Engineering and Technology

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Power conditioning of thermoelectric generated power using dc-dc converters: A case study of a boost converter
(University of Nottingham, 1930) Twaha, Ssennoga; Zhu, Jie; Yan, Yuying
The near exhaustion of non renewable energy resources such as fossil fuels followed by disastrous climatic changes have alerted the world to invest in alternative energy sources. Thermoelectric (TE) technology is responsible for innovating TE devices such as thermoelectric generators (TEGs) which are used to generate electrical energy from heat in an environmentally friendly manner. One of the challenges behind TEG is that they have low efficiency due to low figure of merit. Moreover the power generated is unstable and therefore needs proper power conditioning mechanism before it is connected to the load. The objective of this work is to analyze the performance of a boost dc dc converter connected on TEG system. The simplified models have been used as the basis of TEG design. Results indicate that the converter is able to stabilize and boost the voltage and higher converter efficiencies are achieved at different hot side temperatures.
A microcomputer-controlled solid-state dark adaptometer
(Behavior Research Methods & Instrumentatio, 1981) Omara, Peter A.; Zwick, Harry; Van Sice, Charles W.
A low-cost microprocessor-controlled light-emitting diode (LED) dark adaptometer is described. The apparent intensities of red and green stimuli are controlled by changing the duty cycles of LED display elements which are operated at constant pulse repetition rates. The psychophysical method of limits is used to obtain threshold data. Stimulus parameters and test procedures are under software control. The design utilizes programmable integrated circuitry that may be used with a variety of microprocessors.
Low-Temperature Migration of Silicon in Metal Films on Silicon Substrates Studied by Backscattering Techniques
(Journal of Vacuum Science and Technology, 2000) Hiraki, Akio; Lugujjo, Eriabu
The backscattering method using 2-MeV4He+ ions is employed to obtain microscopic information about solid-solid reaction of Si with thin layers (200∼4000 Å) of Au, Ag, and Al which are vacuum evaporated onto Si crystal substrates. The interesting observation is the migration of Si atoms into these metal films at temperatures (for example, 150°C in Au, 400°C in Ag) well below their eutectic points (375°C for Au and 850°C for Ag). This phenomenon also indicates that at these low temperatures the dislodgment of Si atoms from tightly bound Si crystal does occur. Our experiments clarify that the origin of this effect is the interaction of Si with metals at the interface.
Water sorption and mechanical behaviour of cosmetic direct restorative materials in artificial saliva
(Dental Materials, 2001) Musanje, L.; Shu, M.; Darvell, B.W.
To evaluate the water sorption and mechanical behaviour of a compomer in comparison with those of its nominal forerunners, a filled resin restorative material and a conventional glass ionomer cement. Compomer (Dyract AP) (D-AP), filled resin (SureFil) (SF), and glass ionomer (ChemFlex) (CF) (all Dentsply, Addlestone, UK) restorative materials were tested. Forty bar specimens (26×1.5×1.0mm3) of each material were prepared according to the manufacturer's instructions and randomly distributed into eight groups: dry air (22% RH), saturated water vapour (WV) (100% RH), and five in artificial saliva (AS) at pH6, all at 37°C, as well as untreated control (UC) (23°C, 50% RH). Water sorption was assessed gravimetrically; flexural strength and elastic modulus were determined in three-point bend. The control group was tested at 24h; AS groups were separately tested after 0.5, 1, 3, 6 and 9 months; the other two at 9 months. Mass gain for SF, D-AP and CF in AS was up to 0.17%, 1.2% and 7.0%, respectively. CF showed a marked decrease of strength in AS compared with other groups, followed by a gradual slight rise to a peak at 3 months. Unlike SF and CF, whose flexural strength remained relatively stable, that of D-AP showed a sharp decline from the 1 month peak (P=6×10−7) after 6 months in AS. D-AP also showed a slight decline in flexural modulus from a peak, that of SF was quite stable, while CF showed no peak. The values of flexural strength for both CF and D-AP at 9 months were significantly lower in AS than WV, but SF showed no such difference. Materials intended for service in the mouth must be stored in a realistic medium if the results of testing are to be interpretable. Dyract AP, a compomer, does not seem suitable for application in stress bearing areas as is currently recommended by its manufacturer. The rapid decline in flexural strength after 1 month of exposure to AS and its progressive fall in flexural modulus suggest a progressive deterioration of the material and this necessitates re-examination of the chemistry of compomers, if the behaviour is typical of the class.
