3D Time of Flight Camera

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INDEX

1) Abstract 2) Introduction .....3) Purpose4) Scope5) Overall Description

5.1) Concept.. 5.2) Principle. 5.3) Components Of TOF camera5.4) Working

5.5) Faults In TOF camera.

5.6) Practical Use

5.7) Technologies Developed5.8) Advantages

5.9) Short Comings5.10) Applications

6) Conclusion 7) Bibliography


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new device, its problems and possibilities for its practical application.

INTRODUCTION

What is Time Of Flight?

Time of flight (TOF) describes a variety of methods that measure the time

that it takes for an object, particle or acoustic, electromagnetic or other wave to

travel a distance through a medium. This measurement can be used for a time

standard (such as an atomic fountain), as a way to measure velocity or path length

through a given medium, or as a way to learn about the particle or medium (such as

composition or flow rate). The traveling object may be detected directly (e.g., ion

detector in mass spectrometry) or indirectly (e.g., light scattered from an object in

laser doppler velocimetry).

Time-of-flight cameras are relatively new devices, as the semiconductor

processes have only recently become fast enough for such devices. The systems

cover ranges of a few meters up to about 60 m. The distance resolution is about

1 cm. The lateral resolution of time-of-flight cameras is generally low compared to

standard video cameras, at 320 240 pixels or less. Only one camera reports 484 x

648 pixels of resolution using a standard CCD sensor.The biggest advantage of the

cameras may be that they provide up to 100 images per second.


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Why to use a 3-D TOF camera?

When using a 3D-camera three specific reference values are of increased interest:

the accuracy of coordinates, the resolutionof the recording chip (number of pixels) the maximumrange procurable.

With the development of the latest generation of 3D-cameras, much progress has

been made in regard to thesethree items:

accuracy is now in the range of a few centimetres, resolution goes up to 40k pixels the maximum distance wasraised to about 20m.

In light of these developments the question , whether the 3D-camera may be usedas an instrument for surveying and recording is worth investigating. This may be

done by extrapolating from the technical specifications of currently available 3D-

cameras into the near future, which will most certainly bring higher resolution,

increased accuracy and an expanded maximum distance. Further developments of

this kind will be realised soon as prospective customers exist in the field of

robotics, vehicle construction and many more areas.

The main questions are as follows:

1. Are there any fundamental problems in the process of recording architecture?

2. Which new possibilities for recording of geometry and for visualisation

purposes arise for practical work?


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3. Is the proposed 2D- / 3D-system capable to replace current devices?

PURPOSE

Today, is world of technology. None of the technologies available in any field at

present is complete in itself. The scientists are continuously trying to develop new

one which would be better than the present one.

The 2-D camera has been used for developing pictures, video recording, graphics

modulation, and in many applications for many decades, but now a 3-D time of

flight technology has been developed. A traditional camera delivers a two-

dimensional image of the surrounding world. The 3D-time-of-flight camera (ToF) -

which was developed in the last decade - captures not only two, but three

dimensions in one step by measuring the distance from each pixel of the camera

chip to the object. This way a large number of coordinates is determined for each

exposurea so called frame.

The 3-D time of flight technology is quiet sufficient in producing efficient image

than a 2-D technology. The target source is illuminated using infra red light (

generally), the reflected light is gathered by the image sensor and a cloud of pixels

is generated, each point corresponds to every exposed part of the object.


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The main advantage of using this technology is the speed it provides. Time-of-

flight cameras are relatively new devices, as the semiconductor processes have

only recently become fast enough for such devices. The systems cover ranges of a

few meters up to about 60 m. The distance resolution is about 1 cm. The lateral

resolution of time-of-flight cameras is generally low compared to standard video

cameras, at 320 240 pixels or less. Only one camera reports 484 x 648 pixels of

resolution using a standard CCD sensor. The biggest advantage of the cameras may

be that they provide up to 100 images per second.

