Artificial light at night: a global disruptor of the night-time environment.

Light pollution is the alteration of the natural levels of darkness by an increased concentration of light particles in the night-time environment, resulting from human activity. Light pollution is profoundly changing the night-time environmental conditions across wide areas of the planet, and is a relevant stressor whose effects on life are being unveiled by a compelling body of research. In this paper, we briefly review the basic aspects of artificial light at night as a pollutant, describing its character, magnitude and extent, its worldwide distribution, its temporal and spectral change trends, as well as its dependence on current light production technologies and prevailing social uses of light. It is shown that the overall effects of light pollution are not restricted to local disturbances, but give rise to a global, multiscale disruption of the night-time environment. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Light pollution is skyrocketing.

Data from citizen scientists reveal a worrying growth in light pollution over the past decade.

A linear systems approach to protect the night sky: implications for current and future regulations.

The persistent increase of artificial light emissions is causing a progressive brightening of the night sky in most regions of the world. This process is a threat for the long-term sustainability of the scientific and educational activity of ground-based astronomical observatories operating in the optical range. Huge investments in building, scientific and technical workforce, equipment and maintenance can be at risk if the increasing light pollution levels hinder the capability of carrying out the top-level scientific observations for which these key scientific infrastructures were built. Light pollution has other negative consequences, as e.g. biodiversity endangering and the loss of the starry sky for recreational, touristic and preservation of cultural heritage. The traditional light pollution mitigation approach is based on imposing conditions on the photometry of individual sources, but the aggregated effects of all sources in the territory surrounding the observatories are seldom addressed in the regulations. We propose that this approach shall be complemented with a top-down, ambient artificial skyglow immission limits strategy, whereby clear limits are established to the admissible deterioration of the night sky above the observatories. We describe the general form of the indicators that can be employed to this end, and develop linear models relating their values to the artificial emissions across the territory. This approach can be easily applied to other protection needs, like e.g. to protect nocturnal ecosystems, and it is expected to be useful for making informed decisions on public lighting, in the context of wider spatial planning projects.

Absolute Radiometric Calibration of TESS-W and SQM Night Sky Brightness Sensors.

We develop a general optical model and describe the absolute radiometric calibration of the readings provided by two widely-used night sky brightness sensors based on irradiance-to-frequency conversion. The calibration involves the precise determination of the overall spectral sensitivity of the devices and also the constant G relating the output frequency of the light-to-frequency converter chip to the actual band-weighted and field-of-view averaged spectral radiance incident on the detector (brightness). From these parameters, we show how to define a rigorous astronomical absolute photometric system in which the sensor measurements can be reported in units of magnitudes per square arcsecond with precise physical meaning.

Evaluating Human Photoreceptoral Inputs from Night-Time Lights Using RGB Imaging Photometry.

Night-time lights interact with human physiology through different pathways starting at the retinal layers of the eye; from the signals provided by the rods; the S-, L- and M-cones; and the intrinsically photosensitive retinal ganglion cells (ipRGC). These individual photic channels combine in complex ways to modulate important physiological processes, among them the daily entrainment of the neural master oscillator that regulates circadian rhythms. Evaluating the relative excitation of each type of photoreceptor generally requires full knowledge of the spectral power distribution of the incoming light, information that is not easily available in many practical applications. One such instance is wide area sensing of public outdoor lighting; present-day radiometers onboard Earth-orbiting platforms with sufficient nighttime sensitivity are generally panchromatic and lack the required spectral discrimination capacity. In this paper, we show that RGB imagery acquired with off-the-shelf digital single-lens reflex cameras (DSLR) can be a useful tool to evaluate, with reasonable accuracy and high angular resolution, the photoreceptoral inputs associated with a wide range of lamp technologies. The method is based on linear regressions of these inputs against optimum combinations of the associated R, G, and B signals, built for a large set of artificial light sources by means of synthetic photometry. Given the widespread use of RGB imaging devices, this approach is expected to facilitate the monitoring of the physiological effects of light pollution, from ground and space alike, using standard imaging technology.

Anthropogenic disruption of the night sky darkness in urban and rural areas.

