Understanding the Surface Source Parameters

This page describes the parameters to set when creating a Surface Source.

Intensity Distribution

The Intensity Distribution describes the emission pattern of a light source. You can choose among different distribution profiles:

A Lambertian emission ensures that the source has a uniform distribution. The source theoretically distributes the same amount of light in every direction and has, therefore, the same luminance whatever the observation angle is.
With a Cos distribution, the intensity follows the cosine law. The higher the intensity, the narrower the intensity diagram will appear. You can modify the order of the law to make the rays converge or diverge.
A gaussian distribution follows a gaussian function and can be symmetric or asymmetric.
Intensity files are data measured files that provide an accurate intensity profile.

The supported formats are:

iesna (*.ies)
eulumdat (*.ldt)
XMP maps with conoscopic intensity (*.xmp)

Lambertian Distribution

A lambertian source evenly distributes light in every direction of the half space. The deflection angle(θ) corresponds to the total angle of emission of the light source.


I = A * cos(θ) A: Intensity in propagation axis θ: Deflection angle
Radiation laws and relative intensity diagram, characteristic of a lambertian source emitting on a half sphere.

A source with a lambertian distribution has the same luminance whatever the observation angle is, as illustrated below:



Set-up of emissive source with three radiance sensors.	Radiance map of the lambertian source set-up above. The Luminance is constant no matter the angle of observation.

With the Total Angle:

you can limit the emission cone of your surface source. Output light is set to 0 cd, for deflection angles (θ) bigger than half the Total Angle.
a lambertian source with a total angle set to 0 degree has parallel rays.

Cos - Lambert's Cosine Law

The Lambert's cosine law basically states that the illumination of a surface is proportional to the cosine of the angle between the direction of the incident light and the surface normal.

The cos distribution follows a cosines law at n^th order.

The Total Angle is the maximum angle of emission of the light source.

The N parameter sets the order of the cosines law.


I = A * cosⁿ(θ) A: Intensity in propagation axis θ: Deflection angle n: Order of cos law
Radiation laws of cos function.	Radiation diagram at 2^nd, 3^rd, 4^th and 5^th order compared to a lambertian distribution. The higher the intensity, the narrower the intensity diagram will appear.

A source with cos distribution has a luminance varying according to the observation angle, as illustrated below:



Radiance map of the Cos source

Gaussian Distribution

The intensity distribution of a source can follow a gaussian distribution.




Gaussian distribution laws and relative radiations distribution of gaussian compared to a lambertian distribution.

The Full Width At Half Maximum (FWHM Angle) is used to describe the width of a curve at half its maximum amplitude. It means that the source reaches half its power potential between (0°) the normal of the emitting surface and the FWHM.

It allows you to alter the emission profile of the light source.

As illustrated below:

a small FWHM value tends to restrain and concentrate the light beam.
a large FWHM value results in a broader, more widespread light emission.


	FWHM = 15°	FWHM = 45°
Total Angle = 20°
Total Angle = 75°

If the source is symmetric, then the FWHM Angle is the same on both axes.

If the source is asymmetric, the FWHM Angle can be edited on X and Y.

The axis system of a Gaussian Asymmetric can be global or local:

Global axis: The orientation of the intensity diagram is related to the axis system.
Local axis: The orientation of the intensity diagram is related to the normal at the surface.

If no axis is selected, the axis system is considered local.

Library - Normal to UV Map

Normal to UV map allows you to define the intensity distribution as normal to the selected emissive surface and its orientation on the emissive surface.

Normal to UV map is particularly useful in case of an asymmetrical intensity distribution as it allows you to define accurately its orientation on the surface.

Important: For the Normal to UV map to work you need to create a Texture Mapping on the emitting face. Refer to the Surface Source procedure procedure procedure for more information on how to create a Texture Mapping.

Exit Geometries

The exit geometries represent the geometries present during source measurement (the bulb of a light bulb or the case of a LED) that could potentially influence the optical behavior or intensity distribution of the source.

Selecting exit geometries allows you to define a new emissive geometry to avoid recalculation of the geometry's effect on the source.

For example, if you selected an iesna file (*.ies) corresponding to a light bulb, the bulb geometry is taken into account in the data of the iesna file. To avoid the recalculation of the light bulb's effect, the bulb must be selected as the exit geometry.



Intensity distribution without specific exit geometry.	Intensity distribution with lens defined as exit geometry.

CAUTION: When using a Surface Source in a CPU simulation, rays from the Exit Geometries are only propagated forward according to the emitted rays from the Surface Source.

Timeline

Thanks to the Timeline section you can simulate a flickering Surface source by defining the variation of the source's power in time.

For more information on how to create a flickering Surface source, refer to Dynamic Inverse Simulation.

Flux variation file

Flux variation file (optional) is a *.json file that defines the samples for one period representing the variation of the relative flux of the source with time.

You can create your own Flux variation *.json file thanks to the following Iron python script. The script provides you a way of creating the *.json file, but you can modify it if you want to go further. Follow this procedure to help you create the file thanks to the script.

Important:

If you define a flux variation with identical periods, you only need to define one period in the *.json file. The period will then be repeated N times such as Integration / Period = N.
The flux variation file must begin and end with the same relative flux value.

Example of *.json file content:

{
  "Time": [
  0.000,
  249.9,
  250.0,
  400.0,
  500.0
  ],
  "Relative_flux": [        
  0.0,
  1.0,
  1.0,
  0.0,
  0.0        
  ]
}

Relative lag

Relative lag (percentage, default value of 0%) represents the relative time along the period when the source starts to emit light. That means the relative lag includes a temporal shift of the time period.