Center deviation of optical components Definition and terminology

1 Principles of optical films

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The center deviation of optical elements  is a very important indicator of lens optical elements and an important factor affecting the imaging of optical systems. If the lens itself has a large center deviation, then even if its surface shape is processed particularly well, the expected image quality still cannot be obtained when it is applied to an optical system. Therefore, the concept and testing of the center deviation of optical elements are Discussion with control methods is very necessary. However, there are so many definitions and terms about center deviation that most friends do not have a very thorough understanding of this indicator. In practice, it is easy to misunderstand and confuse. Therefore, starting from this section, we will focus on spherical surface, aspheric surface, The definition of the center deviation of cylindrical lens elements and the test method will be systematically introduced to help everyone better understand and understand this indicator, so as to better improve the quality of the product in actual work.

2 Terms related to center deviation

In order to describe central deviation, it is necessary for us to have an early understanding of the following common sense terminology definitions.

1. Optical axis

It is a theoretical axis. An optical element or optical system is rotationally symmetrical about its optical axis. For a spherical lens, the optical axis is the line connecting the centers of two spherical surfaces.

2. Reference axis

It is a selected axis of an optical component or system, which can be used as a reference when assembling the component. The reference axis is a definite straight line used to mark, check and correct the center deviation. This straight line should reflect the optical axis of the system.

3. Reference point

It is the intersection point of the datum axis and the component surface.

4. The inclination angle of the sphere

At the intersection of the datum axis and the component surface, the angle between the surface normal and the datum axis.

5. Aspheric tilt angle

The angle between the rotational symmetry axis of the aspheric surface and the datum axis.

6. Lateral distance of aspheric surface

The distance between the aspherical surface's vertex and the datum axis.

3 Related definitions of center deviation

The center deviation of the spherical surface is measured by the angle between the normal of the reference point of the optical surface and the reference axis, that is, the inclination angle of the spherical surface. This angle is called the surface inclination angle, represented by the Greek letter χ.

The center deviation of the aspheric surface is represented by the inclination angle χ of the aspheric surface and the lateral distance d of the aspheric surface。

It is worth noting that when evaluating the center deviation of a single lens element, you need to first select one surface as the reference surface to evaluate the center deviation of another surface.

In addition, in practice, some other parameters can also be used to characterize or evaluate the size of the component center deviation, including:

1. Edge run-out ERO, which is called Edge run-out in English. When the component is adjusted, the greater the run-out in one circle of the edge, the greater the center deviation.

2. Edge thickness difference ETD, which is called Edge thickness difference in English, is sometimes expressed as △t. When the edge thickness difference of a component is large, its center deviation will also be larger.

3. Total run-out TIR can be translated as total image point run-out or total indication run-out. In English, it is Total image run-out or Total indicated run-out.

In the early customary definition, the center deviation will also be characterized by the spherical center difference C or the eccentricity difference C,

Spherical center aberration, represented by the capital letter C (sometimes also represented by the small letter a), is defined as the deviation of the geometric axis of the outer circle of the lens from the optical axis at the center of curvature of the lens, in millimeters. This term has been used for a long time It is used for the definition of center deviation, and it is still used by manufacturers so far. This indicator is generally tested with a reflective centering instrument.

Eccentricity, represented by the lowercase letter c, is the distance between the intersection point of the geometric axis of the optical part or assembly being inspected on the node plane and the rear node (this definition is really too obscure, we do not need to force our understanding), in numerical terms On the surface, the eccentricity is equal to the radius of the focal image beat circle when the lens rotates around the geometric axis. It is usually tested with a transmission centering instrument.

4. Conversion relationship between various parameters

1. The relationship between surface inclination angle χ, sphere center difference C and side thickness difference Δt

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For a surface with center deviation, the relationship between its surface inclination angle χ, spherical center difference C and edge thickness difference Δt is:

            χ = C/R = Δt/D

Among them, R is the radius of curvature of the sphere, and D is the full diameter of the sphere.

2. The relationship between surface inclination angle χ and eccentricity c

When there is a center deviation, the parallel beam will have a deflection angle δ = (n-1) χ after being refracted by the lens, and the beam convergence point will be on the focal plane, forming an eccentricity c. Therefore, the relationship between eccentricity c and central deviation is:

          C = δ lf’ = (n-1) χ. lF’

In the above formula, lF’ is the image focal length of the lens. It is worth noting that the surface inclination angle χ discussed in this article is in radians. If it is to be converted into arc minutes or arc seconds, it must be multiplied by the corresponding conversion coefficient.

5 Conclusion

In this article, we give a detailed introduction to the center deviation of optical components. We first elaborate on the terminology related to this index, thereby leading to the definition of center deviation. In engineering optics, in addition to using the surface inclination angle index to express the center deviation, , the edge thickness difference, spherical center difference and eccentricity difference of components are also often used to describe the center deviation. Therefore, we have also described in detail the concepts of these indicators and their conversion relationship with the surface inclination angle. I believe that through the introduction of this article, we have a clear understanding of the central deviation indicator.

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Post time: Apr-11-2024