Images are made up of small square sized pixels. Each pixel (picture element) has a single brightness value. In an 8-bit (bit = binary unit) depth representation of the brightness, the values range from 0-255 which is 2^8. For most images, pixel values are integers that range from 0 (black) to 255 (white).Similarly, a 16-bit means there are 2^16 possible values, from 0-65535. The number of bits is sometimes called the bit-depth.

Images typically describe bits in terms of bits-per-pixel (BPP). For example a grayscale image may have 8-BPP, meaning each pixel can have one of 256 values from 0 (black) to 255 (white). Color images are a little different because they are typically composed of three component images, red (R), green (G), and blue (B). Each component image has its own bit-depth, thus its own brightness. So a typical 24-bit RGB image is composed of three 8-BPP component images, i.e. 24-BPP RGB = 8-BPP (R) + 8-BPP (G) + 8-BPP (B). The dimensions of the original image are say 768 x 512 pixels. Since each pixel requires 3 bytes (1 byte = 8 bits) of information, we can store the image to disk in raw format using 768 x 512 x 3 = 1,179,648 bytes. The bit stream would have length 1,179,648 x 8 = 9,437,184.

Remote Sensing

Prior to the 1960s our view of the earth and the universe was restricted to observations and photographs using visible light. Aerial photography is the original form of remote sensing and remains a widely used method. In the 1960s, technology was developed to acquire images in the infrared (IR) and microwave regions. The term remote sensing refers to methods that employ electromagnetic energy, such as light, heat, and radio waves, as the means of detecting and measuring target characteristics.
The interaction between matter and electromagnetic energy is determined by:

  1. Energy Source or Illumination (A) - the first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest.

  2. Radiation and the Atmosphere (B) - as the energy travels from its source to the target, it will come in contact with and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor.

  3. Interaction with the Target (C) - once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation.

  4. Recording of Energy by the Sensor (D) - after the energy has been scattered by, or emitted from the target, we require a sensor (remote - not in contact with the target) to collect and record the electromagnetic radiation.

  5. Transmission, Reception, and Processing (E) - the energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving and processing station where the data are processed into an image (hardcopy and/or digital).

  6. Interpretation and Analysis (F) - the processed image is interpreted, visually and/or digitally or electronically, to extract information about the target which was illuminated.

  7. Application (G) - the final element of the remote sensing process is achieved when we apply the information we have been able to extract from the imagery about the target in order to better understand it, reveal some new information, or assist in solving a particular problem. While the term ‘remote sensing’ typically assumes the use of electromagnetic radiation, the more general definition of ‘acquiring information at a distance’, does not preclude other forms of energy. The use of sound is an obvious alternative. that remote sensing, in its broadest definition, includes ultrasounds, satellite weather maps, speed radar, graduation photos, and sonar - both for ships and for bats! Scientists say that the Earth itself vibrates at a very low frequency, making a sound far below the human hearing range.

1. Energy Source or Illumination (A)

Region Wavelength Components & Absorbed
Gamma ray < 0.03 nm Oxygen and nitrogen atoms in the thermosphere absorb nearly all x-rays and gamma rays, the most energetic forms of light; the mesosphere and stratosphere screen the remainder.
X-ray 0.03 to 30 nm The energy of these both goes into tearing one of the electrons away from its orbit around the nucleus of a nitrogen or an oxygen atom. Medicine, airport luggage screens
Ultraviolet 30 to 400 nm The shorter the wavelength, the more harmful the UV radiation.
UVC 30 to 280 nm all UVC and most UVB is absorbed by ozone, water vapour, oxygen and carbon dioxide. UVA is not filtered as significantly by the atmosphere.
UVB 280 to 315 nm UVB enhances skin ageing and significantly promotes the development of skin cancer.
UVA 315 to 380 nm 95 per cent of the UV radiation reaching the Earth’s surface, may also enhance the development of skin cancers, contributes to skin ageing and wrinkling, detectible with photo detectors but atmospheric scattering is severe
Visible 380nm to 750nm where the sun’s radiation intensity peaks, RGB are the primary colors
Violet 380 to 450nm
Blue 450 to 495 nm
Green 495 to 570 nm the sun’s emission peaks at 500 nm,
Yellow 570 to 590 nm
Orange 590 to 620 nm
Red 620 to 750 nm
Infrared 750 nm to 1 mm
Near- (NIR) 0.7 to 1.4 \(\mu\)m
Short Wave (SWIR) 1.4 to 3 \(\mu\)m But it is also common to refer to the entire range from 780 nm up to 3000 nm as NIR or SWIR or even reflected infrared. The reflected EM in this band contains n thermal information.
Mid Wave (MWIR) 3 to 8 \(\mu\)m also called Intermediate Infrared (IIR)
Long Wave (LWIR) 8 to 15 \(\mu\)m The earth’s peak radiation is at 9.7\(\mu\)m. The band known as the thermal band.
Far Infrared (FIR) 15 \(\mu\)m to 1 mm These 3 regions are also known as thermal infrared. The “thermal imaging” region, in which sensors can obtain a completely passive image of objects only slightly higher in temperature than room temperature - for example, the human body - based on thermal emissions only and requiring no illumination.
Microwave 1 mm to 1 m Microwaves are the principal carriers of high-speed data transmissions between stations on Earth and also between ground-based stations and satellites and space probes. Can penetrate cloud and fog to acquire images in the active or passive mode without being scattered, absorbed or reflected. Active form of remote sensing - Radar
X band 25 mm to 37.5 mm
C band 37.5 mm to 75 mm
S band 75 mm to 150 mm
L band 150 mm to 300 mm
Radio > 1m
Ultra High Frequency (UHF) 0.1 m to 1 m television broadcasting
Very High Frequency (VHF) 1 m to 10 m propagate mainly by line-of-sight, so they are blocked by hills and mountains, television and radio broadcasting
Frequency Modulation (FM) 2.78 m to 3.5 m
Short Wave Radio 10 m to 80 m
Amplitude Modulation (AM) 200 - 600m radio broadcasting, aircraft navigation