Optical concept of contrast

Contrast, also known as density difference, refers to the black-and-white density difference of negative film or photo image and the brightness difference of the subject. A big contrast between light and shade means a big contrast. The contrast is large, indicating that the photo is hard.

Refers to the contrast between light and shade of photosensitive materials. The brightness change of different parts of the scene is the contrast of the scene; The density change of each part in the image is called image contrast, and the relationship between image density and logarithmic change of scene exposure is called the contrast of photosensitive materials.

Black, white and gray reflect different relative brightness. For color photography, it is more complicated, not only the difference of brightness, but also the color difference and saturation of color. Generally speaking, black-and-white films: low-sensitivity films have large contrast and high-sensitivity films have small contrast; Color film: Low-sensitivity film has small contrast and high-sensitivity film has large contrast.

Contrast refers to the degree of contrast between light and dark in various parts of a scene or image. There are two kinds of scene contrast and image contrast. The former refers to the ratio or logarithmic difference between the maximum brightness and the minimum brightness in the scene; The latter is the difference between the maximum density and the minimum density in the image. The two are closely related. The contrast of an excellent remote sensing image should correctly reflect the original contrast of the shooting scene. This relationship is characterized by contrast coefficient. When the contrast coefficient is 1, it means that the image contrast is equal to the scene contrast; When the contrast coefficient is 1, the image contrast is greater than the scene contrast. Contrast coefficient is one of the characteristics of photosensitive materials and the basis for selecting photosensitive materials correctly. Image contrast is an important index to reflect the quality of remote sensing images. Remote sensing images with good image contrast are beneficial to image analysis and interpretation. There are many factors that affect the image contrast, such as the contrast of the scene itself, the quality of the optical system of the remote sensor, the fog in the atmosphere, the characteristics of photosensitive materials, exposure and photographic processing, etc. Therefore, in remote sensing image processing, whether photochemical processing or digital image processing, contrast enhancement or contrast expansion technology can be used to improve the image interpretation quality by improving the image contrast.