Centroid computation of Gaussian spot is often conducted to get the exact position of a target or to measure wave-front
slopes in the fields of target tracking and wave-front sensing. Center of Gravity (CoG) is the most traditional method of
centroid computation, known as its low algorithmic complexity. However both electronic noise from the detector and
photonic noise from the environment reduces its accuracy. In order to improve the accuracy, thresholding is unavoidable
before centroid computation, and optimum threshold need to be selected. In this paper, the model of Gaussian spot is
established to analyze the performance of optimum threshold under different Signal-to-Noise Ratio (SNR) conditions.
Besides, two optimum threshold selection methods are introduced: T<sub>m</sub>CoG (using m % of the maximum intensity of spot
as threshold), and T<sub>k</sub>CoG ( usingμ<sub>n</sub> +κσ n as the threshold), μ<sub>n</sub> and σ<sub>n</sub> are the mean value and deviation of back noise.
Firstly, their impact on the detection error under various SNR conditions is simulated respectively to find the way to
decide the value of k or m. Then, a comparison between them is made. According to the simulation result, T<sub>m</sub>CoG is
superior over T<sub>k</sub>CoG for the accuracy of selected threshold, and detection error is also lower.