Importance Sampling for a Markov Modulated Queuing Network with Customer Impatience until the End of Service

For more than two decades, there has been a growing of interest in fast simulation techniques for estimating probabilities of rare events in queuing networks. Importance sampling is a variance reduction method for simulating rare events. The present paper carries out strict deadlines to the paper by Dupuis et al for a two node tandem network with feedback whose arrival and service rates are modulated by an exogenous finite state Markov process. We derive a closed form solution for the probability of missing deadlines. Then we have employed the results to an importance sampling technique to estimate the probability of total population overflow which is a rare event. We have also shown that the probability of this rare event may be affected by various deadline values.Importance Sampling, Queuing Network, Rare Event, Markov Process, Deadline

A statistical study of the luminosity gap in galaxy groups

The luminosity gap between the two brightest members of galaxy groups and clusters is thought to offer a strong test for the models of galaxy formation and evolution. This study focuses on the statistics of the luminosity gap in galaxy groups, in particular fossil groups, e.g. large luminosity gap, in an analogy with the same in a cosmological simulation. We use spectroscopic legacy data of seventh data release (DR7) of SDSS, to extract a volume limited sample of galaxy groups utilizing modified friends-of-friends (mFoF) algorithm. Attention is paid to galaxy groups with the brightest group galaxy (BGG) more luminous than \Mr = -22. An initial sample of 620 groups in which 109 optical fossil groups, where the luminosity gap exceeds 2 magnitude, were identified. We compare the statistics of the luminosity gap in galaxy groups at low mass range from the SDSS with the same in the Millennium simulations where galaxies are modeled semi-analytically. We show that the BGGs residing in galaxy groups with large luminosity gap, i.e. fossil groups, are on average brighter and live in lower mass halos with respect to their counter parts in non-fossil systems. Although low mass galaxy groups are thought to have recently formed, we show that in galaxy groups with 15 galaxies brighter than $M_r\ge -19.5$, evolutionary process are most likely to be responsible for the large luminosity gap. We also examine a new probe of finding fossil group. In addition we extend the recently introduced observational probe based on the luminosity gap, the butterfly diagram, to galaxy groups and study the probe as a function of halo mass. This probe can, in conjunction with the luminosity function, help to fine tune the semi-analytic models of galaxies employed in the cosmological simulations.Comment: 11 pages, 11 figures, accepted to PASP journa

Comparative Evaluation of Ultraviolet and Visible Light Transmittance through Prescriptive Ophthalmic Minus Lenses

Background: Wearing spectacles is the most common approach in correcting the refractive errors worldwide. Due to harmful effects of overexposure to solar ultraviolet radiations, the usage of multi-layer coatings in ophthalmic lenses has recently been increased. These lenses can reduce the reflections and hence increase the transmission of visible light; they can also decrease the transmission of ultraviolet rays. This study aims to compare the transmission of ultraviolet (A and B) and visible rays through coated and uncoated prescriptive ophthalmic plastic lenses.Materials and Methods: In this study, 39 minus non-photochromic multi-coated white plastic single-vision lenses; 9 similar lenses but without any coatings were assessed by spectral transmittancemeter for evaluation of the transmission of visible and ultraviolet rays.Results: The transmission of visible light was 97.9%±1.07% for coated lenses and 93.5%±0.54% for lenses without coating. Ultraviolet-A transmission was 12.15%±8.02% for coated lenses compared to 66.27%±23.92% in lenses without coating. The transmission of ultraviolet-B rays was 1.21%±0.4% and 23.0%±15.97% for lenses with and without coatings, respectively.Conclusion: The transmission of visible light was significantly higher in multi-coated lenses compared to uncoated samples; whereas the transmissions of ultraviolet rays in multi-coated lenses were significantly lower than uncoated ones. Therefore, it is recommended that, except for particular cases, prescribed lenses be equipped with this multi-layer coating

