An image is a picture created by a camera on photographic film (called a photograph) or by a remote sensing detector and displayed on a screen or on paper and called simply an image.
A camera takes light energy and records it chemically on film. The film is then processed and the image transferred to paper where we can look at it. We call this final form a photographic image. Most films have chemicals that are sensitive to visible light energy. This means it will record the same images a human eye can see. Camera film can also be chemically sensitive to the "invisible" infrared energy, and can record on the film images that the human eye cannot see.
Scientists have also created very complex detectors that can sense many different wavelengths in the electromagnetic spectrum. These sensitive instruments record the reflected energy as numbers or digits. Sensors record their measurements as numbers from 0 to 255. This digital information is often recorded on magnetic tape, like in a tape recorder or video cassette, or radioed back to Earth. Computers then put these numbers together and make pictures.
Remember, what an image looks like depends on two things: the scale -- the relationship of the size on the photo to the area covered on the ground, and resolution -- the detail and clarity of the image. Each digital number represents a small square area on the ground. This small square area is called a picture element, or pixel, on the image. A remote sensing instrument with a pixel size of 30 meters on a side would at an area on the ground that is 30 meters square, and then assign a code (a number from 0 to 255 representing a shading or color) to it to identify that square. The Denver Museum of Natural History describes it like this:
Imagine how a football stadium and the surrounding parking lot might appear on a satellite image. A football field approximately 100 meters long and 60 meters wide would comprise 6 squares, 30 m by 30 m. The final image would contain 6 pixels showing green (if in visible light) or the infrared signature of living grass (if in infrared and assuming no Astroturf!) surrounded by pixels of a different color or shading representing the stands, parking lot and streets. A single tree (diameter of 5 meters) in a parking lot would not show up since most of the square would be pavement. The one code for that pixel would have to represent a square that is mostly pavement, thereby eliminating the image of the tree itself.The image this process creates is really like a paint-by-numbers picture. The digital number values are assigned colors, and can then be put together on a TV- like monitor, and put on paper to look like a photographic image. The pixel size determines the images resolution. The pixel size and the scale of the image determine the resolution, or detail of the image.
0 = no black, square is entirely whiteThese code numbers are called out to the second student. The second student is the "computer" and takes the information and translates it into an imaging. Using a sheet of graph paper with the same numbers across the top and letters down the side (although the grid size of the squares does not need to be the same) the second student colors in the appropriate square using three contrasting shades of crayon, or shades of grey with a pencil.
1 = less than 1/2 of the square is black (mostly white)
2 = 1/2 or more but not all is black (mostly black)
3 = the entire square is black
0 = no black, leave square white
1 = light gray shading, or lightest crayon
2 = darker grey shading, or darker crayon
3 = darkest grey/black shading or black crayon