Julian Marcel Klabes

Bio: Julian Marcel Klabes is an academic researcher from Technische Universität Darmstadt. The author has contributed to research in topics: Luminance & Digital image processing. The author has an hindex of 2, co-authored 7 publications receiving 16 citations.

Prediction accuracy of L- and M-cone based human pupil light models

Abstract: Multi-channel LED luminaires offer a powerful tool to vary retinal receptor signals while keeping visual parameters such as color or brightness perception constant. This technology could provide new fields of application in indoor lighting since the spectrum can be enhanced individually to the users’ favor or task. One possible application would be to optimize a light spectrum by using the pupil diameter as a parameter to increase the visual acuity. A spectral- and time-dependent pupil model is the key requirement for this aim. We benchmarked in our work selected L- and M-cone based pupil models to find the estimation error in predicting the pupil diameter for chromatic and polychromatic spectra at 100 cd/m2. We report an increased estimation error up to 1.21 mm for 450 nm at 60–300 s exposure time. At short exposure times, the pupil diameter was approximately independent of the used spectrum, allowing to use the luminance for a pupil model. Polychromatic spectra along the Planckian locus showed at 60–300 s exposure time, a prediction error within a tolerance range of ± 0.5 mm. The time dependency seems to be more essential than the spectral dependency when using polychromatic spectra.

The Sternberg Paradigm: Correcting Encoding Latencies in Visual and Auditory Test Designs

Abstract: The Sternberg task is a widely used tool for assessing the working memory performance in vision and cognitive science. It is possible to apply a visual or auditory variant of the Sternberg task to query the memory load. However, previous studies have shown that the subjects' corresponding reaction times differ dependent on the used variant. In this work, we present an experimental approach that is intended to correct the reaction time differences observed between auditory and visual item presentation. We found that the subjects' reaction time offset is related to the encoding speed of a single probe item. After correcting for these individual encoding latencies, differences in the results of both the auditory and visual Sternberg task become non-significant, p=0.252. Thus, an equal task difficulty can be concluded for both variants of item presentation.

Measurement of Circadian Effectiveness in Lighting for Office Applications

Abstract: As one factor among others, circadian effectiveness depends on the spatial light distribution of the prevalent lighting conditions. In a typical office context focusing on computer work, the light that is experienced by the office workers is usually composed of a direct component emitted by the room luminaires and the computer monitors as well as by an indirect component reflected from the walls, surfaces, and ceiling. Due to this multi-directional light pattern, spatially resolved light measurements are required for an adequate prediction of non-visual light-induced effects. In this work, we therefore propose a novel methodological framework for spatially resolved light measurements that allows for an estimate of the circadian effectiveness of a lighting situation for variable field of view (FOV) definitions. Results of exemplary in-field office light measurements are reported and compared to those obtained from standard spectral radiometry to validate the accuracy of the proposed approach. The corresponding relative error is found to be of the order of 3–6%, which denotes an acceptable range for most practical applications. In addition, the impact of different FOVs as well as non-zero measurement angles will be investigated.

Task-related Luminance Distributions for Office Lighting Scenarios

Abstract: For the design of modern office environments, lighting is a central aspect. With regard to current practice, uniform illumination is most often applied in interiors. In this paper, however, further aspects of a more individual approach are investigated, that deliberately violate the usual demands for uniformity by explicitly considering task-related, emotional and psychological effects of lighting. For this purpose, two independent experiments were conducted in an office mock-up setting exploring the impact of spatially variable, non-uniform light distributions on the users’ illumination preferences for the accomplishment of a given task. In the first experiment, three predefined illumination settings were rated by a group of naïve observers. Although the respective light distributions differed in their spatial characteristics, no significant differences were found in the rating scores. In addition, these variations showed no significant effect on the users’ preferred position of task performance. In the second experiment, though, a clearly significant effect could be reported such that, once the users were granted control over the illumination settings, an explicit demand for locally increased illuminance levels at the position of task performance was observed. Furthermore, high rating scores of perceived lighting adequacy indicate the users’ general satisfaction with the degree of visual assistance provided by such a task-related illumination.

