DISCUSSION The final shade of the resin luting cement is highly influenced by the degree of conversion which in turn can be evaluated indirectly by surface hardness measurements whether using Vickers or knop indenter which will give good determination after 24 hours (9). Off course, curing the resin composite polymerization will continue in slow rate, sometimes a week (10,21), and that is why in the current study the measurements were taken after one week.Different CAD/CAM material thicknesses selected in this study represent the average preparation depths recommended by the manufacturer in different situations (1,5,16).Dual curing was introduced to insure full polymerization of resin composite luting cements in areas where light cannot reach with an efficient intensity. Yet, it was mentioned in literature that auto cure alone was incapable by itself to deliver full maturation of the resin composite cement in absence of photo initiated polymerization (15).Color differences can be evaluated visually or by color measurement devices. Most color measurement devices utilized in dentistry use the ?E from the Commission International d’Eclairage CIE (L*a*b*) color system to determine color differences or changes. In this scheme, color is measured in three coordinate dimensions of L*, which represents lightness (from white to black; similar to value), a*, which corresponds to the green-red axis (negative value indicates green; positive value indicates red), and b*, which corresponds to the blue-yellow axis (negative value indicates blue; positive value indicates yellow). Although the used CAD/CAM materials acts as a light barrier during photo initiation of resin luting cements, yet their translucency may play a major role, allowing sufficient delivery of curing wave length to the under lying maturing material. Therefore, a greater degree of conversion leading to harder resin with a more stable color after polymerization (22,23, 26,35). The intensity of the photo initiation unit may have played an important role, as the marked difference in the intensity between both QTH and LED light curing units used may have contributed in the results of the study. Visible light’s wavelength fall in the range between 4/10 to 7/10 micrometer, and CAD/CAM materials and resin cements used, can contain a wide range of filler particles sizes that are larger or smaller than this wavelength(29). Upon falling of light on the surface of filler particle larger than the wavelength of light, light diffuse occur as a result of its refraction and reflection infinitely on the surfaces those particles. A similar phenomenon occurs as a result of the porosity of CAD/CAM materials. The greater the refractive index difference between particle size and matrix of either materials, the greater the refraction and reflection of light, which leads to an opaque effect in CAD/CAM materials (30), and that is why the more translucent material (Vita Enamic) didn’t affect the intensity of the curing light as did the less translucent (28) (Lava Ultimate).Dual cured resin luting cements have a two different amines, one to react with benzoyl peroxide and another to react with camphorquinone. The amine that react with benzoyl peroxide is usually aromatic and more prone to degradation, whereas the coinitiator in the light polymerization system is usually aliphatic and more chemically stable, tending to cause less color variation (37). The color instability of Dual cured resin luting cements is attributed to the presence of unreacted benzoyl peroxide. The reaction between benzoyl peroxide and amine is not very efficient, as it depends on the physical encounter of these molecules for chemical interaction during polymerization. The mobility of the initiators becomes restricted as the cement matures and become more viscous, leaving unreacted radical in the polymer chain. (38)The quality of maturation of a resin luting cement is dependent on the use of a compatible light curing unit and with a proper light exposure time and intensity that is dictated by the thickness as well as the reactivity of the photo-initiator. It was found that hardness of a resin luting cement decreases as the thickness of CAD/CAM material increases (16,39,40). In our study by increasing the distance between the surface of the maturing resin composite luting cement and the photo initiation source, due to increasing the thickness of the CAD/CAM material a drop in the KHN was noticed.Dual cure resin luting cement was capable of producing higher KHN though out the study which may be attributed to progress in polymerization and maturation of the cement after the light initiation time has stopped and till the activity of free radicles is controlled by the growing highly cross-linked polymeric network (24). The extent of resin cement maturation change considerably among products under various curing conditions(27). Clinicians should optimize the materials choice taking into account the curing characteristics of the cements.Upon comparing the effect of different thicknesses of the two tested CAD/CAM materials, type of resin luting cement, and type of photo initiating devices on the hardness of the resin luting cement. A drop in KHN was noticed with increasing the CAD/CAM thickness, this drop is consistent with other studies(11,12) and also in agreement with Koch et al. who stated that the ease at which light flows though ceramics is proportional with their thickness.(13) The thickness of CAD/CAM material was considered as the primary factor determining the light transmittance available for polymerization(14,15).Results of the current study uncovered that increasing the opacity as well as the distance between the light source and the maturing resin luting cement have a negative effect on its ?E and VHN, and this was in agreement with litrature (36).CONCLUSIONWithin the limitation of this study, it could be concluded that, the thickness of the CAD/CAM material and the type of the resin luting cement had a higher effect on the ?E and VHN hence the degree of maturation of resin luting cement than did either the type of tested CAD/CAM materials or the curing units.