About Lidar

 

The Basics: Lidar, or Laser Radar

Almost all Optech products are based on the same technology: laser radar, or lidar.

It works a lot like ordinary radar, except that our systems send out narrow pulses or beams of light rather than broad radio waves. A receiver system times, counts and processes the returning light.

Laser + Receiver System = Lidar (Light Detection and Ranging)

Laser radar depends on knowing the speed of light, approximately 0.3 meters per nanosecond. Using that constant, we can calculate how far a returning light photon has traveled to and from an object:

Distance = (Speed of Light x Time of Flight) / 2

1. Laser generates an optical pulse.
2. Pulse is reflected off an object and returns to the system receiver.
3. High-speed counter measures the time of flight from the start pulse to the return pulse.
4. Time measurement is converted to a distance by using the formula above

 

First/Last-Pulse Measurements

Our counting electronics can operate in two modes:

First-pulse
Measures the range to the first object encountered - in this illustration, the tree foliage.

Last-pulse
Measures the range to the last object - in this case, the ground.

By acquiring first- and last-pulse data simultaneously, Optech's ALTMs can measure both tree-heights and the topography of the ground beneath in a single pass.

 

Are Measurements Affected by ...

Reflectivity of the Object
Not usually. Highly reflective objects may saturate some laser detectors, while the return signal from low-reflectivity objects may occasionally be too weak to register as valid.

Day or Night
No. Laser radar is an "active illumination" technique that, unlike photography, does not depend on ambient illumination. It works during the day or at night.

Sunlight and Reflections/Angle of Measurement
Sometimes. A strong sunlight reflection off a highly reflective target may "saturate" a receiver, producing an invalid or less accurate reading. However, laser measurements are not usually affected by other reflections. Optech's scanning laser instruments scan laser pulses within a preferred range of angles. Instruments are designed to operate in daylight.

Dust and Vapor
Yes. Laser measurements can be weakened by interacting with dust and vapor particles, which scatter the laser beam and the signal returning from the target. However, using last-pulse measurements can reduce or eliminate this interference. Optech systems that are expected to work in such conditions regularly can be optimized for these environments.

Target's Angle of Repose
No. Laser measurements can be made to targets at any angle.

Background Noise and Radiation
The laser is not affected by background noise. Optech products determine baseline radiation levels to ensure that it does not interfere with measurements.

Temperature and Temperature Variations
No. Laser measurements are based on the speed of light and are unaffected by temperature variations. Optech products have a defined operating temperature range, which varies by product.

Vessel Pressure and Off-Gas Layers
No. The laser is unaffected by pressure or vacuum variations, or off-gas layers.

 

Lidar for Hydrographic Measurements (Bathymeters)

The DIAL principle also lies behind lidar bathymetry, a technique that uses two laser wavelengths to measure the depth of coastal waters down to about 50 m, as well as shoreline topography. It works like sonar, but uses light instead of sound. A lidar bathymeter fires co-aligned laser pulses at the water: the infrared wavelength is reflected by the water surface and detected by the receiver, while the blue-green wavelength penetrates the water surface and is reflected from the bottom.

The time difference between the two signals, after accounting for a range of system and environmental factors, determines the water depth.

We can also use this technique to locate objects on the ocean floor, from sunken ships to small targets.

 

Differential Absorption Lidar (DIAL) for Atmospheric Measurements

Optech designs and builds laser radar systems that measure the concentration of certain gases or atmospheric constituents in the atmosphere - ozone, for instance, or aerosols. These systems, called DIALs, work by exploiting the fact that a gas will absorb light emitted at a certain laser wavelength while transmitting light at most others. So if we send two selected wavelengths at the same time in the same direction, a gas will absorb light at one wavelength. When the light returns to our detection electronics, the signal will be stronger for one wavelength than for the other. By comparing the two, we can determine the concentration of the gas at that particular region of the atmosphere - hence differential absorption.

 

Intensity Measurements (ALTM, ILRIS-3D, Vision & Imaging)

Our data processing software can categorize detected laser pulses according to the reflectivity of the target surface. The output has the detail of high-resolution photographs, yet images can be taken at night and the data are already in digital form.