An award-winning, Two-Color Fiber-Optic Infrared Sensor developed by Ward Small IV and others in Livermore's Medical Technology Program (Figure 1, above) not only measures temperatures in this critical temperature regime, but also measures the emissivity, a number between zero and one that indicates how well a target emits infrared radiation, that is, heat. (For example, highly reflective surfaces are generally poor emitters and thus have emissivities close to zero. A piece of black-painted metal, on the other hand, has an emissivity close to one.)"This low-temperature regime--50 to 80°C--is of particular interest in the medical field, where lasers are becoming the tool of choice for therapeutic and minimally invasive surgical procedures," says Small.Most commercial infrared sensors measure surface temperatures of hundreds or thousands of degrees Celsius. The difficulty with measuring surfaces near room temperature is that such surfaces emit infrared signals that are weak and don't easily transmit because of their longer wavelength. The few sensors that do measure in this temperature regime assume an emissivity of 1 or a lack of sufficient spatial resolution. "Ours is the only low-temperature, noninvasive sensor with high spatial resolution that measures both temperature and emissivity," notes Small.Measuring both variables significantly reduces the error that creeps in when measurements are taken in only one infrared wavelength band, or color. This error can be as high as 40% for materials that have a moderately reflective surface.
How It Works
The sensor is part of a temperature feedback system (Figure 2) invented by program researchers investigating surgical applications. The sensor has a single 700-micrometer-bore, hollow-glass optical fiber mounted on one prong of a two-pronged welding handpiece. The other prong is the silica optical fiber that transmits the laser beam that welds the tissue.
While the laser beam is heating the tissue surface, the sensor's hollow fiber collects and transmits the infrared radiation from the surface to a reflective optical chopper, which modulates and splits the signal into two paths--one for each band. Two thermoelectrically cooled mid-infrared photoconductors monitor the chopped signals, which are recovered using lock-in amplification. Using a single optical fiber for both wavelength bands guarantees that the radiation transmitted in each path originates from the same geometric region on the target.
"This means we're getting true values for the temperature and emissivity. This may not be the case in systems that have separate fibers to collect radiation for each band," explains Small.
In addition, the hollow fiber optic has a small numerical aperture, which means it collects radiation coming from one particular direction and, hence, a very small surface area. This configuration has two advantages. First, the distance between the fiber tip and the target surface can be varied without affecting the accuracy of the measurements. Second, the small numerical aperture offers high spatial resolution. "We can accurately monitor the surface temperature of a 0.8-millimeter spot at a distance of 1 centimeter," explains Small. "Other noninvasive sensor systems, because of their design, cannot offer the combination of submillimeter resolution and variable working distance." The variable working distance means that the surgeon can more easily maneuver around the surgical site while still obtaining accurate temperature measurements.
A computer algorithm then calculates the true temperature and emissivity of the target in real time, taking into account reflection of the laser and ambient light from the target surface. Finally, the computer uses this information to open or shut a mechanical shutter, allowing more laser energy to pass if the temperature is too low or cutting off the energy if the temperature is too high.
"Because we use photoconductors that have nanosecond response times and millisecond lock-in integration times, we can accurately monitor most thermal processes," Small notes.
