Thermal imaging & vision systems


Figure 8. Max CRA of an imaging lens.


contributes different optical path lengths to a pixel, which leads to depth measurement inaccuracies. Several strategies in the design process need to be used to reduce stray light, such as optimisation of the anti-reflection (AR) coating and the mechanical aperture, darkening the lens edges and mounting structures, and custom design of the BPF to optimise for wavelength and CRA.


The following are some items that can impact stray light in a system:


Vignetting


Ideally there should not be any vignetting in a ToF lens system. Vignetting cuts off the imaging rays and is sometimes used as a technique to increase the image quality while trading off the brightness of the peripheral fields. However, the cutoff rays often bounce inside the lens system and tend to cause stray light issues.


AR coating


AR coating on the optical elements reduces the reflectance of each surface and can effectively reduce the impact of lens reflections on depth calculation. AR coatings should be carefully designed for the light source wavelength range and the angle range for the incident angles on the lens surfaces.


Number of lens elements Although adding more lens elements


50


a) Lens parameters such as f/# and CRA across the field


b) Light source parameters such as bandwidth, nominal wavelength tolerance, and thermal shift


c) Substrate material properties to low incident angle drift vs. wavelength or low thermal drift vs. wavelength


Microlens array


A ToF backside illuminated (BSI) sensor normally has a layer of microlens array that converges rays incident to the image sensor and maximises the number of photons that reach the pixel modulation region. The geometry of the microlens is optimised to achieve the highest absorption within the pixel region where photons are converted into electrons.


In many lens designs, the CRA of the lens


provides more freedom to achieve the design specifications and better image quality in terms of resolution, it also increases the inevitable back reflections from the lens elements as well as increasing complexity and cost.


Band-pass filter (BPF)


The BPF cuts off ambient light contribution and is essential for ToF systems. The BPF design should be tailored to the following parameters to have the best performance.


increases as the image height increases toward the edge of the sensor, as shown in Figure 8. This oblique incidence leads to absorption loss in the pixel and crosstalk between adjacent pixels when the CRA is too big. It is important to design or choose an imaging lens such that the CRA of the lens matches the specifications of the microlens array it is designed for. For example, the optimal CRA that matches with ADI ToF sensor ADSD3100 is at around 12° at the sensor horizontal and vertical edges.


CONCLUSION


ToF optics have unique requirements to achieve optimal performance. This article provides an overview of a 3D ToF camera optical architecture and design guidelines for the illumination and imaging sub-modules to help design such optical systems and/or choose sub-components. For the illumination sub-module, the key factors are power efficiency, reliability, and the ability of the light source to be driven at high modulation frequency with high modulation contrast. Wavelength selection consideration between 850 nm and 940 nm, as well as how to specify the illumination profiles are discussed in detail. For the imaging sub-module, the lens design considerations including f/#, CRA that matches with the microlens specification, and stray light control are critical to system-level performance.


Analog Devices www.analog.com


April 2025 Instrumentation Monthly


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