Light is a powerful sensing mechanism. A beam of light can be used to detect whether an object is present or absent, understand surface characteristics of an item, and explore the composition of a material by shining light through it. Optical sensing techniques have been prevalent, not just in real-world applications but also in academic science experiments and during microscopy sessions, to sense movement.
 
The simplicity of optical sensing enables object detection in a very straightforward approach. Anyone can mount a photosensor opposite a light source to detect an object. However, optical sensing becomes crucial in extreme medical environments where reliability and consistency are critical for safety and well-being. 
 
Detecting objects by interrupting a light path is simple but measuring characteristics of moving fluid or detecting an air bubble in a tube filled with a certain liquid is more challenging.  The sensor may have to measure the liquid’s characteristics through a transparent barrier, in a low-contrast environment, while compensating for varying levels of ambient light.  Engineers need to evaluate how to best implement the most efficient solution by using standard optical components or developing a custom solution that specifically meets the intended application’s performance and reliability requirements.
 
Choosing an Optical Sensing Technique
The simple presence detect scenario described above is an example of a transmissive technique or absorption-based sensing: the presence of an object between the emitter and a photosensor essentially absorbs the emitted signal.  Well-designed emitter/photosensor pairs can sense on a scale between full transmission and full absorption of the emitter’s signal, which enables a more nuanced characterization of the material in the gap between them – including the presence of fluid.
 
Reflective sensing involves a photosensor next to an emitter, or between a pair of emitters, and measuring the light that is reflected from the object as it moves through the sensor’s domain.  The emitters’ output can be modulated (for example, stepped up or down, or turned on and off at varying rates) to extract more detailed information during the measurement process.  Placing a prismatic reflector at the far side of the sensed object can increase the amount of the emitter’s signal that is reflected to the sensor when no object is present.  This sets a baseline that helps improve the signal-to-noise ratio of the measurement when an object is present.
 
Optical Fluid Sensing Applications
Optical fluid sensing comes in many different types and is dependent on the application purpose.  For example, optical fluid sensors can be used to sense the presence or absence of fluids in a tube, vial or flow line. They can be used to measure fluid levels in tubes and vials, as well as vessels such as storage tanks and reservoirs. Importantly for medical and process-control applications, optical sensors can detect bubbles in flowing liquids, allowing steps to be taken to log issues and avert any potential consequences of irregular flow.
 
More sophisticated optical fluid sensors can be used to identify the color of a fluid, or a change in its color, ideal for medical applications in which it is important to know whether there is blood or saline in an IV line. These powerful devices can measure subtle fluid characteristics as well, such as the fat content of milk.
 
This breadth of applications has led to the development of optical fluid sensors in a variety of form factors. One of the simplest is the ‘slotted switch’, an assembly which consists of an infrared detector and a photosensor, mounted opposite each other across an open gap in a single, sub-compact package. This switch can be used for simple presence/absence detection, to sense when a moving part has reached the end of its allowed travel, or to measure the presence of specific fluid compositional elements. It can sense when a syringe is empty—for example, mounted in a syringe driver used to deliver medicines into an IV line at a steady rate.
 
Slotted switches are usually built using a plastic housing and hermetically sealed discrete sensors and emitters. These types of components have been subject to high-reliability processing to ensure long operating lifetimes.
 
Optical fluid sensors are also available in an immersible format, in which the sensor is submerged within the liquid and measures its reflection. The optical sensor can also be mounted outside a clear tube or vial; the device emits light through a clear window onto the fluid under measurement and then senses its reflection through the window. This type of design does have its challenges, such as the increasing use of low-reflectance materials in the measurement windows, as well as the lack of control over ambient light levels around the sensor.
 
Sensors are also available in housings with standard screw threads, enabling them to mate with threaded fixtures already installed on equipment.
 
Custom Solutions Add Value
Given the versatility of optical fluid sensors and the variety of possible sensing techniques, custom optical sensors add value by combining a solution that delivers the right mechanical fit, sensor performance, cost, and time-to-market. This path also minimizes supply chain complexity—without experience in optical sensing, a development team may not have the supplier relationships and market knowledge to develop and deliver optimal custom solutions in-house.
 
One way to substantially reduce the complexity of developing optical fluid sensing solutions is to use sensing modules that package the key components in a robust sealed assembly. These kinds of devices are optimized to deliver features such as programmable sensitivity, output polarity, and drain select. Ideally, the module includes an emitter and a photosensor, analogue signal-conditioning circuitry, temperature compensation, and automatic gain control for reliable functionality in demanding environments.
 
Illustrated by TT Electronics’ OPB9000, these sophisticated modules can calibrate themselves so manual recalibration is no longer required as the emitting LED ages. Onboard, non-volatile memory enables users to store drive-current values for the emitter, which correspond with the reference level for the photodiode. The feature can be used to create ’set points’ that represent the recognition of a specific fluid property, such as its color.
 
Optical Fluid Sensing Modules in Action
Three sensor types demonstrate versatile performance: an immersible sensor; an external sensor affixed to a clear container such as a vial, test tube, or reservoir; and a sensor to measure fluids in a flow line.
 
This last option has become increasingly important in the medical equipment market. As health services around the world try to shift an increasing proportion of healthcare out of hospitals into homes – for example, in preventative monitoring and post-intervention support, the demand for low-cost equipment is rising. This equipment must be accurate, reliable, and robust, and often may also need to be battery powered and portable – as well as usable by untrained people in difficult conditions.
 
Integrated optical fluid-sensing modules can help meet these needs, saving space over discrete alternatives. They are sealed, which mitigates potential issues with false measurements due to sensor contamination, and patient contamination from spilled liquids. The self-calibration features help make the end equipment easier to maintain, therefore more intuitive to use. And the ambient-light rejection capabilities make the end equipment’s measurements more reliable, despite the fact it is likely to be operating in widely varying lighting conditions.
 
It’s worth noting that optimized sensing modules are designed to meet the stringent regulations imposed by the medical industry on suppliers – these go well beyond the requirements of other industries. Meeting such regulatory requirements equates to significant overhead when developing an in-house optical fluid sensor, especially given the complexity it can introduce to the supply chain as developers try to find qualified component suppliers. Product testing also becomes more complex as certification issues must be overcome.
 
Optical fluid-sensing solutions based on integrated modules have obvious performance and implementation advantages in a wide variety of applications. But it’s addressing market-specific requirements, such as the regulatory regime for medical electronics, which really tips the balance in their favor. It turns out that optical fluid sensing isn’t as simple as it may have seemed back in school.
 
This is part three of our adaptable sensor series, digging deep into portability enabled by these powerful devices. Read Part One: Smart Optical Sensors Drive Medical Device Portability and Part Two: Demystifying Custom Optical Sensors for the full story.
 

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Raymund Chua 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 Chua at raymund.chua@ttelectronics.com or via LinkedIn.