December 2004, Issue 173

Light-to-Frequency Conversion (Part 1)
TSL230R-Based Pulse Oximeter

I grew up reading Spiderman, Superman, the Fantastic Four, and other comics. I always seemed to be able to scrape up the 12 cents or so needed to stay current with new issues. Comics were to me as baseball cards were to most youths at the time. Instead of dreaming of putting on pinstripes or red socks, I wanted colored spandex and the superpowers that were associated with them. Please don’t ask me to reveal my secret identity here. Obviously, that would give those on the dark side an edge.

The back pages of most comic books contain items for sale that feed upon a child’s fantasies: magic tricks, joke items, switchblade combs, and unusual pets—you name it. X-ray glasses have been a popular item for years. A small hand-drawn picture is often the selling point for the product. A curvaceous female is all that’s necessary to get the mind reeling. As you can guess, cheap frames with cardboard lenses won’t exactly allow you to see someone’s bones and anatomical points of interest. However, the medical industry has an arsenal of such machinery at their beckon call.

Seeing into the body brings with it the advantage of noninvasive diagnosis. Luckily for patients, bloodletting pretty much has been replaced as a cure-all. In fact, many of today’s medical procedures don’t require a patient to be opened up at all. Various endoscopic surgical techniques require only a minor incision (or sometimes none at all) to inspect, diagnose, and even repair internal problems. The medical professional has progressed from butcher to miracle worker. Nevertheless, there’s plenty of room for improvement. 

I remember Dr. Kristan coming to my house and using his stethoscope on me when I was sick as a child. He’d place the cold disk on my body and ask me to breath heavily. Connected to the disk were black tubes that ran to his ears. I always asked if I could listen, and he was never too busy to let me. Stethoscopes have not changed much since then; most don’t require a power source for operation. Other equipment, however, is more complicated. From diagnostics to database management, computers and electronics now play an integral role in the medical field.

If you’ve been a hospital patient, an electronic device undoubtedly has monitored your heartbeat. Devices like stethoscopes monitor heartbeats acoustically; other devices measure pressure. Then there are the devices that monitor the light modulation resulting from the pulsing flow of blood. This has become the prevalent technology thanks to the photodiode, which produces a current proportional to the amount of light hitting its PN junction. In this two-part series, I’ll show you how new technology enables healthcare professionals to monitor the light absorbed through living tissue.