Characteristics of thermal infrared scanning image

All objects radiate electromagnetic waves in thermal infrared band at day or night, and the thermal infrared energy radiated by the objects themselves is imaged by scanning, that is, thermal infrared scanning images. 3 ~ 5 micron and 8 ~14 micron are two important thermal infrared atmospheric windows. Because the 8 ~ 14μ m band contains the maximum intensity of radiation flux at the average temperature of the earth's surface, this atmospheric window is often used for thermal infrared scanning imaging.

(A) the characteristics of thermal infrared images

1. It can be imaged day and night.

Thermal infrared scanning images are different from infrared aerial photographs. Infrared aerial photography is photographic imaging, detecting near-infrared band, and the light source comes directly from sunlight, so it must be imaged during the day. Thermal infrared scanning images detect the emission spectrum of ground objects, and all objects on the ground emit infrared rays into space day and night, so infrared scanning sensors can form thermal infrared images.

Thermal infrared images have a great relationship with the temperature difference of ground objects. If the ground object temperature difference is large and the image contrast is large, the image will be clear. On the contrary, the temperature difference between features is small, and the image contrast is also small, so the image is not clear. However, the temperature difference between features is determined by the nature of sunlight and features. Generally speaking, the temperature difference between afternoon and 6: 00 am 1 is the largest, and the imaging effect is the best. However, the thermal infrared images imaged at noon are greatly disturbed by terrain shadows, and the imaging effect is generally not as good as that before dawn.

2. It records the thermal radiation intensity of ground objects.

Thermal infrared images record the intensity of thermal radiation energy of ground objects. The infrared radiation intensity of ground objects is related to temperature, with high temperature, high infrared radiation intensity and light image color. Low temperature, low infrared radiation intensity and dark image tone. Some ground objects are similar in color and difficult to distinguish, but they can be distinguished because of different temperatures and different colors in thermal infrared images. For example, it is difficult to distinguish dolomite from limestone in ordinary aerial photos, but in the thermal infrared image at 6: 00 a.m., dolomite is hotter than limestone because of their different specific heat capacities, thus showing the difference of superior tones.

3. The ground resolution is very low

The ground resolution of the image is mainly determined by the instantaneous field of view angle and imaging height of optical scanning. The smaller the instantaneous field of view angle, the lower the flying height of the aircraft, the smaller the ground resolution unit and the higher the resolution. For infrared scanners, the instantaneous field of view angle is certain, generally 1 ~ 3 mrad. When the altitude is 1000m, the ground resolution unit is 1 ~3m, which is lower than that of ordinary aerial photographs. In addition, the resolution of thermal infrared scanning image is also related to scanning angle. At the same height, with the change of scanning angle, the instantaneous field of view of the ground gradually increases from the middle to both sides on the same scanning line, and the ground resolution gradually decreases.

(2) Characteristics of thermal infrared scanning images

1. Geometric distortion

Because the ground resolution is distorted with the change of scanning angle, the infrared scanning image is also called panoramic distortion, and its formation reason is similar to that of panorama camera. In addition to the scanning system itself, the geometric distortion of thermal infrared scanning images is also related to the temperature of ground objects. Generally speaking, non-heat source features, such as water, mountains, hills, farmland, roads, etc. Most of them are real or close to real, especially in the middle of the image. However, the characteristics with high temperature usually cannot reflect the real shape. Because of the high temperature, the energy radiated to the surrounding space is large, which produces a halo-like phenomenon, and the shape of the ground object is covered, distorted or expanded, or even unrecognizable.

2. Thermal shadow

Visible light shadow belongs to light and shadow, which is divided into umbra and falling shadow. When the light disappears, the light and shadow disappear. The shadow on the thermal infrared image is caused by low temperature, because the surface shadow can not be directly illuminated by the sun, which is usually called thermal shadow. There is no difference between umbra and shadow. Although the light disappeared after the sun went down, the temperature difference between yin and yang did not disappear immediately, and there were still shadows on the thermal image for some time after the sun went down. The duration of thermal shadow varies from place to place and generally disappears before dawn. At this time, the color difference of thermal image is mainly the difference of thermal characteristics of ground objects themselves, which is helpful to interpret rock properties. Thermal images during the day have the characteristics of clear landforms.

3. Temperature resolution

The temperature resolution of thermal image refers to the minimum temperature difference that can be distinguished, or the ability to distinguish the radiation temperatures of two different radiation sources. The temperature resolution of modern thermal infrared remote sensor has reached 0. 0 1℃, while the temperature resolution of thermal infrared scanners used for resource remote sensing is mostly 0. 1 ~ 0.5℃. The temperature resolution of thermal infrared image can be calibrated by gray scale or color scale on the image, and each gray scale color represents a certain temperature range. By comparing the hue or color in the image with the gray scale or color scale, the temperature range of the ground object can be determined. The color thermal image is more striking to the temperature distribution on the ground, and the boundary between the two color areas is the isotherm of the ground.

Step 4: Tone

Tone is an important basis for thermal infrared image interpretation. The hue of thermal infrared image will be affected by the thermal radiation energy of ground objects, imaging time and meteorological conditions.

The thermal radiation energy of ground objects will have a certain influence on the color tone of images. The thermal radiation energy of ground objects is large, the image color is light, the thermal radiation energy of ground objects is small, and the image color is dark. The thermal radiation energy of ground objects depends on their temperature and emissivity. The higher the temperature, the greater the radiation energy, such as blast furnace, fire and other heat sources, the image is often white. The temperature on the land is relatively low, and whether it is imaging during the day or at night, it is a gray tone with different shades. Although some ground objects have the same temperature, they will have different colors due to different emissivity. For example, although the temperature of marble is the same as that of timely, the emissivity of marble is 0. 942, which should be 0. Therefore, the color of marble is lighter, but the color should be darker.

Different imaging time will also have a certain impact on the color tone of the image, such as high heat capacity of water body, good thermal conductivity and small daily temperature difference; However, the heat capacity of land is low and the daily temperature difference is large. During the day, the water temperature is lower than the surrounding land, and the water body is dark; At night, the water temperature is higher than the surrounding land, so it looks like light. During the day, the river is black and at night, the river is white. Another example is green plants, which have strong ability to reflect short-wave infrared, but weak ability to radiate long-wave, and because of the evaporation of water during the day, the temperature is low and the color tone is mostly dark gray; At night, especially in the forest, there is an inversion phenomenon, the temperature is higher than the ground, generally light.

Meteorological conditions will also affect the hue of thermal infrared images, because thermal infrared images record the difference of radiation energy of ground objects, whether the weather is cloudy, sunny or typhoon. If the clouds can cover the sun and the temperature difference between the ground decreases, the image is uniform black-gray tone; The ground wind will make the image produce a unique trailing stripe pattern, and the stripes produced by the wind often appear in the downwind of obstacles on the flat ground; The ground wind will also enlarge the image area of small hot spots a lot, which will affect the correct estimation of their area.