Police, firefighters, and emergency medical personnel are examples of first responders that are utilizing thermal imaging
cameras in a very practical way every day. However, few performance metrics have been developed to assist first
responders in evaluating the performance of thermal imaging technology. This paper describes one possible metric for
evaluating spatial resolution using an application of Spatial Frequency Response (SFR) calculations for thermal imaging.
According to ISO 12233, the SFR is defined as the integrated area below the Modulation Transfer Function (MTF) curve
derived from the discrete Fourier transform of a camera image representing a knife-edge target. This concept is modified
slightly for use as a quantitative analysis of the camera's performance by integrating the area between the MTF curve
and the camera's characteristic nonuniformity, or noise floor, determined at room temperature. The resulting value,
which is termed the Effective SFR, can then be compared with a spatial resolution value obtained from human
perception testing of task specific situations to determine the acceptability of the performance of thermal imaging
cameras. The testing procedures described herein are being developed as part of a suite of tests for possible inclusion
into a performance standard on thermal imaging cameras for first responders.
Police, firefighters, and emergency medical personnel are examples of first responders that are utilizing thermal imaging
cameras in a very practical way every day. However, few performance metrics have been developed to assist first
responders in evaluating the performance of thermal imaging technology. This paper describes one possible metric for
evaluating the nonuniformity of thermal imaging cameras. Several commercially available uncooled focal plane array
cameras were examined. Because of proprietary property issues, each camera was considered a 'black box'. In these
experiments, an extended area black body (18 cm square) was placed very close to the objective lens of the thermal
imaging camera. The resultant video output from the camera was digitized at a resolution of 640x480 pixels and a
grayscale depth of 10 bits. The nonuniformity was calculated using the standard deviation of the digitized image pixel
intensities divided by the mean of those pixel intensities. This procedure was repeated for each camera at several
blackbody temperatures in the range from 30° C to 260° C. It has observed that the nonuniformity initially increases with
temperature, then asymptotically approaches a maximum value. Nonuniformity is also applied to the calculation of
Spatial Frequency Response as well providing a noise floor. The testing procedures described herein are being developed
as part of a suite of tests to be incorporated into a performance standard covering thermal imaging cameras for first
responders.
The use of thermal imaging cameras (TIC) by the fire service is increasing as fire fighters become more aware of the
value of these tools. The National Fire Protection Association (NFPA) is currently developing a consensus standard for
design and performance requirements for TIC as used by the fire service. This standard will include performance
requirements for TIC design robustness and image quality. The National Institute of Standards and Technology
facilitates this process by providing recommendations for science-based performance metrics and test methods to the
NFPA technical committee charged with the development of this standard. A suite of imaging performance metrics and
test methods based on the harsh operating environment and limitations of use particular to the fire service has been
proposed for inclusion in the standard. The performance metrics include large area contrast, effective temperature range,
spatial resolution, nonuniformity, and thermal sensitivity. Test methods to measure TIC performance for these metrics
are in various stages of development. An additional procedure, image recognition, has also been developed to facilitate
the evaluation of TIC design robustness. The pass/fail criteria for each of these imaging performance metrics are derived
from perception tests in which image contrast, brightness, noise, and spatial resolution are degraded to the point that
users can no longer consistently perform tasks involving TIC due to poor image quality.
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