Optical Power Density [Photon Density]
The process of inactivating airborne pathogens using UV-C light involves exposing these harmful microorganisms to UV-C photons from every possible direction. This comprehensive bombardment ensures that the pathogens are effectively neutralized. The key metric in this process is the optical power density at a specific point in space where the pathogen is present.
The Exposure or “dose” a pathogen receives in a device is the combination of the power density and time along the path of the pathogen traveling through the device.
When pathogens absorb UV-C photons, several inactivation mechanisms occur. In the upper UV-C range (230-280 nm), the absorption of photons by the RNA or DNA leads to the formation of uracil or thymine dimers, which disrupt the transcription process and prevent replication. In the far-UV-C range (<230 nm), photons break bonds in the protein layers surrounding the genetic material, further disabling the pathogen from replicating.
Understanding Our Omnidirectional Detector Technology
Our innovative approach to creating an omnidirectional detector leverages both geometrical optics (reflection and refraction) and physical optics (diffraction). This combination allows us to capture light from a wide range of angles effectively.
How It Works:
1. Light Collection: Light enters the detector head from a vast array of angles through strategically placed openings around a near-spherical fused silica substrate. These openings are more numerous in the equatorial region and fewer near the poles, creating a balance in light intake.
2. Reflection and Refraction: the detector, has a highly reflective coating that covers the spherical surface selectively blocking light from entering and reflecting light once inside directing light down an exit shaft. This coating ensures that light entering the openings is efficiently guided towards the detection system.
3. Diffraction Effects: Openings in the equatorial region are designed to enhance diffractive effects. This causes light to bounce around inside the sphere, similar to how light behaves in an optical integrating sphere, ensuring that sufficient light is directed down the exit shaft.
4. Exit Path: Light collected and directed into the shaft is then guided to a traditional photomultiplier tube, which is sensitive primarily to the UV-C spectral range, enabling accurate detection.
5. Optimization: We are continuously refining the size, number, and distribution of the openings to maximize the efficiency of light capture and guidance.
For the Technically Knowledgeable:
Our design utilizes geometrical optics principles such as reflection and refraction, complemented by physical optics principles like diffraction. Light enters the detector head through multiple openings distributed across a near-spherical surface. The internal reflective coating ensures that light is efficiently directed down an exit shaft to a UV-C sensitive photomultiplier tube. The equatorial openings enhance diffractive effects, causing light to reflect within the sphere akin to an optical integrating sphere. The lower hemisphere near the exit axis features reduced reflective coating to allow light to reflect off the northern hemisphere's interior surface, optimizing light direction down the shaft.
For the Non-Tech Savvy:
Think of our detector like a special spherical mirror ball with tiny windows. Light can enter from almost any angle through these windows. Inside, the light bounces around and is funneled down a tube to a sensor that can detect UV-C light, which is important for measuring pathogen-killing light. We carefully place and size these windows to make sure we capture as much light as possible and guide it to the sensor effectively. We're constantly improving this design to make it even better at capturing light from all directions.
Prototype Detector in our Goniometric Test Chamber
Our prototype detector, currently in the goniometric test chamber, represents a significant advancement in measuring optical power density. The detector head, which is only 5 mm in diameter, is mounted on the end of a 40 cm long shaft. This shaft guides light to a photomultiplier housing, where it is converted to a photocurrent proportional to the optical power density.
In the lower left corner of the image, you can see a UV-C LED array. This array can be positioned at any angle around the test fixture ring, allowing us to evaluate the detector's photocurrent response from different angles. Our innovative design has reduced the angle nonuniformity from 4 orders of magnitude with an uncoated detector head to just 2 orders of magnitude.
Our ongoing project at the Cornell Nanoscale Science and Technology Facility is showing great promise. By constructing patterned openings in the reflective coating on the sphere, we aim to further enhance the detector’s performance.
Our prototype detector exemplifies the synergy between advanced materials, optics principles and nanoscale manufacturing. Whether you are familiar with these concepts or new to the field, our design captures light from all angles and accurately measures optical power density, making it a powerful tool in the fight against airborne pathogens.
Funding Acknowledgment:
This material is based in part upon work supported by the National Science Foundation under Grant No. NNCI-2025233. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
