Study Notes on Active Sensors for Computer Vision

Active Sensors for Computer Vision

Overview

  • Institution: KIT - The Research University in the Helmholtz Association, Institute of Photogrammetry and Remote Sensing, Department of Civil Engineering, Geo, and Environmental Sciences.

  • Presenter: Prof. Dr.-Ing. Boris Jutzi.

Basic Concepts of Active Sensors

Active Electro-Optical Measurement (Laser Scanner)

  • Definition: A method of measuring distances using laser technology to obtain precise spatial information.

  • Focus: Overview of scanning laser measurement principles and techniques.

  • Relevant Components: Superimposition of laser data processing, comparison between aerial photography and laser scanning.

Principle of Scanning Laser Measurement

  • Components of a Synchronized System:

    • Positioning:

    • GPS or dGPS: For accurate positioning in three-dimensional space [X,Y,Z].

    • Orientation:

    • INS: Inertial Navigation System used for directional orientation, represented as [α, β, γ].

    • Ranging:

    • Laser Scanner: Core component used for measuring distance.

    • Additional Sensors: Include RGB and Infrared (IR) sensors.

Data Processing Steps

  • Calculation of the Flight Path:

    • Utilizes data from GPS and INS systems.

  • Recording of Strips:

    • Captures data in linear strips for further processing.

  • Combining Strips:

    • Merging recorded data into larger datasets to create comprehensive mapping.

Illustrations and Data Processing

Schematic Illustrations of Laser Data Processing

  1. Illustration 1:

    • Source: TOPOSYS.

    • Depicts the first steps of data processing.

  2. Illustration 2:

    • Examples of height images with colors representing height variations, showcasing DEMs (Digital Elevation Models) for different tracks.

  3. Illustration 3:

    • Fusion of RGB images and LiDAR data showcasing a blended aerial imagery approach.

Aerial Photography vs. Laser Scanning

Advantages of Aerial Photography

  • Captures multi-spectral information in both visible light and near-infrared spectrums.

Advantages of Laser Scanning

  • Object point visibility:

    • Point P must be seen from a singular sensor position, beneficial in areas with heavy vegetation.

  • Independent of illumination conditions:

    • Works well regardless of time of year or weather.

  • Texture independence:

    • Effective on homogeneous surfaces like glaciers, ice, and sand.

Comparison of Data Points

  • Image Results:

    • Semi-Global Matching (SGM) provides around 350 points/m².

    • LiDAR technology yields lower density at approximately 8 points/m² but captures vegetation better.

Influences on Measurement

Radiation Impacts on Measurements

  • Electromagnetic Spectrum:

    • Laser wavelengths impact measurements; specific bands are eye-safe or not.

  • Eye Safety:

    • Wavelengths ranging from 2.6 µm to 100 µm are deemed eye-safe.

  • Source Safety:

    • Established maximum permissible irradiance levels that can affect cornea and retina.

Atmospheric Influences

  • The measurement can be affected by:

    • Extinction:

    • Loss of signal strength due to atmospheric gas absorption and scattering.

    • Turbulence:

    • Impact includes intensity fluctuation, phase faults, and beam widening.

    • Surface Properties:

    • Variability based on object reflection characteristics, categorized into specular, glint-like, and diffuse.

BRDF - Bidirectional Reflectance Distribution Function

  • Describes angle-dependent scattering behavior in different surface conditions.

Measurement Techniques and System Components

Exterior Orientation

  • GPS and Differential GPS (dGPS):

    • Offer high accuracy (under 0.008 grad) at flying speeds of the scanning aircraft (170-300 km/h).

Laser System Characteristics

  1. Channel Principles:

    • Classify based on modulation, detection systems, and optical configurations.

  2. Measurement Methods:

    • Triangulation, phase measurement, and time-of-flight technologies.

Transmitter Principles of Laser

  • Definition:

    • LASER stands for Light Amplification by Stimulated Emission of Radiation, functioning via various mediums like gas, solid, and semiconductor.

    • Historical Development:

    • Einstein's theory (1917) leading to practical implementations (1960).

Construction and Modulation of Active Sensors

  • Types of Modulation:

    • Continuous Wave (CW):

    • Phase differences are measured to derive range.

    • Pulsed Laser Systems:

    • Used for Time-of-Flight (ToF) measurement.

Pulse Detection and Range Measurement Techniques

Range Measurement Techniques

  1. Peak Detection:

    • Identifying maximum amplitude point to determine range.

  2. Leading Edge Detection:

    • Gauging where initially the pulse surpassed a threshold value.

Jitter Issues in Range Determination

  • Variations in determined range due to backscattering differences may exhibit significant errors, termed jitter.

Multiple Reflections in Measurement

  • Understanding separation of first and last pulses can inform on vegetation volumes and similar applications.

Scanning Techniques

  • Different scanning systems and methodologies, with examples from companies such as Riegl and Zeiss, showcasing various optical configurations (rotating mirrors, oscillating mirrors).

Conclusion

  • Overall objective of utilizing active sensors emphasizes improvements in quality and accuracy over traditional measurement systems, with considerations for environmental influences, measurement precision, and technological advancements in sensor capabilities.