Another main feature of TOF camera is that it is quiet simple to use them. As the

whole system is very compact, the illumination is placed just next to the lens, and

no mechanical moving parts are needed.

The 3-D TOF camera provides a better algorithm to calculate distance between

object and the light source. Therefore, this task uses only a small amount of

processing power, again in contrast to stereo vision, where complex correlation

algorithms have to be implemented. After the distance data has been extracted,

object detection, for example, is also easy to carry out because the algorithms are

not disturbed by patterns on theobject.

So, 3-D time of flight technology has a great advantage over the traditional 2-D

technology in image generation and it will be quiet useful in upcoming days.


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SCOPE

Due to fast speed and simplicity of 3-D time of flight technology, it has become

very popular in many fields. Various technologies using 3-D TOF technology:

Automotive applications

Time-of-flight cameras are also used in assistance and safety functions for

advanced automotive applications such as active pedestrian safety, precrash

detection and indoor applications like out-of-position (OOP) detection.

Human-machine interfaces / gaming

As time-of-flight cameras provide distance images in real time, it is easy to track

movements of humans. This allows new interactions with consumer devices such

as televisions. Another topic is to use this type of cameras to interact with games

on video game consoles.

Measurement / machine vision

Other applications are measurement tasks, e.g. for the fill height in silos. In

industrial machine vision, the time-of-flight camera helps to classify objects and

help robots find the items, for instance on a conveyor. Door controls can

distinguish easily between animals and humans reaching the door.


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Robotics

Another use of these cameras is the field of robotics: Mobile robots can build up a

map of their surroundings very quickly, enabling them to avoid obstacles or follow

a leading person. As the distance calculation is simple, only little computational

power is used.


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OVERALL DESCRIPTION

1)Concept of Time Of Flight

Time-of-Flight (ToF) sensors provide a direct way for acquiring 3D surface

information of objects. More recently, applications like gesture recognition or

automotive passenger classification are using ToF sensors and ToF is on its way

to become a component of consumer electronics.

As ToF sensors provide data at rates higher than 15 Hz, they are suitable for

real-time 3D imaging and can also be used as an additional imaging modality in

medicine. We are convinced that ToF technology can contribute to enhance

applications within medicine and suggest several applications within this field.

Time-of-flight cameras are relatively new devices, as the semiconductor

processes have only recently become fast enough for such devices. The systems

cover ranges of a few meters up to about 60 m. The distance resolution is about

1 cm. The lateral resolution of time-of-flight cameras is generally low compared

to standard video cameras, at 320 240 pixels or less.Only one camera reports

484 x 648 pixels of resolution using a standard CCD sensor. The biggest

advantage of the cameras may be that they provide up to 100 images per second.


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2) Time Of Flight Principal

A Time of flight camera (TOF camera) is a camera system that creates

distance data with help of the following principle:

Time of flight Principle (TOF) describes a variety of methods that measure the

time that it takes for an object, particle or acoustic, electromagnetic or other wave

to travel a distance through a medium. This measurement can be used for a time

standard, as a way to measure velocity or path length through a given medium, or

as a way to learn about the particle or medium. The traveling object may be

detected directly or indirectly.

The 3D Time-Of-Flight (ToF) camera, simultaneously delivers gray-levelimages and 3D information of the scene.

Illumination source is in the form of matrix of LEDs. On the basis of reflected ray:

Distance Image (D(i, j)) is computed based on the phase shift between theemitted and reflected signals,

Amplitude Image (A(i, j)) is estimated based on the amplitude of thereflected signal at every pixel location.


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3) Components Of Time Of Flight camera

A time-of-flight camera consists of the following components:

Illumination unit: It illuminates the scene. As the light has to be modulatedwith high speeds up to 100 MHz, only LEDs or laser diodes are feasible.

The illumination normally uses infrared light to make the illumination

unobtrusive.

Optics: A lens gathers the reflected light and images the environment ontothe image sensor. An optical band pass filter only passes the light with the

same wavelength as the illumination unit. This helps suppress background

light.

