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Multi-wavelength airborne laser scanning. ILMF 2011, New Orleans Dr. Andreas Ullrich CTO, RIEGL LMS GmbH. introduction: components of ALS systems full waveform analysis vs. online waveform processing primary and secondary ALS data products

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multi wavelength airborne laser scanning

Multi-wavelength airborne laser scanning

ILMF 2011, New Orleans

Dr. Andreas Ullrich

CTO, RIEGL LMS GmbH

contents
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
components of als systems

RIEGL

LMS-Q680i

RIEGLVQ-820-G

RIEGL VQ-580

RiACQUIRE

DA42-MPP

RIEGL

DR-680

RiPROCESS

IMU & GPS

Flight Guidance

components of ALS systems
state of the art echo waveform digitizing systems
state-of-the-art echo waveform digitizing systems

RIEGL VQ-580

A

RIEGL VQ-820-G

dev

A

W

R

R

Q-560/Q-680i

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]

primary data point cloud1

RIEGL

VQ-580

wavelength 1064 nm

amplitude in dB above detection threshold

RIEGL

VQ-580

wavelength 1064 nm

reflectance in dB above white diffusely

reflecting target

RIEGL

VQ-580

wavelength 1064 nm

pulse shape deviation

from expected pulse shape

primary data: point cloud
images at different wavelengths

1064 nm

visible

visible

532 nm

1064 nm

1550 nm

1550 nm

532 nm

images at different wavelengths
radiometric calibration

actual geometric cross-section of target interacting with laser beam

directivity of backscatteredreflection

reflectance

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

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
multispectral hyperspectral imaging vs multi wavelength als

multispectralimaging

hyperspectralimaging

multi-wavelengthlidar

532 nm

905 nm

1064 nm

1550 nm

hyperspectrallidar

supercontinuum laser (500 nm – 2400 nm)

array of receiver channels and ROIC

400 nm

800 nm

1200 nm

1600 nm

multispectral/hyperspectral imaging vs. multi-wavelength ALS
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)
wavelength selection criteria for ALS sensors
slide12

200

400

600

800

1000

1200

1400

1600

1800

2000

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, 1.06 µm

fiber lasers, Ho-doped, 2.05 µm

frequency-doubled fiber lasers, 532 nm

UV INFRARED

diode

905 nm

solid state

355 nm

532 nm

1064 nm

fiber

532 nm

1064 nm

1550 nm

2050 nm

suitable laser sources

target reflectance versus wavelength

532 nm

905 nm

1064 nm

1550 nm

relative reflectance [%]

wavelength [µm]

target reflectance versus wavelength
background radiation versus wavelength

532 nm

1064 nm

905 nm

1550 nm

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

1000

800

solar irradiance [W/m²µm]

600

400

200

0

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
atmospheric attenuation versus wavelength

atmospheric transmission 20 km, one wayvisibility 23 km, 10 km, 5 km

905 nm

1064 nm

532 nm

1550 nm

transmittance of 1000 feet horizontal air path (sea level)

transmittance [%]

wavelength [µm]

atmospheric attenuation versus wavelength
attenuation in water versus wavelength

ultraviolet

visible

infrared

10 000

1 000

100

100 dB

50 dB

10

10 dB

1

100 dB

1 dB

absorption coefficient [cm-1]

53 dB

0.53 dB

0.1

10 dB

0.1 dB

0.01

100 dB

1 dB

0.01 dB

0.001

10 dB

0.1 dB

0.0001

3.8 dB

0.038 dB

1 dB

0.01 dB

0.1 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10

wavelength [µm]

absorption coefficient of clear seawater

attenuation at depth10 m

attenuation at depth0.1 m

attenuation at depth1 mm

attenuation in water versus wavelength
laser safety considerations

1550 nm

1064 nm

905 nm

532 nm

355 nm

MPE:maximum permissible exposure

parameter: exposure duration / pulse width

laser safety considerations
laser classes nohd enohd

NOHDeNOHD

NOHD, eNOHD

NOHD

eNOHD

RIEGLLMS-Q680i

@ 80kHz

RIEGLVQ-580

@ 50kHz

RIEGLVQ-820-G

@ 100kHz

Laser Safety Standards

0m

1.5m

15m

80m

10m

105m

500m

1600m

1600m

1600m

max. range @ reflectance 20%

max. range @ reflectance 80%

2000m

2000m

2000m

Range [m]

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
conclusions
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