How Do Digital Pens Work? Smart Pen Technology Explained

A digital pen works by tracking where its tip is while you write and sending that path to an app as digital ink. Digital pen technology comes in three main types: an infrared camera in the pen reads a dot pattern printed on special paper, a pad under the page senses the pen by electromagnetic resonance (EMR), or a touchscreen picks up a signal from an active stylus. The method decides what you can write on and how your notes reach your phone.

We make Ophaya smart pens, which use the camera and dot paper method, so our pen is the running example. We also cite what other makers publish about their technology, and we say where another kind of digital writing pen fits better.

Camera pens and dot-pattern paper

This is the smart pen technology most people picture. A small camera beside the refill watches the paper under the tip. Livescribe describes a "high-speed infrared camera" reading micro-dots. The Ophaya pen works the same basic way; we have worked on optical dot-code technology since 2001.

The clever part is the pattern. Brands use different codes. Anoto, the Swedish company that calls itself the original inventor of dot pattern technology (Livescribe is one of its product lines), described its version in a patent first filed in 1999. Dots sit on an invisible grid with lines 0.3 mm apart, and each dot is nudged about 50 micrometers off its grid point in one of four directions, so each dot stores one of four values. A patch of 6 by 6 dots, about 1.8 mm square, has 4 to the power of 36 possible arrangements, enough for any patch the camera sees to pin down exactly where the pen is. The example pen in the patent took a new image every 10 milliseconds.

Neo smartpen's Ncode is a different code with the same goal. Neo says it fits "a large amount of information within a small area of 2mm x 2mm," including the page number or type of book. Its developer kit shows what the pen sends for each point: x and y coordinates, a pressure value and a timestamp, plus section, owner, book and page numbers.

Why each brand's paper only works with its own pen

Plain paper gives the camera nothing to read. Livescribe's support page says "Without the dot enabled paper the smartpen will not know where it is," and Neo says its pen "cannot be used on regular paper or non-Ncode notebooks."

Another brand's paper usually fails too. Codes differ, and a pen's firmware is built to decode its own. Each maker also assigns its own page addresses, which the app has to recognize. And paper is a repeat purchase for every brand in this category, ours included.

The Ophaya pen captures handwriting only on Ophaya dot-code paper and notebooks or on the Ophaya LCD writing tablet. On ordinary paper it still writes ink, but nothing reaches the app. Our guide to using a smart pen on regular paper covers the workarounds.

Electromagnetic resonance: a sensor under the page

EMR flips the arrangement. The pen has no camera, and the sensing lives in a board under the writing surface. Wacom explains it this way: the tablet forms a magnetic field on its surface, electricity runs through a coil inside the pen as it moves, and the tablet picks up the signal coming back, over and over, to follow the pen's path. Because the field powers the pen, it needs no battery or cord.

The same idea works under paper. The Huion Note X10 is an A5 notebook with a sensor board under a replaceable notepad and a battery-free EMR pen that takes ballpoint refills. Huion lists a reading height of more than 10 mm and says a traditional A5 notebook under 10 mm thick can replace the included pages, though it recommends its own replacement notepads.

If writing on paper you already own matters most, this design suits you better than a dot-paper pen, and we don't make one. The catch is that your page can't be bigger than the pad, which goes everywhere with you.

Active styluses on touchscreens

An active stylus writes on glass, with no ink. The screen does the sensing; in Wacom's capacitive version, Active ES, pen and finger share "the same sensor and controller." A stylus only works with screens built for its protocol. Apple Pencil works with iPad, and Apple's pairing help asks you to check that Bluetooth is on. Microsoft says a Surface Pen might work with Windows PCs and other devices that support the Microsoft Pen Protocol (MPP). The Universal Stylus Initiative (USI) publishes an open standard that "defines a standard signaling mechanism and communication protocol between a stylus and a touch-enabled device," so one stylus can work on devices from different makers. Check the device maker's supported list before buying any stylus.

If your notes already live on a tablet and you want to move, recolor and erase ink on screen, an iPad with Apple Pencil does that better than any paper pen.

Pressure, tilt and ink

Pressure is how an app draws a thin line when you write lightly and a heavier one when you bear down. Wacom says its EMR pens detect the force on the tip and the angle you hold them at. Huion lists 8,192 pressure levels for the Note in its pen tablet mode. Apple's comparison page lists pressure sensitivity for Apple Pencil Pro but not for Apple Pencil (USB-C). Neo's FAQ says that when parts of your writing are missing, pressure sensitivity or calibration may be the cause. We wouldn't compare pressure numbers across brands; look at how lines come out in the app.

Ink is the bigger practical difference. Paper pens write real ink, so you keep a paper original even when your phone is dead. An active stylus has no ink, and the digital copy is the only copy.

From pen to phone to text

Bluetooth and offline memory

Ophaya, Neo and the Huion Note all send strokes to a phone or tablet over Bluetooth. The Ophaya pen syncs live to the free Ophaya Pro+ app on iPhone, iPad and Android, so the page fills in as you write. If your phone is in another room, the pen stores about 1,000 pages and sends them when you reconnect. Neo lists the same figure for its M1+; Huion lists 50 pages for the Note.

Audio is the phone's job. The Ophaya app records sound on your phone or tablet, not through a microphone in the pen, and links the recording to what you wrote so you can replay it from your notes.

How handwriting becomes text

Classic OCR reads an image, such as a photo of a page, where only the finished shapes remain. A pen gives the app something richer. Google's handwriting team describes digital ink as "a sequence of strokes and each of those strokes in turn is a sequence of points each with a timestamp attached," which is what its Gboard recognizer works from.

In the Ophaya app, OCR converts your handwriting to editable text, and you can export notes as PDF, Word, PNG, GIF or MP4 (the MP4 includes audio). On iPad, Apple's Scribble converts Apple Pencil handwriting into text. With any tool, check the result before sharing it. A 1 that became a 7 in a client's phone number is an expensive typo. Our guide to converting handwriting to text compares the main options.

Digital pen technologies compared

Older reviews mention one more method. Wacom's Inkling, reviewed by PCWorld in 2011, used a receiver clipped to the top of the page that tracked the pen with ultrasonic and infrared signals. Wacom's support notes say line of sight, sunlight and ultrasonic noise affect its accuracy, and Wacom no longer services it.

Our view: choose by where you want to write. For paper notes that build up in the app as you write, a camera pen fits, and dot paper is part of the deal. If it has to be your own paper, an EMR pad like the Huion Note is the better pick. If you already write on a tablet, use the stylus made for it. Our guide to what a smart pen is covers the basics.

Frequently asked questions

What technology do digital pens use?

Mostly one of three: an infrared camera reading printed dots, electromagnetic resonance with a sensor board, or an active stylus sensed by a touchscreen. Ophaya uses the camera and dot pattern method.

Do digital pens need special paper?

Camera pens do. EMR pads work with ordinary paper laid on the pad, and active styluses skip paper entirely. The Ophaya pen captures on Ophaya dot-code paper and notebooks or the Ophaya LCD writing tablet.

Is a digital pen the same as a stylus?

Not quite. A stylus usually means an inkless pen for a touchscreen. A smart pen or digital writing pen usually writes real ink on paper and sends a copy to an app.

Does the Ophaya pen have a microphone?

The pen itself doesn't. The Ophaya Pro+ app records audio on your phone or tablet and links it to what you wrote, and syncing, OCR and audio recording need no subscription.