Image sensor: This is the heart of the TOF camera. Each pixel measures thetime the light has taken to travel from the illumination unit to the object and

back. Several different approaches are used for timing; see types of devices

above. Driver electronics: Both the illumination unit and the image sensor have to

be controlled by high speed signals. These signals have to be very accurate

to obtain a high resolution. For example, if the signals between the

illumination unit and the sensor shift by only 10 picoseconds, the distance

changes by 1.5 mm. For comparison: current CPUs reach frequencies of up

to 3 GHz, corresponding to clock cycles of about 300 ps - the corresponding

'resolution' is only 45 mm.

Computation/Interface: The distance is calculated directly in the camera.To obtain good performance, some calibration data is also used. The camera

then provides a distance image over a USB or Ethernet interface.


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4) Working of TOF camera

The simplest version of a time-of-flight camera uses light pulses. The

illumination is switched on for a very short time; the resulting light pulse

illuminates the scene and is reflected by the objects. The camera lens gathers the

reflected light and images it onto the sensor plane. Depending on the distance, the

incoming light experiences a delay. As light has a speed of c = 300,000,000 meters

per second, this delay is very short: an object 2.5 m away will delay the light by:

The pulse width of the illumination determines the maximum range the

camera can handle. With a pulse width of e.g. 50 ns, the range is limited to

These short times show that the illumination unit is a critical part of the

system. Only with some special LEDs or lasers is it possible to generate such short

pulses.

The single pixel consists of a photo sensitive element (e.g. a photo diode). It

converts the incoming light into a current. In analog timing imagers, connected tothe photo diode are fast switches, which direct the current to one of two (or

several) memory elements (e.g. a capacitor) that act as summation elements. In

digital timing imagers, a time counter, running at several gigahertz, is connected to

each photo detector pixel and stops counting when light is sensed.


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In the diagram of an analog timer (given below), the pixel uses two switches

(G1 and G2) and two memory elements (S1 and S2). The switches are controlled

by a pulse with the same length as the light pulse, where the control signal of

switch G2 is delayed by exactly the pulse width. Depending on the delay, only part

of the light pulse is sampled through G1 in S1, the other part is stored in S2.

Depending on the distance, the ratio between S1 and S2 changes as depicted in the

drawing. Because only small amounts of light hit the sensor within 50 ns, not only

one but several thousands pulses are sent out (repetition rate tR) and gathered,


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thus increasing the signal to noise ratio.

After the exposure, the pixel is read out and the following stages measure

the signals S1 and S2. As the length of the light pulse is defined, the distance can

be calculated with Medina's formula:

In the example, the signals have the following values: S1 = 0.66 und S2 =

0.33. The distance is therefore:

In the presence of background light, the memory elements receive an

additional part of the signal. This would disturb the distance measurement. To

eliminate the background part of the signal, the whole measurement can be

performed a second time with the illumination switched off. If the objects are

further away than the distance range, the result is also wrong. Here, a second

measurement with the control signals delayed by an additional pulse width helps to

suppress such objects. Other systems work with a sinusoidally modulated light

source instead of the pulse source.

Different denotations exist as far as 3D-cameras are concerned:

The TOF-camera, The PMD-camera The range imaging camera (RIM or flash radar camera)


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They are all targeted at the same type of camera, which is capable of not

only detecting intensity like traditional analogue or modern digital cameras, but

also the distance of each pixel to the mapped part of the object. A more precise

description for the term range imaging camera is the term ToF (TOF)-camera -

indicating that an - optical - signal that has been emitted from the camera is

reflected from the object, and also indicating that the reflected part of the emitted

signal is then returned to the camera.

The flight-time of the signal on its way to the object and back is measured.

The ToF is proportional to the distance. However, this is only a very general

explanation of electronic distance measurement (EDM). Often a modified method

(phase modulation) is used that relies on the modulation of the signal strength (the

amplitude of the emitted light).


