The map
Three ways to ask one question
Light leaves a source, touches your part and arrives at a detector. Physics gives you exactly three things to measure about that trip: the angle it comes back at, the time it takes, and how well it focuses. Every industrial 3D method is one of these three ideas wearing different optics. Hold that map and the seven chapters below become one story.
Measure the angle
Triangulation. A source and a detector sit a known baseline apart; where the light lands reveals how far it traveled. Chapters 1 to 5: point, line, scan, pattern, stereo.
Measure the time
Time of flight. Light moves 300 mm every nanosecond. Time the round trip and distance falls out, at any range. Chapter 6: pulsed and phase.
Measure the focus
Confocal. Split white light into a rainbow of focal depths; only the wavelength focused on the surface returns. Color becomes a ruler. Chapter 7.
Chapter 1 · 1D laser triangulation angle
One point of light
A laser puts a spot on the surface. A lens watches that spot from a known distance to the side, and images it onto a detector. When the surface rises, the spot slides across the detector. Height becomes position. That triangle is the foundation of the next five chapters. Grab the surface and move it.
The lever of the whole chapter. Steeper watching angle: more detector travel per millimeter of height.
Two details separate a diagram from a real sensor. First, the detector is tilted on purpose. The surface moves along an axis the camera views obliquely, so the plane of sharp focus tilts too; tilting the detector until lens plane, detector plane and measurement axis meet in one line (the Scheimpflug condition) keeps the spot focused across the whole range. Turn the tilt off above and watch the spot blur at the ends of travel. Second, the noise floor is speckle: coherent laser light returning from a microscopically rough surface interferes with itself, and the spot's apparent center jitters by micrometers no matter how much light you add. It is the physics, not the electronics, that draws the line.
Chapter 2 · 2D laser triangulation angle
A line of triangles
Chapter 1 measured one point. Now watch that spot stretch into a line: a cylinder lens spreads the beam, the detector gains a second dimension, and the same triangle repeats in every camera column at once. One frame, one whole height profile. Hover the profile and find chapter 1 hiding in each column. The first honest cost of the angle arrives with it: whatever the camera cannot see, it cannot measure.
It spreads on its own the first time. Then it is yours.
Steeper angle: taller line deflection per millimeter, and longer shadows.
Look closely at the camera image: the line is a few pixels wide and roughly Gaussian across its width. The sensor does not read the brightest pixel; it computes the center of gravity of the intensity in each column and lands between pixels. That sub-pixel trick is where micrometer resolution comes from, and it is exactly what a glinting weld bead or a saturated highlight corrupts first.
Chapter 3 · 3D laser scanning angle
Motion makes it 3D
One profile is a slice. Move the part under the line and stack the slices: now you have a surface. The entire trick is knowing exactly where the part was when each slice was taken. That is the encoder's job, and you are about to see what happens without it.
One profile every fixed millimeter of travel, no matter the speed.
Or grab the belt and pull it yourself. The belt drive is deliberately uneven, like a real line.
This is the workhorse of inline 3D inspection, and its resolution budget splits three ways: X and Z come from the optics of chapter 2, but Y is a choice you make at the encoder. Halve the trigger pitch and you double the rows, until the sensor's profile rate caps how fast the belt may run. Speed and point density trade against each other through one number.
Chapter 4 · structured light angle
A pattern instead of a line
Chapter 2's laser line was already structured light at its simplest: one known stripe. Why sweep it when a projector can paint the whole part at once? Stripes flood the surface; a camera at a baseline watches them bend over the shape. Each pixel sees the stripes shift as the pattern steps, and the phase of that shift encodes height. No motion, full field, one catch: phase repeats. Which stripe are you in?
Finer stripes: finer height, and more identical-looking stripes to confuse.
This is the snapshot family: dense clouds of an entire stationary surface in tens to a few hundred milliseconds, with micrometer-class precision on cooperative parts. Its enemies are motion during the sequence, ambient light washing out fringe contrast, and surfaces that fight the pattern itself: mirrors glint, and translucent materials scatter light beneath the surface and blur the stripes from within. Single-shot pattern codes exist for moving scenes; they buy speed with coarser, sparser data.
Chapter 5 · stereo vision angle
What two cameras agree on
Hold a thumb up at arm's length, look past it, and close one eye, then the other. The thumb jumps; the far wall barely moves. That jump is disparity, and it is the only thing stereo measures. Two cameras a known baseline apart play the role of your two eyes. Blink between their views below and watch the near block jump farther than the far plate. Then face the question the whole method rests on: which pixel over there matches this one here?
