307
How Do You Display a Digital Remotely Sensed Image?
Brightness values are scaled to fit a range specified by the sensor.
Again, we’ll discuss more about these various types of ranges in Chapter 11
(with regard to a sensor’s radiometric resolution), but for now, this range of
values represents the energy being recorded on a scale from a low number (indicating very little radiance) to a high number (indicating a lot of radiance)
and measured as a number of bits. For instance, an 8-bit sensor will scale measurement values between the numbers of 0 (the lowest value) and 255 (the
highest value). These values represent the energy being recorded in a range
from 0 to 255 (these values are related to the amount of energy being measured). For instance, all values in a band of an image could possibly only be in
a range such as 50 to 150, with nothing reaching the maximum value of 255.
Each wavelength band that is being sensed is assigned a BV in this range
for each pixel in the image. Brightness values (BVs) can be translated to a grayscale to be viewed on a computer screen. With the range of values associated
with 8-bit imagery, values of 0 represent the color black and values of 255
represent the color white. All other integer values between 0 and 255 are displayed in shades of gray, with lower numbers being darker shades and higher
numbers being lighter shades. In this way, any of the wavelength band measurements (including the wavelengths normally invisible to the human eye)
can be displayed in grayscale on a computer screen (see Figure 10.8). For instance, a near-infrared image would have the brightness value for each pixel
representing the near-infrared measurements at the sensor for that area.
If a sensor is measuring the visible portion of the spectrum and treating
the entire 0.4 to 0.7 micrometer range as if it was one band, the result will be
black-and-white panchromatic imagery. However, remote sensing devices are
capable of sensing several wavelengths of the electromagnetic spectrum simultaneously. Some common remote sensing devices can sense several different
bands at once—sensing 7, 10, or even 36 bands at the same time is currently
done on a regular basis by United States government satellite sensors. Imagery
created by sensing multiple bands together is referred to as multispectral imagery. Some technology can simultaneously sense over 200 bands of the electromagnetic spectrum, producing hyperspectral imagery.
8-bit imagery a digital
image that carries a
range of brightness
values from 0 to 255.
panchromatic
imagery black and
white imagery formed
by viewing the entire
visible portion of
the electromagnetic
spectrum.
multispectral
imagery remotely
sensed imagery
comprised of the bands
collected by a sensor
capable of sensing
several bands of energy
at once.
hyperspectral
imagery remotely
sensed imagery
comprised of the bands
collected by a sensor
capable of sensing
hundreds of bands of
energy at once.
FIGURE 10.8 Pixels of
a sample 8-bit image,
with their corresponding
brightness values and
grayscale shading.
White
255
0
221
255
85
238 170
68
119
170
17
136
68
255
0
136 238
0 221
238 136
170
85
255
119
255
119
68
221
17
238
136
17
221
119
170 85
gray
127
black
0
BV
Greyscale
How Do You Display a Digital Remotely Sensed Image?
Brightness values are scaled to fit a range specified by the sensor.
Again, we’ll discuss more about these various types of ranges in Chapter 11
(with regard to a sensor’s radiometric resolution), but for now, this range of
values represents the energy being recorded on a scale from a low number (indicating very little radiance) to a high number (indicating a lot of radiance)
and measured as a number of bits. For instance, an 8-bit sensor will scale measurement values between the numbers of 0 (the lowest value) and 255 (the
highest value). These values represent the energy being recorded in a range
from 0 to 255 (these values are related to the amount of energy being measured). For instance, all values in a band of an image could possibly only be in
a range such as 50 to 150, with nothing reaching the maximum value of 255.
Each wavelength band that is being sensed is assigned a BV in this range
for each pixel in the image. Brightness values (BVs) can be translated to a grayscale to be viewed on a computer screen. With the range of values associated
with 8-bit imagery, values of 0 represent the color black and values of 255
represent the color white. All other integer values between 0 and 255 are displayed in shades of gray, with lower numbers being darker shades and higher
numbers being lighter shades. In this way, any of the wavelength band measurements (including the wavelengths normally invisible to the human eye)
can be displayed in grayscale on a computer screen (see Figure 10.8). For instance, a near-infrared image would have the brightness value for each pixel
representing the near-infrared measurements at the sensor for that area.
If a sensor is measuring the visible portion of the spectrum and treating
the entire 0.4 to 0.7 micrometer range as if it was one band, the result will be
black-and-white panchromatic imagery. However, remote sensing devices are
capable of sensing several wavelengths of the electromagnetic spectrum simultaneously. Some common remote sensing devices can sense several different
bands at once—sensing 7, 10, or even 36 bands at the same time is currently
done on a regular basis by United States government satellite sensors. Imagery
created by sensing multiple bands together is referred to as multispectral imagery. Some technology can simultaneously sense over 200 bands of the electromagnetic spectrum, producing hyperspectral imagery.
8-bit imagery a digital
image that carries a
range of brightness
values from 0 to 255.
panchromatic
imagery black and
white imagery formed
by viewing the entire
visible portion of
the electromagnetic
spectrum.
multispectral
imagery remotely
sensed imagery
comprised of the bands
collected by a sensor
capable of sensing
several bands of energy
at once.
hyperspectral
imagery remotely
sensed imagery
comprised of the bands
collected by a sensor
capable of sensing
hundreds of bands of
energy at once.
FIGURE 10.8 Pixels of
a sample 8-bit image,
with their corresponding
brightness values and
grayscale shading.
White
255
0
221
255
85
238 170
68
119
170
17
136
68
255
0
136 238
0 221
238 136
170
85
255
119
255
119
68
221
17
238
136
17
221
119
170 85
gray
127
black
0
BV
Greyscale