Atmospheric visibility

Atmospheric visibility

Atmospheric visibility means that people with normal vision can identify the maximum distance of targets with a certain size from the background (sky or floor), also known as the weather visibility. According to the observer and the height of the target, it is divided into three categories: horizontal visibility, strabismic visibility and lead-specific visibility. Atmospheric visibility and aviation, navigation, land transportation, high-altitude photography, astronomical observations, military operations are direct relationships, is a common element that characterizes atmospheric optical properties, and is divided into 10 levels in actual observations.

Measuring the method of atmospheric visibility is generally available, and the measuring instrument of the atmospheric transmitter, laser visibility automatic measuring instrument can also be used. The visibility of visibility is mostly based on artificial visual, normative, and more objective. The atmospheric transmitter is directly measuring the gas column transmittance through the atmosphere passing through the two fixed points, in order to calculate the value of the visibility, this method requires the light beam through a sufficiently long atmosphere, the measured reliability is light source and Other hardware systems work stability, generally only applicable to medium-oriented observations, and low visibility weather such as rain, fog, will cause greater errors due to complex conditions such as water vapor absorption. The laser visibility automatic measuring instrument is to calculate the visibility by laser measurement of atmospheric matters. It is relatively objective and accurate, but this instrument is expensive, high maintenance costs, complex operation, and in rain, foggy day It is difficult to conduct normal observations, so it is difficult to promote. The digital camera method developed by China University of Science and Technology automatically measures the visibility of the instrument system. The image of the selected target and its background is directly taken through the digitized camera, and then transmit the image from the image acquisition card to the computer, and analyze the acquired image , Can automatically obtain the value of visibility. This method measured the visibility value and the visibility value observed with laser radar is relatively close.

White visibility

The vision of the target is L t, the background is brightness is L B, both Differences can be represented by comparison c :

c is a function of distance. The human eye is called a comparison of the smallest brightness comparison value of the distant substantial energy to the distinction ε , for the normal stationary observer of vision, ε average The value is 0.02. When the target at R is satisfied c ( r ) = ε , r is visibility.

Under the uniform assumption of atmospheric level, the basic equation of the day-level visibility R can be derived: μ

μ is Emblem coefficient in the horizon; L h is the sky brightness in the horizon; c * = 喣 (l * tl * b / l * b) 喣 is the target and background Comparison, L * T and L * B inherent brightness are inherent brightness of the target and background. It can be seen from the basic equation that the visibility is related to the optical characteristics of the target, the sky background and the atmosphere. The more turbid atmosphere, the larger the extinction coefficient μ, the smaller the visibility. For black targets with the sky near the skyline, L * b = LH, L * T = 0, so the basic equation can be simplified. If ε = 0.02, the horizontal visibility r = 3.912 / μ (km). If the extinction coefficient can be measured, the visibility can be calculated according to this formula. A extinction coefficient of 0.55 micron wavelength is generally used in the visible optical band. Since the extinction coefficient in the atmosphere decreases with the growth of wavelengths (see atmosphere), the visibility will increase when the wavelength increases. The relationship between the extinction coefficient and visibility and wavelength λ is r

in kilometers, μ λ in kilometers - 1. In actual observations, horizontal visibility can be determined according to the measured atmosphere, but there is no good measurement method for strabismic visibility and lead straightness.

Night visibility

The visibility of nighttime is different. Whenever the night is coming, the brightness of the object and background is greatly reduced, coupled with the contrast of the brightness or color in the night, and cannot determine the visibility according to the brightness contrast. At this time, it is more sensitive to weak light. Whether a light signal in the night can be found, depending on the illuminance of the light source to the observation point. The illuminance generated by the light source of the optical strength is

if e s is the illuminance threshold of the human eye, the corresponding night visibility V will satisfy:

so the night visibility depends on the gas extinction characteristics and intensity of light.