Smooth metal gives the matcher nothing to hold.
The depth arithmetic is one line: Z = f·B/d. Its consequence rules the method: differentiate and the error grows as Z squared. Double the distance and the same sub-pixel matching error costs four times the depth uncertainty. Widen the baseline to fight back and the two views grow so different that matching itself suffers, and the occlusion gap between them widens. Stereo is generous up close, honest at range about what it cannot promise.
Chapter 6 · time of flight time
Ask the time, not the angle
Every triangle on this page dies slowly with distance: the angles flatten and the geometry runs out. Time of flight does not care. Send light out, time the round trip, and distance is time: 300 millimeters every nanosecond, at two meters or two hundred. The catch is what a millimeter costs: 6.7 picoseconds. Fire the pulse and try to catch it.
Drag the target. Light is fast; the clock has to be faster.
Higher frequency reads finer, and wraps sooner.
Time of flight buys range and speed with precision: millimeter to centimeter class where triangulation reaches micrometers. Its systematic errors are physical, not electronic. In a concave corner light arrives by two paths and the pixel reports a blend, so corners bow inward. At a depth edge one pixel straddles near and far and reports a point floating between them, the flying pixel every ToF cloud carries. It is the method you pick when the scene is big, the standoff is long, and a millimeter is good enough.
Chapter 7 · chromatic confocal focus
When the surface fights back
A mirror finish defeats every triangle above: the light obeys the reflection law, leaves at the mirror angle and never reaches the lens. Chromatic confocal gives up on angles altogether. It sends white light straight down through a lens built to focus each color at a different depth, and asks which color comes back in focus. Color becomes the ruler.
Drag the surface up and down.
The price is written in the bars above. The rainbow of focal points spans micrometers to a few millimeters of range, the spot is micrometers wide, and covering an area means scanning point by point, which sends you back to the motion of chapter 3. Chromatic confocal is the specialist you call when the surface is polished, transparent or steep, and the tolerance has a µ in it.
The gauntlet
The same part, seven ways
Here is one deliberately hostile part: a machined step, a polished dome, a black rubber gasket, a glass window and a deep bore. Pick a zone. Every method reports honestly, and the reasons all come from the chapters you just operated.
The reference
The whole map, one table
Everything above, condensed to the figures an engineer compares. Every number is an order-of-magnitude class for the method, never a product spec.
| Method | Returns | Height precision | Working scale | Motion needed | First thing to break it |
|---|---|---|---|---|---|
| 1D laser triangulation | one distance | sub-µm to tens of µm | mm-class ranges, short standoff | none for a point; scan for more | specular or dark surfaces, sharp edges |
| 2D laser profile | height profile Z(x) | µm class | lines of tens to hundreds of mm | none per profile | occlusion shadows, gloss |
| 3D laser scanning | ordered point cloud | µm class in X and Z; Y is the trigger pitch | parts of any length that move past | part or sensor must move, encoded | losing track of the motion |
| structured light | dense cloud, full field | µm to tens of µm | fields of tens of mm to about a meter | none; part still during the patterns | motion mid-sequence, gloss, translucency |
| stereo vision | depth map per shot | mm class, worsening with distance squared | 0.3 m to tens of m | none | surfaces with nothing to match |
| time of flight | depth map per frame | mm to cm | 0.5 m to hundreds of m | none | corners (multipath), depth edges (flying pixels) |
| chromatic confocal | point height, or thickness | nm to sub-µm | window of µm to a few mm | scan for area | leaving the tiny window |
Two cousins earn a mention without a chapter. Photometric stereo lights the part from several directions with one camera and recovers surface slope, not metric height: unbeatable for making scratches and embossing visible, wrong tool for a dimension. Scanning LiDAR is chapter 6's physics swept over big scenes: stockpiles, vehicles and warehouses rather than parts.
The decision
Which method fits your part
Describe your measurement problem in five answers. The ranking below is the physics of this page applied to your part, with every judgment traced back to its chapter. Nothing you choose leaves this browser.
This ranking weighs method physics only: class-level resolution, range, speed, surface behavior and motion requirements. A real selection also weighs price, integration and the specific sensor's datasheet. Treat this as the shortlist you walk into that conversation with.