Abstract—The algorithms and devices used in measuring

luminance have a significant effect on the results to be obtained. The two available methods of measuring luminance have shown that luminance of two identical lighting sources can be totally different owing to certain other factors. Consequently, the current paper examined the effect that distance and a coherent light source may have on a target light source in regards to measured luminance. The results indicate that when the distance from the light source is increased on the outdoor, there is a decrease in measured luminance. For the second objective, the results showed that measured luminance increases by about 3% and 2% when the coherent light source is on and oriented towards the point of measurement at 15˚ and 30˚ respectively.

Index Terms—Luminance, HDR image processing, coherent light source.

I. INTRODUCTION Luminance measurements can be obtained through surface

luminance measurement using HDR (High Dynamic Range) image processing and point luminance measurement using CS-100 luminance meter devices. The quantity of light received at the device is converted into luminance. Normally, Luminance can be classified into luminance that is directly from the light source and luminance that is reflected from object or surfaces such as walls, floors, ceilings and the sorts [1]. Thus, the measured luminance values differ according to the distance from a light source and light source’s orientation, and such should be considered before proposing the criteria forassessing light pollution. Furthermore, when measuring an outdoor light source, according to the luminance measurement method, any light source in the surrounding area may have an influence on the luminance of the targeted light source.

Recently, the Korean government shifted their attention on the effect that light pollution has on visual environment, high energy consumption, view of the night time sky as well as its adverse effects on well-being of plants and animals.Consequently, Korean standards for the purposes of reducing and preventing were put in place. These are to be followed when installingroad lights, lights for residential buildings, commercial advertisements, sports facilities, parks and so forth.

There are several research projects aimed at developing methods to be used in evaluating luminance and illuminance displayed by existing light sources. Also, many researchers

Manuscript received June 1, 2014; revised July 23, 2014. This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (No. 2008-0061908).

The authors are with the Architectural Engineering Department, Kyung Hee University, Yongin, Rep. of Korea (e-mail: gonkim@khu.ac.kr, hongsoolim@khu.ac.kr, jtkim@khu.ac.kr).

are focusing on studying the appropriateness and accuracy of the already existing techniques used to measure different quantities of light such as luminance and illuminance. Therefore, this study examines luminance changes according to the alteration of distance and the relation between the measured light source and its surroundings.

II. LUMINANCE MEASUREMENT THEORIES

A. Algorithm of Luminance Luminance can be defined as the intensity of light

perceived by a human eye. In display and entertainment industries, luminance refers to the intensity of light at which images are displayed.

In buildings, luminance can be definedas the brightness of light radiated from light fixtures or reflected from building surfaces or objects within the building that reaches the human eye. Severe luminance difference by indoor building surfaces are likely to cause imbalance in brightness within a particular area. In addition, diverse building surfaces and objects possessing very high differences in their respective luminance levels lead to visual discomforts or in few cases visual impairment. In order to reduce issues related to visibility therefore, designers try to minimize luminance differences caused by different finishing materials, artificial lighting fixtures, daylighting, irregular space designs and so forth. The CIE (Commission Internationale de l’Eclairage) gives the formula below to be used generally for luminance estimations. ФΩ (1)

is the luminance (cd/m2) Фis the luminous flux or luminous power (lm), is the angle between the surface normal and the specified

direction, is the area of the surface (m2), and Ωis the solid angle (sr).

that is being radiated directly from the light source. However, when dealing with buildings and other enclosed spaces, the luminance reaching an eye after the light has been reflected several times on surfaces and objects should be considered as it also plays a significant role in regards to visibility.

ФΩ (2)

is the 'visual' luminance (cd/m2)

The Impact of Coherent Light Sources On Luminance Measurement

Gon Kim, Hong Soo Lim, and Jeong Tai Kim

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IACSIT International Journal of Engineering and Technology, Vol. 7, No. 4, August 2015

DOI: 10.7763/IJET.2015.V7.815

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IACSIT International Journal of Engineering and Technology, Vol. 7, No. 4, August 2015

The distance from the target surface to the point of measurement is 3 meters, and the angles of coherent light source were changed from 0 degree to 30 degrees. In order to minimize errors in the experiment, CS-100 luminance meter only was used to measure the luminaire influence. The target light source and coherent light source are of a similar nature. In addition, each surface was measured 3 times during the experiment, and the average of the results presented.

Fig. 3. Evaluation of coherent luminaire influence on luminance.

Fig. 4. Experiment using CS-100 in a sky simulator.

Fig. 4 shows the process of performing the experiment in the sky simulator. The matt material was attached on the target surface to reduce the effect of unintended reflection caused by room surfaces, which in turn could have a significant effect on the amount of the luminance measured. Also, in order to reduce the possibility of errors, the CS-100 luminance meter device was firmlyanchored on a tripod to limit movements or shaking

III. RESULTS

A. Luminance Measurement in an Outdoor Environment Depending on the Distance This study focused on the difference displayed by

measured luminance obtained through two different methods. The first measurements were obtained through the use of point luminance meter and the second measurements were obtained through the application of HDRI processing technique. Fig 5 shows the results obtained from outdoor lighting sources that were placed 4 meters away to 10 meters away in an identical measuring condition at night. The

luminance values produced by HDRI processing was higher than those produced by CS-100 by about 1.5 times regardless of the distance. Also, the longer the measuring distance was, the smaller the luminance became. Thus, different algorithms are able to affect the luminance depending on the measuring distance.

Device

Distance HDRI CS-100

4m 42,000cd/m² 26,800cd/m² 10m 32,000cd/m² 21,800cd/m²

Fig. 5. Comparisonof HDRI with CS-100 for surface luminance.

B. Luminance Interference by a Neighboring Light Source Since there may be great differences upon a direct

measurement of the light source, the experiment was conducted at the measurement target surface, which is the same as the one used for other experiments. As explained in previous sections of this paper, this experiment was conducted inside a sky simulator in which two light sources were installed. One light source was considered as our target light source and the other one as the neighboring light source. During this study, we measured the luminance on the target area when the neighboring light source was switched off, switched and tilted to a 15 degree angle, and switched on and tilted at a 30 degree angle facing the luminance meter. The equipment used here was a CS-100 luminance meter positioned facing the front. Fig 6 shows the graph comparing the luminance values when the coherent light source is switched off, switched on and tilted to a 15 degree angle, and switched on and tilted to a 30 degree respectively from left to right.

Fig. 6. Luminance with coherent lighting source.

IV. CONCLUSION This study intended to examine the changes in luminance

caused by alterations in the distance between the point being

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