1. Introduction
The center deviation of optical elements is a crucial indicator in lens-type optical elements, and it is also one of the important factors affecting the imaging quality of optical systems. Even if the surface shape of the lens is processed very finely, if the lens has a large center deviation, the expected imaging effect cannot be achieved in the actual optical system. Therefore, it is of great practical significance to have a deep understanding of the concept, test methods and control measures of the center deviation of optical elements.
However, the definitions and terms of center deviation are numerous and complex, which leads to the lack of in-depth understanding of this indicator by many people, which is easy to cause misunderstanding and confusion. To this end, this section will systematically introduce the definition and test methods of the center deviation of spherical, aspherical and cylindrical lens elements to help everyone better understand this indicator and effectively improve product quality in actual work.
2. Terms related to center deviation
Before discussing center deviation in depth, it is necessary to first understand the definitions of some common terms.
2.1 Optical axis
The optical axis is a theoretical axis, which is the axis of symmetry of an optical element or optical system. For spherical lenses, the optical axis usually refers to the line connecting the centers of two spherical surfaces.
2.2 Reference axis
The reference axis is an axis selected in an optical element or system, usually used as a reference when assembling the element. The reference axis is a fixed straight line used to mark, check and correct the center deviation, which should be consistent with the optical axis of the system as much as possible.
2.3 Reference point
The reference point is the intersection of the reference axis and the element surface, usually used as a reference position for evaluating the center deviation.
2.4 Tilt angle of spherical surface
The tilt angle of the spherical surface is the angle between the surface normal and the reference axis at the intersection of the reference axis and the element surface.
2.5 Tilt angle of aspheric surface
The tilt angle of the aspheric surface is the angle between the rotational symmetry axis of the aspheric surface and the reference axis.
2.6 Transverse distance of aspheric surface
The transverse distance of the aspheric surface refers to the distance between the vertex of the aspheric surface and the reference axis.
3. Related definitions of center deviation
3.1 Center deviation of spherical surface
The center deviation of a spherical surface is usually measured by the angle between the normal of the optical surface reference point and the reference axis, that is, the tilt angle of the spherical surface. This tilt angle is usually represented by the Greek letter χ.
3.2 Center deviation of aspheric surface
The center deviation of an aspheric surface is represented by two main factors: the tilt angle χ of the aspheric surface and the lateral distance d of the aspheric surface.
It should be noted that when evaluating the center deviation of a single lens element, it is usually necessary to select one surface as a reference surface in order to more accurately evaluate the center deviation of the other surface.
3.3 Other parameters that characterize center deviation
In addition to the tilt angle and lateral distance, there are some common parameters that can be used to characterize or evaluate the center deviation of the element:
Edge run-out (ERO): When the element is adjusted, the greater the run-out of its edge, the greater the center deviation.
Edge thickness difference (ETD): It is represented by △t. When the edge thickness difference of the element is large, it usually means that its center deviation is also large.
Total run-out (TIR): It characterizes the total run-out of the image point or indication.
In the early definition, the center deviation can also be expressed by spherical center error C or eccentricity error c:
Spherical center error (C): The degree of deviation between the geometric axis of the outer circle of the lens and the optical axis at the center of the lens curvature, usually in millimeters. This indicator is usually tested by a reflective centering instrument.
Eccentricity error (c): The distance between the intersection of the lens geometric axis on the node plane and the rear node. It is usually tested by a transmission centering instrument.