In 1978, Luxtron (now LumaSense Technologies) invented the field of fiber optic temperature measurement sensors through the creation of its Fluoroptic® technology.
Fluoroptic® optical sensor technology is based on the fluorescence decay time of a special thermo-sensitive phosphorescent (phosphor) sensor, located at the end of a fiber optic cable (see Figure 1). Light generated by an excitation LED is routed through a probe extension and connectors, where it falls on the phosphor sensor located at the probe tip. Older Fluoroptic® temperature systems used xenon flash lamps that need to be replaced over time. Current systems use LEDs that never need to be replaced.
When stimulated with red light from the LED, the Luxtron phosphor sensor emits light over a broad spectrum in the near infrared region (see Figure 2). The time required for the fluorescence to decay is dependent upon the sensor’s temperature. After the LED is turned off, the decaying fluorescent signal continues to transmit through the fiber to the instrument, where it is focused onto a detector. The signal from the detector is amplified and sampled after the LED is turned off.
Figure 1. Phosphor sensor at the end of the fiber optic probe is excited (red glow) by an LED located in the instrument.
The measured decay time is then converted to temperature by the instruments software using a calibrated conversion table. Different calibration tables are used depending on the temperature range and application, but the overall temperature range capability of this optical sensor technology is currently –200 °C to 330 C.
The fact that the excitation light signal and the fluorescent decay signal pass along the same optical path means that the fiber optic probe and sensing tip can be relatively small. This is particularly important in medical research applications. Fiber optic probes as small as 0.5mm diameter (STB probe) are available. Other probe configurations and customized fiber optic temperature sensors are available.
Figure 2. After excitation by the LED light pulse, the emitted light from the sensor decays at a rate that varies precisely with temperature.
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