Image monitoring device, image monitoring method, and recording medium

An image monitoring method comprising:

• calculating, for each of a plurality of blocks set in one frame of a video signal obtained through a lens, a maximum luminance value and a minimum luminance value of a plurality of pixels forming each of the plurality of blocks;
• determining whether or not each of the plurality of blocks has a small luminance difference, by comparing a difference between the maximum luminance value and the minimum luminance value with a first threshold;
• counting a number of blocks having the small luminance difference in the one frame;
• and
• determining whether or not dirt is adherent to the lens, based on the number of blocks having the small luminance difference.

A non-transitory recording medium on which a computer-readable image monitoring program is stored, the computer-readable image monitoring program causing a computer to function as:

• a maximum value and minimum value calculator configured to calculate, for each of a plurality of blocks set in one frame of a video signal obtained through a lens, a maximum luminance value and a minimum luminance value of a plurality of pixels forming each of plurality of the blocks;
• a small-luminance-difference determiner configured to determine whether or not each of the plurality of blocks has a small luminance difference, by comparing a difference between the maximum luminance value and the minimum luminance value with a first threshold;
• a small-luminance-difference-block counter configured to count the number of blocks having the small luminance difference in the one frame;
• and
• a dirt adherence determiner configured to determine whether or not dirt is adherent to the lens, based on the number of blocks having the small luminance difference.