Institute, Geodeetinrinne 2, Masala, 02431,
Finland — (teemu.hakala, juha.suomalainen, sanna.kaasalainen) @ fgi.fi
KEY WORDS: Full waveform, Hyperspectral, LiDAR, Supercontinuum, Reflectance, Spectral indices
ABSTRACT:
We have developed a prototype full waveform hyperspectral LiDAR and investigated its potential for remote sensing applications.
Traditionally hyperspectral remote sensing is based on passive measurement of sunlit targets. These methods are sensitive to errors in
illumination conditions and lack the range information. Our prototype can measure both the range and the spectral information from
a single laser pulse. At this stage, the instrument is optimized for short range terrestrial applications. An active hyperspectral LIDAR
opens up new possibilities for LIDAR data analysis. The lack of spectral information in traditional monochrome LiDARSs rules out
many of the classification techniques available for processing of hyperspectral data. Similarly, passive hyperspectral data does not
allow extensive use of the classifications based on 3D shape parameters. With both hyperspectral and range data available in a single
dataset, the best of the techniques can be applied to form more reliable classification results. The data also allows the mapping of
spectral indices in 3D. As an example a Norway spruce is measured and spatial distribution of several spectral indices is illustrated.
1. INTRODUCTION
Supercontinuum laser sources produce directional broadband
laser pulses by making use of cascaded nonlinear optical
interactions in a nonlinear optical fiber (see Dudley et al., 2006
for a review). The commercial availability of supercontinuum
laser technology has lead into a number of applications in the
recent years, such as those in biomedical optics (e.g., Kudlinski
et al, 2010). Combined with a hyperspectral time-of-flight
sensor, the supercontinuum laser sources can be used for
simultaneous measurement of distance and reflectance
spectrum, which has been the basis for our recent efforts in
development of the hyperspectral LiDAR.
Simultaneous topographic and spectral remote sensing has thus
far been based on passive imaging spectroscopy, fused with
laser scanner point clouds (Jones et al., 2010; Puttonen et al.,
2011; Thomas et al, 2009). Simultaneous geometric and
spectral information can also be retrieved by the means of a
novel approach for photogrammetric point cloud creation from
automatic multispectral image matching (Honkavaara et al.,
2012). Active hyperspectral imaging applications acquire a
spectral signature for every pixel in the image captured by an
imaging detector, but they do not produce range information
(Johnson et al., 1999; Nischan et al., 2003). Range information
is included in multi-wavelength laser scanners that use separate
monochromatic lasers as light sources for each wavelength
(Pfennigbauer and Ullrich, 2011). For these, the wavelength
channels are determined by the light sources.
The hyperspectral scanning LiDAR combines active
hyperspectral imaging and laser scanning with the same
instrument, with no registration problems between data sets.
The hyperspectral LiDAR produces a point cloud and
hyperspectral reflectance: (xy,zR(A), where R(A) is the
backscattered reflectance R as a function of the wavelength A.
The information content of the new type of data is vast and
creates new prospects for improving automatic data processing
and target characterization for laser scanning (LiDAR) data.
We present the design of a full waveform hyperspectral LiDAR
and its first demonstrations in the remote sensing of vegetation.
The concept was first studied with a scanning active
hyperspectral measurement system (Suomalainen et al., 2011),
where the active hyperspectral intensity data was fused with
simultaneous terrestrial laser scanner measurement. This lead to
the development of the scanning LiDAR instrument presented
in this paper (see also Hakala et al., 2012), and enabled us to
study the usage of hyperspectral 3D point clouds in target
classification (Puttonen et al., 2010). To our knowledge, this is
the first full waveform hyperspectral LiDAR producing spectral
3D point clouds and exploiting the supercontinuum laser
technology, and one of the first environmental applications of
supercontinuum lasers.
2. FULL WAVEFORM ACTIVE HYPERSPECTRAL
LIDAR
2.1 The Instrument
We have assembled an optical setup (Fig. 1) for measuring the
time-of-flight and return intensity of a hyperspectral laser pulse.
The supercontinuum laser (NKT Photonics, SuperK) produces 1
ns pulses at repetition rate of 24 kHz and average power of 100
mW. The broadband output laser is collimated using a
refracting collimator (Thorlabs, CFC-5-A). The collimated
beam passes through a beam sampler, which takes a part of the
beam for triggering the time-of-flight measurement. An off-axis
parabolic mirror (50.8 mm diameter, 152.4 mm effective focal
length and 90° off-axis angle) is used as the primary collecting
optic.
The off-axis parabolic mirror is focused to a spectrograph
(Specim, ImSpector V10). A 16-element avalanche photo diode
(APD) array module (Pacific Silicon Sensor) is used to convert
the spectrally separated light to analog voltages. The APD
module has built-in transimpedance amplifiers (Analog
Devices, AD8015) with a bandwidth of 240 MHz producing an
unambiguous resolution of approximately 4 ns. 12-bit analog to
