The signal transfer function (SiTF) is a measure of the
signal output versus the
signal input of a system such as an
infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
system or
sensor
A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal.
In the broadest definition, a sensor is a devi ...
. There are many general applications of the SiTF. Specifically, in the field of image analysis, it gives a measure of the
noise
Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
of an imaging system, and thus yields one assessment of its performance.
SiTF evaluation
In evaluating the SiTF curve, the signal input and signal output are measured
differentially; meaning, the differential of the input signal and differential of the output signal are calculated and plotted against each other. An operator, using computer software, defines an arbitrary area, with a given set of data points, within the signal and
background regions of the output image of the infrared sensor, i.e. of the unit under test (UUT), (see "Half Moon" image below). The average signal and background are calculated by averaging the data of each arbitrarily defined region. A
second order polynomial curve is
fitted to the data of each
line. Then, the polynomial is subtracted from the average signal and background data to yield the new signal and background. The difference of the new signal and background data is taken to yield the net signal. Finally, the net signal is plotted versus the signal input. The signal input of the UUT is within its own spectral response. (e.g.
color-correlated temperature,
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
intensity, etc.). The slope of the linear portion of this curve is then found using
the method of least squares.
SiTF curve
The net signal is calculated from the average signal and background, as in
signal to noise ratio (imaging)#Calculations.
The SiTF curve is then given by the signal output data, (net signal data), plotted against the signal input data (see graph of SiTF to the right). All the data points in the linear region of the SiTF curve can be used in the method of
least squares
The method of least squares is a mathematical optimization technique that aims to determine the best fit function by minimizing the sum of the squares of the differences between the observed values and the predicted values of the model. The me ...
to find a linear approximation. Given
data points
a best fit line parameterized as
is given by:
[ Aboufadel, E. F., Goldberg, J. L., Potter, M. C. (2005).''Advanced Engineering Mathematics (3rd ed.).''New York, New York: Oxford University Press]
:
See also
*
Optical transfer function
*
Distortion
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
*
Minimum resolvable temperature difference
*
Noise equivalent temperature difference
*
Power spectral density
*
Minimum resolvable contrast
*
Signal to noise ratio (imaging)
References
External links
* http://www.electro-optical.com
Signal processing
Image processing
Infrared imaging