Defining visualization operations for temporal cartographic animation design
(International journal of applied earth observation and geoinformation, 2002) Ogao, P.J.; Kraak, M. J.
Cartographic animation has emerged as a potentially effective visualization technique that has an intuitive power in representing dynamic geographical phenomena through its ability to show interrelations amongst geospatial data’s components, location, attribute and time. Whereas cartographic animation has prominently featured in communicating geospatial information, their use as tools for visual exploration has been hampered by lack of the necessary functionality that is capable of allowing users to interact with the dynamic display. In this paper, we outline an approach that defines visualizations operations or basic visual actions that implement a viewer’s task of exploration and characterization of geospatial structures in data or phenomena. The defined operations go along to reinforcing the quest in enabling users to perceive relationships and be able to manipulate geospatial data using more efficient visual tools while keeping low on cognitive demands.
Solution of large-scale weighted least-squares problems
(Numerical linear algebra with applications, 2002) Baryamureeba, Venansius
A sequence of least-squares problems of the form miny G1=2(ATy−h) 2, where G is an n×n positive definite diagonal weight matrix, and A an m×n (m6n) sparse matrix with some dense columns; has many applications in linear programming, electrical networks, elliptic boundary value problems, and structural analysis. We suggest low-rank correction preconditioners for such problems, and a mixed solver (a combination of a direct solver and an iterative solver). The numerical results show that our technique for selecting the low-rank correction matrix is very effective. Copyright ? 2002 John Wiley & Sons, Ltd.
Formation of silicon oxide over gold layers on silicon substrates
(Journal of Applied Physics, 2003) Hiraki, Akio; Lugujjo, Eriabu; Mayer, J. W.
When a single‐crystal substrate of silicon is covered with evaporated gold and heated at relatively low temperatures (100–300°C) in an oxidizing atmosphere, a silicon‐dioxide layer is readily formed over the gold layer. The mechanism and factors controlling this low‐temperature oxide formation have been investigated using backscattering of 2‐MeV He+ ions. The oxide layer is nonuniform in thickness and the initial growth of this layer is proportional to (time)1/2. Both oxidizing ambient and orientation of the substrate influence the growth rate, and the amount of gold determines the final thickness of oxide. A model is proposed to explain the oxide‐growth mechanism.
Polymerization of resin composite restorative materials: exposure reciprocity
(Dental Materials, 2003) Musanje, L.; Darvell, B.W.
To examine whether there is reciprocity between irradiation time and irradiance with regard to the mechanical properties of filled, resin composite restorative materials (RCs). Four visible light-cured RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (1.0×1.5×16.0 mm3) were cured at irradiances (I) ranging from 25–1500 mW/cm2 and irradiation times (t) of 1–3000 s. Six specimens at 250 combinations of t and I were prepared and stored in artificial saliva of pH 6, at 37 °C for 7d before performing three-point bend tests for flexural strength (F), flexural modulus (E) and total energy to failure (W). Contour plots of property value vs. t and I on log scales were prepared. Results. The contour plots showed three regions: unset at low I·t, a plateau corresponding to more or less full property development, and connecting ramp. The boundary between the plateau and the ramp suggests the minimum acceptable exposure. No practical lower limit to irradiance was detected, but there may be no benefit from increasing I beyond about 1000 mW/cm2. The slopes of the contours in the log–log plots provided a test of the hypothesis of reciprocity. These slopes were ∼−1.5 for HR, TC and PR; and ∼−1 for FZ, compared with the expected value of −1. The general hypothesis therefore fails. The existence of localized maxima in property values is further evidence of that failure, even for FZ. Significance. Dentists may use any lamp, including LED sources, and attain satisfactory results providing irradiation time is long enough. Manufacturers ought to supply a graph indicating the minimum acceptable exposure for each product for specified curing lamps. Calculations based on total energy delivered to guide irradiation protocols are invalid and do not recognize product behavior.
Using Java Speech Grammars as Cache Memory for Databases in VoiceXML Applications
(Research Gate, 2003) Obono, S. D. Eyono; Serumaga-Zake, M. J.