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The wavelength of one modulation period depends on the modulation

frequency f. It is calculated with theactual - velocity of light c according to

Wavelength = C/F. Most 3D-cameras emit IR-light modulated with f = 20 MHz. In

the example, the corresponding wavelength of 15m (15m = 300000km/s / 20

x 106/s) is reflected at a distance of about 22m. As the light passes to the object

and back it runs the double distance, so the result needs to be divided by two.

The so-called scale of the distance meter is 15m/2 = 7,5m. The whole

distance is d = (n x wavelength + phase), where n is the even multiple of

wavelength , and phase is the rest of the wavelength which has to be measured, the

so called phase difference between the emitted and the returned signal. Thus it

follows that when a single modulation frequency is utilized, a direct distinct

measurement is possible only up to 7.5m. So a distance of 8.05m would show as

0.55m.

This is a general description how distances can be measured by means of an

amplitude modulated signal. The phase measurementthe determination of . - may

be done in different ways. In a 3D-camera it is done using the PMD-effect

(Photonic-Mixer-Device (Schwarte)). In this case phase is represented by the

difference in charge of two photo-sensitive areas of each respective pixel. Daylight

may cause an overload of one of these areas.

This may cause serious errors in the distance measured by a ToF-PMD

camera, making daylight suppression a fundamental requirement for any 3D-

camera to be used for surveying.


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Above figure shows the difference of images obtained from 2-D & 3-D

camera. It can be shown clearly that images obtained from 3-D camera are of high

resolution than 2-D camera. 2-D camera produces redundant data which makes the

image blurred and the image produced is of low resolution.

Some General Remark

In order to answer the aforementioned main questions it has not been

necessary to utilize the very latest generation of 3Dcameras. However, the device

used for testing needed to meet some minimum requirements, which are sufficient

short-term temperature-stability as well as adequate daylight suppression. Due to

its high performance in the latter area a 3k camera of PMD-technologies ( Siegen,

Germany) was selected. This model, however, is soon to be succeeded by the

camera 2.0 (41k). Figure 1 shows 2.0 - currently being the camera with the highest

pixel-resolution available - 205 x 205 pixels - as well as the model PMD 3k with a

2D-colour-camera placed on top.


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Figure a: 3D-camera 2.0, 41k (PMD-Technologies)

Figure b: 3D-camera 3k (PMD-Technologies) mounted with 2D-camera

On both sides of the 3D-camera's cubes diode arrays are mounted emitting

an amplitude-modulated IR-light that illuminates the whole scene to be captured.

Apart from the distances (resp. coordinates) and the grey-values (intensities)

the signal-amplitudes of the returned signals are registered for each pixel as well.

Additionally, the 2D- / 3D-camera system of Bochum University delivers a colour-

image with higher resolution. The lateral positions of the pixels on the chip of the

3D-camera and of the colour-camera respectively present polar coordinates

together with the distances. They allow the calculation of orthogonal coordinates

of the object.


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5) Typical Faults Of TOF Camera

The emitted light illuminates the whole 3D-scene at once. This is a very

important difference to common electronic distance measurement (EDM). At EDM

the distance measuring ray is precisely focused. The emitted signal amplitude is

D (t0) = Acos wt(0). The remitted light signal comprises the distance information

in terms of a phase delay of the emitted signal: d(t) = k + a cos (wt + phase). The

phase delay contains the distance information. The remitted signal amplitude a is

generally much smaller than the sent amplitude A. Additionally, the returning

signal may be superimposed with a term k which contains various very different

kinds of influences on the signal, like, for instance, a delay that might be caused by

daylight or by temperature effects.

In order to understand the difficulties concerning the application of 3D-

cameras for architectural recording it is necessary to take a closer look at the

instruments faults. Most faults are well-known either from the field of

photogrammetry e.g. distortion of pictures or from electronic distance

measurement (EDM) - like phase shifts caused by optical or electrical

superimposition of the direct signal with error signals (crosstalk). Most of the

faults of 3DPMD- cameras can be sufficiently modelled in a calibration process.

