Abstract: In low light situation, the imaging often suffers dark brightness and low contrast, the image information is not clear, and the existing image enhancement methods can not fully improve the brightness, contrast and clarity of the image. This paper proposes a low light image enhancement method based on equidistant histogram equalization and high frequency highlighting. Firstly, by virtue of the independence of brightness component and color component in HSI color space, the image is converted to HSI color space, and the brightness image is decomposed into low-frequency image and high-frequency image according to Retinex theory and Gaussian filtering. And then, an equal histogram equalization method is proposed for low-frequency image, which divides the frequency of intensity and the dynamic space of low-frequency image into four equal parts, respectively, and piecewise remaps the pixels of low-frequency image using the pixel frequency of local intensity level, and Retinex reconstruction with low-frequency and high-frequency images is performed, so as to improve the brightness and contrast of images. Finally, the enhanced brightness image is transformed to the wavelet domain, and the components of high-frequency wavelet coefficients are increased in a fractional way to improve the clarity of the image and highlight the edges and details of the image. Experiments on low light image datasets show that the images enhanced by the proposed method in this paper are better than some existing methods in enhanced image, average gradient, information entropy and edge extraction. The method in this paper can increase the application value of image more effectively.

Key words: low light image, image enhancement, equidistant histogram equalization, wavelet transform, high frequency highlighting

摘要： 低光照条件下的成像，往往亮度较暗，对比度较低，图像信息不清晰，而现有的图像增强方法对图像的亮度、对比度和清晰度的改善不足。提出一种等分直方图均衡和高频强化的低光照图像增强方法。借助于HSI颜色空间中亮度分量与颜色分量独立的特性，将图像转换到HSI颜色空间，根据Retinex理论并结合高斯滤波将亮度图像分解为低频图像和高频图像；针对低频图像提出一种等分直方图均衡方法，将低频图像的灰度级频次和动态空间分别四等分，利用局部灰度级的像素频次对低频图像的像素进行分段重映射，再由低频图像和高频图像进行Retinex重构，以改善图像的亮度和对比度；将增强后的亮度图像转换到小波域，以分数阶的方式提升高频小波系数的分量，以改善图像的清晰度和凸显图像的边缘和细节。在低光照图像数据集上的实验显示，经该方法增强后的图像，在增强效果图、平均梯度、信息熵以及边缘提取上，均优于部分现有方法，该方法能更有效地提高图像的应用价值。

关键词: 低光照图像, 图像增强, 等分直方图均衡, 小波变换, 高频强化