Introduction: What is structured light?
Structured light is a pre-determined intensity pattern that is projected at a certain Field Of View (FOV) to enable 3D measurement of objects in the FOV. The way the reflected pattern is distorted by the object when sensed through a camera can be used to calculate the spatial position of each point in the FOV.
As there are typically few lasers that generate a 3D structured light pattern useful as-is for structured light applications, some beam shaping is typically required. In this article we review common structured light laser pattern and structured light applications and show how diffractive optical elements can generate such structured light patterns.
While simple binary diffractive gratings often have most energy in the first two orders (+-1), more complex periodic gratings can generate energy distributions that allocate most of the input laser energy equally to a desired number of output orders. These are often called Damman gratings or Fan out gratings. By creating surface structures that are periodic in both x and y, i.e made of cells that are replicated over the entire surface, custom diffraction gratings can also split the light to two-dimensional beam arrays.
Typical structured light laser patterns
Some typical structured light patterns include:
- Grid lines – crossed grid lines are one of the most classic structured light laser patterns, used for 3D sensing of surfaces that have curvature in both axes (x-Y) of the FOV.
- Parallel lines- parallel lines are a useful structure d light pattern in applications where the depth changes are one dimensional in the FOV, for example in metrology of cylinder sides or tire sides.
- Ordered spots array- such structured light patterns offer the best resolution in depth sensing, but due to their ordered nature tend to require complicated algorithms to assign the position to each area.
- Random spot arrays – these structured light patterns are similar to ordered arrays, with the added benefit that their random nature enables simpler location encoding in the pattern.
Typically, such structured light patterns are generated at wavelengths from visible to Infra-red, by using a diffractive optical element placed after the laser to create a structured light source.
Diffractive optical elements for structured light laser beam shaping
Diffractive Optical Elements (DOEs) are flat, transmissive window-like elements that can shape laser beams going through them using diffraction effects. They do this by creating a controlled phase delay over the laser beam front, resulting in controlled diffraction to pre-designed orders.
DOEs used for structured light generation are often period grating like structures, that can generate any desired distribution of orders. They are insensitive to centration of beam size, making them robust and easy to integrate in structured light laser sources.
Other types of diffractive optical elements used in structured light laser applications are beam shaping diffusers. These elements can generate lines, line arrays and grids as well as more exotic distributions required for specialized structured light applications such as in-tube sensing.
Structured light applications that benefit from Holo/Or’s high quality DOEs
Holo/Or offers high end etched fused silica and polymer-on glass diffractive optical components to diverse structured light applications in many high-end markets. These include:
- Large scale machinery automations such as LIDAR for autonomous farming equipment, self-driving forklifts and automatic warehouse systems
- Aerial and space-based mapping and collision avoidance
- Precise wafer metrology
- Medical analytic equipment
These structured light applications share a need for small-to-medium series (thousands to tens of thousands), fast development cycle times, high reliability of the structured light source and a need for a tailored specific pattern per application. These characteristics are at the core of Holo/Or’s capabilities.
Our expertise in structured light sources design extends to more than just the diffractive optics– we can design solutions incorporating refractive optics with DOEs to achieve large FOV or 360degrees coverage.
Conclusion
Structured light applications are becoming more common in many fields, as automation and digitization are strong trends in most industries. Structured light laser beam shaping is required to shape laser light into application specific illumination patterns, including dot arrays, lines, grids, and random dots scatter. This shaping is done by DOEs , thin diffractive optical elements that can split laser beams into arrays and shape them to desired illumination distributions. Of special interest to Holo/Or are various high end applications such as large scale machinery , medical analytical equipment, wafer metrology and aerial mapping, where our advantage in flexibility, know-how and fast product development comes to the fore.