The growing emissions of artificial light to the atmosphere are producing, among other effects, a significant increase of the night sky brightness (NSB) above its expected natural values. A permanent sensor network has been deployed in Galicia (northwest of Iberian peninsula) to monitor the anthropogenic disruption of the night sky darkness in a countrywide area. The network is composed of 14 detectors integrated in automated weather stations of MeteoGalicia, the Galician public meteorological agency. Zenithal NSB readings are taken every minute and the results are openly available in real time for researchers, interested stakeholders and the public at large through a dedicated website. The measurements allow one to assess the extent of the loss of the natural night in urban, periurban, transition and dark rural sites, as well as its daily and monthly time courses. Two metrics are introduced here to characterize the disruption of the night darkness across the year: the significant magnitude (m1/3) and the moonlight modulation factor (γ). The significant magnitude shows that in clear and moonless nights the zenithal night sky in the analysed urban settings is typically 14-23 times brighter than expected from a nominal natural dark sky. This factor lies in the range 7-8 in periurban sites, 1.6-2.5 in transition regions and 0.8-1.6 in rural and mountain dark sky places. The presence of clouds in urban areas strongly enhances the amount of scattered light, easily reaching amplification factors in excess of 25, in comparison with the light scattered in the same places under clear sky conditions. The periodic NSB modulation due to the Moon, still clearly visible in transition and rural places, is barely notable at periurban locations and is practically lost at urban sites.

Wavefront aberration statistics in normal eye populations: are they well described by the Kolmogorov model?

This Letter studies the statistics of wavefront aberrations in a sample of eyes with normal vision. Methods relying on the statistics of the measured wavefront slopes are used, not including the aberration estimation stage. Power-law aberration models, an extension of the Kolmogorov one, are rejected by χ2-tests performed on fits to the slope structure function data. This is due to the large weight of defocus and astigmatism variations in normal eyes. Models of only second-order changes are not ruled out. The results are compared with previous works in the area.

Zernike analysis of all-sky night brightness maps.

All-sky night brightness maps (calibrated images of the night sky with hemispherical field-of-view (FOV) taken at standard photometric bands) provide useful data to assess the light pollution levels at any ground site. We show that these maps can be efficiently described and analyzed using Zernike circle polynomials. The relevant image information can be compressed into a low-dimensional coefficients vector, giving an analytical expression for the sky brightness and alleviating the effects of noise. Moreover, the Zernike expansions allow us to quantify in a straightforward way the average and zenithal sky brightness and its variation across the FOV, providing a convenient framework to study the time course of these magnitudes. We apply this framework to analyze the results of a one-year campaign of night sky brightness measurements made at the UCM observatory in Madrid.

Estimating the eye aberration coefficients in resized pupils: is it better to refit or to rescale?

In order to work in a consistent way with Zernike aberration coefficients estimated in different pupils, it is necessary to refer them to a common pupil size. Two standard approaches can be used to that end: to rescale algebraically the coefficients estimated in the original pupil or to refit them anew using the wavefront slope measurements available within the new one. These procedures are not equivalent; they are affected by different estimation errors that we address in this work. Our results for normal eye populations show that in case of reducing the pupil size it is better to rescale the original coefficients than to refit them using the measurements contained within the smaller pupil. In case of enlarging the pupil size, as it can a priori be expected, the opposite holds true. We provide explicit expressions to quantify the errors arising in both cases, including the expected error incurred when extrapolating the Zernike estimation beyond the radius where the measurements were made.

Visual Strehl performance of IOL designs with extended depth of focus.

PURPOSE: The use of monofocal, bifocal, or multifocal intraocular lenses (IOLs) is a common option to restore the refractive power of the eye in aphakic patients after cataract surgery. In this article, we study the predicted performance of two new designs of IOL, both with extended depth of focus: the quartic axicon and the light sword optical element (LSOE). These elements provide continuous focal segments spanning the range of dioptric power required for presbyopia compensation. METHODS: The performance analysis is based on the visual Strehl ratio (SR) computed in the spatial frequency domain, a neuro-optical quality metric that takes into account the effects of both the optical transfer function and the neural contrast sensitivity function. Furthermore, the classical SR and compound modulation transfer function were calculated. Some conventional transmittances of commercially available IOLs are also analyzed. RESULTS: The LSOE design has a more homogeneous behavior than other available solutions, providing a more uniform image quality over a significant fraction of the required addition range. CONCLUSIONS: The angular average of the visual SR computed in the spatial frequency domain and compound modulation transfer function indicate that both designs of extended depth of focus elements provide better optical quality on the whole addition range, except at the end points, than the discrete focus designs. The LSOE performed better than the quartic axicon in terms of uniformity of image quality within the same range.

