Ming Ronnier Luo

The impact of LED on human visual experience

High flexibility of LED offers possibility to adjust parameters of lighting, like CCT (correlated color temperature), luminance, and spectral power distribution so that lighting can be made to have positive effects on people. This experiment investigated the effects of available lightings on visual performance and atmosphere feeling. Lightings in the experiment included: LED, energy-saving lamp, tungsten, and a smart lighting product from Philips. 10 participants were asked to complete the experiment tasks and a questionnaire. The experimental tasks include: critical flickering frequency (CFF), calculation, proofreading, reading comprehension and a concentration test (d2 test). They were designed to test the visual performance of participants. The questionnaire was made up of categorical judgment of the atmosphere of lighting environment. It was designed to study the influences of lighting on peoples perception of atmosphere in the environment. The result revealed that Hue-energetic (6300K, 150cd/m2) made people performed best and that CCT and perception of brightness affected peoples feeling of activeness and comfort. Furthermore, it also confirmed that blue light could trigger energetic arousal.

Improved computation of the adaptation coefficient in the CIE system of mesopic photometry

New values of parameters a and b are proposed for the CIE system of mesopic photometry MES2 [CIE Publication 191:2010], because from the original values this model may have no solution or multi-solutions. From the new values of parameters a and b it is shown that the CIE MES2 system has a unique solution. The difference however, between the original and the new values of parameters a and b is very small and the changes do not affect previous conclusions based on the MES2 model. To compute such a solution, we propose a Bisection-Newton method which exhibits fast convergence (8 iterations in the worst case), and improves the fixed-point method recommended by the CIE MES2 system, which has convergence problems for high values of the photopic luminance and very high values of the scotopic/photopic ratio. Comparative results for the fixed-point method, the Bisection method, the Newton method, and the Bisection-Newton method, in terms of the number of iterations necessary for convergence and the computation time used, are reported.

New spectral data for skin colours

Two experiments were conducted for collecting skin database at the Universities of Liverpool and Leeds (UK), and Zhejiang University (China). Overall, 235 subjects from 4 different skin groups (Caucasians, Chinese, South Asian and Dark) were recruited. Each was measured using 4 types of colour measuring methods (tele-spectroradiometer, spectrophotometer, digital camera and visual assessment) including 6 instruments and 2 sets of colour charts. The results from the former two types are summarised here. The results were analysed in terms of skin colour distribution, repeatability and inter-instrumental agreement between 4 skin groups in CIELAB coordinates and spectral domain. Introduction Skin colour has been one of the most extensively studies over the years. It has been involved with many applications such as photography, display, imaging, printing, medical, lighting, etc. It is important to reproduce skin colours to make them either preferable or accurate. So, many experiments were conducted using different colour measuring instruments. According to the target measured, they can be divided into non-contact and contact methods. For contact method, spectrophotometers are normally used, including a light source, a grating and a detector. They have the illumination/viewing geometry of either diffuse integrating sphere or 45o:0o. For measuring skin colours, caution should be taken to apply constant pressure to the surface because skin colour varies with different pressure applied [1]. Different from cosmetic, printing and imaging industries which are interested on the colour management and colour specification, the medicals are also interested in the pigments in the skin. Spectrophotometers are normally used to measure colours at a fixed skin location to ob tain the haemoglobin concentration [2,3] . The non-contact methods include tele-spectroradiometer (TSR), camera and visual assessment. They have been widely used for measuring skin colou rs. The visual assessment was also used by means of reference colours presented by a fan deck or a colour chart. A typical example is that De Rigal [4] measured skin colours to design skin colour charts as a visual aid for evaluating skin whitening product. It provides a fast and inexpensive method to evaluate the effect of clinic treatment or to find a cosmetic product to match the skin colour in stores. TSR measures the spectral power distribution (SPD) of a colour illuminated by a source, and spectrophotometer measures the spectral reflectance of a surface colour. More recently, colour calibrated digital cameras were used to measure colours of objects [5]. It can obtain the colour information of the whole region rathe r than a selected location like the other measuring methods. Although the above methods have been used, there is a lack of research to investigate their differences. Because of the importance of skin spectral database, ISO ISO/TR 16066-2003 Graphic Technology – Standard object colour spectra database for colour reproduction evaluation (SOCS) [6] provides a database including 51182 sets of spectral reflectance, for which 8213 of them are skin colours. There are 6 skin groups, provided by 5 organisations. Each subject was measured at forehead, cheek, neck, zygomatic region and arm. However, they did not define the instruments and their measuring conditions used. It can be found that the colour distribution covers a very large co lour gamut. So, it is not able to be used for further research. With the above in mind, the CIE has established a technical committee, TC 1-92 Skin Colour Database. It is aimed to investigate the uncertainty in skin colour measurement, to recommend protocols for good measurement practice, and to evaluate skin colour measurements that according with these protocols covering different ethnicity, gender, age and body location. This paper summarises the results of the two datasets, which carried out at Liverpool and Leeds Universities, UK, and Zhejiang University, China. The objectives are to report the performance of repeatability and inter-instrument agreement, to reveal the colour distributions for each instrument, and to compare the measuring results between two sites having same type of instrument, such as between two TSRs or between two SPs. It is hoped to provide a general understanding of colour variation using different instrument for measuring different skin groups. Experimental Data Two datasets accumulated at Liverpool/Leeds Universities and Zhejiang University, called UK data and China data, respectively. Table 1 shows the details of the data collected from each site. It includes number of subjects, male and female, of each skin group and the number of locations measured for each subject. Four measurement methods were used: TSR, spectrophotometer (SP), digital camera, and visual assessment. Only the measurement results from SP and TSR are investigated here, because these instruments were used in both experiments and are more widely used to accumulate skin colour database. Table 2 describes the specification about the 4 instruments used. Table 1 shows that there are 4 times more subjects participated in the UK experiment than that of China experiment. It can alsobe found that there is also a shortage of female subjects for the South Asian and Dark skin groups in both data. Table 1 The number of subjects in the UK and China experiments. UK data Chinese data Skin group Male Female Male Female

