Light & Science
- Jardins de les Dones de Negre
- Nº1, Floor 2
- Sant Adrià de Besòs
- 08930 - Barcelona
- Dr. Josep Carreras
- Director of the Lighting Group
- (+34) 933 562 625
Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. Instead, LEDs emit nearly monochromatic light, making them highly efficient for coloured light applications such as traffic lights and exit signs. However, to be used as a general light source, white light is needed. White light can be achieved with LEDs in three ways:
- Phosphor conversion, in which a phosphor is used on or near the LED to convert the coloured light to white light;
- RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light; and
- A hybrid method, which uses both phosphor-converted and monochromatic LEDs.
The potential of LED technology to produce high-quality white light with unprecedented energy efficiency is the impetus for the intense level of research and development currently supported by the U.S. Department of Energy.
The number of white light LED products available on the market continues to grow, with new generations of devices becoming available about every four to six months. Some of these products perform very well, but their quality and energy efficiency still vary widely. New standards, test procedures, and ENERGY STAR® criteria have been released—with more in development—that will enable buyers to make informed decisions when evaluating LED lighting.
How is LED lighting different from other energy-efficient lighting technologies?
LEDs offer the potential for cutting general lighting energy use nearly in half by 2030, saving energy and carbon emissions in the process. Their unique characteristics—including compact size, long life and ease of maintenance, resistance to breakage and vibration, good performance in cold temperatures, lack of infrared or ultraviolet emissions, and instant-on performance—are beneficial in many lighting applications. The ability to be dimmed and to provide colour control are other benefits of LED lights.
One of the defining features of LEDs is that they emit light in a specific direction. Since directional lighting reduces the need for reflectors and diffusers that can trap light, well-designed LED fixtures can deliver light efficiently to the intended location. In contrast, fluorescent and "bulb" shaped incandescent lamps emit light in all directions; much of the light produced by the lamp is lost within the fixture, reabsorbed by the lamp, or escapes from the fixture in a direction that is not useful for the intended application. For many fixture types, including recessed downlights, troffers, and undercabinet fixtures, it is not uncommon for 40 to 50% of the total light output of fluorescent and incandescent lamps to be lost before it exits the fixture.
How energy efficient are LEDs?
Two aspects of energy efficiency are important to consider: the efficiency of the LED device itself (source efficacy); and how well the device and fixture work together in providing the necessary lighting (luminaire efficacy). How much electricity is used to provide the intended lighting service depends not only on the LED device, but also on the lighting fixture design. Because they are sensitive to thermal and electrical conditions, LEDs must be carefully integrated into lighting fixtures. The efficiency of a poorly designed fixture that uses even the best LEDs will be only a fraction of what it would be if the fixture were well-designed, and the design can also affect lumen maintenance. Learn more about LED energy efficiency
Energy performance of white LED products continues to improve rapidly. DOE's long-term research and development goal calls for cost-effective, warm-white LED packages producing 224 lumens per watt by 2025. This chart shows typical luminous efficacies for traditional and LED sources, including ballast losses as applicable.
Do LEDs provide high quality lighting?
Key aspects of lighting quality are colour appearance (whether a white light appears more yellow/gold or more blue) and colour rendering (the ability of the light source to render colours, compared to incandescent and daylight reference sources). Learn more about LED colour characteristics
• Colour appearance. Colour appearance is communicated using correlated colour temperature (CCT) on the Kelvin (K) scale. For most interior lighting applications, warm-white (2700K to 3000K) and in some cases neutral-white (3500K to 4000K) light is appropriate. Cool-white LEDs with very high CCT (bluish in appearance) tend to offer higher efficacy at low cost, but may not meet user expectations for colour. An increasing number of high-efficacy LED products are available in warm-white or neutral-white colours, to the point where many have surpassed CFLs. Two light sources with identical CCTs can render object colours very differently due to the differences in spectra. While CCT provides an indication of whether a light source may appear yellowish or bluish in colour, Duv is an additional metric that can help identify sources with excessively greenish or pinkish hues.
• Colour rendering. The colour rendering index (CRI) measures the ability of light sources to render colours, compared to either incandescent reference sources if warm in colour, or daylight reference sources if cooler in colour. The leading high-efficiency LED manufacturers now claim a CRI of 80 or higher for phosphor-converted, warm-white devices. In general, a minimum CRI of 80 is recommended for interior lighting, with CRIs of 90 or higher indicating excellent colour rendering. The CRI has been found to be inaccurate for RGB (red, green, blue) LED systems because it's poor at predicting the quality of the appearance of saturated colour objects, and doesn't correspond well to human perception of colour quality. As a supplement to CRI, a lamp's R9 value describes how closely it renders a saturated red colour sample, relative to the reference illuminant. CCT and CRI together only get you in the ballpark for selecting and matching lamp colours. A number of new colour-rendering metrics have been proposed in recent years, but none have been widely adopted as of yet. In the meantime, colour rendering of LED products should be evaluated in person and in the intended application if possible.