Certain errors may also be suppressed, e.g. daylight via the SBIcircuit

(Suppression of Background Light Intensity). The short time random inaccuracy of

neighboring pixels may nowadays be estimated to generally not exceeding 1cm

in a distance-range of some meters. However, two fundamental influences on the


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distances have a systematic nature. They can considerably compromise the general

accuracy. Both are resulting from unwelcome reflections of the distance measuring

signals.

The first influence may be characterized as internalsuperimposition, the

second as external superimposition. Superimposition in general terms means that

the periodic amplitude modulated signal, which is characterized by frequency,

amplitude and phase , is superimposed by a signal of the same frequency, but with

another phase and generally much lower amplitude.

By this process the resulting signal may deviate significantly from the

undisturbed one in regards to phase. The amount of the phase shift depends on the

phase difference between both signals as well as on the ratio of the amplitudes of

the signals as in the following figure:

Signal a with phase p is superimposed by a small reflected- signal b with

phase p'. Resulting is a signal with phase p + t. The influence of b depends on its

individual phase. A phase shift is equivalent to an error in distance. The grave

influence of internal and external superimposition respectively on distances


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measured with the 3D-camera has to be regarded in more detail in order to access

the possibilities to use the tool for architectural recording. Internal superimposition

effecting distance shadows that encircle the foreground like a small ribbon.

In literature internal superimposition is also characterised as scattering:

Small parts of incoming signals are reflected in the camera itself, overlaying the

directly reflected incoming signals on their way to the pixel. Thus the stronger

direct signal is changed in phase by the reflected parts belonging to neighbouring

pixels. Scattering predominantly effects neighboring pixels with very different

phases . As the lens of the camera has to be focused to a fixed distance, e.g. 5m,

the blurring effect favours scattering at distances which are shorter or longer than

the one the camera is focused on. However, efforts and advances made by the

manufacturers in order to minimize the influence of scattering raise hope that

scattering will be overcome in the future. Mathematical models also help to

minimize it (Mure-Dubois, 2007).

External superimposition seems to be the most dangerous error. It might

very well ultimately prove to fundamentally limit the possibilities to use 3D-

cameras for architectural recording. The reason for the occurrence of this effect lies

in the fact that the array of diodes emitting the IR-Signal always illuminates the

whole scene all at once. Small parts of diffusely reflected light from different parts

of the object may superimpose the directly reflected signals on their way back to

the camera. The possible influence of diffuse reflections on a wall, causing a phase

shift of a directly reflected signal, can be depicted only schematically as in

following figures:


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Possible influence of diffuse reflection on direct measurement

External superimposition effecting a phase delay, respectively a distance shift

The impact of the effect is demonstrated in following figure. Appropriate

measurements were made to a vertical edge, formed by two flexible planes. In the

experiment the angle between the planes was modified while the position of the

edge itself did not move. The smaller the angle between the planes, the more

deformed the edge will appear. However, the distances measured more or less

exactly into the centre of the corner, into the intersection of the planes, are those

which are less deformed by the superimposition. The amount of error thus may

reach considerable amounts.


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Interaction between topography of the architecture

and the measurement with a)d) increasing deformation withsmaller angle; e),f) same angle but f) one plane less reflecting

The topography of an area can also mean the surface shape and features

themselves or in a broader sense, topography is concerned with local detail in

general. Here the feature and shapes of the reflecting surfaces are concerned,

showing changes occurred in images as the position of the surfaces is changed. It

can be easily observed that as the angle between the planes decreases the image

gets more deformed keeping the position of edges constant.


