Temperature-Sensitive Coating Sensor Based on Hematite
- Created: Tuesday, 01 November 2011
This inexpensive, robust sensor system enables easier measurement and interpretation for optical detection.
A temperature-sensitive coating,
based on hematite (iron III oxide), has
been developed to measure surface temperature
using spectral techniques. The
hematite powder is added to a binder
that allows the mixture to be painted on
the surface of a test specimen. The coating
dynamically changes its relative spectral
makeup or color with changes in
temperature. The color changes from a
reddish-brown appearance at room temperature
(25 ºC) to a black-gray appearance
at temperatures around 600 ºC.
The color change is reversible and
repeatable with temperature cycling
from low to high and back to low temperatures.
Detection of the spectral
changes can be recorded by different
sensors, including spectrometers, photodiodes,
and cameras. Using a-priori
information obtained through calibration
experiments in known thermal
environments, the color change can
then be calibrated to yield accurate
quantitative temperature information.
Temperature information can be
obtained at a point, or over an entire
surface, depending on the type of equipment
used for data acquisition.
Because this innovation uses spectrophotometry
principles of operation,
rather than the current methods, which
use photoluminescence principles,
white light can be used for illumination
rather than high-intensity short wavelength
excitation. The generation of
high-intensity white (or potentially filtered
long wavelength light) is much
easier, and is used more prevalently for
photography and video technologies. In
outdoor tests, the Sun can be used for
short durations as an illumination
source as long as the amplitude remains
relatively constant. The reflected light is
also much higher in intensity than the
emitted light from the inefficient current
methods. Having a much brighter
surface allows a wider array of detection
schemes and devices. Because color
change is the principle of operation, the
development of high-quality, lower-cost
digital cameras can be used for detection,
as opposed to the high-cost imagers
needed for intensity measurements with
the current methods.
Alternative methods of detection are possible to increase the measurement sensitivity. For example, a monochrome camera can be used with an appropriate filter and a radiometric measurement of normalized intensity change that is proportional to the change coating temperature. Using different spectral regions yields different sensitivities and calibration curves for converting intensity change to temperature units. Alternatively, using a color camera, a ratio of the standard red, green, and blue outputs can be used as a self-referenced change. The blue region (575 nm), so a ratio of color intensities will yield a calibrated temperature image.
The new temperature sensor coating is easy to apply, is inexpensive, can contour complex shape surfaces, and can be a global surface measurement system based on spectrophotometry. The color change, or relative intensity change, at different colors makes the optical detection under white light illumination, and associated interpretation, much easier to measure and interpret than in the detection systems of the current methods.
This work was done by Timothy J. Bencic of
Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. LEW-18761-1