raytracing of simple 1D gratings is done following Ludwig 1973 https://doi.org/10.1364/JOSA.63.001105
for this approach, rays now have the additional attribute "order", which is taken into account when hitting an object specified as optical grating
using this it is possible to simulate multiple orders of diffraction at one grating by generating rays with the same wavelength but different order
gratings are defined by their type (reflection, transmission with diffraction at 1st surfache, transmission with diffraction at 2nd surface), by their line spacing, their line direction as specified by a hypothetical set of planes intersecting the body and generating the lines as intersecion cuts, and also have the attribute "order". Additionally, for transmission gratings a refractive index should be provided.
diffraction at a grating object can be specified to be calculated using the order defined by the ray, or by the hit grating, allowing for multiple diffractions of different orders at multiple gratings beeing hit in the path of a single ray.
note that due to the specific type of this approach to simulate diffraction gratings, one ends up very fast with very many rays or even sunrays, which heavily increases calculation time.
also note that errors in the code might of course be present, however in my testing diffraction (at least for reflection and transmission grationgs without taking into account different indices of refactions) is simulated accurate.
enjoy! Thanks to the creator of the fantastic OpticsWorkbench!
this illustrates a simple reflection grating with 500 lpm hit by sunray. Planes with normal 010 indicate the set of intersecting planes used to define the grating lines direction.
this shows the same body, defined as transmission grating. Note that the diffraction happens at the 2nd surface as specified by the grating type. Differences in refractive indices are taken into accound.
screenshots show that also non-planar bodies can be used, in the example of a concave reflection grating. A stepfile for a thorlabs concave mirror is used and set as a grating with 500 lpm. Sunray of 1st and 2nd order are shown.
an example of a simple echelle spectrometer using a R2 52.91 lpm grating and a set of sunrays from order -47 to -82 (each order comprises ~5-10 nm, sampled by 15 rays around a center wavelength from blue to red) and a flint glas prism. Collimation and camera optics are thorlabs step files and a transparent absorber shows the resulting echelle spectrum. Entrance into the spectrometer design is by a 50 mu slit. This is a example with very long calculation time due to the high number of rays. Note that the sign of the order is not intuitive. If an error occurs, stating that complex numbers are not supported, while diffraction with this order is considered valid by the user, try to change the sign of the order.
Geometrical optics for FreeCAD.
Performs simple raytracing through your FreeCAD objects.
Optics workbench is available through the FreeCAD Addon Manager
Expand to view manual installation instructions
cd ~/.FreeCAD/Mod/
git clone https://github.com/chbergmann/OpticsWorkbench.gitOnce Optics workbench is installed, FreeCAD will need to be restarted. When you restart FreeCAD, "Optics Workbench" workbench should now show up in the workbench dropdown list.
- Create some FreeCAD design objects. For 2D simulation Sketches will do the job.
- Select one or more of your design objects and then create Optical Mirror to let your objects act as mirrors
- Select one or more of your design objects and then create Optical Absorber to let your objects act as black walls for the light rays
- Select one or more of your design objects and then create Optical Lenses to let your objects act as lenses. Lenses should be closed shapes. Select a material in the Lens properties or provide a refraction index.
- Add some source of light (Ray, Beam).
A single ray for raytracing
Parameters:
- Power: On or Off
- Spherical: False=Beam in one direction, True=Radial or spherical rays
- BeamNrColumns: Number of rays in a beam per row
- BeamNrRows: Number of rays in a beam per column
- BeamDistance: Distance between two beams
- HideFirstPart: Hide the first part of every ray that comes from the source and goes to the first point of reflection/refraction/nirvana
- MaxRayLength: Maximum length of a ray
- MaxNrReflections: Maximum number of reflections. This prevents endless loops if the ray is inside a mirror box.
A bunch of rays with different wavelengths of visible light.
The rays overlap. If they hit a lens, they will be dispersed. See the Example - Dispersion.
A row of multiple rays for raytracing
Parameters: see Ray. BeamNrColumns must be > 1 to get a beam
Rays coming from one point going to all directions in a 2D plane
Parameters: see Ray. BeamNrColumns must be > 1 and BeamNrRows=1 and Spherical=True to get a radial beam
Rays coming from one point going to all directions
Parameters: see Ray. BeamNrColumns and BeamNrRows must be > 1 Spherical=True to get a spherical beam
The FreeCAD objects in parameter Base will act as mirrors
Select some FreeCAD objects, then create Optical Mirror
After a ray or beam has been added, a parameter 'Hits From Ray/Beam...' will appear. This is a counter of how many reflections you have from each ray. Do not modify this value.
The FreeCAD objects in parameter Base will swallow the rays of light
Select some FreeCAD objects, then create Optical Absorber
After a ray or beam has been added, a parameter 'Hits From Ray/Beam...' will appear. This is a counter of how many rays have hit this absorber. Do not modify this value.
'Hit coordinates from ... (read only)' records the position of each LIGHT RAY when it hits. This way, it is possible to visualize the image on the absorber in a XY diagram.
To show an XY plot, open a python console and type:
import OpticsWorkbench
OpticsWorkbench.plot_xy(App.ActiveDocument.Absorber)
The FreeCAD objects in parameter Base will act as lenses
Select some FreeCAD objects, then create Optical Lens
The Refration Index has to be provided. The parameter Material contains a list with pre defined refraction indexes.
After a ray or beam has been added, a parameter 'Hits From Ray/Beam...' will appear. This is a counter of how many refractions you have from each ray. Do not modify this value.
Switches off all Rays and Beams
Switches on and recalculates all Rays and Beams
generates the screenshot above
see issues on Github
Please offer feedback or connect with the developer via the dedicated FreeCAD forum thread.
GNU Lesser General Public License v3.0 (LICENSE)