STL vs PLY files: what your dental lab needs from your scanner
An STL file holds the shape of a scan and nothing else. A PLY file can hold colour as well. Here is what each gives a dental lab, and which to send.
By the Tusko team
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An STL file holds the shape of a scan and nothing else. A PLY file holds the shape and can hold the colour of every point on it.
For most cases, send whichever your lab asks for. If it has not said, a colour file plus a clear Rx gives the technician the most to work with.
This guide explains what each format contains, what a lab does with the difference, and how to export a set of files that arrives whole.
What each file carries
Two columns compare the formats. An STL file holds the surface shape and has no standard way to hold colour, so it looks like a single-colour model. A PLY file holds the shape and can hold colour for every point, so it looks like the scan on screen.
STL
The shape, and nothing else
- Yes: Holds the surface shape
- No: Has no standard way to hold colour
- Looks like a single-colour model
- Enough for night guards, splints and models for printing
PLY
The shape, and the colour of every point
- Yes: Holds the surface shape
- Yes: Holds colour, per point
- Looks like the scan as you saw it on screen
- Suits crowns, bridges and veneers, if your scanner exports it
What an STL file contains
STL is the older format. It was first documented by 3D Systems in 1988, for an early kind of 3D printing.
The Library of Congress describes it as “the surface of an object as a triangular mesh”. Each triangle has three corners and a direction it faces.
That is all. The same description says STL “does not have a standard way to support multiple colors for an object or any other appearance characteristics”.
An STL file of a mouth is a single-colour model. Teeth and gum are the same shade, and only the shape tells them apart.
What a PLY file contains
PLY came out of the Stanford Computer Graphics Laboratory in the 1990s. It was designed to store scanned objects.
The Library of Congress description starts the same way: a list of points and a list of faces that join them.
The difference is what each point can carry. Colour values are among the properties the description calls “often used”.
A PLY file from an intraoral scanner can look like the mouth. Enamel, gum and a dark preparation each show in their own colour.
The two side by side
| STL | PLY | |
|---|---|---|
| Holds the surface shape | Yes | Yes |
| Holds colour | No standard way | Yes, per point |
| First documented | 1988, 3D Systems | 1990s, Stanford |
| Text and binary versions | Both | Both |
| Read by design software and printers | Almost everywhere | Widely |
| Looks like | A single-colour model | The scan as you saw it on screen |
Neither format records who the patient is, which tooth is which, or what you want made. That is still the job of the Rx.
What a lab does with colour
Colour does real work at the bench. It answers questions the shape alone leaves open.
Where the margin runs. Tooth and tissue differ in colour at the edge of a preparation. On a single-colour model the technician has only the contour to go on.
What is tooth and what is not. Blood, a retraction cord or a bit of old filling can read as surface on an STL. In colour they are plain.
Marks you made. Some dentists mark contacts or draw on a tooth before scanning. Those marks exist only in a colour file.
What colour does not do is settle the shade. Scan colour depends on the scanner and the light, so send a shade tab photo as well.
For work where none of that matters, such as a study model or a simple appliance, an STL is enough.
Which to send for each kind of work
Your lab’s preference comes first. Where it has none, this is a reasonable start.
| Work | A sensible choice |
|---|---|
| Crowns, bridges and veneers | Colour, if your scanner exports it |
| Implant cases with scan bodies | Either, since the lab reads position from the shape |
| Night guards and splints | STL |
| Models for printing | STL |
| Removable work | Ask the lab |
A third format: OBJ
Some scanners also export OBJ. It was developed by Wavefront Technologies around 1990, and it handles colour differently.
The Library of Congress description says an OBJ uses “an associated Wavefront Material Template Library (MTL) file to define both colors and textures”.
In practice that means a colour OBJ is more than one file. Send every file in the set, or the lab opens a grey model.
If your scanner offers both, PLY keeps the colour inside a single file, which leaves less to go missing.
If the lab opens a grey model
A colour scan that arrives grey has lost its colour on the way. Three causes are worth checking.
- The export was set to STL, which carries none
- An OBJ was sent without the files that go with it
- The colour option was switched off in the export settings
Export again with colour on, and send the whole set. Ask the lab to confirm what it sees.
Scanner formats and open formats
Scanners also save scans in a format of their own. It may hold more than an exported file does, and only that maker’s software reads it.
If your lab uses the same system, sending through the scanner’s own channel can be the fullest transfer. Ask the lab before you assume.
If it does not, export an open format. An intraoral scanner can generally export STL, and many export PLY or OBJ too.
Check your scanner’s manual for the export options. Menu names change between software versions.
File size
A colour file is larger than a plain one, and a full arch is larger than a quadrant.
Both formats come in a text version and a compact binary one. Use binary if you are given the choice.
A large file is a reason to avoid email. Use a route that sends it whole and tells you it arrived.
What to ask your lab
Five answers settle the question for good.
- Which format do you prefer for crown and bridge work, and for appliances?
- Do you want colour when it is available?
- Should the upper, lower and bite scans come as separate files or as one set?
- How do you want files named?
- How should they be sent, and how will you confirm they arrived?
Write the answers down and keep them beside the scanner. A new assistant should not have to ask again.
Ask again when you change scanners, or when the lab changes its software. Either can change which format works best between you.
Export a set that arrives whole
A file format is only part of the transfer. Missing and mismatched files are the other part.
- Check the scan before you export. Look for holes, and make sure the margin is clear all the way round.
- Export every part. The prepared arch, the opposing arch and the bite. A case with two arches and no bite cannot be articulated.
- Keep the bite relationship. Use the export that saves the arches in occlusion, if your scanner offers it.
- Do not shrink the file. A reduced mesh is smaller and loses detail at the margin.
- Name files so they explain themselves. Patient initials, arch and date are enough.
- Send the Rx with them. Files with no Rx sit until someone works out whose they are.
- Keep your own copy until the case is seated. A resend is easy to arrange, and a rescan is another appointment.
How to send intraoral scans to a dental lab without losing files covers the transfer step by step.
List the files on the Rx as well. Our dental lab prescription checklist shows where.
On Tusko, the scans are attached to the case with the Rx, so the lab opens them together. See how it looks for practices and for labs.
Does an STL file include colour?
No. The Library of Congress format description says STL has no standard way to carry colour or other appearance details. An STL scan is a single-colour surface.
Is a PLY file more accurate than an STL?
Not in itself. Both describe the surface as a mesh, and the accuracy comes from the scan. PLY can add colour to each point, which helps a technician tell tooth from tissue.
Can a lab work from an STL alone?
Yes, for a great deal of work. Design software and printers read STL. The lab loses the colour, so anything that depended on seeing it needs a note or a photo.
Can a lab read shade from a colour scan?
Treat scan colour as a guide to anatomy. Send the shade the usual way, with a shade tab in a photo and the guide named on the Rx.
Sources
Related guides
A dental lab prescription has to name the patient, the teeth, the restoration, the material, the shade and the date. This checklist covers every case type.
A paper lab slip and a digital Rx ask for the same details. Here is what changes for the practice and the lab when the Rx goes digital, and what does not.
Phone tag between a dental practice and its lab costs staff time on both sides and days on the case. Here is how to count it, and how to cut it.