Currently, computer graphics programs (CG) as a tool for designing lighting installations allows us to move to a fundamentally new approach to assessing the quality of lighting: to move away from designing based on illuminance values to designing based on synthetic images or lighting design. Modern CG programs allow us to calculate the spatial-angular distribution of luminance (SADL). Still, some criterion is needed for the assessment, which will allow us to uniquely determine the quality of lighting in terms of luminance distribution. In this paper, we consider constructing a physiological model of the visual sense scale for assessing the quality of lighting according to SADL based on an experiment to assess visual sense from the luminance of a light source in the field of view on a uniform background with different luminance. As a result of the work done, it is proposed to use a scale of sensations in the form of a certain space. The axis of space is the threshold contrast for the resulting sense of discomfort, depending on the observation conditions. All other sensations are separated from the axis at some distance. Threshold contrasts, at which visual sensations arise within the framework of the proposed scale, can be used in the future to assess the storm in the lighting scene using a new criterion and will allow analysing the quality of lighting using CG methods.
Currently, computer graphics programs (CG) have become the main tool for designing lighting
installations (LI). The use of CG determined a fundamentally new approach to design: the synthetic
image of the projected installation allowed us to move away from designing for illuminance values to
designing images – lighting design. In the modern understanding of lighting design, the design is
conducted on the spatial-angular distribution of luminance (SADL) [
Human-centric lighting (non-human-oriented lighting) is a modern motto that is not yet feasible in
practice. We know little about the non-visual effect of light on a person. While we have studied the
visual effect well, we do not know how to express it unambiguously through the photometric
characteristics of the light field [
The modern understanding of high-quality lighting is based on normalized parameters: quantitative
and qualitative. The quality of lighting (in terms of light distribution) is related to the concept of visual
comfort. The simplest definition of visual comfort sounds like the absence of visual discomfort. On the
one hand, some lighting conditions can cause discomfort. On the other hand, there is a positive sense
of comfort that occurs after eliminating all sources of discomfort. Currently, the assessment of indoor
lighting is carried out based on illuminance values. At the same time, the eye reacts to luminance. A
highly uneven (arbitrary) distribution of luminance in the scene and the presence of a wide range of
luminance in the field of view can be a source of discomfort [
Currently, there is no exact expression for evaluating the quality of lighting with an arbitrary
luminance distribution, including glare light sources and glare. In [
Studies [
The modern approach to assessing the discomfort of lighting is based on the calculation of the
unified glare rating (UGR), which allows you to make a forecast of the blinding effect of lighting
installations on the psychophysical scale of the discomfort scale. In fact, UGR is a measure of the sense
of this scale. The UGR values are linked to the actual values of the luminance of the light source (LS)
and the luminance of the adaptation (Lad), as well as to the size and position of the source compared to
the observation line. When calculating UGR, the average background luminance is taken as the
adaptation luminance, without considering the luminance of the light sources. Values below 10 indicate
that the lighting conditions will be perceived as comfortable, and above 28 there will be a blinding
effect for most observers. At the same time, UGR does not allow considering reflected glare, and has a
limit on the size of light sources. According to [
It should be noted that the visual perception of lighting depends on the adaptation of the visual
system, on the environment and is determined by our current knowledge and our experience. The study
of visual perception is based on simple tasks reduced to the maximum number of controlled parameters.
The classical one in this case is an experimental setup with a glare light source on a uniform background,
creating a luminance difference in the observer's field of view [
In connection with recent studies [
Since the sensitivity of the eye varies depending on the luminance level, for each interval of the
scale, the variance also increases with the increase in the average luminance value. In [
The purpose of this work is to build a physiological model of the scale for evaluating lighting scenes according to SADL based on empirical data obtained from an experiment to evaluate the reaction of observers depending on the luminance of a glare light source. The paper solves the problem of choosing a metric and finding the numerical boundaries of the intervals of subjective categories in units of standard deviation for a certain probability value.
An experimental installation like the installation of Lekiesh and Guth but with a large range of
luminance and on modern LED light sources [
According to the definition of the Commission Internationale de l'eclairage (CIE), background luminance is formed by the luminance of all objects in the observer's field of view, except for glare sources and glare. In our case, the average background luminance was found using the Aftab Alpha program from three images of the scene captured in the format. NEF with different exposure settings +1E, 0 and -1E (see Fig. 1). These images were converted to the HDR format (images with an extended luminance range). To normalize the images by luminance, the measured luminance values at several points of the scene were used using the Konika Minolta LS-100 luminance meter. The uniformity of the background luminance Lb = Lmax/Lav of this surface is 75 %. In the installation, the light source varied in the range from 67 to 56000 cd/m2 and Lad-from 21 to 190 cd/m2.
The observer smoothly changed the luminance of the light source at a constant background luminance and recorded the LS readings. Between measurements with different Lad levels, a pause of 5 minutes was made so that the observer's eyes had time to rest.