Obtaining Absolute Scene Luminance Using HDR Imaging

The absolute measure of scene luminance is needed in many applications, e.g., color image reproduction (especially outdoor), surveillance images, light appearance, and glare prediction. Currently, commercial luminance cameras are being used, which are very expensive and have limitations in dynamic range. An efficient and inexpensive method is needed to obtain absolute luminance of each of pixels in the scene. In the current study, a method was proposed to obtain absolute scene luminance (XYZ map) at each pixel using digital RGB camera and high dynamic range (HDR) imaging. The proposed model was calibrated using a tele-spectroradiometer (TSR) by means of average luminance at a small spot. A good match of results was obtained between TSR and the proposed method.

A study on lighting uniformity for LED smart lighting system

A psychophysical experiment was conducted to study lighting uniformity across a desk top and the wall as well. The data were used to evaluate three uniformity metrics. Four attributes were scaled including uniformity, comfort, brightness and contrast perceptions using a categorical judgment method. Twenty-one subjects were asked to rate the attributes under different degree of lighting uniformity. The results showed that using the ratio between minimum and average to evaluate the uniformity performed the best, and a ratio of between 0.80 and 0.90 were consistently to provide accepted comfort across the desk top. Also, the contrast of lighting is significantly negatively correlated with uniformity. In the same illuminance level, more uniform lights will make subjects feel more comfortable. And in the same uniformity level, brighter lights will also make subjects feel more comfortable. Different desk tops and different illuminance levels will not significant influence the uniformity perception.

Performance Comparison of Uniform Color Spaces by Integrating into a Tone Mapping Operator

Typically, tone mapping operators are used to map luminance of high dynamic range (HDR) scenes to that of a displayable range. The goal of this study was to compare the performance of different uniform color spaces (CIELAB, ICTCP, and Jazbz) by integrating them into a tone mapping operator. The Contrast Limited Adaptive Histogram Equalization-based Tone Mapping Operator (CLAHE-TMO) was used here. For this purpose, the HDR radiance map of a given scene was first constructed from multiple exposures of the scene captured by using an ordinary RGB camera. The radiance map was then transformed to a uniform color space for the application of TMO on its lightness channel. A psychophysical experiment was conducted using pair comparison method to compare performance of different uniform color spaces. Tone mapped images were compared in four different contexts: preference, naturalness, contrast, and texture details. The experimental results are reported here.

