《应用光学》课程教学课件 chapter5 Selection of Imaging Rays.pptVIP

Why do we have to select imaging rays? The position of the imaging rays will determine the diameters of each of the elements in the optical systems. According to, The product of the slope aperture angle and the refractive index nu in object space is called the numerical aperture and is expressed by NA. For microscope, n’=1 To ensure the diameter of emergent rays , we can get D’≥1mm thus, NA≥Γ/500 Methods of increasing NA: NA=nu 1. To enlarge u; 2. To Increase n. We can immerse the objective into liquid with high index, such as oil . NA is always wanted to be larger. 1.NA= D’ Γ/500. Suppose D’ is constant，if we want to increase Γ, we need to enhance NA. Suppose Γis constant , if we want to increase D’, we also need to enhance NA. 2.NA is related with the resolution of a microscope. The larger the NA is, the higher the resolution is. For example, if a microscope adopts an eyepiece with the magnifying power of 15x, and an objective with the lateral magnification of 3x, then the magnifying power of the microscope will be: Generally the NA of a 3x objective lens is 0.1, and the eyepieces with NA =5X,6X,10X,15X, can all match with the objective with NA=0.1. 2. Selection of rays in the measuring microscope 1) The microscope which can measure the height of objects: 2y 2y′ As the distance between the objective and the reticule is fixed, the corresponding β is then a constant . we can get the height y of object by measuring the height of image y’. y=y′/β The object plane must be positioned precisely in order to make the image plane fall just on the reticule, or else, there will be some errors. A1 B1 A B B′ A ′ B1′ A1′ y1′ y′ If the object plane is not positioned precisely , The cause of errors: What we measure now is the distance between the centers of the two blur spots, and the centers is the intersections of the chief ray and image plane. As the aperture stop is still on the