In our installation, when the luminance of the source increased, the luminance of the field around it increased, which affected the background luminance, and, so, the adaptation luminance. At the same time, the background luminance at the background luminance Lb = 128 cd/m2 increased by no more than 5 cd/m2, which is 3.9 %. Therefore, this slight increase in the background luminance was not considered. The light source luminance may also contribute to the adaptation luminance. Since the luminous adaptation with increasing luminance occurs within a few minutes, and the speed of measuring threshold sensations was from 15 to 30 sec., the eye did not have time to adapt to the changing luminance of the light source. The light source luminance was also not considered in the calculation of the adaptation luminance since it was impossible to estimate it.
To study the discomfort from a lighting installation in a lighting scene, the following five categories were used as a psychometric scale of visual perception: "barely noticeable", "comfortable", "uncomfortable", "unpleasant" and "unbearable". So, the "barely noticeable" criterion is the threshold for detecting a signal. This category served as a reference point for constructing the scale. The category "unbearable" is a pain threshold or a threshold at which further performance of a visual task is impossible, or rather, it is the boundary value of the scale, respectively. The most difficult category is "comfortable", since it is difficult to describe it and, therefore, each observer formed his own understanding of comfort. Nevertheless, all observers were guided by the description of a comfortable sunbeam. A clearer category is "uncomfortable". It was associated with the description of a "buzzing fly". The "unpleasant" category meant that the IP was distracting from seeing the palette.
The calculation of the contrast at which a particular sensation occurred was performed according to the formula: (1) Lad
Lad С L LS Lad С 1 ,
1 С1
L
S .
Lad where the ratio of the luminance of the light source to the luminance of the background can be expressed as follows:
In this case, the contrast С1 does not find the visibility of the object against a certain background, but the sensation that the light source causes while in the field of view. In our case, each sensation can be described by its value С1(i), where i is the ordinal number of the category.
The results were obtained in an experiment with a glare source found 10° above the line of sight. The average threshold values of С1 for each sensation and standard deviations were obtained under the condition of a uniform background, on which there is one glare source, and for a simple visual task – seeing the palette - are shown in Table 1. Table 1 shows that the largest standard deviations are seen in the category of the "comfortable" scale. This is since a comfortable luminance contrast is a very subjective concept. Whereas "uncomfortable" contrasts have a smaller spread.
We will construct graphs of the inverse normal distribution of the probability of sensations for the luminance of the adaptation Lad = 21 cd/m2 (see Fig. 2). The graphs are linear in nature, which speaks in favour of the normal distribution.
The linearity is preserved, as can be seen from Fig. 2 for different luminance levels. Only the slope of the curve changes. With an increase in the adaptation luminance, the angle of inclination of the line decreases. Thus, we will take the category of uncomfortable contrast as the centre of the scale. We will analyse the distribution of other curves of sensations compared to discomfort. To do this, we divide the value of the threshold contrast of each sensation by the value of the uncomfortable contrast for each observer separately. And we will plot the curves of sensations with the centre at the point of discomfort (see Fig. 3). Note that by increasing the luminance of the adaptation, their angle of inclination changes when passing through discomfort. That is, the sense between the sense is unbearable and uncomfortable, the distance in the space of the scale decreases with increasing the adaptation luminance.
That is, the scale of sensations will be tied to the adaptation luminance and cannot be expressed by a single number for the entire space. Figure 4 shows how the distance between the axis of uncomfortable contrast, taken as the centre of reference, and other sensations changes depending on the adaptation luminance. The curves above the axis of discomfort can be given by equations with a high degree of correlation. The curves under the axis responsible for the discomfort can be approximated with a lower degree of correlation.
The obtained threshold values of contrasts, see Table 2, found in the space of the visual perception scale can be further used as threshold values for evaluating the continuous SADL of lighting across the scene in a new quality criterion for simple visual tasks, such as reading a sign or viewing the surrounding space.
As a result of the work conducted, a new scale of visual perception was built by luminance contrasts with a probability of 50 %. That is, for 50 % of observers, with a certain contrast, one or another sensation will arise according to the categories of the constructed scale. This scale can be used to find the quality of indoor lighting by SADL, expanding the functionality of modern methods of modelling CG. This scale reflects the relationship between sensations compared to the contrast that causes a sense of discomfort. This sensation is the simplest for observers and is in the centre of the scale. Therefore, "uncomfortable" or uncomfortable contrast is chosen as the centre of reference. This scale is obtained for a simple classical case-the observation of a glare source on a uniform background. Nevertheless, it allows us to find the nature of the relationship between sensations in numbers. Therefore, these values can be used to evaluate the lighting quality of simple scenes with a similar luminance distribution, that is, with a single source in the field of view. Thus, the first approximation to the assessment of the quality of lighting according to the SADL is made.
Typical office visual tasks while working at a computer are different from reading a sign or seeing the surrounding space. Therefore, the next stage of the study will be an experiment to assess the lighting conditions during the performance of typical office visual tasks.
Three experiments are planned: reading black text on a white sheet installed on a turned-off computer screen; reading black text on the white screen of the included computer screen; reading white text on the black screen of the turned-on computer screen.
All three visual tasks are selected as typical office tasks, that is, imitation of reading printed documentation on white sheets of paper, the work of an editor and the work of a designer in the AutoCAD program. Conducting this experiment will expand the functionality of the new scale of visual perception and will allow evaluating a larger number of lighting scenes according to the SADL at the modelling stage in the CG programs.