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6) Practical Use Of TOF Camera

The examples prove the wide range of possibilities inherent in the new

methodology, which can be applied to a variety of tasks that currently require

several different measuring devices, like the electronic-distance-meter,

tacheometer, laserscanner or camera. The possibilities of the new system are

demonstrated in regards to distance-measurement, angle-measurement, taking

images, measuring point clouds or using it like a video-camera at a high frame

rate. The 2D- / 3D-system has all the fundamental capabilities of common

contemporary measuring devices. Will it be possible to discard these instruments

in favour of the 3D-camera in the future? Will it be possible to use it efficiently

despite the handicap provided by external superimposition? The following

experiments try to find some answers.

User interface of the 2D- / 3D-system


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The 2D- / 3D-system in exchange to a tacheometer

With the tacheometer polar coordinates are measured in reference to the

vertical: the zenith distance, the horizontal angle and the distance. In architectural

surveying it is often used to establish a network or to capture single points.

Whether the 3D-camera can replace a tacheometer is largely dependent on the

orientation of the camera and on its accuracy. To prove this, the 2D- / 3D-camera-

system was mounted on a tacheometer tripod and leveled with a bubble-tube. The

3D camera was used for distance measurement, the 2D-camera for angle

measurement because of its higher angle resolution. Different types of special

targets were developed. E.g. in various sectors rods were marked with reflecting

film or reflecting spheres.

Targets for automatic identification of single points

Although the cameras angle resolution is rather poor, distances could still

be detected sufficiently well, especially as far as measurements to spherical-

reflectors are concerned. In order to carry out angle-measurement well-defined

points in the same line e.g. the reflecting spheres could be chosen automatically.

Thus automated extraction and identification of reflecting points of high intensity

was possible. Distances of more than 20m were reached, however with rapidly

decreasing accuracy down to + 5cm. Faults caused by excessive intensity of the

signal amplitude at the reflecting points had to be considered. If necessary, these

strongly reflecting points can be used for identification purposes only, while less


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reflecting adjacent (red) spheres may be utilized for angle measurement

respectively for distance measurement. The latest generation cameras, however,

should yield better results. It will also be easier to measure distances longer than

half a wavelength, because increased switching times of the measuring frequency

make it possible to determine n in the equation d = (n x + .) using different

frequencies. The present means of interaction using a measuring rod is a typical

modus operandi of the 3D-camera. It cannot be realized with other instruments this

way. Only here different targets can be reached all at once. The eventual

replacement of the tacheometer by the 3D-camera seems a likely future course for

certain areas, e.g. for documenting the advancing process of measurements at an

archaeological excavation site, or for topographical purposes at short distances. In

these applications of measurements to reflecting targets faults caused by external

superimposition may be sufficiently suppressed. It has also been proved that a

network can easily be referenced by moving a GPS-antenna across the field of

sight with the camera positioned on a tripod. Single-frame exposure time and

GPS-coordinate measurement were synced.

Scanning with the 3D-camera

With the 3D-system being used as a scanner special attention should be paid to two

different working modes: In the first one, the camera is mounted on a tripod to be

turned around a fixed e.g. vertical - axis, whereas in the second one the camera

may be moved freely around an object or moved about in a room. In both cases

every single frame delivers a cloud of thousands of points that are fixed to the


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camera system. In either of the two modes good starting points for matching are

obtained by the procedurethanks to the high frame rate and the frames

overlapping heavily. However, external superimposition may cause considerable

deformations and will deteriorate the quality of matched point-clouds in any case.

To minimize the influence the point-clouds of structures scanned with the 3D-

system should preferably only be neighboring small reflecting areas. Figure gives

an example of a suitable object. The point cloud was constructed from 50 frames,

taken from only three positions with the camera on a tripod. This unfavourable

way of recording can easily be identified by the characteristic structure of points

forming rows.


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Visualization

Photorealistic texturing requires geometry data and pre-rectified images. The

images used for photorealistic modeling were taken with the 2D-camera and

matched with the coordinates taken by the 3D-camera. Mapping and rectifying of

the images was done here using the technology and the algorithms of photo

tacheometry (Scherer, 2006). Figure shows an image of a three-dimensional work

of art consisting of coloured planes facing each other in different angles. In this

example no effects by external superimposition on the coordinates were to be

expected. Changes in colour result from to the lower quality of the 2D video

camera.

