Dennis Dan Corell

Antireflective sub-wavelength structures for improvement of the extraction efficiency and color rendering index of monolithic white light-emitting diode.

We have theoretically investigated the influence of antireflective sub-wavelength structures on a monolithic white light-emitting diode (LED). The simulation is based on the rigorous coupled wave analysis (RCWA) algorithm, and both cylinder and moth-eye structures have been studied in the work. Our simulation results show that a moth-eye structure enhances the light extraction efficiency over the entire visible light range with an extraction efficiency enhancement of up to 26 %. Also for the first time to our best knowledge, the influence of sub-wavelength structures on both the color rendering index (CRI) and the correlated color temperature (CCT) of the monolithic white LED have been demonstrated. The CRI of the monolithic white LED could be improved from 92.68 to around 94 by applying a cylinder structure, and the CCT could be modified in a very large range with appropriate design of the cylinder structure.

Light Emitting Diodes as an alternative ambient illumination source in photolithography environment

We explored an alternative light emitting diode (LED) - based solution to replace the existing yellow fluorescent light tubes (YFT) used in photolithography rooms. A no-blue LED lamp was designed and a prototype was fabricated. For both solutions, the spectral power distribution (SPD) was measured, the colorimetric values were calculated, and a visual comparison using Gretagmacbeth colorcharts was performed. The visual comparison showed that the LED bulb was better to render colors despite a low color rendering index (CRI). Furthermore, the LED bulb was tested in a photolithography room and there was no exposure to the photoresist even after 168 hours illumination.

'No blue' LED solution for photolithography room illumination

This paper explored the feasibility of using a LED-based bulb as the illumination light source for photolithography room. A no-blue LED was designed, and the prototype was fabricated. The spectral power distribution of both the LED bulb and the yellow fluorescent tube was measured. Based on that, colorimetric values were calculated and compared on terms of chromatic coordinates, correlated color temperature, color rendering index, and chromatic deviation. Gretagmacbeth color charts were used as a more visional way to compare the two light sources, which shows that our no-blue LED bulb has much better color rendering ability than the YFT. Furthermore, LED solution has design flexibility to improve it further. The prototype has been tested with photoresist SU8-2005. Even after 15 days of illumination, no effect was observed. So this LED-based solution was demonstrated to be a very promising light source for photolithography room illumination due to its better color rendering in addition to energy efficiency, long life time and design flexibility.

A color management system for multi-colored LED lighting

A new color control system is described and implemented for a five-color LED light engine, covering a wide white gamut. The system combines a new way of using pre–calibrated lookup tables and a rule-based optimization of chromaticity distance from the Planckian locus with a calibrated color sensor. The color sensor monitors the chromaticity of the mixed light providing the correction factor for the current driver by using the generated lookup table. The long term stability and accuracy of the system will be experimentally investigated with target tolerance within a circle radius of 0.0013 in the uniform chromaticity diagram (CIE1976).

PHOSPHOR CONVERTED LASER DIODE LIGHT SOURCE FOR ENDOSCOPIC DIAGNOSTICS

(04/11/2019) Phosphor converted laser diode light source for endoscopic diagnostics In order to provide light sources for endourology and on-site testing of the light source, we are developing a portable endoscope light source prototype based on a phosphor converted laser diode. A small emitting area from the phosphor material excited by a laser diode enables coupling of the generated white light into thin optical fibres. The development involves designing optics for optimizing the light extraction efficiency and guiding of light to the area of interest. In this paper we compared the developed light source to the current standard in endoscopy – xenon arc lamps. Detailed spectral analysis of illuminance, CRI and CCT at two power levels and two distances for both the PC-LD and the xenon light source was performed. The obtained results verified that the developed light source is suitable for endoscopy illumination and the first pre-clinical trials will be performed shortly.

Investigation of Saturation Effects in Ceramic Phosphors for Laser Lighting

We report observations of saturation effects in a Ce:LuAG and Eu-doped nitride ceramic phosphor for conversion of blue laser light for white light generation. The luminous flux from the phosphors material increases linearly with the input power until saturation effects limit the conversion. It is shown that the temperature of the phosphor layer influences the saturation power level and the conversion efficiency. It is also shown that the correlated color temperature (CCT), phosphor conversion efficiency and color rendering index (CRI) are dependent both on the incident power and spot size diameter of the illumination. A phosphor conversion efficiency up to 140.8 lm/W with CRI of 89.4 was achieved. The saturation in a ceramic phosphor, when illuminated by high intensity laser diodes, is estimated to play the main role in limiting the available luminance from laser-based lighting systems.

