Interpolated focus, explained simply
How the software keeps a whole part sharp from a few measured points, so no defect slips through soft across a large scan. No focusing at every tile. In plain language.
- Interpolated focus measures true focus at a few points on the part, fits a surface through them, and predicts the right height at every tile in between. No focusing at every tile.
- A part is never flat or square to the lens. It sits at a slight tilt in the fixture, and it has its own warp, step heights, and surface relief. One fixed height goes soft at the edges.
- You choose how many points and where they go. A few for a flat, tilted part. A dense grid for a warped or stepped part. Corners first, then the features you are inspecting.
- A heat map colors the surface by height. Even color is level. A gradient is a tilt. A bullseye is warp or bow.
- Under the hood it is a surface fit. A plane needs three points. Add terms to follow curves. Extra points give a best fit that shrugs off one bad measurement.
01 What it is
Interpolated focus lets the software keep a large part sharp without focusing at every tile. You measure true focus at a handful of spots. It fits a surface through those points and reads off the right height everywhere in between, so every feature you are inspecting stays in focus edge to edge.
Think of it as a survey. Measure the height at a few spots, draw the terrain between them, and you can predict the height anywhere you did not measure. Here the terrain is the surface of your part, and the survey stakes are the few places you focused by hand or by autofocus.
Once the surface is set, every tile in the scan gets its own predicted height. No single compromise height that only works in the center. The far corner of the part gets the height the far corner actually needs.
The alternative is to run autofocus at every tile. That works, but it is slow, and slow is expensive when you are inspecting a large part or a full lot. A surface pays the focusing cost once, up front, then coasts across the whole scan.
Pros
- Every defect stays sharp corner to corner on a tilted or warped part
- No autofocus at every tile, so faster scans and higher throughput
- Built from just a few measured points
- Repeatable: set it once for a part type, reuse it across identical parts
Cons
- Assumes the surface between points is smooth
- One bad measured point warps the whole fit
- Predicting past the outer points is unreliable
- Costs a little setup time per part type
02 Why one focus height fails
A part looks flat. It rarely is. It sits at a slight angle in the fixture or on the stage. It has its own warp, machining marks, coating thickness, and step heights, whether that is solder and traces on a board, a plated surface, or the relief on a fracture face. On a failure sample, the surface you care about is the least flat of all.
Set one focus height and inspect a small field, and you will never notice. Scan a large part and the edges tell on you. The plane you focused on in the center has drifted away by the time you reach the far corner. Those tiles come back soft, and a soft tile is a missed crack, pore, or particle.
The tilt does not have to be large. A slope of a single degree is enough to pull the edge of a big scan out of focus. The bigger the part, the worse it gets.
The fix is to stop describing focus as one number and describe it as a surface instead. Then every tile gets its own height, and the whole part stays sharp edge to edge.
03 Placing the points
This is the part you control. You decide how many points to measure and where to put them.
Match the count to the shape of your part. A flat part with a slight tilt needs only a few points. A plane through three or four spots describes it fine. A warped, stepped, or curved part needs a denser grid, because a plane cannot follow a curve.
More points is not always better. Past a certain density, extra points stop describing real shape and start chasing noise in each measurement. Use enough to capture the shape, not so many that you overfit.
Where you put them matters as much as how many. Anchor the corners first, so the fit is pinned at the edges. Then add points over the features you are inspecting, the weld, the die, the fracture, the connector, and anywhere the height changes fast, like a step or an edge. Keep the area you care about inside the ring of points. The surface is only trustworthy between the points you measured. Past the outer edge it is a guess.
04 The heat map
Once the surface is fit, the software colors it by height. That heat map is the fastest read you have on the shape of your part and whether to trust the fit.
Even color across the whole map means a level part. A single height would nearly have done the job.
A smooth gradient from one side to the other is a tilt. The part is not sitting square in the fixture. Normal, and exactly what the surface is there to correct.
A bullseye or dome means the part is warped or bowed. Common in thin parts, boards, and coated surfaces. A plane cannot describe that, so switch to a curved fit.
A speckled, noisy map means your points disagree with each other. Look for a mis-focused point or a specular hot spot, fix it, then add a few points to confirm.
The colors show relative height, not absolute thickness. The mid tone is your reference plane. Warmer is higher, cooler is lower. Read that way, the same map also tells you the part’s warp and coplanarity at a glance.
05 The math, lightly
You do not need the math to use this. But it helps to know what the software is doing.
Every method solves the same problem. Find a surface, z = f(x, y), that agrees with your measured points. They differ only in how much the surface is allowed to bend.
The simplest is a tilted plane. Three points define it. Solve for the slope in x, the slope in y, and the height:
Give it more points than it needs and there is no exact answer that hits them all. Least squares finds the surface closest to all of them at once. The payoff of the extra points is that one bad measurement no longer throws the whole fit. The surface averages through it, and helps you spot the outlier.
To follow curves, add terms. An x·y term lets the surface warp (bilinear). Squared terms let it bow (biquadratic). Higher orders follow more complex shapes (bicubic). The rule is simple: an n-order fit needs (n + 1)² points. A plane needs 3, bilinear 4, biquadratic 9, bicubic 16.
06 How to use it well
Short version.
Anchor the corners first, then add points over the features you inspect. Keep the area of interest inside the points.
Match point density to shape. A few for a tilt. A grid for a warp. Not so many that you chase noise.
Read the heat map before you run the scan, not after. Even color is fine. A bullseye means switch to a curved fit. A speckled map means fix a bad point. Catch a tilted fixture or a warped lot before you commit a batch to the scan.
Pair it with autofocus. The surface gives autofocus a close starting height at every tile, so the search is short and never wanders off looking for a peak on a reflective surface.
And do not trust the surface past the outer points. Interpolation between your points is solid. Extrapolation past them is a guess.
None of this needs a software background. It needs someone willing to lay out what the software is actually doing instead of leaving it as a black box. Keeping a whole part sharp for inspection is the software’s job, not yours to chase tile by tile.
This is how focus interpolation works in Nebula. You place the points, Nebula fits the surface and shows you the heat map, and the scan holds focus across the whole part so no defect slips through soft. It is hardware-agnostic, so it runs on the inspection scope you already have instead of locking you to one vendor’s stack.
The one-page version, free
Everything above on one page you can keep at the inspection station. The cross-section that shows why one height fails, how to place your points, the heat map read, the four surface shapes in 3D, and the math. No email required.