Demystifying Custom Optical Sensors

Custom sensors are playing an increasing role in innovation and growth within the medical device arena. This article is the first in a series on the how adaptable sensors enable highly integrated devices, offering the performance, connectivity, and portability necessary to benefit patient care worldwide.
 
The advent and adoption of the Internet of Things (IoT)—coupled with stringent semiconductor standards required by the medical industry—continues to fuel the importance of optical sensors as the baseline building block for a complete solution. At the component level, medical devices and solutions must deliver performance and reliability unmatched by many other highly engineered systems and sectors. Already complex and mission-critical, medical applications are further challenged by the industry’s incredibly diverse range of performance requirements. Devices may need to perform at a patient’s home, on an emergency vehicle, in a surgical theater, in remote or rugged geographic regions, or perhaps “just” nonstop at a busy metropolitan hospital.
 
Since each application is unique, integrating commercial off-the-shelf (COTS) sensor components does not often produce the desired outcome given the broad operating parameters that must be accommodated. COTS solutions may provide too wide or too little detection sensitivity and could lead to a false reading or inconsistent results. In patient diagnosis and care, this is a non-starter. Designing the optimum sensor solution with the appropriate sensitivity range, opto-electrical performance and footprint becomes critical to ensure precise, consistent, and reliable performance. These custom optical sensor solutions are fine-tuned with compatible LEDs, sensors, housings, and with factory matched testing and configuration to produce the desired results for a turn-key solution. Adding cables, connectors, passive circuit components, and custom PCBs often complements and enhances the flexibility and reliability of the desired solution. Meeting all these technical challenges—while keeping application performance at the forefront of design—is no simple feat. Understanding key steps and options can help guide the process.
 
Optical Sensors as Building Blocks
Options for creating an optimized sensor solution are quite broad. They range from simply procuring paired discrete COTS emitter and sensor components to higher-level custom assemblies. Let’s take a quick look at the building blocks across the continuum of optical sensor solutions.
 
Discrete Components, Diverse Options
The discrete sensor is the most basic element of the sensor solution but requires thoughtful selection of its accompanying components. Matching an appropriate LED with the sensor element illustrates the challenge: multiple sensor options include basic photo transistors, photo diodes, and even smart detectors that complement the sensor element itself. Features such as temperature-compensation, auto-gain control, or autonomous decision-making capabilities could be added with this type of intelligent sensor. Ultimately, the right sensor depends on specific application needs, as well as performance requirements such as wavelength, sensitivity, footprint of the device, and much more.
 
A discrete emitter is typically part of the design as well. LEDs are used most often, with vertical-cavity surface-emitting lasers (VCSELs) used less frequently depending on parameters such as wavelength, optical output power, angular optical output, and forward voltage. Pairing of the emitter and sensor is part of the consideration and is also impacted by wavelength, power levels, and sensitivity to mechanical alignment.
 
Housing Impacts Overall Performance
The housing (or packaging) of the sensor solution is the next step up the design hierarchy, with application requirements driving a broad range of optical and mechanical housing options. Designers must be aware of the mechanical stack-up tolerances involved with each design, along with considerations such as spacing between the sensor and emitter, aperture size, through-hole vs. surface mount, and mounting bracket. It’s important to recognize that discrete components and housing design contribute to overall system performance. The customized optical sensor solution design must be developed to accommodate a spectrum of design choices and performance demands, ranging from variability in mounting location and detection distance, to types of media being detected and environmental conditions.
 
Pre-Attached Wiring and Connectors Add Value
Since optical sensors are often mounted remotely from the system’s primary printed circuit board (PCB) or microcontroller, proper connectivity is critical for input power and communications. In these cases, wire length and connector types are important considerations in designing a robust solution. Procuring custom assemblies with pre-attached wiring and connectors fosters plug-and-play compatibility. This simplifies the assembly process of the final product, reducing costs through improved manufacturing efficiencies and accelerates time-to-market.
 
Applications May Demand Higher-Level Assemblies
Medical applications may have optical sensor needs that go beyond simple detector and emitter design techniques. Multiple sensors and/or emitters, other passive and active components (such as a voltage regulator), or even other mechanical components may require placement on a common PCB. Additional wiring and connectors, or perhaps a common substrate with multiple discrete die components, would be necessary. Higher-level assemblies can take on many different forms, dictated by application requirements for functionality, performance, and size.
 
Real, tangible benefits of customized optical sensor assemblies are shown in the summary table below.
 

VALUE	DETAILS
Simplification	Optimizes sensor performance and efficiency, increasing reliability and consistency Reduces supply-chain complexity and procurement cycle times Improves lead times and efficiencies; faster time-to-market
Customization	Best physical form, fit, and function Tailored solutions increase peace of mind Cost-effective, even for low-volume, high-mix applications
Reliability	Factory matched calibration and testing ensures consistent performance Field-proven solutions designed to operate in challenging environments Maximizes initial return on investment

 
With an estimated 23 billion IoT connected devices installed in 2018, finding an off-the-shelf solution for a specific optical sensor application is beyond challenging. It requires optical, electrical, and mechanical engineering expertise to design a solution that performs consistently and reliably over time and across a variety of operating conditions. In this landscape, the need for customized optoelectronic sensor solutions is becoming not only a reality, but a necessity due to the complexities of integrating compatible light emitters and sensors, managing the trade-offs of optical power and sensitivity, and balancing stack-up tolerances and overall costs. Reliable, customized performance—even in low-volume, high-mix applications—adds strong competitive value and return on investment in a market characterized by growth and a global appetite for exciting new connected health applications.
 
This is part one of our adaptable sensor series, digging deeper into portability enabled by these powerful devices. Read part two here. Read part three here.

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Raymund Chua, global product line director at TT Electronics, manages all aspects of the product life cycle for TT’s optoelectronics business. His team supports the company’s global role in providing sensor solutions that bridge the analog physical world with the realm of digital computing, including sophisticated sensing applications in medicine, industry, aerospace, and transportation. Connect with Raymund at [email protected] or via LinkedIn.