It is important to retain enough visibility of the opposite passenger's face on the street at nighttime for the passengers to walk with feeling of security. There are several previous studies identifying the effects of street lighting on visibility of the opposite passenger's face using scale models. However in those studies, the effects of the light except for the street lighting such as headlight of cars or motorbikes, spilled light from shops or houses and etc., that exist in actual situations, were not considered.
Recently, the number of LED street lighting is increasing rapidly. It is necessary to evaluate the effects of LED street lighting on the passengers' visual environment properly in actual situations.
In this study, subjective experiment was conducted to identify the effects of changing the light source of the street lighting on the passengers' visual environment at nighttime in an actual situation. Three different streets within the distance of 500 m from the south exit of JR Tsudanuma station were chosen to be evaluated. The light source of the street lighting on the two of them was changed from high pressure mercury lamp to LED, and there was no street lighting on the other one (only security lighting illuminated the street). The experiments were conducted three times. Each experiment was conducted on the same day of the week with almost the same age of the moon. University age young people with no visual impairment nor eye diseases were chosen as the subjects. The number of the subjects ranged from 41 to 60.
The experiment was started at 8 p. m. and finished before 10 p.m. In the beginning of the experiment the subjects were instructed about the experiment, and after the instruction a subject started to walk alone after another at intervals of 2 minutes. After enough walking to adjust their eyes, he/she reached to the target street. He/she evaluated the impression of the light environment of the street with eight items, such as brightness of the street, visibility of steps, feeling of security and etc., at the entrance of the street. Then he/she went forward the street and judged visibility of the opposite passenger's face, preference of color appearance and easiness for distinguishing colors at two points on the street, the point with the highest road illuminance and that with the lowest road illuminance. After the evaluation at the two points, he/she went through the street and answered whether the light environment of the street was suitable or not as the street for walking at night. Continuously, the subjects walked and evaluated the light environment on the other two target streets in the same way.
From the results of the measurement, it was identified that average color rendering index Ra and S/P ratio became higher by changing the light source of the street lighting from high pressure mercury lamp to LED. The results of the subjective experiment showed that spacious brightness of the street at nighttime was related stronger to mesopic luminance than to photopic luminance, i.e. spacious brightness at nighttime was effected by spectral power distribution much or less. In addition, visibility of the opposite passenger's face was improved by changing the light source of the street lighting from high pressure mercury lamp to LED. It was also identified that LED street lighting caused significantly higher glare than high pressure mercury lamp street lighting. However, there was no significant difference in the impression of the light environment of the street or the suitability as the light environment for walking at night between the two kinds of light sources.