Detectors used in illumination design

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This lesson provides an introduction and an information hub for detectors in illumination systems. In this lesson, the various detectors and how-to's for detectors on an illumination system will be described. The detector is the end point of an illumination system, and arguably where all of the work done prior comes into fruition.

Authored By Katsumoto Ikeda

Introduction: What detectors do

There are six different types of detectors in OpticStudio. All detectors can display either radiometric units in Watts, or photometric units in Lumens, much like in the discussion of units in the article Performance targets of illumination design. Detectors can be used to evaluate the illumination system that we are constructing. Depending on the system, we can measure the uniformity of a flat surface, the color properties of the surface as if the human eye viewed it, or the angular intensity of a light source.

The non-sequential rays from the light source must be traced to generate any analytical results. The detector is empty when created, that is to say, that the data in each pixel/voxel is initially zero. Detectors then accumulate energy based on tracing analysis rays, and energy is accumulated until the detectors are cleared. Further, data obtained on a detector can be used for optimization, and we can optimize based on data from a single pixel, or average data over the detector.

Just as the light source is the start of any illumination design, the detector is what brings the design process together into a quantifiable result. These results are needed for the analysis of the design, and future improvements of the design.

Different detectors

• Detector Color: A flat rectangular detector with an arbitrary number of pixels. This detector can record and display incoherent illumination data defined by a tristimulus response. This detector can accurately record and display the color of illumination. This detector is one of the more used Detector types in examples of the Knowledge Base and also in application.

• Detector Polar: A section of a sphere, or a full sphere, that collects angular (far-field) intensity data. Data collected by this detector may be exported into source data files such as IESNA and EULUMDAT. Use of this detector is explained in the article How to use the polar detector and IESNA/EULUMDAT source data

• Detector Rectangle: A flat rectangular detector with an arbitrary number of pixels. This detector can record and display incoherent, coherent, point spread function, polarization, and other data. This detector is one of the most commonly used detectors in terms of the analysis it provides, but it is limited to a flat rectangular shape. This detector is one of the more used Detector types in examples of the Knowledge Base and also in application.

• Detector Surface: A circular or annular detector with an arbitrary number of pixels in the radial and angular directions. The surface may follow a plane, sphere, conic asphere, or aspherical shape. The surface detector can only record incoherent irradiance data.
• Detector Volume: A rectangular volume with an arbitrary number of voxels in the local x, y, and z directions. The detector volume may be nested within or straddle any other object. Multiple detector volumes may also be superimposed, and rays passing through the individual voxels are all illuminated.

There is a good example of an analysis of the volume detector explained in the article How to show detector volume data in 3D

• Objects as detectors: Most objects of arbitrary shape may be used as a detector that records incoherent irradiance data.

(The full list of features for detectors can be found in The Setup Tab...Editors Group (Setup Tab)...Non-sequential Component Editor...Non-sequential Detectors of the Help files.)

A comment on resolution and noise

There is a trade-off between spatial resolution and energy resolution. The common question is: "I have a rectangle of a given size. How many pixels should I use?" If we assume that the detector is uniformly illuminated, and assume that every ray, striking the detector, carries the same flux:

• The signal on the detector goes as the number of rays per pixel
• The statistical noise goes as the square root of the number of rays per pixel

For a detector with 10 x 10 pixels, a total of 100 pixels, the resolution vs. noise is as follows.

We can see that 10000 rays are not sufficient for a 10 x 10 detector, and 100000 is a more reasonable number of rays to trace. A 2% result is about the best you can do in the lab. Generally, it is not a wise use of time to try to force simulation results to be significantly better than this.

Let's see what happens when we increase the resolution to 100 x 100:

This result shows that for a detector with 100 x 100 pixels, to get the same noise level as the 10 x 10 detector, we need 100 times as many rays. This shorthand calculation can be used to estimate the number of rays we need to have a certain amount of noise to signal ratio for our simulation. When iterating a design, it is good practice to spend "just enough" time for the simulation so that the lower ray count can be considered. Other methods such as smoothing the detector and average out the rays can be used in the intermediate stages. Since smoothing is effectively reducing the number of pixels, it is better to reduce the number of pixels rather than use a high-resolution detector with smoothing if you are not willing to spend the time for the ray trace.

Useful articles about detectors

There are some useful examples of detectors in the current Knowledgebase archives.

How to use the polar detector and IESNA/EULUMDAT source data

Although this article primarily illustrates how to create a source file in the IESNA/EULUMDAT format, the introductory section shows how to use the polar detector to generate the source files. This article is an excellent resource for people who want to use the polar detector to create light sources.

The polar detector used to create light sources is a method to create a fixed ray data set through a ray trace. When the light source is complex, such as a modeled light source, the ray trace of the model alone may take a long time. By creating a separate light source, we can use the results of the polar detector as a source file and use more processing resources on the optical components after the modeled light source.

How to show detector volume data in 3D

This article shows how to create a CAD model to represent Detector Volume data in 3D. Although a ZPL macro is used in the article to create the data, the use of the Detector volume is explained in detail. The Detector Volume object in OpticStudio is a handy tool for detecting light inside a volume. It uses volume pixels, or voxels, to detect light. However, the OpticStudio interface currently allows these voxels to be viewed only in 2-D planes. This article will show you how to use OpticStudio to create intricate CAD models of the voxels in a Detector Volume, and a sample ZPL macro is given for use in creating these CAD models automatically.