VoiceXML applications capture tokens input by users and match them against a specified grammar. But the Java Speech Grammar Format does not provide for a mechanism to capture tokens unmatched by a grammar. In this paper, we show how can a VoiceXML application capture tokens unmatched by a Java Speech Grammar. We also show how to dynamically update a Java Speech Grammar so that it can recognize tokens previously unmatched. We subsequently show how such captured tokens can interact with a relational database and how to transform a grammar into a cache memory for the database.
Aspects of water sorption from the air, water and artificial saliva in resin composite restorative materials
(Dental Materials, 2003) Musanje, L.; Darvell, B.W.
Primarily to establish whether artificial saliva (AS) at 37 °C is essential as a clinically relevant environment for testing filled, resin composite restorative materials. The effect of other storage conditions was also investigated for comparison and controls: desiccation, exposure to the laboratory atmosphere, high humidity cabinet, saturated water vapor, and deionized water. Two visible light-cured products were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein). Bar specimens (26×1.5×1.0 mm3) were cured at five overlapping spots for 60 s per spot and randomly distributed into groups of six. Trial 1: one group of each material was exposed first to atmospheric air at 24 °C, ∼50% RH (24WV50), then to water vapor at 37 °C, ∼97% RH (37WV97), and then immersed in deionized water at 37 °C (37DW). Trial 2 used three groups of each material, one first exposed to 37WV97 followed by 37DW, the other two were immediately immersed in 37DW or artificial saliva (37AS). Trial 3: two groups of each material were vacuum desiccated at 37 °C, then exposed to 37 °C, ∼100% RH (37WV100), then immersed in 37DW or 37AS. Trial 4: four groups of HR were treated similarly to Trial 3; one was left under desiccation, and another in 37WV100 for the remaining period. Three-point bend tests for flexural strength (F), flexural modulus (E), and total energy to failure (W) were performed at the end of Trials 2–4. Environmental moisture absorption was substantial at 24WV50(c. 0.2%), at least 40% of that in 37DW (HR: c. 0.7%, TC: c. 0.5%). Saturation was achievable in 37WV100. Mass loss on desiccation (HR: c. 0.4–0.5%, TC: c. 0.25%) was reversible in 37WV100. There were some significant effects of exposure conditions on mechanical properties (e.g. F for HR: after desiccation, 85.7±1.4 MPa; after 37WV100, 73.2±3.6 MPa; difference: p<0.0002), but overall the results were unclear. After a rapid gain in mass, there was a gradual loss in both 37DW and 37AS for both materials, slightly more in 37AS than 37DW. Water vapor absorption is substantial, hence attention must be paid to the laboratory working environment and conditions of storage and testing, i.e. temperature and RH must be stated to assist interpretation of data and comparisons between studies. Test conditions need to be standardized and with reference to normal oral conditions, immediate immersion in artificial saliva at 37 °C is the preferred treatment for these materials, whatever time of testing is chosen.
Optical density and depth of cure in visible light-cured filled-resin dental restorative materials
(POLYMER PREPRINTS-AMERICA, 2004) Darvell, B. W; Musanje, L.
The identification of the correct exposure (irradiance × time, I.t) of a direct-placement, visible light-cured, filled-resin dental restorative material in order to achieve “complete” reaction is of continuing concern. Essentially, in order to attain the intended set of mechanical properties, and thus (presumably) maximize the service life of the restoration, the degree of conversion of reactable vinyl groups must also be maximized. This involves, typically, a diketone-amine photosensitized free-radical polymerization system, irradiated by light of wavelengths in the region of 400 – 500 nm, and then in which a kinetically-complicated series of processes occur in a highly viscous, non-isothermal medium, approaching the glassy state as reaction proceeds. Reaction does not come to a stop on cessation of irradiation but continues for some time afterwards. There is economic pressure on the dentist to minimize the time spent performing the irradiation, and thus a concomitant drive to increase the irradiance to achieve this, frequently on the erroneous assumptions that total energy is the criterion and that reciprocity holds
Training for Improved Decentralized Service Delivery – A case study from Uganda
(WEDC, Loughborough University, 2004) Niwagaba, B.C.; Nalubega, M.; Kulabako, R.
