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 products 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 synchronized with the cameras. Synchronization 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 synchronization a coded IR flash is sent out by a tracking camera. The active markers’ electronics recognize 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.
Active Marker
- Single LED
- Big active spherical marker
- Big active flat marker
In ART tracking systems four types of LED-based active markers are used, depending on the application:
Single LEDs:
- simple and robust markers providing visibility up to long distances within a ca. +/- 60° angular range of visibility
- can be covered with acrylic protection sheets
- the dimension for the single LED is 3,4mm x 2,8mm
Single LEDs with diffuser sphere:
- for optimum angular range of visibility at limited distances
Big active spherical markers:
- several single LEDs per marker, covered with light scattering spheres
- provide visibility from all sides and up to very long distances
- Diameter 50mm, weight 50g
Big active flat markers:
- several single LEDs per marker, covered with light scattering surface
- provide tracking up to very great distances, within an angular range of visibility less than 180°
- suitable for outdoor tracking
- special features: magnetic base for easy positioning on metal surfaces
- diameter 30mm
All active markers provided by ART are controlled by special electronics and need power supply.
Passive Markers
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.
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 |