Research On Characteristics Analysis And New Design Of Non-rotationally Symmetric Phase Masks Used To Extend The Depth Of Field

As is widely known,any imaging system has limited depth of field whose corresponding concept is called depth of focus in image space.Only the objects within the depth of field can be imaged clearly,and other parts being out of the range will be blurred in the image,which leads to the loss of information.The depth of field can be improved to a certain extent by simply decreasing the radius of aperture plane. However,this will sacrifice the system resolution.In 1995,Dr.E.R.Dowski in University of Colorado proposed a novel technique called wave-front coding and solved the dilemma encountered in the conventional imaging system wonderfully.By introducing a suitable phase mask into the aperture plane,the imaging system can be made insensitive to focus errors,defocus related aberrations and the factors causing defocus,such as temperature variation and mechanical instability and so on.It is this modification that greatly extends the depth of field.However,the captured images by wave-front coding system are purposely coded intermediate images and have to be decoded to generate the clear images by using image restoration techniques.Being different from the conventional imaging system,no matter what the object distance is, the defocused point spread function is nearly the same.So,the objects at different distances will be uniformly blurred and this makes the digital post processing easier.In a word,the wave-front coding system contains two pars:optics and digital filtering.Excellent phase masks and digital restoration methods are the keys to the amazing performance of wave-front coding system.So far,many kinds of phase masks have been suggested,including rotational symmetrical and non-rotational symmetrical.Considering the better performance of non-rotational symmetrical phase masks in enlarging the depth of field,the dissertation majors in several aspects following below,in which the methods adopted and results obtained demonstrate the novelties:First,by analyzing the characteristics of existing phase masks considering five main aspects,useful instructions are provided when one decides to use the wave-front coding technique,and this is the first time that comprehensive assessments are given. At the same time,it is worth noting that the rarely discussed question of the influence of fabrication precision on system characteristics has been paid special attention and the corresponding results are not only interesting,but also meaningful.Second,studies on analytical approximate computation of optical transfer function corresponding to the wave-front coding system have been carried out and a method based on fitted polynominal equation is suggested to be used to simplify the complicated mathematical derivations.This method is especially useful to the phase masks whose approximate defocused transfer function can not be obtained by using the computation routine suggested by E.R.Dowski.Third,by introducing the concept of composite phase masks,new phase masks which can be called sinusoidal-added phase masks have been designed and this enriches the family of phase masks.Lots of simulated and experimental results demonstrate that these new phase masks not only own the ability of enlarging depth of filed,but also are less sensitive to the orginal ones.Besides those,sinusoidal phase mask that is rarely refered in the literatures has been thoroughly investigated and very meaningful results have been obtained as well.Fourth,based on singlet wave-front coding system,which is low cost and has poor imaging quality,the effectiveness of wave-front coding technique and characteristics of several phase masks have been tested.This is very good for underlying application of wave-front coding technique in real cases and demonstrasts that this technique is still effective in extending the depth of field,even on the condition that imaging quality of the original system was relatively poor.