handbook of modern sensors

492 16 Temperature Sensors
Fig. 16.23. A typical transfer function of a LM35DZ semiconductor temperature sensor; (Courtesy
of National Semiconductors, Inc.).
quickly. Also, they are needed for determining temperatures at tough hostile environments
when very strong electrical, magnetic, or electromagnetic fields or very high
voltages make measurements either too susceptible to interferences or too dangerous
for the operator. Also, there are situations when it is just difficult to reach an object
during a routine measurement. In addition to the infrared methods of temperature
measurements, there are sensors that are contact by nature but still use photons as
carriers of thermal information.
16.4.1 Fluoroptic Sensors
These sensors rely on the ability of a special phosphor compound to give away
a fluorescent signal in response to light excitation. The compound can be directly
painted over the measured surface and illuminated by an ultraviolet (UV) pulse while
observing the afterglow. The shape of the response afterglow pulse is function of
temperature. The decay of the response pulse is highly reproducible over a wide temperature
range [13,14]. As a sensing material, magnesium fluoromagnetite activated
with tetravalent manganese is used. This is phosphor, long known in the lighting
industry as a color corrector for mercury vapor street lamps, prepared as a powder
by a solid-state reaction at approximately 1200 ◦ C. It is thermally stable, relatively
inert, and benign from a biological standpoint, and insensitive to damage by most
chemicals or by prolonged exposure to ultraviolet UV radiation. It can be excited to
fluoresce by either UV or blue radiation. Its fluorescent emission is in the deep red
region, and the fluorescent decay is essentially exponential.
To minimize cross-talk between the excitation and emission signals, they are
passed through the bandpass filters, which reliably separate the related spectra (Fig.
16.24A). The pulsed excitation source, a xenon flash lamp, can be shared among a
number of optical channels in a multisensor system. The temperature measurement
is made by measuring the rate of decay of the fluorescence, as shown in Fig. 16.24B;
that is, a temperature is represented by a time constant τ which drops fivefold over the