keywords: sources

This demo shows how to setup temporally varying sources.

Summary

In the scene, a combination of blinking and modulating sources are setup. One is always on, two use the "blinking" parameters (frequency and time offset) and three are assigned power spectral density (PSD) descriptions to define modulation. To represent what might be observed in a large area with a 3-phase power grid, the three modulating sources are out of phase with each other by 120 degrees. The scene is then observed with a 2D array camera employing a 2400 Hz read-out rate so that the modulation of the AC sources can be observed.

Related Materials

The following demos, manuals and tutorials can provide additional information about the topics at the focus of this demo:

Details

A detailed description of how sources are configured is discussed in detail in the "User-Defined Sources Manual" (External Link). This demo is a minor expansion of the "Sources1" demo. Rather than a single source we have 4 and the configuration for each is slightly different. The modulation aspect of the source is an additional attribute assigned in the material database file. For the DC (steady, unmodulated) source, the configuration is identical to the source in the "Sources1" demo. The AC (modulating) sources has a power spectral density (PSD) description.

Important Files

This section highlights key files important to the simulation.

Important
 The older GDB method for defining the geometric location and orientation of the sources is not the recommended method but is used here for historical reasons and test coverage. The GLIST point source method is now the recommended method.

  • The constant and PSD modulated sources are defined in 4 different "light" GDB files:

    • geometry/Light_DC.gdb (the unmodulated source)

    • geometry/Light_AC_L1.gdb (the phase #1 modulated source)

    • geometry/Light_AC_L2.gdb (the phase #2 modulated source)

    • geometry/Light_AC_L3.gdb (the phase #3 modulated source)

  • Each "light" GDB file contains just the source as a single vertex facet (polygon).

  • Each "light" GDB file has a unique material ID for the 4 different light descriptions.

  • The two "blinking" sources are defined using the <basesource> approach in the GLIST file.

  • The top-level description of the source’s spatial and spectral is defined in the materials/demo.mat file (see material ID #100, 101, 102 and 103).

  • The spectral output of the source is described in the 40w_2700k.int file. The format of this file is described in the DIRSIG documentation (External Link). In brief, the spectral units are in microns and the radiant intensity values are in Watts per steradian per micron.

  • Some key options regarding the source modeling are define in the source.options file. These options are discussed in more detail in the "Sources1" demo.

Setup

The Scene Geometry

The point source and flat surface it is illuminating is placed in the scene via an ODB file (see geometry/demo.glist). That ODB file inserts the 4 "light" GDB files and the GDB file for the flat surface (see geometry/Plate.gdb). Each source hovers 5 meters above the surface.

The Source Description

Each source’s spatial (angular), spectral and modulation configuration can be inspected in the materials/demo.mat file. Material ID #100 is the DC source material and it will have the following properties:

  • The Radiant Intensity filename is 40w_2700k.int

    • This file has a spectral shape derived from a 2700 Kelvin blackbody curve. The total integrated power (spectrally integrated from 0.4 to 14.0 microns and angularly integrated over a sphere) is 40 Watts. The brightness is 496 lumens, and the Overall Luminous Efficacy is 12.562 lumens per watt. These characteristics match up well with a standard, commercial 40 Watt tungsten incandescent light bulb.

  • The Shape Factor is 0

    • This makes it an omni-directional source.

  • The Normalize Shape option is enabled.

Important
 Enabling the Normalize Shape option is extremely important if you want to have traceable radiometry. This option is available so that it can be disabled in order to provide compatibility with DIRSIG3.

The AC (modulated) source materials (ID 101, 102 and 103) have the same general setup as the DC (unmodulated) source material (ID #100), except they have an extra variable (PSD_FILENAME) that assigns the modulation description:

MATERIAL_ENTRY {
    ID           = 101
    NAME         = 60 Hz AC Point Source (Phase L1)
    EDITOR_COLOR = 1, 1, 1

    OPTICAL_DESCRIPTION = SOURCE
    INTENSITY_FILENAME  = 40w_2700k.int
    PSD_FILENAME        = materials/sources/phase_L1.psd
    SOURCE_SHAPE        = 0
    NORMALIZE_SHAPE     = TRUE
}

There are three unique PSD files because although we want all three AC sources to modulate at 60 Hz, we want to have them on three different phases. The contents of the phase_L1.psd file is shown below:

<psd frequencyunits="hertz">
  <entry>
    <frequency>60</frequency>
    <magnitude>1.0</magnitude>
    <phase>0.00000000</phase>
  </entry>
</psd>

The <psd> element can contain one or more <entry> elements that define the magnitude and phase for a given frequency. Complex frequency structures can be described using multiple frequency entries. The phase_L2.psd and phase_L3.psd files are the same, except they have a <phase> of 2.09439510 radians (120 degrees) and 4.18879020 radians (240 degrees), respectively.

Important
 The combined magnitude of all frequencies can exceed 1 at a given time, which will result in an "amplification" of the sources power. There is no mechanism at this time to automatically normalize the modulation described in the PSD.

The "blinking" sources use the relevant parameters in the material file:

MATERIAL_ENTRY {
    ID           = 104
    NAME         = 60 Hz Blinking Point Source (Phase 0)
    EDITOR_COLOR = 1, 1, 1

    OPTICAL_DESCRIPTION = SOURCE
    INTENSITY_FILENAME  = 40w_2700k.int
    BLINK_FREQUENCY     = 60
    BLINK_OFFSET        = 0
    SOURCE_SHAPE        = 0
    NORMALIZE_SHAPE     = TRUE
}

MATERIAL_ENTRY {
    ID           = 105
    NAME         = 60 Hz Blinking Point Source (Phase 1)
    EDITOR_COLOR = 1, 1, 1

    OPTICAL_DESCRIPTION = SOURCE
    INTENSITY_FILENAME  = 40w_2700k.int
    BLINK_FREQUENCY     = 60
    BLINK_OFFSET        = 0.008333333
    SOURCE_SHAPE        = 0
    NORMALIZE_SHAPE     = TRUE
}

The only different in the two descriptions is the BLINK_OFFSET, which is set to half of a 60 Hz period for the second source.

The Sensor

A simple 320 x 240 array sensor with spectral output (via the "raw" capture method) is placed 100 meters above the scene (directly over the point source). The focal plane has a broad band visible (pan) spectral response. The focal plane is read out at 2400 Hz so that the modulation of the AC sources can be observed. There is a single task that has a duration of 1/60 = 0.016666667 seconds, which allows the phase L1 source to modulate a full cycle.

The Options

The user-defined source related options are discussed in more detail in the User-Defined Sources Manual. For this demo (like in the "Sources1" demo) we override the default contribution rejection threshold because this scene uses a 40 watt bulb at some distance from a dark (18% reflector) surface, which results in a very small contribution. Without lowering this threshold, the expected reflected radiance would be too small to be considered important and the source would be ignored. Therefore, the default value of 1.0e-05 was changed to 1.0e-07.

Important
 The options file and threshold is only relevant for DIRSIG4. For DIRSIG5, the radiance threshold part of the convergence threshold should be changed to 1.0e-07.

Results

The time animation below shows the output of the simulation. The source in the upper-left is the DC source, the middle two are the blinking sources and the remaining three are the PSD modulated sources.

video
Figure 1. Time animation of the 60 Hz modulating sources captured at 2400 Hz.