digital converters (SP Devices, ADQ412), with 1 GHz sampling 
rate, are used to digitize 8 of the 16 available spectral channels. 
An average of 10 pulses is saved to improve signal to noise 
ratio and to reduce the amount of data. 
  
Fig.l. The optical setup: A laser pulse (A) is collimated and 
sent to a 2D scanner setup (B). An off-axis parabolic 
mirror (C) is used as a primary light collecting optic. 
A spectrograph (D) disperses the colors of the return 
and trigger (E) pulses to an APD array, which 
converts the light to analog voltage waveforms. 
The scanning geometry is defined by the two rotators (Newport 
URS75BCC and URS100BCC) with an absolute accuracy of 
+0.0115°. The rotators are attached to each other, with one 
performing the azimuth rotation and the other sweeping the 
laser over the target area in vertical passes. Due to uncertainty 
in timing, the accuracy of the elevation angles is 0.1°. 
2.2 Calibration and Data Processing 
A monochromator (Oriel, Cornerstone 74125) was used to 
calibrate the spectral responses of the APD elements. The 
current configuration produces spectral Full Width at Half 
Maximum (FWHM) of about 19 nm for each element and 
spectral range of 470-990 nm. However, the sensitivity of the 
APD array and the laser intensity below 550 nm are low, and 
therefore the first channel is selected to be at 542 nm followed 
by 606, 672, 707, 740, 775, 878 and 981 nm. 
The transmitted pulse energy of the SuperK laser source may 
vary slightly. To take this into account, an average waveform of 
all spectral channels is calculated and a Gaussian peak function 
is fitted to the trigger part of the waveform and the waveforms 
are normalized with the transmit pulse intensity. Similarly, the 
return echo positions are detected from a mean waveform, 
averaged over all spectral channels. Once the return echo 
positions and widths are determined from the mean waveform, 
the hyperspectral intensities are extracted by fitting Gaussian 
peak heights to the spectral waveforms (Fig. 2). 
    
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 
    
Backscattered Reflectance 
  
Ti 40 
me of Flight /ns 50 
Fig. 2. Result waveforms for each channel after post processing. 
Gaussian peaks are fitted to each of the measured 
echoes. The first echo is produced by target spruce 
and the second by black cotton background canvas. 
The intensity is converted into reflectance by applying the 
distance and spectral calibration. During the calibration 
measurements, waveforms are collected using a 99% Spectralon 
as a reference target at various distances. The echo intensities 
are normalized with the intensity of the Spectralon echo at the 
same distance, producing “backscattered reflectance”. As the 
backscattered reflectance spectra are combined with the 
corresponding time-of-flight and concurrent scanner 
orientation, a hyperspectral point cloud (x, y, z, R(4)) is 
produced. 
The accuracy of the measurement and the Gaussian fitting was 
tested by acquiring waveforms of 100 pulses reflected from a 
Spectralon panel at a 6-meter distance. The distance and the 
backscattered reflectance spectrum were retrieved individually 
for each pulse. The precision of ranging (standard deviation) 
was found to be 11.5 mm. The precision of backscattered 
reflectance of a single waveform was found to be better than 2% 
for spectral channels within the range of 600—800 nm and better 
than 5.596 for all channels. The quality of the fit is affected by 
the return pulse intensity, and thus lower precision is expected 
for targets that are darker or further away from instrument. 
Higher precision can be reached by averaging over a number of 
measurement points. For both range and reflectance, the 
absolute accuracy is expected to be lower than the precision due 
to uncertainty in calibration. 
The instrument does not have a strictly defined maximum range 
of measurement, as the performance of the waveform echo 
detection decreases slowly with the fading echo intensity. The 
current configuration is focused to approximately 12-meter 
distance. Measurements have shown that high quality point 
clouds can be measured from targets within 10-meter range and 
bright targets can be detected even from over 20 meters. 
2.3 Results and Discussion 
A Norway spruce (Picea abies) (Fig. 3) was measured in 
laboratory using the full waveform hyperspectral LiDAR. The 
bottom branches of the 2-meter spruce had suffered from lack of 
light and were in various stages of drying and dying, while the 
top branches had healthy new growth. In addition to the LiDAR 
measurement, reference spectra were acquired using a passive 
spectrometer (Avantes, AvaSpec 3648) and a quartz-tungsten- 
halogen light source.