Image processing based on the Retinex theory is known as a method for compressing the dynamic range of an imaging device. Since the dynamic range of an imaging device is smaller than the dynamic range of the natural world, the dynamic range is compressed by separating, compressing, and resynthesizing an illumination light component on the basis of the Retinex theory.
Single-scale Retinex (SSR) is known as a type of image processing based on the Retinex theory. In the Retinex theory, incident light I is defined as the product of illumination light L and reflectance R (I=RL). Based on this formula, log R in logarithmic space is used as the output value of the image (see paragraph 0007 of Patent Document 1).log R=log I−log L The above formula shows that, assuming that corrected illumination light L′ is a uniform illumination light environment (L′=1), an output value I′=R is converted into logarithmic space which is easy to visually recognize. Thus, it is possible to obtain a value from which the influence of changes in the illumination light has been eliminated.