The introduction of decentralization and decentralized service delivery in Uganda placed an increased demand for qualified personnel at the districts and lower levels. Despite this, there were no efforts in the past to enlighten students and trainers at Makerere University about the existing gaps and opportunities available in districts so that the training could be tuned to the needs of the districts. In the year 2000, the Department of Civil Engineering in Makerere University started a pilot project to attach students to the technical departments of the district local governments to undertake internship training for a period of 8 weeks. The students were supervised by staff from districts and the Department of Civil Engineering, Makerere University. The project was financed by the Rockefeller foundation and the World Bank. Since then, 72 students (11 during the pilot and 61 during the full implementation phase) have undergone internship training in 9 districts in Uganda. Eleven (11) Makerere staff and over 90 district staff were involved in the project. A training manual was developed, relevant research to solve technical problems and assist the districts in improved service delivery was identified, and efforts are under way to evaluate the project and quantify the impact.
Effects of strain rate and temperature on the mechanical properties of resin composites
(Dental Materials, 2004) Musanje, L.; Darvell, B.W.
To evaluate the effects of strain rate and temperature on the mechanical properties of resin composite restorative materials (RCs) and to investigate the construction of temperature—strain rate equivalence ‘master curves’. Four visible light-cured resin composite RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy Condensable (PR) (Kerr, Orange, CA, USA). Bar specimens were cured for 50 s at an irradiance of 500 mW cm−2 and were randomly distributed into groups of six for each type of material. All specimens were stored in artificial saliva at pH 6, for 7 d. The specimens tested at 12, 24 and 37 °C were stored at the corresponding temperature but those tested at 0 °C were stored at 24 °C. Three-point bend tests for flexural strength (F), flexural modulus of elasticity (E) and total energy to failure (W) were performed at cross-head speeds (XHS) of 0.1, 1.0, 10, 50 and 100 mm min−1 for all materials as well as at 0.01, 0.03, 0.2 and 0.5 mm min−1 for some materials. There was a common pattern of behavior across materials. At constant temperature, F showed a slight variation with cross-head speed, with a broad peak in the region of 1–10 mm min−1. E, on the other hand, showed a more marked and steady increase with XHS at all temperatures except at 0 °C, where it tended to level off above about 10 mm min−1. In contrast, the values of W showed a decline with increasing XHS, except at 37 °C where an initial rise followed by a decline was observed. At constant XHS, increase in temperature caused a small, but highly significant (P<10−3) decline in F but a marked decline in E. W, again in contrast to F and E, showed a general increase with temperature. A master curve model for the temperature–strain-rate equivalence was fitted to the E and W data (all P<10−5) and the fitted parameters interpreted in terms of strain rate and temperature sensitivity.
Geochemical Significance of Seepage Oils and Bituminous Sandstones in the Albertine Graben, Uganda
(Journal of Petroleum geology, 2004) Lirong, Dou; Dingsheng, Cheng; Rubondo, Ernest N. T.; Kasande, Robert; Byakagaba, Abdul; Mugisha, Frank
Geochemical analysis of surface samples comprising three seepage oils, two bituminous sandstones and five shales collected from the Ugandan portion of the Albertine graben (a Cenozoic rift basin in Eastern Africa) indicates that the basin may have some petroleum potential. The analyses show that seepage oils from the Kibiro and Paraa localities were derived from Types I or II1 source rocks, deposited in semi-deep or deep-water lacustrine environments containing abundant freshwater algae. However, the seepage oil from Kibuku is derived from a Type III source rock. A comparison of the seepage oils and the bituminous sandstones suggests that there are at least two sets of mature or threshold-mature source rocks in the Albertine graben — one in the Upper Tertiary and the other in the underlying (pre-Upper Tertiary) succession. According to molecular indicators of maturity, the Kibiro seepage oil was generated during peak maturation, whereas the Paraa and Kibuku seepage oils are of lower or threshold maturity. This work was completed under the supervision and guidance of Qingping Li, Chief Engineer of CNODC; Xiaoguang Tong, Chief Geologist of CNODC; and Reuben Kashambuzi, Commissioner, PEPD, MEMD, Uganda. The Embassy of the People's Republic of China in Uganda assisted with the completion of the project. Professors Tieguan Wang and Lihua Chen reviewed the research results, respectively. CNODC and the Ugandan authorities (PEPD) provided the favourable conditions for this project. Journal review was by Roger Swart (Namcor, Namibia) and Andrew Carr (Advanced Geochemical Systems Ltd) whose comments on the manuscript are acknowledged with thanks.
