What markers do we use?
On the following pages you will find a description of the technology of our active and passive markers. Please refer to our product sites to learn about the markers and targets we provide.
Active markers are infrared light emitting elements, mostly LEDs. All active markers provided by ART are just emitting radiation when the tracking cameras are sensitive, thus have to be synchronised with the cameras. Synchronisation can be done by a wired connection between the tracking system and the electronics controlling the active markers, but can also be done in a wireless way: For wireless synchronisation a coded IR flash is sent out by a tracking camera. The active markers’ electronics recognise the coded flash and activate the LEDs.
Passive markers are retroreflective, i.e. they reflect the incoming IR radiation into the direction of the incoming light.
Both active and passive markers will give the 3DOF (3 degrees of freedom), and can be either spherical or flat.
In ART tracking systems four types of LED-based active markers are used, depending on the application:
| Single LEDs:
||Big active spherical markers:
|Single LEDs with diffuser sphere:
||Big active flat markers:
All active markers provided by ART are controlled by special electronics and need power supply.
AMEK (Active Marker Evaluation Kit) for active flat marker
The active target set for active flat markers consists of the active marker station (AMS1), the external IR receiver and the single active flat markers. The active marker station is powered by rechargeable batteries which can be recharged with the supplied battery charger.
AMC (Active Marker Controller) for active spherical markers
The active target set for active spherical markers consists of the active marker controller (AMC) and the single active spherical markers. The markers can be synchronised optically or via cable. Up to 6 active spherical markers can be connected to the AMC.
The passive markers most commonly used in tracking systems, including ART’s, are retroreflectors (see picture above). These markers reflect incoming IR radiation back in the direction of the incoming light. More precisely the IR radiation is back-reflected into a narrow range of angles around the source of the incoming light. These markers are mostly spheres covered with retroreflecting foils, but can also be stickers made from retroreflecting material.
Retroreflecting sheets or foils available on the market can be based on two different optical principles:
- Three mirrors, arranged in 90° angles to each other, reflect light in the described way. Mostly foils with arrangements of lots of very small mirrors in a plane are used.
- Glass balls (with appropriate refraction index) focus incoming light onto the opposite surface of the ball. A layer of microscopic glass balls, carried by a reflecting material, acts as a retro reflector. These foils can be fabricated on a flexible carrier material, thus they are widely used for equipping spherical markers with retroreflecting surfaces.
Spherical markers are plastic spheres with a precisely centric M3 threaded insert, which are hand-coated with a highly retro-reflective film, for the most accurate optical tracking possible (see picture above).
They are also available as a coated version with a robust surface. We provide these coated markers in the following sizes: 14 mm, 16 mm, 20 mm.
Available sizes are:
|Size [mm]||Size [1/64 in]|
Flat markers are pre-cut circular pieces of self-adhesive retro-reflective film.
Available sizes are:
|Size [mm]||Size [1/64 in]|
Robust Ring Markers
Ring markers can be fitted on almost any cylindrical object such as torque wrenches or screw drivers. Sometimes tracking of tools with flexible parts is required, in order to find the exact position of the tool tip. With special calibration and tracking algorithms it is possible to account for the motion of the extension and track the tool tip with a precision of less than 5 mm.
Our markers in comparison
|Marker type||Visibility||IR Range||Description|
|Passive spherical||from all sides||4 – 10 m, depending on tracking camera type and marker size||sensitive surface|
|Passive spherical, coated||from all sides||3 – 7 m, depending on tracking camera type and marker size||robust surface|
|Passive flat||limited: +/- 45°||4 – 10 m, depending on tracking camera type and marker size||cheap, more robust surface possible|
|Active, single LED||limited: +/- 60°||up to 10 m||electronics and power supply necessary, can be covered with acrylic protection film|
|Active, single LED with diffusor sphere||for optimum angular range of visibility||up to 4.5 m||electronics and power supply necessary|
|Big active spherical (several single LEDs per marker)||provide visibility from all sides||up to 20 m||electronics and power supply necessary, suitable for outdoor tracking|
|Big active flat(several single LEDs per marker)||angular range of visibility is limited+/- 90°||up to 20 m||electronics and power supply necessary, suitable for outdoor tracking, magnetic base for easy positioning on metal surfaces|