## LiDAR and ToF Cameras – Technologies explained

Autonomous cars and mobile machines are in need of ranging sensors. In the last decades a bunch of technologies where introduced and many technological terms are used in this context: Lidar, Time-of-Flight (ToF) cameras, Stereo, Radar, Laser scanners and many more. Some of us might be confused about the sheer amount of different wordings.

The graph below gives an overview of various ranging technologies and how they are related to each other. This article focuses on a comparison of Lidars and ToF cameras.

## The principles

Principally sensors measuring the time of flight emit some kind of signal and measure the delay at the receiver. The delay is proportional to the distance of an object.

Lidars, Radars and Interferometry are a well establish technologies. Radar uses electromagnetic waves, interferometry uses coherent light, which is hard to apply on out-of-laboratory use-cases.

In previous years Lidars where only available as scanners, however this changed with the introduction of Flash Lidars. With having Lidar cameras on the market, there might be confusion about what the technological differences are.

### Time-of-Flight

In general, Time-of-Flight describes a measurement principle without specific technology in mind, that is measuring the travel time of a signal and derive the travel distance. Radars, Lidars, Light modulation sensors and other technologies use this principle.

Due to historical reasons however, the term Time-of-Flight is usually applied to Lidars and cameras using as technological term.

### Lidar – Emission of Light Pulses

Lidar stands for Light-Detection-and-Ranging. The term is used for sensors that emit pulses and measure the time delay between emission and reception of these pulses (see below figure). The principle is comparable to Radar, however using light instead of electromagnetic waves (which is actually only another frequency domain, but let’s not be silly here).

This working principle is used in various devices, e.g.:

• 2D Laser scanners (e.g. Sick, Leutze, …)
• 3D Laser Scanners (Velodyne, …)
• 3D architecture Laser scanners (e.g. Leica Geosystems, …)
• Flash Lidars (Continental, …)

The particular implementation may vary, but the measuring principle remains the same:

Send out a pulse, detect it at receiver side and derive the distance from the time delay between emission and reception.

While conventional Lidar technology continuously scans the environment (like a machine gun), newer approaches are able to flash the entire scene with one beam and measure the time delay for a whole pixel matrix at once (Flash Lidar).

Flash Lidar might be a big leap for automotive applications, because scanning requires complicated mechanical or electronical (in case of phased arrays) technology and usually reduces the frame rate. However illuminating the whole scene introduces new systematical problems. ToF cameras have to suffer of some of these.

How are ToF cameras different from Lidars?

### ToF cameras – Detecting phase shift

A newer technology does not use single pulses, but continuously illuminates the whole scene with a modulated light (laser or LED). The receiver integrates this signal for a certain amount of time and derives the distance from the signal’s phase shift on receiver side. The time-of-flight and thus the distance is proportional to the signal’s phase shift.

Companies and institutes call these cameras also Time-of-Flight (ToF) cameras. As we already have seen, Time-of-Flight may refer to various technologies, thus this term may lead to some confusion.

The working principle in short:

Modulate a light source and measure the phase shift of the modulation between emitter and receiver.

One main advantage of ToF cameras compared to Lidars is the much simpler pixeldesign – often realized with standard CMOS processes. Also the system integration and scalability is far simpler to achieve. Thus ToF cameras are in principle much cheaper to produce and can be easily scaled.

Following a list of some ToF camera manufacturers:

• Industrial cameras from ifm (O3D, O3X, O3M)
• Consumer cameras from PMDTec (one of the initiators of this technology)
• Mesa Imaging (one of the initiators of this technology)
• Panasonic
• Microsoft Kinect 2

## Comparing Lidars and ToF Cameras

As explained above, the main difference between Lidars and ToF cameras is:

• Lidars use light pulses to measure the time-of-flight
• ToF cameras use a continuous wave to derive the time-of-flight from its phase shift.

Pros

LidarToF Camera
Better in dusty/foggy environments (does not apply to Flash Lidar)Much smaller form factor, easier integration and miniaturization
Better accuracyLower price
Less systematical errors No rolling shutter effect, range data for all pixel synchronious (applies also for Flash Lidar).
Usually higher lateral resolution
Higher framerate
Mechanical robustness (applies also to Flash Lidar or phased array Lidars)

Cons

LidarToF Camera
More expensiveLower accuracy
Usually a scan line only. Stacking of multiple devices to get 3DBad performance with fog and dust
Usually rolling shutter effectSystematic errors due to stray light and multi path (applies also to Flash Lidar)
Bulkier formfactorLower measurement range
Lower lateral resolutionMotion artifacts at objects with high lateral speed.

## O3D and Frequency modes

The ifm O3D series of cameras and sensors provide parameters to change the frequency mode. What is a frequency mode and which settings to choose?

## What means frequency?

The word frequency refers to the modulation frequency of the illumination. The measurement principle of O3D, O3M and O3X cameras uses a continuous modulation of the illumination. Principally it’s a square waveform.

The O3D provides three different “modulation frequencies” or “modulation channels” (both terms have same meaning): 28, 29 and 30 MHz. When using the single frequency mode, one of these frequencies or channels can be chosen. When using multiple devices close by, it is advisable to set each of the devices to different modulation frequencies. This way interferences can be minimized.

The principle is analogous to receiving different radio channels at different frequencies – in case of ToF cameras in the form of light.

## What are frequency modes?

The standard frequency mode is “single frequency”. In this mode, the O3D uses only one modulation frequency for measurement. However, that way, the possible measurement range is only up to 5m. Every object at a distance of 6m is measured at a distance of 1m again. Why is that?

As mentioned, the measurement principle uses a continuous wave. A wave is periodical, with repeating values after each 360°. Therefor the measurement value is repeating. See below sketch.

For measurement of object distances, the distances shown in above figure need to be divided by 2, because of the way forth and back.

If you are interested in a more detailed explanation, see subheading The Ambiguity Problem for more details.

To be able to measure beyond 5m (10m light path), the O3D supports multiple-frequency modes. Internally the camera takes multiple images with two or even three different modulation frequencies and combines the data to one frame without ambiguities.

Here is a list of measurement ranges with no ambiguity:

Unambiguity rangeParameter string
single frequencyUp to 5munder5m_[exposure_mode]
double frequencyUp to 30mupto30m_[exposure_mode]
triple frequencyBeyond 30mmorethan30m_[exposure_mode]
The frequency mode can be set via custom software. Therefor a parameter string is specified in the form of [frequency-mode]_[exposure_mode]. See the right column of the table. The exposure modes are discussed in another post.

Whenever the O3D uses multiple frequencies for each frame, be aware of potential interferences with other devices.

## The Ambiguity Problem

Example: Let’s assume a modulation frequency of 30MHz. Given the speed of light with $c_0=299 792 458 m/s$, we get a wave length $\lambda_{mod}=\frac{c_0}{f_mod} = \frac{299 792 458 m/s}{30 000 000 Hz} \approx 10m$

That means, that our light wave has a wave length of 10m. Beyond 10m it repeats. When measuring, the light wave needs to travel forth and back, thus the distance to objects without ambiguity is 5m.