Elsevier

Optics and Lasers in Engineering

Volume 62, November 2014, Pages 46-56
Optics and Lasers in Engineering

Study on the optical reflection characteristics of surface micro-morphology generated by ultra-precision diamond turning

https://doi.org/10.1016/j.optlaseng.2014.04.017Get rights and content

Highlights

  • A new method for valuating reflector surface finish turned by SPDT is provided.

  • We get the relationship among surface finish, optical defect and turning condition.

  • The analysis is conducted by using scattering theory and interference theory.

  • The method can help to characterize machining defects by SPDT.

  • The method can help to obtain more appropriate turning conditions.

Abstract

Ultra-precision single point diamond turning technology has been widely adopted to machine high precision reflectors. Diamond turned surfaces have special micro-morphology, which affect reflecting performance greatly. To reduce optical defects, the relationship between optical defects, surface micro-morphology and turning conditions will be analyzed theoretically, using the interference method and integral method, for turning reflectors. And turning experiments will be conducted to validate the analysis. The study can provide a new methodology for evaluating surface finish and it is with guiding significance for production and processing.

Introduction

The ultra-precision single point diamond turning (SPDT) has been used to generate optical surfaces directly for a long time [1], [2], [3]. Compared with grinding and polishing, the high efficiency and precision repeatability is achievable. To get a better surface quality and machine surface with a higher efficiency, there are considerable researches [4], [5], [6] engaging in analyzing diamond turned surface micro-morphology and improving surface quality, and selecting more suitable turning parameters to get a better surface.

Optical surfaces have specific optical parameters. For reflectors, one kind of optical device, it is important to assess the reflecting performance. Machining factors, such as tool wear, vibrations, materials effect, etc., affect the performance. Li et al. [7], [8] systemically analyze how different machining factors, such as spindle speed, feedrate, tool condition selection affect surface micro-morphology turned by fly cutting, fast tool servo, slow tool servo, tool broaching, and affect reflectivity and diffraction. This research provides a thought for optics industry on how to decrease optical defects by optimizing machining parameters.

Conical reflector is one kind of reflector. It has the functions of generating omnidirectional imaging, that is, it can produce full circle 360° panoramic image [9], so it has been used in many fields such as mechanical, medical, astronomy [10], [11], [12]. With the requirement of high-quality imaging, the high surface precision and good roughness of conical surface is necessary. In conical reflectors machining, optical defects of conical reflectors are detected, which impede machining efficiency improving. How to decrease optical defects is critical.

To analyze what factors cause reflector optical defects in machining, interference theory and random rough surface scattering theory is used. Many typical studies have been focused on the rough surface scattering and they have displayed various methods to analyze the surface scattering [13], [14], [15]. And there are methods using in some commercial electromagnetic simulation software, like COMSOL.

The purpose of this study is to provide a methodology on how to analyze optical defects of reflectors both theoretically and practically, and how to select turning conditions to turn reflectors with high quality, and reduce rejection rate. Conical reflector is used as a major example to conduct analysis. In Section 2, several typical defects of conical reflector are described. Multiline phenomenon is analyzed both theoretically and experimentally in Section 3. Interference method based on interference theory and integral method based on rough surface scattering theory are used in analyzing multiline defect. And stray light and blurred line phenomena are analyzed preliminary in Section 4. In the last section, conclusion and future work are shown. The study provides a new thought to assess machined optical surface and help to machine surface with better surface finish.

Section snippets

Problems description

As shown in Fig. 1, it is a device to detect optical defects of conical reflector. Fig. 1(a) is the schematic diagram to detect defects. Light source is laser diode that emits laser with 650 nm wavelength shown in Fig. 1(b). The performance of the reflecting image on the screen is supposed to be one clear and uniform line. Several typical optical defects will be discussed below.

The defective reflecting image on the screen can be observed when the conical surface of reflector is not good enough.

Analysis of multiline phenomenon

Multiline phenomenon is analyzed in this section. It is caused by regular and periodic surface micro-morphology. Firstly, the turned surface model is built. Secondly, scattering of ideal SPDT surface is analyzed using interference theory to calculate critical feedrate (μm/r) that can machine reflectors without multiline, and the integral of random rough surface scattering theory based on Kirchhoff approximation [13] is used to calculate the surface scattering characteristic precisely. Finally,

Analysis of stray light and blurred line phenomena

In this section, stray light and blurred line defects are analyzed. These two phenomena are with similar mechanisms in our research. When turning conditions is not good enough, these two phenomena will appear. Since turning conditions is complex, including parameters selection, environment, etc., it is conducted a preliminary and brief analysis.

Conclusions and future work

In this paper, several kinds of conical reflector׳s optical defects by SPDT are analyzed. The scattering characteristic of a reflector surface is closely related with its surface micro-morphology, and micro-morphology relies on machining conditions. We provide a new methodology to evaluate surface micro-morphology via its optical characteristics, which can be characterized by several parameters, such as scattering distribution function. This methodology can also help to characterize the

Acknowledgment

The authors appreciate supports of the National Basic Research Program of China (973 Program, Grant no. 2011CB706700), the National Natural Science Foundation of China (Grant no. 51275344). Support is also acknowledged by the 111 Project (B07014).

References (19)

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