Spectral reflectance estimation of organic tissue for improved color correction of video-assisted surgery

Abstract: With video-assisted surgery devices becoming more common in all fields of diagnostics and therapy, the question of how well such systems are able to reproduce surface colors of organic tissue arises, especially in cases where a proper distinction of different kinds of tissue—not only through their texture but also through their color—might be crucial for the success of a surgery. Since modern devices are usually made of a highly efficient, multispectral LED light source in combination with some light-guiding structures and a digital camera system, an approach of optimizing these systems’ color reproduction properties based on the estimation of in-situ spectral reflectances is proposed. Following the International Organization for Standardization (ISO) standard procedure of colorimetric characterization, an initial color correction matrix was determined first by solving a linear least-mean-squares optimization problem for a small set of artificial color samples mapping the corresponding camera responses onto the samples’ tristimulus values. This initial matrix was then used as a starting point for a second, nonlinear optimization, which makes use of the estimated reflectance spectra in order to minimize the average squared color difference between the color corrected and the actually perceived tristimulus values of the individual organic tissue samples. Compared to the ISO standard, which only knows simple color patches to be used for the nonlinear optimization, a significant enhancement in the color reproduction of organic tissue could be confirmed with the newly proposed method being applied instead.

A Unified Formula For Light-adapted Pupil Size

Abstract: The size of the pupil has a large effect on visual function, and pupil size depends mainly on the adapting luminance, modulated by other factors. Over the last century, a number of formulas have been proposed to describe this dependence. Here we review seven published formulas and develop a new unified formula that incorporates the effects of luminance, size of the adapting field, age of the observer, and whether one or both eyes are adapted. We provide interactive demonstrations and software implementations of the unified formula.

Light, Colour and Vision

Abstract: on his own subject; in this book the reader will not be disappointed. Moreover, the translation from the original French, with the inclusion of some new material, suffers from few of the defects so frequently seen, for it reads fluently and spontaneously as if it were the original text. The first chapter deals with the nature of light and its relation to other forms of radiant energy: a succinct and interesting review, followed by a full description of different types of illuminants including the various types of artificial lighting and natural lighting from the sun. This is followed by a description of the physiology of vision and the functioning of the eye shown by experiments carried out on the human eye by physical methods. This part comprises a full discussion of such problems as photometry, the illumination of the retina, luminance efficiency and the duality of the retina, colorimetry and the trivariance of the retina, colour vision and its anomalies, and the various thresholds, time effects, and the spatial interactions affecting vision. The second section of the book is more physiological and speculative. It is introduced by a description of the relevant anatomy of the eye, the photochemistry and electrophysiology of vision, the various theories of colour vision, and a physiological interpretation of the visual thresholds. The book is easy and good to read and gives a fair and critical exposition of its subject. It should be useful not only to the ophthalmologist but also to the physiologist and physicist.

Melanopic Limits of Metamer Spectral Optimisation in Multi-Channel Smart Lighting Systems

Abstract: Modern indoor lighting faces the challenge of finding an appropriate balance between energy consumption, legal requirements, visual performance, and the circadian effectiveness of a spectrum. Multi-channel LED luminaires have the option of keeping image-forming metrics steady while varying the melanopic radiance through metamer spectra for non-visual purposes. Here, we propose the theoretical concept of an automated smart lighting system that is designed to satisfy the user’s visual preference through neural networks while triggering the non-visual pathway via metamers. To quantify the melanopic limits of metamers at a steady chromaticity point, we have used 561 chromaticity coordinates along the Planckian locus (2700 K to 7443 K, ±Duv 0 to 0.048) as optimisation targets and generated the spectra by using a 6-channel, 8-channel, and 11-channel LED combination at three different luminance levels. We have found that in a best-case scenario, the melanopic radiance can be varied up to 65% while keeping the chromaticity coordinates constant (Δu′v′≤7.05×10−5) by using metamer spectra. The highest melanopic metamer contrast can be reached near the Planckian locus between 3292 and 4717 K within a Duv range of −0.009 to 0.006. Additionally, we publish over 1.2 million optimised spectra generated by multichannel LED luminaires as an open-source dataset along with this work.

Deep learning-based pupil model predicts time and spectral dependent light responses.

Abstract: Although research has made significant findings in the neurophysiological process behind the pupillary light reflex, the temporal prediction of the pupil diameter triggered by polychromatic or chromatic stimulus spectra is still not possible. State of the art pupil models rested in estimating a static diameter at the equilibrium-state for spectra along the Planckian locus. Neither the temporal receptor-weighting nor the spectral-dependent adaptation behaviour of the afferent pupil control path is mapped in such functions. Here we propose a deep learning-driven concept of a pupil model, which reconstructs the pupil’s time course either from photometric and colourimetric or receptor-based stimulus quantities. By merging feed-forward neural networks with a biomechanical differential equation, we predict the temporal pupil light response with a mean absolute error below 0.1 mm from polychromatic (2007 $$\pm$$ 1 K, 4983 $$\pm$$ 3 K, 10,138 $$\pm$$ 22 K) and chromatic spectra (450 nm, 530 nm, 610 nm, 660 nm) at 100.01 ± 0.25 cd/m2. This non-parametric and self-learning concept could open the door to a generalized description of the pupil behaviour.