A Guide to Optical Sensors and their Role in Medical Products

Increasingly, custom sensors are playing a key role in growth and innovation within the medical device arena. This article explains how adaptable sensors allow highly integrated devices, to provide the connectivity, performance, and portability required to assist patient care globally.

The advent and adoption of the Internet of Things (IoT), together with the stringent semiconductor standards required by the medical industry, continues to power the significance of optical sensors as the baseline building block for a complete solution. Medical solutions and devices must provide reliability and performance at the component level, unmatched by many other highly engineered sectors and systems.

Already mission-critical and complex, medical applications are challenged further by the industry’s incredibly diverse scope of performance requirements. Devices may need to be used in a surgical theater, on an emergency vehicle, at a patient’s home, in rugged or remote geographic regions, or perhaps “just” nonstop at a busy hospital.

As every application is different, often integrating commercial off-the-shelf (COTS) sensor components does not give the outcome desired because of the broad operating parameters that must be accommodated. COTS solutions may supply too little or too wide detection sensitivity and could result in inconsistent results or a false reading. This is a non-starter in patient diagnosis and care.

Creating the optimum sensor solution with the appropriate opto-electrical performance, sensitivity range, and footprint becomes crucial for consistent, precise, and reliable performance. These custom optical sensor solutions are fine-tuned with compatible LEDs, housings, sensors, and with factory matched configuration and testing to give the desired results for a turn-key solution.

Adding passive circuit components, connectors, cables, and custom PCBs often enhances and complements the reliability and flexibility of the desired solution. It is no easy task to meet all of these technical challenges while keeping application performance at the forefront of design. Understanding the choices and key steps can help to guide the process.

Sensor solutions consider a spectrum of options, including packaging choices, discrete components, wiring and connector needs, and higher-level assemblies which may be made up of multiple sensors, passive and active components.

Sensor solutions consider a spectrum of options, including packaging choices, discrete components, wiring and connector needs, and higher-level assemblies which may be made up of multiple sensors, passive and active components.

Optical Sensors as Building Blocks

Choices for creating an optimized sensor solution are vast. They range from higher-level custom assemblies, to simply procuring paired discrete COTS emitter and sensor components. Below outlines the building blocks across the continuum of optical sensor solutions.

Discrete Components, Diverse Options

The most basic element of the sensor solution is the discrete sensor, but it requires thoughtful selection of its accompanying components. Pairing an appropriate LED with the sensor element illustrates the challenge: numerous sensor options include basic photo diodes, photo transistors, and even smart detectors which complement the sensor element itself.

With this type of intelligent sensor, features like auto-gain control, temperature-compensation, or autonomous decision-making capabilities could be added. The right sensor ultimately depends on specific application requirements, in addition to performance needs like sensitivity,  wavelength, footprint of the device, to name a few.

Typically, a discrete emitter is part of the design too. LEDs are most often utilized with vertical-cavity surface-emitting lasers (VCSELs), used less often depending on parameters such as angular optical output, optical output power, wavelength, and forward voltage. Pairing of the sensor and emitter is part of the consideration and is also affected by power levels, wavelength, and sensitivity to mechanical alignment.

Housing Impacts Overall Performance

The next step up the design hierarchy is the housing (or packaging) of the sensor solution, with application needs powering a wide scope of mechanical and optical housing choices. Designers must be knowledgeable of the mechanical stack-up tolerances that come with each design, along with considerations such as spacing between the sensor and emitter, through-hole vs. surface mount, aperture size, and mounting bracket.

It is crucial to acknowledge that housing design and discrete components contribute to the overall system performance. The customized optical sensor solution design must be developed to accommodate a spectrum of design choices and performance demands, ranging from detection distance and variability in mounting location, to varieties of media being detected and environmental conditions.

Pre-Attached Wiring and Connectors Add Value

Proper connectivity is vital for input power and communications as optical sensors are frequently mounted remotely from the system’s primary printed circuit board (PCB) or microcontroller. In these instances, connector types and wire length are crucial considerations for designing a robust solution.

Acquiring custom assemblies with pre-attached wiring and connectors fosters plug-and-play compatibility. This decreases costs through better manufacturing efficiencies and accelerates time-to-market by simplifying the assembly process of the final product.

Applications May Demand Higher-Level Assemblies

Medical applications may have optical sensor requirements which go further than simple detector and emitter design methods. Other active and passive components (such as a voltage regulator), multiple sensors and/or emitters, or even other mechanical components may need placement on a common PCB.

Further connectors or wiring, or perhaps a common substrate with multiple discrete die components, would be needed. Dictated by application requirements for functionality, performance, and size, higher-level assemblies can take on many different forms.

In the summary table below, tangible advantages of customized optical sensor assemblies are shown.

VALUE DETAILS
Simplification
  • Optimizes sensor performance and efficiency, increasing reliability and consistency
  • Reduces supply-chain complexity and procurement cycle time
  • 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 approximately 23 billion IoT connected devices installed in 2018, finding an off-the-shelf solution for a specific optical sensor application is nearly impossible. Electrical, optical, and mechanical engineering expertise is required to design a solution that performs reliably and consistently over time and across a number of operating conditions.

In this landscape, the requirement for customized optoelectronic sensor solutions is becoming not only a reality, but a necessity because of the complexities of managing the trade-offs of sensitivity and optical power, integrating compatible light sensors and emitters, and balancing overall costs and stack-up tolerances.

Customized, reliable performance – even in high-mix, low-volume applications – adds powerful competitive value and return on investment in a market which is characterized by growth and a global appetite for exciting new connected health applications.

This information has been sourced, reviewed and adapted from materials provided by TT Electronics plc.

For more information on this source, please visit TT Electronics plc.

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