Signal-to-noise ratio and aberration statistics in ocular aberrometry.

We define a signal-to-noise ratio (SNR) for eye aberrometry in terms of the sensor geometry, measurement noise, and population statistics. The overall estimation error is composed of three main contributions: the bias in the estimated modes, the truncation error, and the error due to the noise propagation. This last term can be easily parametrized by the proposed SNR. We compute the overall error as well as the magnitude of its three components for a typical sensor configuration, population statistics, and different SNR. We show that there are an optimum number of Zernike aberration modes to be retrieved in each case.

Closed-loop adaptive optics with a single element for wavefront sensing and correction.

We propose a closed-loop adaptive optical arrangement based on a single spatial light modulator that simultaneously works as a correction unit and as the key element of a wavefront sensor. This is possible by using a liquid crystal on silicon display whose active area is divided into two halves that are respectively programmed for sensing and correction. We analyze the performance of this architecture to implement an adaptive optical system. Results showing a closed-loop operation are reported, as well as a proof of concept for dealing with aberrations comparable to those typically found in human eyes.

Centroid displacement statistics of the eye aberration.

We discuss a method for the study of the spatial statistics of the ocular aberrations, based on the direct use of the Hartmann-Shack centroid displacements, avoiding the wavefront reconstruction step. Centroid diagrams are introduced as a helpful aid to visualize basic properties of the aberration datasets, and slope-related second-order statistical functions are applied to check the compatibility between the experimental data and different models for the aberration statistics. Preliminary results suggest that no single power-law spectrum (e.g., Kolmogorov's) is able to represent the whole range of spatial statistics of individual eye fluctuations and that more elaborated models, including at least the contribution of a relevant defocus fluctuation term, are required. This centroid-based approach allows for an easier intercomparison of results between laboratories and avoids the bias and information loss associated with the estimation of a reduced number of Zernike coefficients from a much wider slope data set.

Pupil tracking with a Hartmann-Shack wavefront sensor.

We present the theoretical background and experimental validation of a pupil tracking method based on measurement of the irradiance centroid of Hartmann-Shack aberrometric images. The experimental setup consists of a Hartmann-Shack (HS) sensor forming over the same camera the images of the eye's pupil and the aberrometric image. The calibration is made by comparing the controlled displacements induced to an artificial eye with the displacements estimated from the centroid of the pupil and of the HS focal plane. The pupil image is also used for validation of the method when operating with human eyes. The experimental results after calibration show a root mean square error of 10.45 mum for the artificial eye and 27, 10, and 6 mum rms for human eyes tested using Hartmann-Shack images, with signal-to-noise ratios of 6, 8, and 11, respectively. The performance of the method is similar to conventional commercial eye trackers. It avoids the need for using separate tracking devices and their associated synchronization problems. This technique can also be used to reprocess present and stored sets of Hartmann-Shack aberrometric images to estimate the ocular movements that occurred during the measurement runs, and, if convenient, to correct the measured aberrations from their influence.

Reconfigurable Shack-Hartmann sensor without moving elements.

We demonstrate wavefront sensing with variable measurement sensitivity and dynamic range by means of a programmable microlens array implemented onto an off-the-shelf twisted nematic liquid crystal display operating as a phase-only spatial light modulator. Electronic control of the optical power of a liquid lens inserted at the aperture stop of a telecentric relay system allows sensing reconfigurability without moving components. Results of laboratory experiments show the ability of the setup to detect both smooth and highly aberrated wavefronts with adequate sensitivity.

Green laser pointers for visual astronomy: how much power is enough?

