Popularity of Light Detection and Ranging Scanners Creates Need for Clarity in Standards
As tripod-mounted light detection and ranging (lidar) scanning gains acceptance and popularity in the engineering and surveying markets, users are demanding more uniformity and clarity in the hardware specifications issued by vendors. The beam diameter, beam divergence and eye safety class are among the determining factors in the selection of a laser scanner. Unfortunately, these specifications are also the most misunderstood by users, and the most discontinuous among scanner manufacturers. This fact highlights the requirement for the establishment of a clear set of industry wide standards. The standards that ASTM International Committee E57 on 3D Imaging Systems is developing will provide the framework from within which end users can adequately compare the varying parameters of different models.
This article discusses the system factors that determine the spot size of a laser beam at a given range and options for focusing the beam. This will enable users to understand the implications of choosing different hardware to suit their applications, and to ask the right questions before purchasing a laser scanner. An examination of laser eye safety designations is also presented.
Laser Spot Size
The term laser spot size refers to the diameter of the laser beam at the point of reflection. The laser spot size affects the maximum resolution, and in extreme cases it can affect the accuracy of the data. Generally, its effect on the accuracy and resolution of the laser data is muted by other factors, including scanner calibration, atmospheric effects and angular resolution.
A small spot size is desirable for good resolution and accuracy. However, there are cases where a larger spot is more useful. For example, when trying to penetrate foliage with last-pulse technology, a larger spot is advantageous because it is more likely that some of its energy will penetrate the foliage and reach the intended target.
Laser Wavelength The wavelength of the laser does not affect the laser spot size at range, but it does determine the maximum achievable beam divergence and spot size at the laser source. Different wavelengths inherently have different focus and detection characteristics. The diffraction limit determines the focusing capability of light at a given wavelength. For example, visible light at 532 nanometres (green) is up to three times easier to focus than infrared light at 1,500 nm. However, visible light is subject to stricter safety regulations, as discussed later.
Beam Diameter at Source Beam diameter at source is measured as the laser beam exits the scanner optical subassembly. The size of the spot beyond the source either increases (using a collimated configuration) or decreases (using a focused configuration). For example, a user can calculate the spot size of a laser point at any range of a collimated laser scanner by using the angular beam divergence and adding the spot size at source as a constant. The spot size can be calculated the same way if the beam is focused, but the focal distance must also be known.
Beam Divergence Beam divergence refers to the rate at which the laser spot size increases as the range from the source increases. The beam divergence can be expressed in microradians (mrad) or in micrometers per unit range (µm/m), and is a necessary parameter to calculate the spot size at range. However, as of December 2005, beam divergence is not listed in all manufacturers’ published specifications.
The influence of beam divergence on laser spot size becomes more pronounced as the range from the origin increases. For example, the collimated spot size at 800 m would be larger than the source spot size by the divergence value multiplied by 800. Assuming that divergence is 170 µm/m, the spot size would increase by 14 centimetres.
There are two popular optical configurations for tripod-mounted laser scanners: focused and collimated. It is important to understand the strengths and weaknesses of each when choosing a system.
Focused Laser Optics Focusing the laser spot at a specific distance is a solution used by some time-of-flight laser manufacturers. A focused design works so that the laser spot size is smaller at a predetermined target range (e.g., 50 m) from the source, known as the focal distance. This method of operation is advantageous if the spot size is at its smallest at the typical working range of the lidar scanner. Beyond the focal distance, however, the laser spot size diverges. At twice the focal distance, the spot size has increased to the source spot size again. At four times the focal distance, of course, the spot is three times the source spot size. The figure helps to understand this concept. What is gained in optimization for one application is lost in versatility for others.
Collimated Optics A collimated optical design minimizes the divergence of the beam. The collimated design has useful performance across a wide range, whereas the focused system out-performs the collimated design within its focal region but quickly fades outside of it. Collimated systems are ideal for applications requiring a flexible system that can accommodate large differences in range.
Eye Safety Classes and System Designations
Eye safety and system designation is one of the most ambiguous areas for tripod-mounted lidar technology. For instance, the implications of deploying a Class 1 system rather than a Class 3R system are not immediately apparent to the customer, but there is an impact. Furthermore, there are three different published eye safety standards: the International Electrotechnical Commission’s IEC 60825-1, Safety of LAser Products; FDA/CDRH, Requirements for Laser Products; and ANSI Z-136.1-2000, Safe Use of Lasers. Fortunately, the standards published by each group are similar.
Laser wavelength and optical power (pulse energy, pulse duration and pulse repetition frequency) determine the eye safety designation of the system. Generally speaking, the human eye is more vulnerable to visible light compared to infrared. To maintain the lowest possible safety class, the maximum power output of a visible laser must be lower than that of an infrared laser. The power difference means that the Class 1 infrared lidar is superior in both safety and range to visible light lidars.
The IEC International Standard
IEC 60825-1 has been used as the primary source material. In this classification, the following categories are listed in increasing order of hazard: Class 1, Class 1M, Class 2, Class 2M, Class 3R, Class 3B and Class 4. The most common designations for commercial tripod-mounted lidar scanners are: Class 1, Class 1M, Class 2 and Class 3R.
An important concept for understanding the definition of these classes is (un)aided viewing. Unaided viewing is with the naked eye. Aided viewing assumes collective/ magnifying optics that focus incoming light into the eyes, such as binoculars or a total station.
Class 1 This is the safest class, and can be deployed anywhere with no precautions. Class 1 laser products are practically liability-free because they are safe for both aided and unaided viewing. Areas of operation require no access control or signs. This is the ideal class for a tripod-mounted lidar scanner.
Class 1M, 2 and 3R Class 1M lasers pose a hazard if aided viewing of the beam occurs for extended periods. Warning signs and precautions to avoid aided beam viewing are required.
Class 2 and 3R lasers emit in the visible spectrum. It is assumed that the human eye’s blink aversion response protects the naked eye from damage. Furthermore, the scanning motion of the laser limits the total laser exposure at any single point. The visible light may pose a distraction hazard in high-traffic areas. Even at 3R, signs and full access control of the survey area are the only requirements, along with the suggestion that the beam path should be raised or lowered from typical human eye level.
Understanding the meaning of tripod-mounted lidar scanner specifications is a crucial step toward choosing the right system. A small laser spot size is the basic indicator of good resolution and accuracy, but it is not critical for applications requiring foliage penetration. Because the spot size changes, divergence specifications and spot size at source are necessary to understand how the beam evolves with range. Optical configurations are also important, because focused optical configurations work best at their optimized range, but are outperformed by collimated configurations with their long ranges.
Wavelengths in the infrared spectrum are harder to focus and diverge more quickly, but are also the safest. Visible light lasers are easier to focus and diverge more slowly, but are subject to more restrictive safety class ratings and shorter maximum ranges.
The three published eye safety standards can be confusing, but for the most part they are very similar. Class 1 lasers are ideal for tripod-mounted lidar scanners because they can be deployed anywhere without safety precautions.
With demand for tripod-mounted lidar scanners growing, it is now important for consumers to educate themselves and become aware of the important system specifications. The industry will benefit from well-informed customers who can sort out the discord among manufacturer specifications.//