Lower tear meniscus in computer reading task with and without soft contact lens

Dry eye is the major contributor to computer vision syndrome. The reductions in the blink rate and amplitude have been considered as the possible mechanisms leading to this condition. The purpose of this study is to evaluate the changes in the lower tear meniscus heights and areas in the computer reading task, in dry eye subjects, with and without soft contact lenses. In this quasi-experimental study, 39 subjects (with mean age of 21.2 ± 2.4 years), with dry eye, were enrolled.  The lower tear meniscus height and area (TMH, TMA) of the right eye was captured by Spectral Optical Coherence Tomography before and after 45 minutes of reading task behind desktop 17” Cathode Ray Tube monitors. The same procedure was performed after the insertion of the hydrogel soft contact lenses with 58% water content. The results of TMHs, TMAs were analyzed with a repeated measures analysis of variance (ANOVA) with two within-subject factors.: The mean ± standard error of  TMH and TMA pre and post computer tasks were 283.6± 25.4 µm, 0.029±0.005 mm2, 297.6±20.7 µm , 0.036 ± 0.005 mm2 respectively. After contact lens insertion, the mean TMH and TMA pre and post computer task were 231.6 ± 10.6 µm, 0.020±0.0019 mm2, 213.0±9.0 µm, 0.018 ± 0.0011 mm2, respectively. Repeated measures analysis of variance showed that the changes in TMH (P=0.86) and TMA (P=0.37) after computer task, were not statistically significant, while the insertion of the soft contact lens had reduced the TMH and TMA significantly (P<0.001 for both). After 45 minutes of reading task behind desktop monitors, the tear volume has not changed, with and without high water SCLs. A reduction in tear volume might be expected with longer duration of computer task or at late hours of the day. The creation of a more stable tear film should be considered in the management strategies for shorter duration of work behind desktop monitors

Comparison of an Italian Chart with an Iranian Chart in Visual Acuity Measurement

Background: Taking visual acuity is an important part of an eye routine examination. This study was conducted to compare visual acuity measured using an Iranian digital tumbling E chart with visual acuity measured using an Italian digital tumbling E chart as a familiar foreign chart.Materials and Methods: This study was carried out on 200 participants (400 eyes; healthy n=40, glaucoma n=40, retinal disorders n=40, post lasik n=40, corneal n=40) with mean age 44.28 years. Visual acuity of both eyes of half of the subjects (n=100, n=20 from each group), was first measured using the Italian digital chart and then measured using Iranian digital chart. Visual acuity of the rest of subjects (n=100, n=200 from each group) was first measured using the Iranian digital chart and then measured using the Italian digital chart. Measured visual acuities were recorded in logMAR notation.Results: The mean of the Iranian and Italian Tumbling E chart is 0.280±0.012 and 0.277±0.012, respectively. Paired t-test used to evaluate the mean difference between two groups (p=0.721) indicated that there was no statistically significant difference between means of the two tests. Spearman correlation coefficient for the results of both tests was 0.942 (p<0.001), which was statistically significant and could be categorized as a strong positive correlation.Conclusion: The two digital Tumbling E charts acted similarly at different levels of acuity in different disorders. However, the Iranian chart requires a more accurate design for optotypes of the lower acuity lines in order to obtain more accurate measurement of visual acuities in healthy subjects

Comparison of Two Marketed Hydroxypropyl Methylcellulose Based Artificial Tear Drops in Young Patients with Dry Eye Syndrome

Background: Artificial tears have been among the first line of therapy in management of Dry Eye Syndrome (DES). This study was conducted to compare a local artificial tear with an imported one in reduction of DES. This comparison would help to evaluate the cost and benefits of each drop in the proper management of DES.Materials and Methods: In this double-blind randomized clinical trial study, a total 65 students meeting our inclusion criteria for DES entered the study. The OSDI questionnaire, TBUT, corneal and conjunctival staining and Schirmer test, were performed. The patients were divided into two groups by block randomization. Group 1 received first drop and group 2 received second drop. Both groups were instructed to use the drops 4 times a day for 14 days. The same tests were performed by the same examiner who was blind to the treatment type after two weeks. Repeated measured ANOVA was used to analyze the data.Results: A total of 58 patients completed the study. In both groups, after the intervention, the OSDI scores (P<0.001), TBUT score (P=0.041), corneal (P<0.001) and conjunctival staining scores (P<0.001) showed improvement in compare to those before the intervention. However, the Schirmer test score did not show significantly difference before and after intervention. In comparing two groups the OSDI scores, the TBUT score, the corneal and conjunctival staining scores and the Schirmer scores did not show statistically significant difference.Conclusion: The two artificial tears equally reduced the symptoms and signs of DES in two weeks

How Much of Hazardous Blue Light is Transmitted By Spectacle Lenses?

This is a Letter to the Editor and does not have an abstract