Basic Illumination

Basic Illumination

Illumination is an important parameter for imaging applications such as photography, microscopy, and machine vision. Whether from sunlight, light bulbs, LEDs, or lasers the characteristics – intensity, spectral distribution, and directionality – influence an optical system’s performance. Additionally, surfaces can either diffuse light uniformly or reflect it specularly, helping or hindering performance. Utilizing the proper light source for the application begins with an understanding of the final illumination necessary and the tradeoffs between different source types.
In the 3DOptix cloud-based simulation tool, the light source object has multiple parameters that are necessary to simulate different source types.  We will overview the light source object and some ways to analyze the illumination profile.
We will first compare the light sources available in 3DOptix and their illumination profile:
  1. Point Source
    • Half Cone Angle (Elliptical Far Field), 10×10 degrees
    • Wavelength – Gaussian Distribution
      • 520 nm center wavelength
      • 10 nm spectral width
      • 500-540 nm min/max range
      • 1 nm step size
    • Power, 1 W
    • Unpolarized
  1. Plane Wave
    • Circular, 5 mm radius
    • 520 nm wavelength
    • Power, 1 W
    • Polarized – Linear, 90 degree
  1. Detector, Plane Wave
    • Spot: Coherent Irradiance
    • Analysis Rays: 1 million
    • 200×200 pixels
  1. Detector, Plane Wave
    • Spot: Coherent Irradiance
    • Analysis Rays: 1 million
    • 200×200 pixels
You can see the image of our optical system below. The 3DOptix simulation file can be downloaded to see additional information about the optical system such as component spacing and analysis detectors.
First, we will look at the illumination profile of the two sources. Since both sources have “flat top” profiles the illumination pattern will look identical, but the point source will have diminishing power density as it is diverging.
Now that we have set up two different light sources, let’s look at an illumination application with two sources that will act as field points for a light source.
The new optical system will consist of the following components:
  • Edmund Optics 08-068, plano-convex
  • Edmund Optics 08-068, plano-convex
  • 2x Light sources
    • Plane wave
    • Circular, 0.1 mm radius
    • Polarized – 0 and 90 degrees
  • Detector, image plane
    • Spot: Incoherent irradiance
    • Analysis Rays: 1 million
    • 200×200 pixels
Typically, we will have a more complex optical system with multiple light sources or field points which makes it difficult to see each source’s illumination contribution at the image plane.  To help visualize each light source or field point we can change the source color.  Now we can clearly see where each source interacts with our optical system.

Since the PLANE WAVE sources are polarized in opposite orientations (0 and 90 degrees), we can also use polarization-sensitive detectors in the analysis to isolate polarized components of the illumination. This can be important to determine any scattering or reflections that cause polarization effects, such as Brewster’s angle.

Next, we will move on to the POINT SOURCE and analyze illumination using diffuse scattering. To create the scattering surface a flat metal plate will be inserted and the source and detector placed at conjugate positions angled 20 degrees. The red dot is the POINT SOURCE.
We will select the ENABLE SCATTERING checkbox and set transmittance to zero and reflectance to 10%; the absorbance automatically adjusts to 90%. The Lambertian scattering is isotropic and there are no additional parameters. Gaussian scattering has a few parameters we can change to generate the profile we desire.
To see the difference between the scattering profiles and the effect on the illumination we will create two Gaussian profiles, 5- and 10-degree SIGMA X/SIGMA Y, and Lambertian profile.
We can visually see the difference between the illumination profiles in the plots above. As the Gaussian scattering angle increases, we get more even illumination that starts to appear as Lambertian scattering.
For illumination profiles typically there is a minimum or maximum value that is desired to fulfill the optical system specifications. To simulate this, we can go into the analysis window and set the minimum and maximum ADJUST COLOR MAP RANGE to the values we need.
Now we can observe only the pixels with the illumination that fits our criteria. Additionally, we can download the pixel data to further analyze the spatial size of the illumination profile. This is a great way to compare the illumination coverage and uniformity of different configurations or optical systems.
Modeling illumination by different sources is an important step in optical design due to the varied performance at the image plane. Generating illumination using both the light source and different scattering profiles of the surface under test can help drive better solutions overall.

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Available on January 30th, 2023