PURPOSE: Green laser pointers with output powers in the tens to hundreds of milliwatt (mW) range, clearly exceeding the limiting 5 mW of American National Standards Institute class 3a (International Electrotechnical Commission class 3R), are now easily available in the global market. They are increasingly being used in public sky observations and other nighttime outreach activities by educators and science communicators in countries where their use is not well regulated, despite the fact that such high power levels may represent a potential threat to visual health. The purpose of this study was to determine the output power reasonably required to perform satisfactorily this kind of activities. METHODS: Twenty-three observers were asked to vary continuously the output power of a green laser source (wavelength 532 nm) until clearly seeing the laser beam propagating skyward through the atmosphere in a heavily light-polluted urban setting. Measurements were conducted with observers of a wide range of ages (9 to 56 years), refractions (spherical equivalents -8.50 to +1.50 diopters), and previous expertise in using lasers as pointing devices outdoors (from no experience to professional astronomers). Two measurement runs were made in different nights under different meteorological conditions. RESULTS: The output power chosen by observers in the first run (11 observers) averaged to 1.84 mW (+/-0.68 mW, 1 SD). The second run (17 observers) averaged to 2.91 mW (+/-1.54 mW). The global average was 2.38 mW (+/-1.30 mW). Only one observer scored 5.6 mW, just above the class 3a limit. The power chosen by the remaining 22 observers ranged from 1.37 to 3.53 mW. CONCLUSIONS: Green laser pointers with output powers below 5 mW (laser classes American National Standards Institute 3a or International Electrotechnical Commission 3R) appear to be sufficient for use in educational nighttime outdoors activities, providing enough bright beams at reasonable safety levels.

Characteristic functions of Hartmann-Shack wavefront sensors and laser-ray-tracing aberrometers.

It is shown that the aberration estimated at any point of the pupil using wavefront slope aberrometers such as Hartmann-Shack wavefront sensors or laser ray tracers is a spatial average of the actual aberration weighted by a characteristic function that depends on the aberrometer design and on the estimation procedure. This characteristic function, whose explicit form is given here for wavefront slope aberrometers using either modal or zonal estimators, may be useful in analyzing some basic aspects of the aberrometer performance. It is also instrumental in establishing the links between the statistical properties of the actual and the estimated aberrations. Explicit formulas are given to show in terms of this function how the bias arises in the first- and second-order statistics of the retrieved aberrations. This approach is mathematically equivalent to the analysis of the effects of modal coupling (cross-coupling and aliasing). It may provide, however, some complementary insight.

Efficient compensation of Zernike modes and eye aberration patterns using low-cost spatial light modulators.

Off-the-shelf spatial light modulators (SLMs) like those commonly included in video projection devices have been seldom used for the compensation of eye aberrations, mainly due to the relatively low dynamic range of the phase retardation that can be introduced at each pixel. They present, however, some interesting features, such as high spatial resolution, easy handling, wide availability, and low cost. We describe an efficient four-level phase encoding scheme that allows us to use conventional SLMs for compensating optical aberrations as those typically found in human eyes. Experimental results are obtained with artificial eyes aberrated by refractive phase plates introducing either single Zernike terms or complex eye aberration patterns. This proof-of-concept is a step toward the use of low-cost, general purpose SLMs for the compensation of eye aberrations.

Measurement and compensation of optical aberrations using a single spatial light modulator.

We describe a compact adaptive optical system using a spatial light modulator (SLM) as a single element to both measure and compensate optical aberrations. We used a low-cost, off-the-shelf twisted nematic liquid-crystal display (TNLCD) optimally configured to achieve maximum phase modulation with near constant transmittance. The TNLCD acts both as the microlens array of a Hartmann-Shack wavefront sensor and as the aberration compensation element. This adaptive setup is easy to implement and offers great versatility.

Direct transformation of Zernike eye aberration coefficients between scaled, rotated, and/or displaced pupils.

In eye aberrometry it is often necessary to transform the aberration coefficients in order to express them in a scaled, rotated, and/or displaced pupil. This is usually done by applying to the original coefficients vector a set of matrices accounting for each elementary transformation. We describe an equivalent algebraic approach that allows us to perform this conversion in a single step and in a straightforward way. This approach can be applied to any particular definition, normalization, and ordering of the Zernike polynomials, and can handle a wide range of pupil transformations, including, but not restricted to, anisotropic scalings. It may also be used to transform the aberration coefficients between different polynomial basis sets.