A vision experiment comparing the brightness of direct current light and pulsed light of red (640 nm) and green (550 nm) colors was conducted. The frequency of the pulsed current is 100 Hz and the duty ratio varies between 10% and 90% with an interval of 10%. The Talbot-Plateau law holds for green light but fails for red color when the duty ratio is smaller than 70%. For red light, the maximum enhancement factor is 1.17, which appears at the condition of 20% duty ratio. The results show that the sensitivity of the human eye on pulsed light changes when the spectrum and duty cycle are different.
This paper introduces a new colorimetric measurement method for the transition width of the precision approach path indicator. The measurement system consists of a spectrometer, a fiber probe, a moving means and a ruler. The spectrometer is used to measure the chromaticity coordinates to distinguish the white and red light. The fiber probe is the input of the spectrometer. It is fixed to the moving means, which can move along with the upright rule. The precision approach path indicator certain distance away projects the light to the fiber probe. By moving the fiber probe crossing the transition sector up and down, the chromaticity coordinate of the light moves from the white area to the red area. The intermediate distance of the fiber probe is the width of the transition sector. Use the ruler to measure it and then calculate it to angle. With the measurement distance of 10 meter and the precision of the ruler 1 millimeter, the precision of the system can be 21 seconds of arc. Compared with the traditional measurement methods, the method introduced in this paper is more precise and it strictly accords with the ICAO standard Annex 14.
Lifetime is one of the most important characteristics of white LEDs for the solid state lighting industry and end users.
The measurement uncertainties should be controlled well to ensure consistent measurement results. This paper gives
uncertainty analysis in the measurement for the L50 lifetime of white LEDs. The exponential model is assumed for LEDs’
light output degradation, and an Eyring model is used for accelerated life test. The influences of photometric
measurement instruments, measurement duration and interval, junction temperature, input current, current accelerating index and activation energy are analysed. The analysis method introduced in this paper can be referenced for other related analysis, and the results are important to the practices in LED lifetime measurement.
Pulsed operations can be used to improve the efficiency of light-emitting diodes (LEDs) on human perception. We conducted an experiment asking 46 observers to compare the brightness of a viewing area that was separated into two similar parts and illuminated by both pulse-modulated LEDs and DC-driven LEDs. Using filters, we eliminated the influence of color shift on human perception. The results show that the efficiency improves more when the duty ratio decreases. In addition, the perception enhancement has a linear relationship with duty ratios ranging between 20% and 80%. When the duty ratio is close to 100%, there is almost no effective enhancement.
The relationship between the reliability of GaN LEDs and their junction temperatures and ideal factors is investigated. 20
groups of both blue and white GaN LEDs are tested. Their ideal factors and junction temperatures under 700mA
operating current are measured. The measurement methods are introduced. After the measurement, 700mA high current
accelerated life test is carried out on the LEDs. Analysis results show that the reliability of the LEDs is strongly
dependent on their junction temperatures and ideal factors. For most of the unreliable LEDs with their 50% ALT life less
than 400 hours, their ideal factors are higher than 10, or the junction temperatures of the blue LEDs under 700mA are
higher than 130°C, and the junction temperatures of the white LEDs under 700mA are higher than 120°C.
Traditional PAPIs commonly use halogen lamps as their light sources, which have short lifetime. LEDs have much
longer lifetime and thus can enhance the reliability of PAPIs. In this paper, a new design of a PAPI based on LED light
sources is introduced. White and red LEDs are used to emit white and red beam separately. The white and red LEDs are
separated into two cases. In each case there is a single lens in front of the LED, separately. The LEDs are placed exactly
at the focal planes of the lens. The white LED is fixed below the optical axis to project the white light in the upper part,
and the red LED is fixed oppositely. Baffles are used to form a sharp separation of the beam. Optical simulation is
carried out to verify the design. The measurement results of the finally realized model show that it meets the
requirements of the technical criterion.
This article analyzes the efficacy of the light sources and their limitations in theory and in technology. The LED's spectra were simulated by a Gaussian model to calculate the efficacy. The conventional light sources have been compared with LEDs; the results show that significant increase of LEDs' internal quantum efficiency and extraction efficiency is essential for LED application in general lighting.