2.9. LUMINOUS INTENSITY
In photometry, luminous intensity is a measure of the
wavelength-weighted power emitted by a light source in a particular direction,
based on the luminosity function, a standardized model of the sensitivity of
the human eye. The SI unit of luminous intensity is the candela (cd), an SI
base unit. Photometry deals with the measurement of visible light as perceived
by human eyes. The human eye can only see light in the visible spectrum and has
different sensitivities to light of different wavelengths within the spectrum.
When adapted for bright conditions (photopic vision), the eye is most sensitive
to greenish-yellow light at 555 nm. Light with the same radiant intensity
at other wavelengths has a lower luminous intensity. The curve which measures
the response of the human eye to light is a defined standard, known as the
luminosity function. This curve, denoted V(ë) , is based on an
average of widely differing experimental data from scientists using different
measurement techniques. For instance, the measured responses of the eye to
violet light varied by a factor of ten.
Luminous intensity should not be confused with another
photometric unit, luminous flux, which is the total perceived power emitted in
all directions. Luminous intensity is the perceived power per unit solid
angle. Luminous intensity is also not the same as the radiant intensity,
the corresponding objective physical quantity used in the measurement science
of radiometry.
Units
One candela is defined as the luminous intensity of a
monochromatic 540 THz light source that has a radiant intensity of
1/683 watts per steradian, or about 1.464 mW/sr. The 540 THz
frequency corresponds to a wavelength of about 555 nm, which is green
light near the peak of the eye's response. Since there are about
12.6 steradians in a sphere, the total radiant intensity would be about
18.40 mW, if the source emitted uniformly in all directions. A typical
candle produces very roughly one candela of luminous intensity.
In 1881, Jules Violle proposed the Violle as a unit
of luminous intensity, and it was notable as the first unit of light intensity
that did not depend on the properties of a particular lamp. It was superseded
by the candela in 1946.[15]
Intensity Histogram
In an image processing context, the histogram of an image
normally refers to a histogram of the pixel intensity values. This histogram is
a graph showing the number of pixels in an image at each different intensity
value found in that image. For an 8-bit greyscale image there are 256 different
possible intensities, and so the histogram will graphically display 256 numbers
showing the distribution of pixels amongst those greyscale values. Histograms
can also be taken of colour images either individual histograms of red, green
and blue channels can be taken, or a 3-D histogram can be produced, with the
three axes representing the red, blue and green channels, and brightness at
each point representing the pixel count. The exact output from the operation
depends upon the implementation it may simply be a picture of the required
histogram in a suitable image format, or it may be a data file of some sort
representing the histogram statistics.[15]
The working operation.
The operation is very simple. The image is scanned in a single
pass and a running count of the number of pixels found at each intensity value
is kept. This is then used to construct a suitable histogram.
Binary Images
Binary images are images whose pixels have only two possible
intensity values. They are normally displayed as black and white. Numerically,
the two values are often 0 for black, and either 1 or 255 for white. Binary
images are often produced by thresholding a greyscale or colour image, in order
to separate an object in the image from the background. The colour of the
object (usually white) is referred to as the foreground colour. The
rest (usually black) is referred to as the background colour. However,
depending on the image which is to be thresholded, this polarity might
be inverted, in which case the object is displayed with 0 and the background is
with a non-zero value.
Some morphological operators assume a certain polarity of the
binary input image so that if we process an image with inverse polarity the
operator will have the opposite effect. For example, if we apply a closing
operator to a black text on white background, the text will be opened.
Intensity of images.
Intensity images measure the amount of light impinging on a
photosensitive device. The input to the photosensitive device, typically a
camera, is the incoming light, which enters the camera's lens and hits the
image plane. In a digital camera, the physical image plane is an array which
contains a rectangular grid of photosensors, each sensitive to light intensity.
The output of the array is a continuous electric signal, the video signal. The
video signal is sent to an electronic device called frame grabber, where it is
digitised into a 2D rectangular array of integer values and stored in a memory
buffer.
The interpretation of an intensity image depends strongly on
the characteristics of the camera called the camera parameters. The parameters
can be separated into extrinsic and intrinsic parameters. The extrinsic
parameters transform the camera reference frame to the world reference frame.
The intrinsic parameters describe the optical, geometric and digital
characteristics of the camera. One parameter, for example, can describe the
geometric distortion introduced by the optics.
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