Introduction LED light sources are more and more widely used in the automotive industry due to their energy saving, diverse shapes and high performance. Recently, LED lights modulated by time signals (such as PWM modulation) have been used for additional functions of vehicle lights, such as a combination of taillights and brake lights, dimming control of interior lighting, dynamic signal lights, and the like.
Audi A8 time signal modulation LED light
However, a light source modulated by a time signal produces some visual illusion when perceived by the human eye. The visual illusion caused by phenomena such as light quality or stroboscopic is called Temporal light artifact (TLA). CIE defines it as the phenomenon that the observer perceives the light or the spectral composition of the light in a certain environment, and the visual perception changes. And the CIE classifies the TLA by whether the observer's eyes move and whether the environment contains perceptible movements. Among them, flicker is a visual phenomenon that causes static observers to be unstable due to light stimuli whose brightness or spectral distribution fluctuates with time in a static environment. The Stroboscopic effect is a phenomenon in which the motion of an object observed by a static observer exhibits a difference from its actual motion under illumination with a certain frequency. There is another TLA known as the phantom array effect. When you glance at a fast-flashing point source in the dark, you will see a series of light sources that extend in space, a phenomenon known as the “phantom array effect†(Hershberger, 1987).
The phantom array effect is usually related to eye movement. In the field of automotive lighting, the phantom array effect is called the bead string effect, and the instantaneous discrete light pulses are mapped at different positions in the observer's retina, and the image formed on the retina is like a string of beads. A German article in 2007 mentions two cases other than the CIE classification (Brückner, 2007): the eye moves, the line of sight moves from the front target to the LED taillights of the test vehicle that is driving; the eye looks at the front target without moving, and there is When the vehicle under test with LED taillights passes by. The phantom array effect occurs in both cases. Therefore, definitions can be extended to include not only non-static observers and static environments, but also static observers and non-static environments (non-static sources). Both the movement of the eye and the movement of the light source can cause the image of the discrete source to form on the retina, creating a phantom array effect.
The research content of this paper is not moving the eyes. Is it only possible to move the light source to produce a phantom array effect similar to that seen by eye movement? And what is the relationship between the threshold frequency at which the phantom array effect is caused by the movement of the eye and the threshold frequency at which the illuminant array effect is caused by the movement of the light source?
experimental method
The LED light source is controlled by PWM, and the PWM modulation frequency can be controlled by the Bluetooth on the mobile phone. The rotation speed of the rotary disk equipped with the LED light source can be controlled by the DC motor through the power supply. The red LED is fixed on the edge of the turntable, and the observation position is 50cm from the turntable and the viewing angle is 40 degrees. The experiment used red LED, color coordinates (0.691, 0.308), brightness: 252.87 cd/m2. A total of 11 participants (2 females & 9 males) participated in the trial, with an average age of 28.7 years. A total of 10 kinds of PWM frequency and 4 kinds of rotation speeds were set in the experiment. In addition, the 0 rotation speed was set as the control group, which was the case where only the movement of the eyes caused the phantom array effect. The experiment was carried out in a dark room with no other light source. The subjects were tested under various PWM frequencies at five different speeds after dark adaptation, and the threshold frequency at which each subject produced a phantom array effect at each speed was determined.
Experimental result
The figure shows the threshold frequency at which each subject produces a phantom array effect at each speed.
Shown in the table is the average threshold frequency at which the phantom array effect is produced at each speed.
From the experimental results, it can be seen that the threshold frequency of the phantom array effect is proportional to the rotational speed. In addition, the speed of the light source when the threshold frequency of the fantasy array effect generated by the rotation of the light source and the threshold frequency of the light source rotation speed of 0 (the eye movement produces the array effect) is the closest is counted, and the histogram shows most people most. The speed is close to 120 rpm. The angular velocity of the human eye is generally 116.7 rpm to 166.7 rpm. Therefore, it can be concluded that the threshold frequency when the human eye moves is substantially the same as the threshold frequency when the light source moves, and is within the speed range of the eye sway.
Analysis conclusion
From the above results, we can conclude that:
1. The threshold frequency at which the phantom array effect is generated is proportional to the rotational speed of the light source.
2. Moving the light source can produce a phantom array effect similar to moving the eye.
Prospective to the future
1. The threshold frequency varies greatly among individuals. It is necessary to interpret the individual difference phenomenon of threshold frequency according to the study of individual eye saccade speed and visual sensitivity.
2. Research on other influencing factors of the threshold frequency of the phantom array effect, such as the geometric size, brightness, and color of the light source.
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