In the visualization of built structures the level of detail (LOD) is

characterized by four degrees. LOD3 describes models of buildings that have been

texturized on the outside. In these cases the general geometry recording resolution

goes down to a few decimeters. LOD4 is reserved to describe the most detailed

models, the interior of buildings as well as detailed textures. The visualization of

the niche in example figure 12 shows that - in spite of superimposition-effects -

planes can be extracted sufficiently well to build a LOD3-model.


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7) Technologies Developed for Time Of Flight Camera

Several different technologies for time-of-flight cameras have been developed.

Pulsed light source with digital time counters

There are devices with a pulsed laser and a custom imaging integrated

circuit with a fast counter behind every pixel. These devices produce depth values

for each pixel on every frame. Typical image sizes are 128 x 128 pixels. Ranges up

to 22,000 feet with an eye-safe narrow beam have been achieved. Detectors are

typically InGaAs (indium-gallium-arsenide) devices.

RF-modulated light sources with phase detectors

Photonic Mixer Devices (PMD) and the Swiss Ranger works by modulating

the outgoing beam with an RF carrier, then measuring the phase shift of that carrier

on the receive side. This is a compact, short-range device. This approach has a

modular error challenge; ranges are mod the maximum range, which is the RF

carrier wavelength. With phase unwrapping algorithms, the maximum uniqueness

range can be increased. The Swiss Ranger has ranges of 5 or 10 meters, with 176 x

144 pixels. The PMD can provide ranges up to 60m. Illumination is pulsed LEDs,

rather than a laser. The demodulation is usually achieved by gating the sensor in

synchrony with the light source modulation, so in essence they are range gated

imagers.


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Range gated imagers

This is the most promising technology, invented by Antonio Medina. The

phase detector is the gate or shutter in the camera. The gate allows collection of

portions S2 and S1 of the received light pulse S. The portions are dependent of the

time of arrival, and range is derived from them according to Medina's equation, z

=R(S2-S1)/2S+R/2 for an ideal camera. R is the camera range, determined by the

round trip of the light pulse. The Z-cam and Canesta 3D cameras are range-gated

systems.

Similar principle is used in the ToF camera line developed by FraunhoferInstitute of Microelectronic Circuits and Systems and TriDiCam. These cameras

employ photo detectors with fast electronic shutter used as a gated integrator and

pulsed lasers.

There are also "range gated imagers", which are not 3D cameras. The gate is

open to collect the totality of the reflected pulse and nothing else. Anything outside

a specified distance range can be suppressed. Those are useful for seeing through

fog. A pulsed laser provides illumination, and an optical gate allows light to reach

the imager only during the desired time period.


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8) Advantages of Time Of Flight Camera

Simplicity

In contrast to stereo vision or triangulation systems, the whole system is very

compact: the illumination is placed just next to the lens, whereas the other systems

need a certain minimum base line. In contrast to laser scanning systems, no

mechanical moving parts are needed.

Efficient distance algorithm

It is very easy to extract the distance information out of the output signals of

the TOF sensor, therefore this task uses only a small amount of processing power,

again in contrast to stereo vision, where complex correlation algorithms have to be

implemented. After the distance data has been extracted, object detection, for

example, is also easy to carry out because the algorithms are not disturbed by

patterns on the object.

Speed

Time-of-flight cameras are able to measure the distances within a complete

scene with one shot. As the cameras reach up to 100 frames per second, they are

ideally suited to be used in real-time applications.