Effects of resin formulation and nanofiller surface treatment on the properties of experimental hybrid resin composite
(Biomaterials, 2004) Musanje, Lawrence; Ferracane, Jack L.
This study evaluated the effects of nanofiller surface treatment and resin viscosity on the early and long-term properties of experimental hybrid composites. Three resin formulations (low, medium and high viscosity) were prepared by varying the ratio of TEGDMA:UDMA:bis-GMA (47:33:16wt%; 30:33:33wt%; 12:33:51wt%). Composites contained 71.3wt% silanated strontium glass (1–3μm) and 12.6wt% of either silanated or unsilanated silica (OX-50; 0.04μm). Specimens (n=10) for flexural strength, flexural modulus, fracture toughness and Knoop hardness were tested after 24h, 1 and 6 months exposure to water at 37°C. Degree of conversion (DC) was determined 24h after photoinitiation using FTIR. Resin viscosity only had a marginal influence on the mechanical response of composites but it can be adjusted to achieve a balance between DC and mechanical properties. Adding non-bonded nanofiller to hybrid composites had no systematic effect on DC. Non-bonded nanofillers had no significant effect on the long-term properties of hybrid composites.
Geological Conditions and Petroleum Exploration Potential of the Albertine Graben of Uganda
(Acta Geologica Sinica‐English Edition, 2004) Lirong, Dou; Dingsheng', Cheng; Rubondo, Ernest N.T.; Kasande, Robert; Byakagaba, Abdul; Mugisha, Frank
The Albertine Graben in western Uganda is a Mesozoic-Cenozoic rift basin with petroleum exploration potential. A fundamental evaluation of petroleum potential of the graben is given based on field research, data processing of gravity and magnetism, analysis of graben structure, geochemistry, reservoir and composition research. The basin has a double-layered framework and a large thickness of sediments. Gravity highs shown in a residual anomaly map might indicate central uplift zones. There exist at least two sets of mature or low-maturity source rocks corresponding to a certain source rock in the Cretaceous or Paleogene and Neogene strata. The graben has basement rock with potential reservoirs and Tertiary sandstone reservoirs and thus has petroleum exploration potential.
Cryptosporidiosis and Microsporidiosis in Ugandan Children with Persistent Diarrhea with and Without Concurrent
(The American Society of Tropical Medicine and Hygiene, 2005) Bakeera-kitaka, Sabrina; Tumwiine, James K.; Ketiniinwa, Addy; Ndeezi, Grace; Downning, Robert; Xiaochun, Feng; Akioshi, Donna K.; Tzipori, Saul
Cryptosporidium spp. and Enterocytozoon bieneusi are enteric pathogens that have emerged as significant causes of persistent diarrhea (PD) in immunologically compromised individuals particularly in association with HIV/ AIDS. We conducted a cross-sectional study on the clinical epidemiology of E. bieneusi and Cryptosporidium in children with PD, with and without HIV/AIDS, attending Uganda’s Mulago National Referral Hospital. Two hundred forty-three children aged < 60 months, admitted between November 2002 and May 2003 with PD (> 14 days), were analyzed for HIV status and CD4 lymphocyte counts, and stools were screened for the presence ofE. bieneusi and Cryptosporidium by microscopy and positive samples genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. Eighty (32.9%) of the children were excreting E. bieneusi, and 76 (31.3%) were excreting Cryptosporidium. Ninety-one of the 243 children had HIV, of who 70 (76.9%) had E. bieneusi, versus 10 (6.6%) of the 152 without (odds ratio 47.33; 95% CI 19.88 to 115.97), while 67 (73.6%) had Cryptosporidium, versus 9 (5.9%) without (odds ratio 44.36; 95% CI 18.39 to 110.40). Children with counts < 25% CD4 cells were more likely to have either E. bieneusi (odds ratio 7.42; 95% CI 3.77 to 14.69) or Cryptosporidium (odds ratio 6.45; 95% CI 3.28 to 12.76) than those with higher CD4 percentages. However, only HIV status was independently associated with either Cryptosporidium or E. bieneusi. Among the 243 children with PD, 67 (27.8%) were infected with both enteric pathogens, with HIV being the only independent predictor of coinfection. Finally, some 81% of HIV-infected children with PD excreted one or both organisms, compared with only 10% of children with PD testing negative for HIV. Seventy-four percent of isolates were C. hominis, the anthroponotic species, 17% were C. parvum, the zoonotic species, and 8% were a mixture of the two or others.
