SCM 330 Ocean Discovery through Technology Area F GE

Sensors - Physical Optical Sensors: Solar Radiation Sensors Apparent Irradiance Radiance Inherent Absorption Scattering LASERS LIDAR LLS Theory Application Sensor

Definitions Spatial quantities: Temporal quantities:

Why the sky is blue? Air molecules scatter blue light light disproportionately Blue light is scattered out of the beam, leaving yellow light behind, so the sun appears yellow.

Absorption of light varies Visible spectrum Penetration depth into water Wavelength 1 km1 m1 mm1 µm1 nm UV X-ray Radio Microwave IR 1 mm 1 km 1 µm 1 m Penetration depth into water vs. wavelength Notice that the penetration depth varies by over ten orders of magnitude! Water is clear in the visible, but not elsewhere.

Basic equation E(z) = E(0) exp(-Kd( )*z) Kd( ) is the diffuse attenuation coefficient for downwelling irradiance (1/m). Optical depth = Kd( )*z Euphotic depth = 1% light depth 1 = 100 exp(-Kd( )*z) Kd( )*z = ln(0.01) = 4.6

Sensors - Physical Optical Sensors: Solar Radiation Sensors Apparent Irradiance Radiance Inherent Absorption Scattering LASERS LIDAR LLS Theory Application Sensor

Properties that depend both on inherent optical properties (IOPs) and on the light field in which they are measured. In practice, every optical measurement is dependent on the light field used for the measurement, but instruments for IOP measurements provide their own controlled light field rather than relying on ambient light. Apparent Optical Properties (AOPs)

The Irradiance A light wave’s average power per unit area is the “irradiance.” Irradiance detector is a sensor which measures the flux of light incident on a surface. Spectral irradiance is abbreviated E(λ) where λ is the center wavelength of the detector. Calibration units common for our irradiance sensors are µW. cm -2. nm -1 for narrow bandwidth detectors

Used for measuring upwelling or downwelling sunlight, a properly designed cosine collector, will report irradiances that are proportional to the cosine of the angle of incidence. This diagram is a side view of a cosine collector, showing the relationship between the collector, occluding rings, and photodetectors.

Radiance detector is a sensor which measures the flux of light flowing in a specified direction per unit solid angle. Spectral radiance is abbreviated L(λ) where λ is the center wavelength of the detector. Submersible radiance sensors are normally oriented to measure the nadir (upwelling) radiance, abbreviated L u (λ). The calibration unit of radiance used in our radiance sensors is µW. cm -2. nm -1. sr -1.

Radiance is a function of viewing angle, and other radiometric quantities such as irradiance can be calculated by integrating the radiance over a range of angles.

Sensors - Physical Optical Sensors: Solar Radiation Sensors Apparent Irradiance Radiance Inherent Absorption Scattering LASERS LIDAR LLS Theory Application Sensor

Inherent optical properties (IOP) These properties depend only on the characteristics of the water, not the radiation field Absorption, a( ) Scattering, b( ) Forward and backscattering, bb Only backscattering is relevant for attenuation of downwelling irradianc

Relationship between IOP and AOP Kd( ) = (a( ) + bb( ))/µ The mean cosine, µ, is the average cosine of the zenith angle of all light in a given layer

What absorbs radiation in the ocean? Water CDOM Suspended sediments Phytoplankton What backscatters light in the ocean? Water Suspended sediments Small phytoplankton and bacteria Large particles primarily scatter light forward

a + b = c Absorption + Scattering = Attenuation

Beam Attenuation - Transmissometers

Absorption Attenuation Meters

Backscattering Meters

Sensors - Physical Optical Sensors: Solar Radiation Sensors Apparent Irradiance Radiance Inherent Absorption Scattering LASERS LIDAR LLS Theory Application Sensor

LASER Light Amplification by Stimulated Emission Radiation Mechanism for this postulated by Einstein in 1917 LASER is a device that produces and amplifies light

In a medium with many excited atoms, spontaneous emission will result in random anisotropic light output

The Laser A laser is a medium that stores energy, surrounded by two mirrors. Photons entering the medium undergo stimulated emission. As a result, the irradiance exiting from the medium exceeds that entering it. A partially reflecting output mirror lets some light out. A laser will lase if the beam increases in irradiance during a round trip: that is, if I 3 > I 0.

The Helium- Neon Laser Energetic electrons in a glow discharge collide with and excite He atoms, which then collide with and transfer the excitation to Ne atoms, an ideal 4-level system.

Navigation Channels Nautical Charting Shoreline Mapping Coral Reef Mapping Rapid Environmental Assessment Hurricane Response

Red Laser for Surface Green Laser for Bottom All distances based on Time differences

Laser Line Scan