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temperature by illuminating the surface with an IR laser and measuring the amount of the known laser
light that is reflected. The total IR during illumination is the sum of the reflected laser light and the
thermally emitted IR. If care is taken that only short pulses or low power laser light is used, to avoid
heating the measured surface, this will yield the reflectance at the wavelength of the laser and thus its
emissivity which, for opaque surfaces, is one minus reflectance.
Heat transfer to and from the detector by means other than thermal radiation exchange with the
monitored surface will require that the temperature of the detector be regarded as only representing the
temperature of the monitored surface rather than being the same as the monitored surface. Heat transfer
from the detector to the instrument can be via conduction or radiation and is accommodated by calibrating
the instrument against a black body of known temperature. When this is done, the temperature of the
instrument is usually monitored and circuitry may be set up to have the reference junction for the detector
monitor the instrument casing temperature. Alternatively the actual photon flux can be measured by
electronic means using photodiodes, photoresistive cells, or other such electronic photon sensors sensitive
to IR. When this is done, narrow band filters are often used to limit the response of the detector to a
particular wavelength of IR radiation to avoid counting visible photons that may be reflected from the
surface or to limit the response to a particular wavelength where atmospheric interferences are minimized.
Optical components are often employed to limit the field of view of the detector so that a defined
portion of the surface to be measured is brought to focus on the detector. As long as the entire detector
sees the surface of interest, the distance between the detector and the surface is not important, except as
it relates to IR absorption by the H 2 O, CO 2 , or other IR active gasses in the air.
A variation of the IR techniques discussed above is the disappearing filament pyrometer. This device
superimposes the image of a tungsten filament whose temperature is a known function of current through
the filament onto the view through a telescope. The walls of a furnace or other incandescent surface are
observed through the telescope while adjusting the filament current until it just disappears in the
background glow. The temperature of the filament then matches the temperature of the incandescent
surface and can be determined from the current through the filament. A simple refinement is to put a
narrow band red filter in the telescope so that the color is the same for both the target and the filament,
and it becomes a single wavelength brightness comparison rather than radiation color comparison. If
the emissivity of both the filament and the surface is unity, this can be very accurate. If not equal to one,
when a monochromatic filter is used, only the emissivity at that one wavelength needs to be known for
accurate temperature determinations.

Microscale Temperature Measurements
As the microelectronics industry surges forward with increasingly higher operating frequencies and
increasingly smaller device dimensions, measurement techniques with high spatial and/or temporal res-
olution are becoming increasingly important. Few techniques are available that can actually measure
temperature on a microscale, i.e., sub-micron spatial resolution and/or sub-microsecond temporal reso-
lution. However, many techniques that are being developed concentrate on observing the differential
temperature on a microscale. The transient thermoreflectance technique, for example, utilizes a femtosec-
ond pulsed laser to heat and probe the transient reflectance of the sample to enable observation of thermal
transport on a sub-picosecond time scale. The technique involves relating the measured reflectivity changes
to temperature changes using the material’s complex index of refraction (Rosei and Lynch, 1972).
The three most common methods of observing microscale thermal phenomena include thin film
thermocouples, thin film microbridges, and optical techniques. Nanometer scale thermocouples are typ-
ically used in conjunction with an atomic force microscope (AFM). This technique is nondestructive since
the AFM brings the probe into contact with the sample very carefully. Thin film microbridges are patterned
metallic thin films, usually thinner than 100 nanometers with a width that depends on the application.
This technique relies on the fact that the electrical resistance of the microbridge is a strong function of
temperature. The microbridge must be deposited onto the sample surface, therefore the technique is
neither noncontact nor nondestructive. Optical techniques typically use a laser as the heating source

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