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9) Shortcomings in Time Of Flight Camera

Background light

Although most of the background light coming from artificial lighting or the

sun is suppressed, the pixel still has to provide a high dynamic range. The

background light also generates electrons, which have to be stored. For example,

the illumination units in today's TOF cameras can provide an illumination level of

about 1 watt. The Sun has an illumination power of about 50 watts per squaremeter after the optical bandpass filter. Therefore, if the illuminated scene has a size

of 1 square meter, the light from the sun is 50 times stronger than the modulated

signal.

Interference

If several time-of-flight cameras are running at the same time, the cameras may

disturb each others' measurements. There exist several possibilities for dealing with

this problem:

Time multiplexing: A control system starts the measurement of theindividual cameras consecutively, so that only one illumination unit is active

at a time.

Different modulation frequencies: If the cameras modulate their light withdifferent modulation frequencies, their light is collected in the other systems

only as background illumination but does not disturb the distance

measurement.


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Multiple reflections

In contrast to laser scanning systems, where only a single point is

illuminated at once, the time-of-flight cameras illuminate a whole scene. Due to

multiple reflections, the light may reach the objects along several paths and

therefore, the measured distance may be greater than the true distance.


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10) Applications Of TOF Camera

Automotive applications

Time-of-flight cameras are also used in assistance and safety functions for

advanced automotive applications such as active pedestrian safety, precrash

detection and indoor applications like out-of-position (OOP) detection.

Human-machine interfaces / gaming

As time-of-flight cameras provide distance images in real time, it is easy to track

movements of humans. This allows new interactions with consumer devices such

as televisions. Another topic is to use this type of cameras to interact with games

on video game consoles.


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Measurement / machine vision

Other applications are measurement tasks, e.g. for the fill height in silos. In

industrial machine vision, the time-of-flight camera helps to classify objects and

help robots find the items, for instance on a conveyor. Door controls can

distinguish easily between animals and humans reaching the door.

Robotics

Another use of these cameras is the field of robotics: Mobile robots can build up a

map of their surroundings very quickly, enabling them to avoid obstacles or follow

a leading person. As the distance calculation is simple, only little computational

power is used.
http://en.wikipedia.org/wiki/File:TOF_Kamera_Boxen.jpghttp://en.wikipedia.org/wiki/File:TOF_Kamera_Boxen.jpghttp://en.wikipedia.org/wiki/File:TOF_Kamera_Boxen.jpghttp://en.wikipedia.org/wiki/File:TOF_Kamera_Boxen.jpg


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CONCLUSION

On the one side, fundamental difficulties using 3D-cameras for architecturalsurveying are to be outlined, and on the other side possibilities for the use of a

2D- / 3D-system replacing traditional measuring instruments are to be shown.

The most important results from these first experiences are as follows:

- When modelling structures the geometrical accuracy depends on the structures

themselves and on the geometrical relation between the camera and the object. The

strong interdependence caused by external superimposition has to be taken into

account.

- Geometrical correctness can often not be guaranteed.

- A sufficient resolution of point-clouds can be reached only with a large number

of frames; the most efficient way to arrange them will have to be examined.

- The aforementioned method of recording with the 3D-camera is suited for

recording and visualization with LOD3-quality.

- The combination of a 3D-camera with a 2D-camera of higher resolution yields

improved coordinate quality.


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- The large variety of possibilities to combine the different functionalities may

allow to replace traditional measuring instruments and measuring methods under

certain conditions.

Although many instrumental parameters will see additional improvements in

the future, the external superimposition error is inherent in the technology itself. It

may cause uncontrollable alterations in the distances measured. Determining

accurate boundaries for widespread and common use of 3D-cameras in surveying

might prove to be a long way.


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BIBLIOGRAPHY

www.wikipedia.org/wiki www5.informatik.uni-erlangen.de/.../time-of-flight-tof-group www.mesa-imaging.ch/pdf/Application_SR3000_v1_1.pdf www.artts.eu/publications/oprisescu_et_al_ieee.pdf www.springerlink.com/index www.cs.unc.edu/~lguan/publications/guan_pollefeys_eccv08.pdf

3D Time of Flight Camera

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