The Optical Filter Trade off Required for Remote Sensing of LiDAR

The Optical Filter Trade off Required for Remote Sensing of LiDAR

LiDAR is short for light detection and ranging; a technology that uses pulsed lasers to correctly calculate distances, while accurately detecting the size and shape of objects.

LiDAR can resolve objects as small as a few centimeters more than 100 meters away, and this high resolution coupled with the capacity to produce accurate three-dimensional images have seen the technology become essential in numerous applications.

Examples of these applications include construction, agriculture, autonomous vehicles and automobile crash avoidance, environment, oil and gas exploration, surveying and pollution modeling.

The applications make use of lasers that emit light at a specific wavelength, working alongside standard silicon detectors which respond to everything from 300 nanometers to 1200 nanometers.

High-quality optical filters form an important component of the detector’s design, so ideally, they only see the reflected laser light. Integrating filters so that they are part of a design can pose a challenge, so being able to properly choose and integrate filters can save result in time and money savings.

Seeking out informed advice at the beginning of the design process and making some basic changes, companies can see cost and efficiency savings while still achieving their goals.

How LiDAR Works

LiDAR works on the same principle as radar. Both technologies measure distances via the emission of a signal — laser light for LiDAR and high-frequency radio transmissions for radar.

Both systems work by reflecting this signal off a distant object, back to the system. Electronic circuitry is used to measure the time between sending and receiving the signal, using this measurement to calculate the distance.

A LiDAR system will send laser pulses before measuring the time they take to reach the object and travel back after reflection. This is accomplished by either clocking the time or by detecting a  phase shift in the returned light, then using that to calculate time.

The system will rapidly pulse a laser of a certain wavelength - generally 532, 905, 1064, 1054, or 1550 nanometers, with different wavelengths lending themselves better to specific applications.

For example, the 905 nanometer wavelength is commonly used in autonomous vehicles, while the 532 nanometer wavelength generates a green laser that is well suited for operation under water.

While custom gas-based detectors exist which are able to work with specific  wavelengths, LiDAR equipment designers generally opt for silicon-based broadband detectors which react to wavelengths between 300 and 1200 nanometers. The primary motivation for this choice is cost, as these devices are comparatively inexpensive and, as such, do not have a great impact on a project’s budget.

However, the broadband nature of the detectors does cause some issues. A unit will react to many other light wavelengths and sources, not just the laser in question.

For a broadband device to work with a narrow range of wavelengths, LiDAR applications need filters which can eliminate extraneous wavelengths, allowing them to focus on the specific laser wavelengths of interest. The equipment is then able to avoid unnecessary information and false readings, allowing for quicker and more efficient analysis.

Filter Considerations

LiDAR systems use filters that are, in principle at least, no different than a pair of sunglasses. The filter manufacturer coats a transparent base (called the substrate), with thin films of appropriate substances that only permit the laser’s light wavelength to pass through, simultaneously blocking others.

There are multiple practical characteristics to consider, however. These include:

Highly Transmissive at Desired Wavelengths

The more light at the wavelength of the laser that the detector can collect, the greater its sensitivity. Because the full range of light will be passing through the filter, a high level of transmission at the specific wavelength is ideal.

Deep and Broad Blocking

While the filter is required to pass light at the wavelength of the laser, it must also block as much other light as possible. If this does not occur, the sensor may detect the extra light and provide false readings.

Field of View

Light reflected from a LiDAR system can return directly to the detector. For many applications however, such as autonomous vehicles, light may be received over a wide angle range.

The physics of multi-layer interference films suggests that the spectral shape of the filter for light passing through at a higher angle of incidence will shift it to a shorter wavelength.

For example, a filter designed to transmit at 905 nanometers may shift transmission below 900 nm for light incident at a large angle. This may necessitate the design of a wider transmission band in order to account for angle shift resulting from a wide field of view. This can have adverse effects on the signal-to-noise ratio.

Surface

It is important to address any potential defects on an optical surface, but only in the context of the light passing through them. While a defect may interfere with light in the visual spectrum, this may not be an issue for the longer wavelengths used in LiDAR. Unnecessary correction in these cases will cause an increase in costs.

Operating Conditions

LiDAR systems generally operate outside of a standard lab temperature, but factors like temperature and humidity can impact on coatings’ performance. A good LiDAR filter function regardless of what environment it is operating within.

Signal to Noise Ratio

Like any communications system, there is always some level of background noise. The filter must possess sufficient signal to noise ratio to pass enough of the actual signal content to the detector. A combination of high and narrow transmission with deep and broad blocking work together to facilitate this.

Size

Larger filters are more expensive and difficult to manufacture.

Structural Considerations

Manufacturers create filters in layers. Layer materials chosen, and the methods of depositing these on the substrate, can impact upon on all of the above characteristics.

All these characteristics represent a set of compromises. Wider view angles will result in the detector being able to see a wider set of wavelengths, therefore reducing the range of blocking.

Smaller filters mean that less light can pass through them, but this does result in a reduction in cost. Meanwhile, a coating which enhances the signal to noise ratio may not perform adequately in the environment within which the LiDAR system be operating.

These compromises offer a range of unplanned consequences. Should designers focus too much on one feature such as wavelength selectivity, the resulting product may underperform in another area such as field of view.

Working with a specialist filter vendor can help both designers and engineers to develop better applications, identifying problems before they occur.

Working with the Filter Vendor

In order to avoid potential problems, it is advisable to start working with a vendor’s filter experts as early in the project as possible. This allows for maximum flexibility.

The process should be approached with an eye to real-world conditions and solutions, instead of what may appear best in theory. Focusing on ideals can increase costs.

For example, over-specification can be a considerable barrier to better and faster results, and tighter specifications can make it harder to construct a filter that meets the required characteristics needed for actual use.

A key example is the size of the filter, where it may be substantially more costly to purchase large filters and cut them down to the ideal size instead of ordering small individual filters.

Wavelength blocking is a further example. Should light be outside of a detector’s ability to react, then the filter does not need to keep this from reaching the detector. Altering the size and positioning of a filter in the optical path may affect the operation of the whole device, potentially enhancing performance while lowering costs.

It is important to keep an open mind when considering different solutions. The objective is to acquire the desired results from a new device while balancing business and technical needs, rather than prioritizing design choices that may have become routine.

Overall, it is a good idea to treat a filter vendor as a partner and collaborator. Experts  in filter design and manufacturing are best placed to raise vital questions based on their knowledge and experience, and this can significantly help make an application idea become reality.

This information has been sourced, reviewed and adapted from materials provided by Iridian Spectral Technologies.

For more information on this source, please visit Iridian Spectral Technologies.

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