Hydrolight is a radiative transfer numerical model that solves the time - independent radiative transfer equation. In order to compute the radiance, Hydrolight requires the inherent optical properties of the water body (IOP's), a chlorophyll profile and the water depth. Hydrolight can be applied in different cases:
- It can be run with modeled input values to generate in – water light fields, which are then used as input in primary productivity models or mixed – layer dynamics.
- It can be run with the IOP’s of different water types to simulate in – water light fields for the purpose of selecting and designing instruments for use in various water types.
- It can be run with assumed IOP’s as input in order to obtain estimates of the signals that would be received by various types or configurations of remote sensors, when flown over different water bodies and under different environmental conditions.
- It can also be used to isolate and remove unwanted contributions to remotely sensed signatures. As spectrometers detect not only the water leaving radiance in which we are interested, but also the sky radiance reflected upward by the sea surface. As Hydrolight separately computes all of these components, it can be used to correct the detected signature for surface reflection effects.
- It can be used in different configurations (IOP’s and boundary conditions) when analyzing experimental data in order to tighten up an existing parameterization.
- It can be used to simulate optical signatures for the purpose of evaluating proposed remote – sensing algorithms for their ability to simulate different environments or for examining the sensitivity of algorithms to simulated noise in the signature.
- It can be used to characterize the background environment in an image. When attempting to extract information about an object in the scene, all of the radiance of the natural environment may be considered noise, with the radiance from the object being the signal. The model can then be used to compute and remove the environmental contribution to the image.
- Hydrolight can be run with historical (climatological) or modeled input data to provide estimates about the marine optical environment during times when remotely sensed or in situ data are not available.
Due to the difference in scales in the ocean ( Lh >>> Lv ), we can consider the ocean as consisting of optically independent patches of waters. We thus obtain patches of homogeneous water body whose optical properties vary only with depth. We can then independently apply a one dimensional radiative transfer model at the center of each patch in order to simulate the entire, horizontally inhomogeneous water body. In the analysis of imaging spectrometer data, one might even apply such a model to the water patch associated with the pixel in the image.
Main characteristics of Hydrolight
- Time – independent
- Horizontally homogeneous IOP’s and boundary conditions
- Arbitrary depth dependence of IOP’s
- Wavelengths between 350 & 800nm
- Capillary – wave – air – water surface
- Finite or infinitely deep (non – Lambertian) bottom
- Includes all orders of multiple scattering
- Includes Raman scatter by water
- Includes fluorescence by chlorophyll and CDOM
- Includes internal sources such as bioluminescence
- Does not include polarization
- Does not include gravity waves or white caps
The fundamental quality that describes the time - independent, 1D light field in the ocean is the spectral radiance L(z,θ,Φ,λ)in (W/m2.sr.nm), with z the depth, θ the zenithal, polar angle of the sun, Φ the azimuthal angle of the sun and λ the wavelength. An overview of the most important optic properties & their relationship to other variables is given by the figure below. This figure also learns us we will have to provide Hydrolight with incident radiance, sea state, bottom condition, absorption coefficient a, scattering coefficient b and phase function.
The Hydrolight standard quad layout has a nominal angular resolution of Δθ = 10º and ΔΦ = 15º. For mathematical reasons, there is no quad centered on the equator.
How to install Hydrolight
- The Lahey Fortran 95 express compiler is a pre - requisite to Hydrolight. It has to be installed and registred before installing Hydrolight.
- Install Hydrolight.
- Paste the "novoteste.for" - file and the "teste.for" - file in the batch - map of the maincode directory.
- Paste the "runlist" - file into the run directory
- Paste the "Inovoteste" and the "Iteste" - files into the batch - map of the run - directory
How to use Hydrolight
When first using Hydrolight you will need to read the disclaimer and accept the license agreement.
Then you will have to define a root name in the run identification form. It will be used for all files generated by Hydrolight in this run. You can also enter a descriptive title in this form.
The parametrization of Hydrolight can be changed by selecting "Change limits"
Then we have to select the most accurate calculation method for calculationg IOP's. The total IOP's of a water body are built up as a sum of IOP's attributable to the various components of a water nody. Thus the total absorption coefficient is given by:
Although recent papers have proven that this assumption is no longer essential, Hydrolight distinguishes case 1. waters in with a high concentration of chlorophylle (in comparison to other particles) and case 2. waters in which in organic substances are dominant. The models proposed here will use different calculation methods to calculate the absorption a and the light scattering b. Absorption can be defined as the process in which radient (light) energy is converted into chemical (e.g. photosynthesis) or thermal energy. Lights scattering is the proces wherein the light changes direction (elastic scattering ) and / or wavelength. In case only the wavelength is modified we speak of inelastic scattering (e.g. Raman - scattering , chlorophylle fluorescence). It is thus paramount to understand the behaviour of light in the studied water column before selecting a calculation method and a phase function.
The ABCONST - option will consider a fixed total light absorption coefficient a and a fixed light - scattering coefficient b at a single wavelength for a homogeneous water body. This option should only be used in very shallow, homogeneous waters, where we can consider absorption and diffusion as constants (swimming pool case). It is particularly useful for studies in which the depth is measured as non - dimensional optical depth ζ rather than as geometric depth z in meters. When selecting this option we will also have to define a total light - scattering phase function (see also the figure below). Scattering is often modeled by a power law dependence on wavelength: