First-photon imaging is a photon-efficient,computational imaging technique that reconstructs an image by recording only the first-photon arrival event at each spatial location and then optimizing the recorded photon information.The optimization algorithm plays a vital role in image formation.A natural scene containing spatial correlation can be reconstructed by maximum likelihood of all spatial locations constrained with a sparsity regularization penalty,and different penalties lead to different reconstructions.The l1-norm penalty of wavelet transform reconstructs major features but blurs edges and high-frequency details of the image.The total variational penalty preserves edges better;however,it induces a"staircase effect,"which degrades image quality.In this work,we proposed a hybrid penalty to reconstruct better edge features while suppressing the staircase effect by combining wavelet l1-norm and total variation into one penalty function.Results of numerical simulations indicate that the proposed hybrid penalty reconstructed better images,which have an averaged root mean square error of 12.83%,5.68%,and 10.56%smaller than those of the images reconstructed by using only wavelet l1-norm penalty,total variation penalty,or recursive dyadic partitions method,respectively.Experimental results are in good agreement with the numerical ones,demonstrating the feasibility of the proposed hybrid penalty.Having been verified in a first-photon imaging system,the proposed hybrid penalty can be applied to other noise-removal optimization problems.