Widely available and accurate testing will be an important tool for identifying those needing treatment and for controlling the COVID-19 pandemic. Testing is required to answer two important questions: 1) Am I infected? and 2) Do I have immunity? However, as we move forward with opening up society, there is also a third question: Are any of the people with whom I have come into contact infected? Due to the highly contagious nature of the SARS-CoV-2 virus and the unique characteristics of the COVID-19 disease, obtaining accurate answers to these questions with the aid of noninvasive, optical technology will be critical for targeting treatment. Identifying infectious and immune people is especially challenging because some infected people have mild or no symptoms. Also, virus production varies through the progression of the disease, and there is a multiday latency period between infection and the appearance of symptoms. Several days are required for the presence of antibodies to be measurable in a person’s blood. Frequent, convenient, noninvasive, and inexpensive diagnostic testing will help to ensure the safety of workers, facilitate school openings, screen passengers boarding an aircraft, verify the health status of patients preparing for outpatient procedures, and secure a virus-free environment for all. Recently, several research groups, with the support of the Natural Sciences & Engineering Research Council of Canada, the National Science Foundation, and the U.S. National Institutes of Health, have demonstrated that the internal sensors in smartphones are capable of measuring the output of health diagnostic tests, with similar limits of detection and quantitation as laboratory-based instruments. Fundamentally, diagnostic tests involve mixing patient test samples (such as serum or nasal swab media) with liquid reagents, which results in a change of color or generation of light in proportion to the concentration of the material of interest. Because modern smartphones all incorporate rear-facing cameras whose image sensors have been optimized for low-light sensitivity (in the visible part of the spectrum) and high spatial resolution, the cameras in smartphones have been shown to be effective instruments for detecting fluorescence, luminescence, and subtle changes in liquid absorption spectra. To enable a smartphone to provide functionality that it was not originally engineered for, a compatible diagnostic instrument can use an inexpensive clip-on device or snap-in cradle that incorporates illumination sources (LEDs or laser diodes), lenses (for close-up photography), heaters (for driving chemical reactions), and wireless communications (to control the components). To fully realize point-of-care diagnostics, a cartridge is required that contains the test sample, includes the required reagents for the test, and provides any mixing/incubation steps that mimic the laboratory-based protocol that would otherwise be performed by a technician. Cartridges incorporate microfluidic flow channels, reservoirs, and reaction compartments that can be inexpensively mass-produced from plastics, enabling the cartridges to be single-use disposable items. For tests that are detected by smartphone cameras, calibrators and alignment features are needed to adjust the readings appropriately for the characteristics of image sensors in a variety of phone models. To achieve regulatory approval, steps for performing the test must be simplified to the greatest extent possible. However, the potential impact of self-testing using mobile devices would be substantial. Modern smartphones contain powerful computing capabilities that can rapidly perform image processing to convert pictures of the test cartridge into data. Through a smartphone’s integral capability for wireless communications, the data can be securely used by cloud-based service systems for a variety of purposes. For example, a telemedicine provider can communicate directly with the patient by video, use information from the patient’s full medical record — including test results — and make recommendations for further treatment. Contact-tracing apps will be able to track in-person proximity to others during the period prior to a positive test. Patients under quarantine will know when they are considered safe to come into contact with their family. This vision can only be realized if the tests themselves are accurate. False-negative results provide a route for an infected person to inadvertently expose others. For COVID-19 diagnostics, some false-negative tests stem from the difficulty associated with extracting the virus from nasal passages via a deeply inserted swab, or due to a virus concentration that is below the detection limit of the sensing technology. Engineers, chemists, and biologists are researching and developing alternative sample media (such as saliva instead of nasal swab media), and new assay methods with greater robustness and sensitivity. It is highly likely that a much greater frequency of COVID-19 testing is in our future, and point-of-care approaches using the sensing capabilities of our mobile devices should play an important role in that development. Meet the author Brian T. Cunningham is a Donald Biggar Willett Professor in Engineering in the Department of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign. The views expressed in “Biopinion” are solely those of the author and do not necessarily represent those of Photonics Media. 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