We have investigated the feasibility of employing quantum dot (QD) phosphor-based light-emitting diodes (LEDs) in aviation applications that request Night Vision Imaging Systems (NVIS) compliance. Our studies suggest that the emerging QD phosphor-based LED technology could potentially be superior to conventional aviation lighting technology by virtue of the marriage of tight spectral control and broad wavelength tunability. This largely arises from the fact that the optical properties of semiconductor nanocrystal QDs can be tailored by varying the nanocrystal size without any compositional changes. It is envisioned that the QD phosphor-based LEDs hold great potentials in cockpit illumination, back light sources of monitor screens, as well as the LED indicator lights of aviation panels.
The essence of Night Vision Imaging Systems (NVIS) compliance in aviation lighting technology is to ensure that the pilots can successfully operate avionic systems when wearing NVIS goggles under cockpit illumination conditions. This paper reports the feasibility of employing quantum dot (QD) phosphor-based light-emitting diodes (LEDs) in aviation applications that request Night Vision Imaging Systems (NVIS) compliance.
NVIS technology refers to the application of photoelectron systems that perform image information transformation, enhancement, processing, display, and other physical processes. At the heart of such a system is an image intensifier which enhances the infrared signals under low-level-light (LLL) conditions. The third generation NVIS uses Gallium Arsenide photocathode image enhancement tube. The maximum response falls in the near-infrared wavelength. A typical product is the AN/AVS-6 LLL night vision system [
While pilots rely on the visible illumination to read aviation panels and display screens in cockpits, the light sources also produce some radiation in near-infrared (IR) regime [
The aviation standard usually uses a set of parameters to characterize the cockpit lighting for NVIS compatibility, that is, NVIS radiance, color limits, and visible light transmittance [
The NVIS radiance can always be calculated by the following formula:
The basic requirements of NVIS radiance is
The colors and color limits are determined by the following formula:
These lighting colors and limits are designated as “NVIS GREEN A,” “NVIS GREEN B,” “NVIS YELLOW,” “NVIS RED,” and "NVIS WHITE" [
Colloidal compound QDs have recently been introduced to the white LED technology as a new family of phosphor materials with many superior properties [
In the present work, white emission from QD phosphor-based LEDs is achieved by depositing a layer of solution-processed QD film, composed of multi-color Cd(S, Se)/(Zn, Cd)S quantum dots, on top of the emissive surfaces of blue-emitting nitride LEDs [
When QD phosphor-based LEDs are used as the light sources in the aircraft cockpit, it is important to make the LED output compatible with NVIS requirements. This sets rigorous limits on the emission spectra of the LEDs, especially on the reddish side of the band. For traditional white fluorescent lighting systems employed in the aircraft cockpit, the NVIS Radiance Class B (NRB) is measured ~1 × 10−6 (0.1 ft), which is much higher than the specified threshold (
By selecting the QD size, it is possible to precisely control the residual emission of the white light QD phosphor-based LEDs in near-IR regime. Hence it is feasible to fabricate high-luminous-efficiency white QD-LED chips that are compatible with NVIS without any filters. The spectral overlap between the output of QD phosphor-based LEDs and that of the night vision systems can be minimized. These QD phosphor-based LEDs hold great potentials in cockpit illumination, back light sources of monitor screens, as well as the LEDs indicator lights of aviation panels.
In the present study, the NRB of a QD phosphor-based LED was calculated to illustrate the NVIS-compatibility of QD phosphor-based LEDs. In this device, the blue emission of 450 nm-peak wavelength of an InGaN QW LED is mixed with the yellow-orange luminescence of CdSe/CdS/ZnS core-shell QDs to produce “complimentary white” in the QD phosphor-based LED output. Figure
(a) The luminescence spectrum of yellow CdSe/CdS/ZnS core-shell QDs that is used in the calculation. (b) The luminescence spectra of Y3Al5O12:Ce3+ phosphor that is used in the calculation.
In a proof-of-concept experiment, we have fabricated QD phosphor-based LEDs by mist-depositing orange-emitting CdSe/CdS/ZnS core-shell QDs over the emissive surface of InGaN QW LEDs [
Absorption and luminescence spectra of orange-red-emitting CdSe/CdS/ZnS core-shell QDs used in the proof-of-concept experiment. The inset shows the HRTEM image of an individual QD.
The QDs were dispersed in toluene solution for mist deposition, and the thickness of the QDs was precisely controlled by varying the deposition time, the flux of the carrier gas, and the concentration of the QDs solution. A deposition rate of ~100 nm/min for the formation of QDs films over the emissive surface of nitride LEDs is used in the present work. In processing white QD-LEDs, a type of InGaN-QW-based blue chips was used with a peak emission at
The NVIS radiance of a QD-LED and a commercial white LED.
Model | UCS 1976 chormaticity coordinates | Tested NVIS(W/( | Specified NVIS(W/( |
---|---|---|---|
QD-LEDs | |||
Thickness ~400 nm | |||
QD-LEDs | |||
Thickness ~300 nm | |||
QD-LEDs | |||
Thickness ~150 nm | |||
QD-LEDs | |||
Thickness ~50 nm | |||
Commercial White LED | |||
(LL-HP70MWG) |
(a) The output spectra of QD phosphor-based LEDs with different QD film thicknesses. The inset shows a portion of the emissive surface of the white QD phosphor-based LED. (b) The output spectrum of a commercial LED. The inset shows a portion of the white emissive surface of the commercial LED.
It is evident from our measurement results that the NRB of the commercial white LEDs falls in the range of 10−7, which is significantly off the NVIS specification without filters. On the other hand, the NVIS radiance of QD phosphor-based LEDs was found as low as
In summary, our calculation and proof-of-concept experiment suggest that white QD phosphor-based LEDs can be tailored to exhibit higher compatibility with NVIS than commercial LEDs. It is therefore envisioned that the QD phosphor-based LEDs hold great potentials in cockpit illumination, back light sources of monitor screens, as well as the LED indicator lights of aviation panels.
The work at University of Shanghai for Science and Technology is supported by NSFC under Grant 61078007, Shanghai Municipal Education Commission and Shanghai Education Development Foundation under Shu Guang Project 10SG46, Science and Technology Commission of Shanghai Municipality under Grants 11530502200 and 1052nm07100, and Program for New Century Excellent Talents in University. The work at the Penn State University is being supported by the National Science Foundation under Grants CMMI-0729263 and ECCS 0824186.