A Novel Approach to Skin Coloration on Displays

Cosmetics are used as makeup to enhance the impact of appearance. Individuals like to make strong impression on others from their appearance. Current study is based on facial make-up in two color colorimetric attributes “whiteness” and “hue composition” in CIELAB color space. A workflow was implemented to achieve the facial makeup based on color image processing. From facial images against a fixed gray background, skin colors were extracted and transformed to tristimulus XYZ values via a camera characterization model. The XYZ values were used to calculate CIELAB L*, a*, b* color coordinates. They were then converted to L*, \(D_{ab}^{*}\), \(h_{ab}\) color attributes to render whiteness and hue. Using the workflow, the images based on different facial whiteness level and hue angle were successfully rendered.

Effects of LED lighting on office work performance

This experiment was carried out to study the impact that illuminance and CCT have on office work performance. The lighting system consists of ten LED light panels hung on the ceiling. There were 5 different lighting conditions including three CCTs and three illuminance levels (3501×6500K, 5501×6500K, 7501×6500K, 7501×4000K, 7501×80 00K). Fourteen participants took part in the experiment. Testing methods included visual acuity, attention, typing, reading, critical flicker frequency (CFF), ECG, emotion and lighting appearance questionnaire, and eye fatigue questionnaire. The results confirmed that 750 lux gave the highest task performance with less eye fatigue. At this level, the 6500K condition achieved the highest work efficiency. Hence the overall best work performance occurred at 6500K–7501×. On the other hand, 4000K condition was considered to be the most comfortable lighting since it caused the lowest visual acuity reduction and achieved the highest relaxation and comfortable score

The impact of dynamic light with different CCT ranges and frequencies on human alertness

Light could have an acute effect on human performance, including task performance, alertness and phase shift. Most studies have investigated the alerting effect of static light. The aim of the present study is to investigate the effect of dynamic light of changing frequency and CCT range on human alertness and task performance. Ten participants including 7 males and 3 females took part to perform six phases of dynamic light by varying their CCT range and CCT frequency. There are six experimental phases: CCT ranging from 4000K to 10000K, with frequency at 2h, 1h, 0.5h, and CCT ranging from 6000K to 12000K, with frequency 2h, 1h, 0.5h. EEG alpha, beta power, 2-back and 3-back task and questionnaires were used to measure each subject. The results showed that, power in alpha range was significantly, i.e. lower after exposure in high CCT condition than low CCT condition. Beta power was also significantly higher in high frequency condition than low frequency. For both 2-back and 3-back tasks, participants perform significantly better in high CCT condition, but no marked difference between different frequency. It was also found that there is a significant different effect on the questionnaire score between different CCT ranges. In general, participants felt more alert in higher CCT range condition and higher CCT frequency conditions. The results can be used to overcome drowsiness after lunch, to increase alertness and to improve work efficiency.

Comparing Skin Colours Between Different Skin Groups

This study investigates the basic characteristic of human skin. 47 subjects from 4 groups (Chinese, Sub-Asian, Caucasian and the Dark) were recruited. All the skin colours were measured in terms of spectral reflectance using a d:8° spectrophotometer. And they included eight parts of body locations in total: forehead, right cheek, left cheek, hand back, fist back, palm, inner forearm and outer forearm. The data revealed certain pattern for human skin colour. All the data of the four skin colour groups fit a particular curve in the \( L^{*} - C_{ab}^{*} \) plane, and that is very similar to the definition of ‘Whiteness’ and ‘Blackness’ as defined by NCS, and ‘Depth’ and ‘Vividness’ as defined by Berns. The result also showed that Chinese group and Caucasian group had very similar pattern: the sunburnt part of the skin had lower Lightness value and higher Chroma value, or lower Whiteness. However, for the Dark skin group, the data show reversed pattern: sunburnt area will have lower Lightness value and lower Chroma value, and all the colours were located on the ‘Blackness’ scale. The area where the ‘Whiteness’ and ‘Blackness’ scales cross each other is the region for Sub-Asian, where they have the highest Chroma values. The difference of different skin colour groups also appears in the shape of spectral reflectance. Dark skin colour not only means lower amplitude, but also has a flatter curve shape, while for light skin colour, the spectral reflectance has a very clear ‘W’ shape. ‘W’ shape reflects the feature of human skin spectral reflectance, and that is the result of the absorption and reflection of the haemoglobin and melanin. Future work includes the study for the reason of the particular pattern shown in \( L^{*} - C_{ab}^{*} \) plane and its application in various areas.