Frequency Adjustment of Microelectromechanical Cantilevers using Electrostatic Pull Down
(Journal of Micromechanics and Microengineering, 2005) Kafumbe, S. M. M.; Burdess, J. S.; Harris, A. J.
In this paper, a new way of actively tuning the resonant frequency of vibrating microelectromechanical devices by electrostatically adjusting the length of the resonating structure is explored. Variations in micromachining processes cause submicron differences in the size of fabricated micromachined devices, which lead to frequency variations in resonators. For radio frequency (RF) applications where high frequency selectivity and low noise frequency manipulation are key performance issues, micromachined resonators need to output a fixed frequency if they are to replace current off-chip, passive devices. This motivates the investigation of post-fabrication techniques that compensate for fabrication defects and errors, and shift the resonant frequency to its designed value. A simple universal analytical model has been developed to investigate the different states a cantilever undergoes during pull-in due to an applied voltage. The beam's natural frequencies throughout these states have been plotted. It is shown that the frequency can be changed by a factor of 4 during pull down, and thereafter increased proportionally with actuation provided an initial minimum voltage was applied.
Towards A Sanitation Selection Algorithm For Enhancing Decentralized Service Delivery
(WEDC, Loughborough University, 2006) Niwagaba, B. C.; Kinobe, J. R.; Atwine, E.; Kisaka, J. N.
In Uganda, sanitation coverage is estimated at 53% and 39% for urban and rural areas respectively. The national coverage is 41%. Lack of proper sanitation potentially leads to environmental health problems, which in many cases cost lives and impact on health of a community and family income as more money is spent on medication. This leads to a vicious circle of poverty. The objective of this study was to collect information on the current practices in selection of sanitation arrangements and use it to develop a simple algorithm for use by decision makers and district staff to advise households on selection of appropriate sanitation systems. Currently, there is no streamlined criterion used. People select systems based on what they are used to. Consequently, traditional pit latrines are the commonest sanitation system used. These toilet systems however, are disadvantageous due to: difficult soils (rocky, collapsing formations and areas with high water table); when full, require that new pits are dug, which is expensive and in the dense settlements this is inhibited by lack of space for new pits. As a starting point, we have proposed a simple algorithm that can be used by decentralized districts to give guidance to households in the selection of sanitation systems. The principle of the sanitation ladder, where people choose from the whole range of options, and select systems based on site conditions, affordability as well as user acceptance and perceptions applies. At the next phase, we intend to carry out detailed consultations to get specific information on user preferences, develop costs for all categories and package the information in an easy to use document for awareness creation, advocacy and promotion of sanitation.
Curing-light attenuation in filled-resin restorative materials
(Dental Materials, 2006) Musanje, L.; Darvell, B.W.
To characterize the attenuation of the curing light in filled resin restorative materials (FRRMs) to aid understanding of curing depth. One hundred and eighty materials of various shades from several manufacturers were tested in various ways. One set (66 materials) was used to determine the applicability of Lambert's Law using a quartz-tungsten-halogen curing light (Optilux 400, Demetron Research) by measuring the transmitted light with a dental radiometer (Cure Rite, EFOS) for successive thicknesses of ground 10mm diameter specimens from 3 to 0.5mm in 0.5mm steps. A second set (17 materials) were similarly tested with separate specimens from 1 to 5mm in thickness using a transmission densitometer (DT1405, RY Parry) fitted with a curing-light dichroic filter. For a third (overlapping) set (165 materials), the 1mm pure (reflectance-free) optical density (D1 value) was determined from two specimens, ∼1 and ∼2mm thick using the densitometer as above. From D1 the critical thickness (xCRIT), identified as depth of cure (DoC) for an excess surface exposure factor of 2, was calculated. Lambert's Law was found to hold with no evidence of appreciable differential absorption effects. Attenuation coefficient and D1 were significantly correlated (P<1×10−13). D1 varied between about 0.23 and 0.72, for corresponding xCRIT values of 1.3 and 0.4mm. There was no correlation between D1 and reflectance (P>0.09), and no systematic effect due to shade letter, but a highly significant (P<7.5×10−8), but weak (−0.066mm/unit), correlation between shade number and D1.

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