Absolute and Relative Positions in QUADOA® Optical CAD
The position reference of an element in QUADOA® Optical CAD defines what its position values refer to: with “Absolute”, they refer to the origin of the parent’s coordinate system, which for an element outside any assembly is the zero point of the global coordinate system; with “Relative to previous”, they refer to the coordinate system of the previous element. The same value of 20 mm in Z therefore places a lens at different locations, and relative positions follow the previous element when it moves.
Key points
- Absolute: the position refers to the origin of the parent’s coordinate system, i.e. the global zero point for elements outside assemblies.
- Relative to previous: the position refers to the coordinate system of the previous element, for example the last surface of the previous lens.
- With a relative reference, an element moves along when the previous element moves.
- Three more options exist: Relative To Sibling, Absolute (Relative Z) for stack mounts and By ID for branched light paths.
- Positions can be converted later without moving the elements.
Terms used in this article
- Position reference
- The option that defines the coordinate system a position value refers to.
- Global zero point
- The origin of the global coordinate system.
- Previous coordinate system
- The coordinate system of the element before the current one, typically the last surface of the previous lens.
What does the reference “Absolute” mean?
With Absolute, a position of Z = 20 mm means that the lens is 20 mm away from the zero point of the global coordinate system in Z direction. (Inside an assembly, Absolute refers to the origin of the assembly’s local coordinate system instead.) The position does not depend on other elements.
What does “Relative to previous” mean?
With Relative to previous, the same 20 mm refer to the coordinate system of the previous element. In the video, this is the second surface of the first lens, so the second lens moves to the right when the reference is switched.
In the manual this option is called Relative To Preceding. The preceding item is the next item above in the tree of the Optical Design Editor; it does not have to belong to the same element or assembly. The first surface of an element cannot be set to relative. This behavior corresponds to the relative surface positions of classical spreadsheet-based optical design software.
Because the reference is the previous element, the lens follows it: if the first lens is moved, for example upward in Y, the second lens moves with it. Switching back to Absolute moves it back, because it again refers to the global zero point.
Which other reference options are there?
| Reference | Position refers to | Typical use |
|---|---|---|
| Absolute | The local coordinate system of the parent (global if there is no parent) | Elements at fixed coordinates |
| Relative To Preceding | The preceding item in the tree, typically the last surface of the previous lens | Air spaces between consecutive lenses |
| Relative To Sibling | The preceding sibling inside the same assembly | Elements that should follow another element of the same group |
| Absolute (Relative Z) | X, Y and rotation absolute; Z is the sum of the Z positions of the preceding items in the same assembly | Tolerancing lenses in a stack (fill) mount separated by spacer rings: thickness errors shift the following lenses along Z without unwanted tilts or decenters |
| By ID | Any position item above it in the Optical Design Editor, chosen by its identifier | Multi-path systems, for example a light path that branches at a beam splitter |
Absolute or relative: which should I use?
| Absolute | Relative to previous | |
|---|---|---|
| Reference | Origin of the parent’s coordinate system (global zero point if there is no parent) | Coordinate system of the previous element |
| If the previous element moves | Element stays in place | Element moves along |
The right choice also matters for tolerancing. You can convert between the options at any time without moving the element; see converting positions and dependencies.
Video tutorial: absolute and relative positions
This 2-minute video shows the difference between the reference options Absolute and Relative to previous.
Video length: 2:25 min · Also on YouTube: Absolute and Relative Positions in QUADOA® Optical CAD
Frequently asked questions
Absolute positions refer to the origin of the parent’s coordinate system, which is the zero point of the global coordinate system for elements outside assemblies. Relative to previous positions refer to the coordinate system of the previous element, for example the last surface of the previous lens.
The position value stays the same but now refers to a different coordinate system, so the lens is placed at a different location.
Its position is defined Relative to previous, so it follows the coordinate system of the previous element. Switch to Absolute if it should stay fixed.
Yes. Use Convert Position in the right-click menu of the Optical Design Editor; the values are recalculated so the lens stays in place.
Absolute (Relative Z). X, Y and rotation stay absolute, while the Z position adds up from the preceding items in the assembly, so thickness tolerances move the following lenses without introducing tilts or decenters.