A white–cyan-red LED system for low correlated colour temperature lighting

A white LED complemented by cyan and red LEDs is a good candidate for achieving high colour rendering at low correlated colour temperatures. This is usually very difficult with commercially available white LEDs. In addition, the system is able to replace incandescent lighting in many applications; for example, the lighting for museum display cases. To investigate and optimize the colour and light distribution properties, both spectral and geometrical modelling are used. Mapping of the possible combinations of LEDs is used to locate the optimal solutions within the colour gamut, with emphasis on chromaticity and colour rendering indices. A geometric optical model is used to design and optimize the homogeneity of the colour and light intensity distribution as a function of angle. The resulting system produces diffused homogeneous white light with a tunable correlated colour temperature from 2000 K to 2400 K. Within this range the white light is characterized by a high general colour rendering index (Ra > 90), special colour rendering indices for saturated red objects (R9 > 85), and low chromaticity distance (Duv) from the Planckian locus (Duv < 2 × 10−3).

Assessment of filament led bulbs with respect to temporal light artefacts

(07/10/2019) Assessment of filament led bulbs with respect to temporal light artefacts Temporal light artefacts, abbreviated TLAs (including flicker, stroboscopic effect and phantom arrays), i.e. undesired time modulation in luminance from a light source, has shown to be a threat to wider SSL adoption especially related to dimming functions and low-quality LED products. This is due to the effects that both noticeable and unperceivable TLAs have on human perception and wellbeing. In the present work a number of filament LED bulbs, currently available on the market, are assessed primarily with respect to TLAs, but also with respect to photometric, colorimetric and efficiency properties. The investigation shows that only one of the 10 investigated lamps can be considered flicker free. The rest of the lamps the TLAs are of such a magnitude that none of them fulfil the suggested recommendations currently available.

Comparing the light quality of retrofit LED products

The rapid development in flux and efficiency of LEDs has resulted in a flooding of the lighting market with solid-state lighting (SSL) products. These are regarded as advantageous alternatives—with superior quality and energy efficiency—to traditional light sources. We found, however, that there are large variations in the quality of these products, and some are not better than the ones they are supposed to replace. To help consumers make informed decisions and avoid products with unwanted performance characteristics, we conducted a two-year study from January 2010 to January 2012 to investigate SSL products on the Danish market.1 The focus was on SSL products for replacement of incandescent lamps and halogen spotlights. We collected 266 SSL replacement lamps from lighting companies in Denmark and random samples from stores in the Copenhagen area. Our sample consisted of 144 230V AC lamps (95 directional, i.e., with the luminous flux emitted within an angle of less than 120, and 49 non-directional) and 122 12V DC lamps (99 directional and 23 non-directional). We tested each light source once for luminous flux, power consumption, and spectral light quality—characteristics essential to long-term satisfaction and therefore technology adoption by consumers— with an integrating sphere spectrometer, using standard measurement guidelines.2 We then selected 48 products for a long-term maintenance test, with several measurements conducted at approximately 1000-hour intervals. The European Union LED Quality Charter (EU-QC) is a voluntary program to promote energy savings in the residential lighting sector.3 We used EU-QC requirements as a reference for evaluation of the light sources in our survey. The efficacy criteria for inclusion under the program increase in steps every year from 2011 to 2015, depending on the color rendering index (CRI)4 of the light source. The measurements of these quantities Figure 1. Luminous efficacy as a function of the time of the first measurement, for the tested directional and non-directional light sources. The colored lines show the requirement for inclusion in the European Union Quality charter (EU-QC). The black lines show a backward extrapolation of the EU-QC requirements. CRI: Color rendering index.

Light quality and efficiency of consumer grade solid state lighting products

The rapid development in flux and efficiency of Light Emitting Diodes (LED) has resulted in a flooding of the lighting market with Solid State Lighting (SSL) products. Many traditional light sources can advantageously be replaced by SSL products. There are, however, large variations in the quality of these products, and some are not better than the ones they are supposed to replace. A lack of quality demands and standards makes it difficult for consumers to get an overview of the SSL products. Here the results of a two year study investigating SSL products on the Danish market are presented. Focus has been on SSL products for replacement of incandescent lamps and halogen spotlights. The warm white light and good color rendering properties of these traditional light sources are a must for lighting in Denmark and the Nordic countries. 266 SSL replacement lamps have been tested for efficiency and light quality with respect to correlated color temperature and color rendering properties. This shows a trade-off between high color rendering warm white light and energy efficiency. The lumen and color maintenance over time has been investigated and results for products running over 11000 h will be presented. A new internet based SSL product selection tool will be shown. Here the products can be compared on efficiency, light quality parameters, thus providing a better basis for the selection of SSL products for consumers.