There are large variations between different previously published lightness difference experimental data sets. Two hundred and eight pairs of matt and glossy paint samples exhibiting mainly lightness differences were accumulated. Each pair was assessed about twenty times by a panel of fourteen observers using the grey scale method. The results were used to derive a new lightness difference formula (CII), and to a large extent, a possible new CIE lightness difference formula (CMC99). Both formulae were found to be more accurate than the typical deviation of an individual assessment from the mean of a panel of 20 observers, and outperformed the existing formulae using the present data set. The new CMC99 lightness difference formula is integrated into the new CIE colour difference equation CIEDE2000. The results also showed that special attention should be paid to measuring very dark samples. This is caused by poor instrument repeatability and inter‐instrument agreement in this colour region.

Ultrasonic machining (USM) technique has long been used for fabricating various patterns and drilling holes on brittle materials. However, the surfaces generated by USM are normally rather rough and covered by deep penetrated cracks. This has greatly limited USM being used in micro-machining and fine machining. This research aimed to study the surface integrity of the USMed surface and develop a feasible way to minimize the scattered cracks so that good surface finish could be achieved. Machining parameters such as type and concentration of abrasive particles, grit size, and feed rate were systematically investigated to check their influences on the surface obtained. A ‘multi-stage’ micro-USM process was developed in this study and surface with Ra value better than 0.2m was achieved using the proposed process.

The glass molding process (GMP) is regarded as a very promising technique for mass producing high precision optical components such as spherical/ aspheric glass lenses and free-form optics. However, only a handful of materials can sustain the chemical reaction, mechanical stress and temperature involved in the glass molding process. Besides, almost all of these mold materials are classified as hard-to-machine materials. This makes the machining of these materials to sub-micrometer form accuracy and nanometer surface finish a rather tough and expensive task. As a result, making mold life longer has become extremely critical in the GMP industry. The interfacial chemical reaction between optical glass and mold is normally the main reason for pre-matured mold failure. This research aimed to investigate the interfacial chemical reaction between various optical glasses, different anti-stick coating designs and several mold materials. The results showed that glass composition, coating design (composition, microstructure, thickness), environment (vacuum, air or in protective gas), reaction temperature and time could all have profound effects on the interfacial chemical reaction. Based on the results, a design developed specially for certain glasses is more likely to be the viable way of optimizing the effect of the protective coating.

Rotary ultrasonic machining (RUM) is considered to be a very effective and relatively accurate way to drill deep holes in brittle materials. Although brittle fracture (micro chipping) is the dominant material removal mechanism utilized by the RUM process, poor surface roughness and deep penetrated cracks are the consequence if the machining parameters are not properly controlled. To ensure the quality of the generated surface and to improve the process efficiency, efforts have been made in this study to correlate the material removal mechanisms, surface integrity and tool wear involved in the RUM process. Diamond-impregnated tools were used in the experiment and the ultrasonic vibration frequency was kept at 20 kHz. Three major material removal modes namely, impact mode, grinding mode and erosion mode were found to be the dominant removal processes at the tool tip, around the diamond wheel and around the steel sleeve respectively. It was also found that, during the grinding/erosion processes, the bonding material of the wheel was first eroded away and left big part of diamond grits well-exposed. Pull-out and/or fracture are normally the consequence of these exposed diamond grits due to the lack of support and protection.

physical and mechanical properties, is one of the most important materials used in the mechanical, telecommunication and optoelectronic industty. However, high hardness value and extreme briuleness have made diamond a yery dicacult material to be machined by conventional rnechanicat grinding and po]ishing. In the present study, thc microwave CVD method was employed to produce epitaxial diamond films on silicon single crystal. Reactive ion etching (RIE), laser ablation and thermo-chemical polishing experiments were then conducted on the obtained diamond fiIms, The underlying material removal mechanisms, microstructure ofthe machinedsurface and related machining conditions were also investigated. Lt was tbund that during the laser ablation, peaks of the diamond grains were removed mainly by the photo-thermal effects introduced by excimer laser. The diamond structures ofthe protruded diamond grains were transfbrmed by the laser photonic energy into graphite, amorphous diamond and amorphous carbon which were remoyed by the subsequent laser shots. Laser ablation could improve the surface rouglmess from above 1pm to around O.1pm in few minutes' time in this study, However scanning would be required to coyer a large area, and, as a consequence, it could be very time consuming. Thermo-chemical polishing, in the other hand, was proved to be able to remove the diamond film very effectively (4.8pm deep of diamond film was removed in 30 minutes when polishing at 5500C and 5.7mfs) and the removal rate increased with pQlishing speed, temperature and pressure. Gases such as 02, 02 1 CF4, 021 SF6 were used as the reactive gases in the RIE experiments and it was fbund that 02 and 021SF6 offered better results. Howeyer, the obtained eteh rates were higher at areas like grain boundary and sharp corners which made further improvement ofsurface rouglmess very difficult.