Non-sequential (NSQ) Mode is one of the most intimidating but powerful parts of Zemax. Non-Sequential Mode gives you the freedom to design complex real-world effects like scattering, stray light, ghost images, fluorescence, LED illumination, or even complex opto-mechanical assemblies.
Sequential Mode is like following a recipe step by step. You chop, mix, cook, and plate in a fixed order. Light travels through a strict sequence going from -Z to +Z.
NSQ is like walking into a buffet. You can wander in any direction, pick what you want, skip steps, or even go back for seconds. Rays in NSQ do the same: they reflect, scatter, split, or pass through objects in any order.
In this post, we will break down NSQ in simple terms, show how to set up your first simulation, and highlight a few common pitfalls beginners run into. By the end, you will see it is not as intimidating as it first looks - and you will be ready to start experimenting with your own NSQ models.
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The Key Difference - Objects vs. Surfaces
The first hurdle for beginners is realizing that NSQ does not use surfaces the way SEQ does. In SEQ, you are building a lens one surface at a time: light enters a surface, bends according to Snell's law, then moves on to the next in a fixed order that you control at all times.
In NSQ, that structure disappears. Instead you place objects in a 3D CAD environment. These objects are divided into three buckets: Sources, Objects, Detectors, which can be lenses, mirrors, prisms, light guides, CAD parts, Detectors, or various light sources.
PRO TIP: EVERY OBJECT STARTS AT (0,0,0) in cartesian (X,Y,Z).
Think of it like this:
- Sequential Mode: You are tracing a train on a track - the order is fixed.
- Non-Sequential Mode: You are flying a drone through a room filled with furniture - it can bump into, bounce off, break apart into pieces, or fly through anything in any order.
Let's dive into Zemax and begin designing in Non-Sequential Mode.
START: Create a Light Source
Initialize the System Explorer is the first step.
Every NSQ simulation starts with a source. This is where your rays originate, and Zemax gives you many options depending on what kind of system you are modeling.
| Source Diffractive | A source with the far-field diffraction pattern of a defined UDA. |
| Source Diode | An array of diodes with separate X/Y distributions |
| Source DLL | A source defined by an external user supplied program. |
| Source Ellipse | An elliptical surface that emits light from a virtual source point. |
| Source EULUMDAT File | A source defined by lamp data in an EULUMDAT format file. |
| Source Filament | A source in the shape of a helical filament. |
| Source File | A user defined source whose rays are listed in a file. |
| Source Gaussian | A source with a Gaussian distribution. |
| Source IESNA File | A source defined by lamp data in an IESNA format file. |
| Source Imported | A source defined by the shape of an imported object. |
| Source Object | A source defined by the shape of another object. |
| Source Point | A point source that radiates into a cone. The cone may be of zero width or be extended up to a full sphere if desired. |
| Source Radial | A radial symmetric source based upon a spline fit of arbitrary intensity vs. angle data. |
| Source Ray | A point source aligned with direction cosines. |
| Source Rectangle | A rectangular surface that emits light from a virtual source point. |
| Source Tube | A source in the shape of a cylindrical tube. |
| Source Two Angle | A rectangular or elliptical surface that emits light into a cone with distinct angles in the X and Y directions. |
| Source Volume Cylinder | A volume source in the shape of a cylinder with an elliptical cross section. |
| Source Volume Ellipse | A source in the shape of an elliptical volume. |
| Source Volume Rectangle | A volume source in the shape of a rectangle. |
- Select your source. A good start is to make your source the (0,0,0) point to start. Remember every object you insert into the Non-Sequential Component Editor will be relative to the (0,0,0) point, which for this example is our source.
- # Layout Rays: 10 - 50 rays is more than sufficient.
- # Analysis Rays: 100k is a good start.
- Define Beam.
- Customize your Source for your application. Select the Object Properties -> Sources. Define the Polarization or keep it random. Make it an Array.
Insert Object
There is many choices. I suggest heading to the Help Manual and reviewing the summary table. The Setup Tab » Editors Group (Setup Tab) » Non-sequential Component Editor » Non-sequential Geometry Objects » Summary of NSC Objects.
Lets keep it simple a start with a Lens. Notice that if we define the Z Position of the Lens at 5.000 mm then the distance from the Source point to the first surface of the Lens will be 5 mm (cursor is indicated by the red dot).
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We can interesting Objects like beam splitters using the Polygon Object. Go to Object Properties -> Type -> General -> Data File and select some pre-defined objects.
Next, if we choose to go with a beam splitter, we need to define the transmission and reflection characteristics.
Select the Object Properties -> Coat/Scatter -> Face -> Coating.
IMPORTANT: We selected the coating. Why did nothing happen? You MUST enable scattering and splitting the the NSC 3D Layout, Settings -> check Split NSC Rays -> check Scatter NSC Rays (polarization will auto check).
Now we see the TRUE Power of Non-Sequential Mode. This first Lens in our system has a GNARLEY Ghost Image. REAL LENSES DO THIS, ESPECIALLY ASPHERES WITH SMALL RADII. We need to fix this! We can apply an AR coating the front and back faces of our Lens Object. Click the Lens Object -> Object Properties -> Coat/Scatter -> Face -> Coating -> AR.
Much Better! I can allude to some expert features here for Scattering Analysis - which will not be covered in this blog. We can see how strong our back-scatter rays are from that first Lens. Click System Explorer -> Non-Sequential -> Minimum Relative Ray Intensity -> Change from 1.0E-03 to 1.0E-02 to 1.0E-01. Here we can see that the back-scatter is two orders of Magnitude weaker then the main beam.
Add a material to the Polygon Objects. Now we can add some mirrors. There are many different ways to do the same thing in NSQ. For the mirrors I will use the Off-Axis Object.
For the Mirror we need to make the coating reflective (at minimum, we are only using the basic features for coat/scatter). Click Object Properties -> Face -> Front Face -> Face Is -> Reflective.
For the splitter, Object 7, use ideal I.50 coating for both front and back faces.
NOTE: Just because we mitigated the GHOST Image above does not mean it is completely gone. It just means that the Relative Ray Intensity is lower than what we are tracking. Beam Splitter Cubes are notoriously scatter prone because of their geometry. AR coatings will help, but there will always be ghosting, and scattering. This is where other advanced techniques need to be employed.
Detectors
Detectors are the primary analysis tool in Non-Sequential Mode. They collect the light so you can analyze it.
| Detector Color | A flat rectangular detector with an arbitrary number of pixels. This detector can record and display incoherent illumination data defined by tristimulus response. This detector can accurately record and display the color of illumination |
| 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. |
| 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 is the most powerful detector in terms of the analysis it provides, but it is limited to a flat rectangular shape. |
| Detector Surfce | 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 all will be illuminated by rays passing through the individual voxels. |
| Objects as detectors | Most objects of arbitrary shape may be used as a detector that records incoherent irradiance data. |
Here we will add a Detector Rectangle and do some analysis.
Once the Detector is added and placed in the right location we much define the width and the number of pixels. Use something like 512x512 to start.
IMPORTANT: Light will go through the Detector. To make the rays terminate at the Detector set the material to "ABSORB".
Open a Detector Viewer. Click Analyze -> Detector Viewer.
Now we MUST run a Ray Trace. Click Analyze -> Ray Trace.
IMPORTANT: After clicking "Ray Trace" make sure you check "Split NSC Rays" & "Scatter NSC Rays" (polarization will auto check). Then click Clear & Trace.
Use the Detector Viewer Settings to change to Coherent Irradiance. This is another example of the power of NSQ in Zemax.
There are many other features to explore in NSQ. This Blog should get you started exploring on your own!
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