a multipurpose 3D Monte Carlo Code
is a natural 3D forward
The SimulO GUI works on windows OS only (XP, Seven). The Graphical User Interface allows creating easily a simulation setup and run it. A version of SimulO simulation engine works also on Linux platformes. It could read simulation setup created by hands or with the GUI and run it extensively by using linux cluster for example.
In SimulO, the user can compose complex optical devices by positioning and sizing any number of elementary objects. Presently three elementary shapes are available: sphere, cube, and cylinder.
In addition to the topologic setup, SimulO also allows the definitions of the optical properties of each object. Volumes occupied by each elementary object are homogenous in term of bulk optical properties, while different optical properties can be defined for each face of the object. Four bulk properties must be set: the refractive index of the material (used for the calculation of Snell’s and Fresnel’s laws), the absorption and scattering coefficients, and the scattering phase function. For the latter, the user can presently select one of the built-in phase functions (pure water, isotopic, Henyey-Greenstein, Fournier-Forand) or upload their own using a text file, which will be interpolated at appropriate angles. To simulate a volume with two different types of scatterers (water and particles for example), SimulO accommodates the use of two scattering functions which can be weighted with their respective probability by setting different scattering coefficients. The surface properties (bidirectional reflectance function) used to describe each face of an elementary object can be set to transparent, specular or Lambertian reflection (with a given reflection probability). An object’s face can also be defined as light source. In this case, photons are emitted from points randomly distributed over the area of this face, with a direction obtained relative to the local normal to the face. The user can select whether the angular direction of an emitted photon is (i) normal to the face, (ii) given by a Lambertian distribution or (iii) randomly distributed with a given divergence around the normal direction.
To understand the optics of the simulated device, SimulO provides several types of counting tools to obtain the information regarding a simulation. By default, SimulO records: the number of collisions on each face of the elementary objects, the average pathlength of photons, the average number of scattering events per photon, and the number of photons absorbed in the volume and on each face of an object. It is also possible to add several specific tools to know: the average pathlength of photons which are absorbed by a specific face, the angular distribution of photons reaching a specific face or the number of photons reaching a specific face within a specified angular range centered on the normal.
SimulO software is registered by the French "Agence pour la Protection des Programmes" under the number 68993.
is a multipurpose
SimulO was originally developed to study Point Source Integrating Cavity Absorption Meter (PSICAM) devices. Therefore, the first simulations used to validate the code deal with the PSICAM configuration. A PSICAM is modeled in two steps with SimulO. Firstly, a sphere with the appropriate radius is drawn and the internal face of this sphere is set to Lambertian reflector (with the correct absorption probability for the material modeled). The bulk properties are set to those of the sample examined. Secondly, another small sphere is placed at the center of the first one, and the external face of this sphere is set as a photon emitter. Kirk (1997) derived theoretical formulae for both the average pathlength and the number of collisions on the internal face of the cavity when the sample is purely absorbant. Different simulations were performed with SimulO considering various cases of sphere radius, cavity wall reflectivity and sample absorption coefficient. Results from SimulO were always in agreement (difference smaller than 0.4%) with equations (4) and (5) in Kirk (1997).
The second geometry used to validate the code is the
Kirk (1992) provided interesting results from
Reproduction of the Fig 3. of Kirk(1992) with SimulO.
SimulO can also be used to calculate the self shading error of a sensor embedded in a given structure (like a boat, a buoy or profiler) (Gordon 1992).
To estimate this error with SimulO, we use the backward
(i) A first simulation with an infinitely small sensor and thus without any shading. In this simulation, the amount of photons in all the sky directions are recorded.
(ii) A second simulation is done with the sensor embedded in the deployment structure which is assumed to be perfectly black. The amount of photons in all the sky directions are also recorded.
The ratio between the amount of photons recorded in these two steps, for all sky's directions, allow the determination of the shading effect due to the structure.
To validate SimulO and this two steps approach, we present several comparisons with the results obtained by Piskozub (2004). This study reports estimations of the the self-shading measurement error on the upwelling irradiance which are caused by the presence of a typical cylindrical housing of an optical instrument as a function of the housing dimensions and of the optical parameters of seawater. The comparison between Piskozub and SimulO results is presented hereafter.
Reproduction of the Fig 1. of Piskozub(2004). The self-shading error dependence for the absorption coefficient (a) for instruments with two radii, a flat disk and cylindrical shape (b=0.3 m-1). Solid and dashed lines represent results from Piskozub (2004) and SimulO, respectively.
The agreement between these results is good with a difference of 2% in average.
SimulO can be used to estimate the self shading of a large range of devices. As an example, some simulations were made to estimate the self shading error for a upwelling radiance (Lu) sensor embedded in a glider.
Simulation with SimulO of the Lu self shading for a Glider. The shape of the glider (body and wings) clearly appear.
The computation of underwater radiances and
irradiances is also
a typical problem that can be addressed using a
These problems are:
1: An unrealistically simple problem.
2: A base problem using realistic inherent optical properties for the sea water.
3: The base problem but with stratified water.
4: The base problem but with atmospheric effects.
5: The base problem but with a winded blown sea surface.
6: The base problem but with a finite depth bottom.
7: A problem involving Raman scattering.
Here we present a comparison based on problems 2 and 3 of Mobley (1993). Problems 1, 4 and 6 can also been studied with SimulO, but not problems 5 and 7 as the wind-blown surface or Raman Scattering are not yet implemented in SimulO
Comparison between Mobley(1993) and SimulO results. Problem 2 for ω0=0.9
Comparison between Mobley and SimulO for the stratified problem
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- Kirk J.T.O
- Gordon H. R., Ding K. (1992). Self-shading of in-water optical instruments, Limnol. Oceanogr., 37(3),49 I-500
- Kirk, J.T.O. (1997) Point-source integrating-cavity absorption meter: theoretical principles and numerical modeling, Appl. Opt. 36, 6123-6128.
- Piskozub, J. (2004). Effect of 3-D instrument casing shape on the self-shading of in-water upwelling irradiance, Opt. Express 12, 3144-3148.
- Mobley, C.D., Gentili, B., Gordon, H.R., Jin, Z., Kattawar, G.W., Morel, A., Reinersman, P., Stamnes, K. and R.H. Stavn (1993). Comparison of numerical models for computing underwater light fields, Applied Optics, 32, 7484-7504. (Get PDF Reprint)