1. Multi-wavelength airborne laser scanning
ILMF 2011, New Orleans
Dr. Andreas Ullrich
CTO, RIEGL LMS GmbH

2. introduction: components of ALS systems
full waveform analysis vs. online waveform processing
primary and secondary ALS data products
discussion multi-spectral, hyper-spectral, multi-wavelength
selection criteria for laser wavelength
availability of laser sources
target properties
signal attenuation, background radiation
laser safety
classification of multi-wavelength data / systems
conclusions
contents

3. components of ALS systems
RIEGL VQ-820-G
RIEGL VQ-580
RiACQUIRE
RIEGL
LMS-Q680i
DA42-MPP
RiPROCESS
RIEGL
DR-680
IMU & GPS
Flight Guidance
components of ALS systems

4. state-of-the-art echo waveform digitizing systems
RIEGL VQ-820-G R A W R A
RIEGL VQ-580
Q-560/Q-680i
dev
Full Waveform analysis
range: R [m]
amplitude: A [LSB and linearized]
echo width: W [ns]
On-Line Waveform Processing
range: R [m]
calibrated amplitude: A [dB] calibrated reflectance: r [dB]
pulse shape deviation: dev [1]
state-of-the-art echo waveform digitizing systems

5. primary data: point cloud
RIEGL LMS-Q680i, wavelength 1550 nm
dry conditions wet snow
primary data: point cloud

6. primary data: point cloud
RIEGL
VQ-580
wavelength nm
pulse shape deviation
from expected pulse shape
RIEGL
VQ-580
wavelength nm
reflectance in dB above white diffusely
reflecting target
RIEGL
VQ-580
wavelength nm
amplitude in dB above detection threshold
primary data: point cloud

7. images at different wavelengths
1064 nm
visible
visible
532 nm
1064 nm
1550 nm
1550 nm
532 nm
images at different wavelengths

8. radiometric calibration
Laser Radar Cross Section (LRCS)
cross section  in [m²]
area-normalized cross section values in [m²m-2] or [dB]
by laser footprint area: 
by illuminated object area: 0
actual geometric cross- section of target interacting with laser beam
reflectance
directivity of backscattered reflection
Radiometric calibration of small-footprint airborne laser scanner measurements: Basic physical concepts, Wagner, W., ISPRS Journal of Photogrammetry and Remote Sensing, 65, 2010.
radiometric calibration

9. radiometric calibration

10. multispectral/hyperspectral imaging vs. multi-wavelength ALS
400 nm
800 nm
1200 nm
1600 nm
multispectral imaging
hyperspectral imaging
multi- wavelength lidar
532 nm
905 nm
1064 nm
1550 nm
hyperspectral lidar
supercontinuum laser (500 nm – 2400 nm)
array of receiver channels and ROIC
multispectral/hyperspectral imaging vs. multi-wavelength ALS

11. wavelength selection criteria for ALS sensors
pulsed time-of-flight laser ranging: best performance wrt maximum range, measurement speed, ranging precision and accuracy
selection of wavelength
availability of suitable laser and detector
reflectance of objects
attenuation of atmosphere and background radiation
laser safety
laser requirements
short pulse width (multi-target resolution, high precision)
high peak power (maximum range)
good beam quality (beam divergence, spatial resolution)
high pulse repetition rate (point density)
narrow spectral width (background rejection)
detector requirements
high bandwidth (corresponds to pulse width)
high sensitivity (maximum range)
low noise (high precision)
airborne laser scanning makes use of pulsed time-of-flight laser ranging (best figure of merit taking into account maximum range, measurement speed, ranging precision and accuracy)
traditionally high-power mid-pulse-repetition rate monochromatic sources in use
selection of wavelength governed by availability of suitable laser and detector, but also by reflectance of objects, attenuation of atmosphere and background radiation, laser safety
requirements on laser: short pulse width (multi-target resolution, high precision), high peak power (maximum range), good beam quality (beam divergence, spatial resolution), high pulse repetition rate (point density), narrow spectral width (background rejection), etc.
requirements on detector: high bandwidth (corresp. pulse width), high sensitivity (max. range), etc.
wavelength selection criteria for ALS sensors

12. solid state lasers (fundamental wavelength), Nd:YAG, 1064 nm
200
400
600
800
1000
1200
1400
1600
1800
2000
UV INFRARED
diode
905 nm
solid state
355 nm
532 nm
1064 nm
fiber
532 nm
1064 nm
1550 nm
2050 nm
diode lasers, 905 nm
solid state lasers (fundamental wavelength), Nd:YAG, 1064 nm
solid state lasers (harmonics), Nd:YAG, 532 nm, (355 nm)
fiber lasers, Er-doped, 1.55 µm
fiber lasers, Yt-doped, µm
fiber lasers, Ho-doped, µm
frequency-doubled fiber lasers, 532 nm
suitable laser sources

13. target reflectance versus wavelength
532 nm
905 nm
1064 nm
1550 nm
relative reflectance [%]
wavelength [µm]
target reflectance versus wavelength

14. background radiation versus wavelength
solar spectral irradiance at zenith sun angle 60° at sea level
1400
corresponds to spectrum of sun light
absorption due to ozone (O3) , water vapor (H2O),
oxygen (O2),
carbon dioxide (C02)
1200
532 nm
1000
800
solar irradiance [W/m²µm]
600
1064 nm
400
905 nm
1550 nm
200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
wavelength [µm]
background radiation versus wavelength

15. atmospheric attenuation versus wavelength
transmittance of 1000 feet horizontal air path (sea level)
532 nm
1064 nm
905 nm
1550 nm
atmospheric transmission 20 km, one way visibility 23 km, 10 km, 5 km
transmittance [%]
wavelength [µm]
atmospheric attenuation versus wavelength

16. attenuation in water versus wavelength
absorption coefficient of clear seawater
attenuation at depth 10 m
attenuation at depth 0.1 m
attenuation at depth 1 mm
wavelength [µm]
absorption coefficient [cm-1]
10 000
1 000
100 10 1
0.1
0.01
0.001
0.0001
visible
infrared
ultraviolet
0.1 dB
1 dB
10 dB
0.01 dB
100 dB
53 dB
0.53 dB
50 dB
10 dB
100 dB
1 dB
0.1 dB
0.01 dB
3.8 dB
0.038 dB
attenuation in water versus wavelength

17. laser safety considerations
MPE: maximum permissible exposure
1550 nm
1064 nm
905 nm
532 nm
355 nm
parameter: exposure duration / pulse width
laser safety considerations

18. Laser Classes / NOHD / ENOHD
RIEGLLMS-Q680i
@ 80kHz
RIEGL VQ-580
@ 50kHz
RIEGL VQ-820-G
@ 100kHz
Laser Safety Standards
NOHD, eNOHD
NOHD eNOHD
NOHD
eNOHD
EN60825
21CFR
class 1
class I
class 1M -
class 2
class II
class 2M
class 3R
class IIIA
class 3B
class IIIB
class 4
class IV 0m
1.5m
15m
80m
10m
105m
500m
1600m
1600m
1600m
max. reflectance 20%
2000m
2000m
2000m
Range [m]
max. reflectance 80%
NOHD (nominal ocular hazard distance): distance beyond which exposure becomes less than maximum permissible exposure (MPE)
extended NOHD: includes the possibility of optically-aided viewing
Laser Classes / NOHD / ENOHD

19. classification of multi-wavelength ALS
description
same area
common platform
common scanner
same IFOV
synchronized pulses
data set from two different campaigns X
data from several laser scanners on same platform
several LIDARs sharing the same scanner
co-axial beams having thus the same instantaneous field-of-view
additionally pulses of LIDARs are synchonized
increasing sensor/system complexity
increasing flexibility
classification of multi-wavelength ALS

20. for hydrography, ad 532 nm LIDAR
select scanner model (wavelength) according to target characteristics, mission requirements, laser safety requirements, ...  wide variety of applications covered by eye-safe 1550 nm ALS scanners (e.g., RIEGL LMS-680i and RIEGL VQ-480)
for special applications, e.g., forest health investigations integrate two or more scanners with different wavelength on a single platform  providing flexible “multi-wavelength” system (e.g., RIEGL VQ-480 at 1550 nm and RIEGL VQ-580 at 1064 nm)
for hydrography, ad 532 nm LIDAR
regardless of wavelength: echo-digitizing pulsed time-of-flight systems provide utmost accuracy, multi-target resolution and calibrated (calibratable) amplitudes and target’s cross-section
conclusions