基于调制传递函数的光学低通滤波器评价模型与实验研究

Study of modular transfer function-based optieal low-pass filter evaluation model and experiment

为了量化评价光学低通滤波器(optical low pass filter,OLPF),利用调制传递函数(modulation transfer function,MTF)建立了一个包含OLPF的二维数字光学成像系统模型,并以此模型为基础构建了OLPF的评价函数.应用该评价函数,计算三种典型OLPF的滤波特性,计算结果表明随着理想光学系统空间截止频率的增大,三种OLPF的滤波性能均有先微弱增强,后迅速减弱的趋势;当光学系统F数大于4.4时,采用嵌入"两层四点"式OLPF的数字成像系统的低通滤波效果优于另外两种,而当光学系统F数小于4.4时,采用嵌入"三层八点"式OLPF的数字成像系统效果最好;在光学系统F数为2.8时,"三层八点"式的OLPF相对于"两层四点"式和"三层四点"式的滤波性能分别改善了28.3%和37.5%.最后,三种OLPF的成像滤波实验表明"两层四点"式和"三层四点"式的滤波图像仍有部分莫尔条纹和伪彩色残余,而"三层八点"式的滤波图像的莫尔条纹和伪彩色均抑制得较好,实验结果与理论计算相符合.

To evaluate the performance of optical low pass filter （OLPF） accurately, a new 2D digital imaging system model including an OLPF was created for the first time using the modular transfer function （MTF）. An evaluation function was also established based on this model. To manifest the model and its evaluation, three typical OLPFs were studied. The results show that the A value of each filter undergoes a slow and slight decrease and then rapidly increases when the ideal optical system cutoff frequency is increased. The optieal system embedded with the 2-chip-4-point type OLPF created a better image than those with the other two OLPF＇s when the F number is greater than 4.4; whereas the OLPF of 3-chip-8-point type worked best when the F number is lower than 4.4. When the F number of the optical system is 2.8, namely, with a spatial cutoff frequency of 649 lp/mm, the filter of 3-chip-8-point type performs better than that of the 2-chip-4-point and 3-chip-4-point types. The Δ value of the first filter is 28.3 % and 37.5 %, lower than that of the other two filters, respeatively. That is, the filter performance is 28.3 % and 37.5% higher. Finally, spatial frequency test shows that the moiré fringe and false colors can be seen in the last two pictures, while in the first, these effects are well eliminated. The theoretical calculations agree with the experimental results.