1. An apparatus for enhancing an image, comprising: a pre-processing unit configured to generate an inverted image by inverting a luminance component estimated from a red (R) channel among color channels of a red, green, and blue (RGB) color space of an input image; a channel merging unit configured to generate a merged luminance (L) channel by merging an L channel of an International Commission on Illumination (CIE) L*a*b* (CIELab) color space of the input image and the inverted image; a contrast enhancement unit configured to enhance contrast by expanding a histogram of the merged L channel; and a color restoration unit configured to generate a restored image by combining the merged L channel whose contrast has been enhanced and chromaticity components of the input image and converting a resultant color image to the RGB color space.