Freeform optics: introduction.

This feature issue of Optics Express highlights 28 state-of-the-art articles that capture a snapshot of the recent developments in the field of freeform optics. As an introduction, the editors provide an overview of all published articles, which cover a broad range of topics in freeform optics. The wide variety of applications presented here demonstrates that freeform optics is a growing and vibrant field with many more innovations to come.

Spatial and Temporal Variability in Emissions of Fluorinated Gases from a California Landfill.

Emissions of twelve (hydro)chlorofluorocarbons (F-gases) and methane were quantified using large-scale static chambers as a function of cover type (daily, intermediate, final) and seasonal variation (wet, dry) at a California landfill. The majority of the F-gas fluxes was positive and varied over 7 orders of magnitude across the cover types in a given season (wet: 10-8 to 10-1 g/m2-day; dry: 10-9 to 10-2 g/m2-day). The highest fluxes were from active filling areas with thin, coarse-grained daily covers, whereas the lowest fluxes were from the thick, fine-grained final cover. Historical F-gas replacement trends, waste age, and cover soil geotechnical properties affected flux with significantly lower F-gas fluxes than methane flux (10-4 to 10+1 g/m2-day). Both flux and variability of flux decreased with the order: daily to intermediate to final covers; coarser to finer cover materials; low to high fines content cover soils; high to low degree of saturation cover soils; and thin to thick covers. Cover-specific F-gas fluxes were approximately one order of magnitude higher in the wet than dry season, due to combined effects of comparatively high saturations, high void ratios, and low temperatures. Emissions were primarily controlled by type and relative areal extent of cover materials and secondarily by season.

Erratum to: Six years ahead: a longitudinal analysis regarding course and predictive value of the Strengths and Difficulties Questionnaire (SDQ) in children and adolescents.

Erratum to: Eur Child Adolesc PsychiatryDOI 10.1007/s00787­014­0640­x Unfortunately, the names of two authors, Ulrike Ravens- Sieberer and Fionna Klasen, were omitted in the original publication of the article. Please find the correct author list below:Andreas Becker · Aribert Rothenberger · Alexander Sohn · Ulrike Ravens-Sieberer · Fionna Klasen · The BELLA study group

Nonparametric inference in hidden Markov models using P-splines.

Hidden Markov models (HMMs) are flexible time series models in which the distribution of the observations depends on unobserved serially correlated states. The state-dependent distributions in HMMs are usually taken from some class of parametrically specified distributions. The choice of this class can be difficult, and an unfortunate choice can have serious consequences for example on state estimates, and more generally on the resulting model complexity and interpretation. We demonstrate these practical issues in a real data application concerned with vertical speeds of a diving beaked whale, where we demonstrate that parametric approaches can easily lead to overly complex state processes, impeding meaningful biological inference. In contrast, for the dive data, HMMs with nonparametrically estimated state-dependent distributions are much more parsimonious in terms of the number of states and easier to interpret, while fitting the data equally well. Our nonparametric estimation approach is based on the idea of representing the densities of the state-dependent distributions as linear combinations of a large number of standardized B-spline basis functions, imposing a penalty term on non-smoothness in order to maintain a good balance between goodness-of-fit and smoothness.

Six years ahead: a longitudinal analysis regarding course and predictive value of the Strengths and Difficulties Questionnaire (SDQ) in children and adolescents.

BACKGROUND: Scientifically sound and valid information concerning course and prediction of mental health problems in children and adolescents in the general population is scarce, although needed for public mental health issues and daily clinical practice. OBJECTIVES: The psychopathological profiles of children and adolescents were analysed using the parent version of the Strengths and Difficulties Questionnaire (SDQ-P) in a longitudinal setting, also investigating the predictive value of the SDQ-scores. METHODS: SDQ's total psychopathological difficulties, emotional symptoms and hyperactivity-inattention scores of n = 630 children and adolescents (age 6-18;11 years) were examined along four assessment measurement points (T0-T3) over 6 years, using data from the BELLA study. According to the English normative data, the participants were categorized as "normal", "borderline" or "abnormal" based on their SDQ-scores. Groups remaining within categories were descriptively determined by means of frequency analysis, a subsequent graphical evaluation displayed the transitions from T0 to T3 concerning the different categorical classifications. Finally, ordered probit regression was used to examine whether age, gender, socio-economic status (SES) and baseline impact-score (IS) correspond to the SDQ-predicted classification. RESULTS: As expected, low SES and high SDQ-IS were associated with significantly increased scores on all examined SDQ-scales. Regarding the long-term aspect of SDQ-scores it could be shown that most children and adolescents remained "normal" over a measurement period of 6 years, while only a small number of children and adolescents steadily remained "abnormal" or newly developed mental health problems, respectively. For example, on the "hyperactivity-inattention"-scale, only 1 % of the children and adolescents changed from "normal" to "abnormal" (T0-T3), whereas on the "emotional symptoms"-scale, 7 % changed from "normal" to "abnormal" (T0-T3). In general, the SDQ-category "borderline" and specifically the subscale "emotional symptoms" change in both directions. Abnormal SDQ-scores at baseline, SES, gender and IS were related to the prediction of the SDQ-sores at T3. CONCLUSION: An SDQ-screening of children and adolescents may help for early detection, prediction and treatment planning. Also, these results may contribute to a better understanding of the course of mental health problems in childhood and concurrently may allow a better psychoeducation and prevention.