How long do LEDs last?
Unlike other light sources, LEDs usually don't "burn out"; instead, they get progressively dimmer over time (a process called lumen depreciation). LED useful life is typically based on the number of operating hours until the LED is emitting 70 percent of its initial light output. Good-quality white LEDs in well-designed fixtures are expected to have a useful life of 30,000 to 50,000 hours or even longer. A typical incandescent lamp lasts about 1,000 hours; a comparable CFL lasts 8,000 to 10,000 hours, and the best linear fluorescent lamps can last more than 30,000 hours. Learn more about LED lifetime and reliability
A primary cause of lumen depreciation is heat generated at the LED junction. LEDs do not emit heat as infrared radiation like other light sources, so the heat must be removed from the device by conduction or convection. Thermal management is arguably the most important aspect of successful LED system design.
Are LEDs cost-effective?
Costs of LED lighting products vary widely. Good-quality LED products currently carry a significant cost premium compared to standard lighting technologies.
However, costs are declining rapidly. Recent industry roadmapping indicates prices for warm white LED packages have declined by about one-third, from
approximately $18 to $12 per thousand lumens (kilolumens, or klm) from 2010 to 2011. Prices are expected to decline significantly, to approximately $2/klm
by 2015. It is important to compare total lamp replacement, electricity, and maintenance costs over the expected life of the LED product.
Learn more about cost-effectiveness trends.
Partly extracted from the SSL U.S. Department of Energy website https://www1.eere.energy.gov/buildings/ssl
The past two decades have seen an explosion of research into the human non-visual response to light. Light not only enables humans to see, but also affects cognition, mood, hormone balance and biological rhythms, and therefore influences health and productivity, both at the individual and societal levels. At the same time,developments in lighting technology have now made it possible to tailor the lighting environment to maximize task performance, with striking examples of colour-tuned fluorescent and SSL systems in retail, healthcare, and architectural design domains http://www.colorkinetics.com/showcase.
Many of these solutions for specific environments have been motivated by common conceptions of emotional responses to the colour of light, which have not been cross-validated with studies demonstrating the effects of light spectra on alertness, cognition and circadian rhythm. Advances in the intelligent control of SSL – such as those proposed by the HI-LED project -- now enable the lighting industry to go beyond these solutions. In particular, digitally-controlled multi-channel LED/OLED light sources provide the unprecedented opportunity to control the spectral composition, intensity, and timing of exposure to light in a multitude of environments, from clinical settings to classrooms to workplaces.
Lighting in the domestic and traditional office environments nonetheless remains largely traditional, with some general resistance to the current forms of compact fluorescent lighting recommended by governmental regulations. For maximum take-up of the new energy-efficient SSL technologies, there is a need to move the concept of intelligently, dynamically controlled light into the home and workplace environment. The capability to do so now exists, but key questions remain about the optimal light spectra and irradiance levels for optimizing human health and performance, and the answers to these questions will ultimately fine-tune the design of LED/OLED sources.
Conversely, the unprecedented availability of real-time spectral tuning of multi-channel LED/OLED sources provides the optimal testing system for probing both visual and non-visual responses to light.
Our aim is therefore to employ novel multi-channel LED/OLED modules with real-time spectral tuning to determine the optimal duration, irradiance levels, and, most importantly, relative spectral content of illumination for maximising mood and cognitive performance, while minimising disruption to biological rhythms and retinal function. The development of OLED channels to span the “green gap” will be important in developing illuminations that achieve the desired balance between visual and non-visual stimulation. The light source modules developed for this purpose will also be capable of generating and effectively instantaneously switching between an almost infinite range of narrow- and broad-band light spectra at any desired irradiance. Our further aim is then to provide knowledge feedback for the realization and demonstration of a final luminaire prototype that meets these optimisation criteria for human health and performance.
Plants have a completely different sensitivity to light colours than humans (see figure below).
Sensitivity of the eye (left) which partly determines energy efficiency of a light source and photosynthesis efficiency (right) which also determines the efficiency in horticulture lighting.
With regard to plant growth, light is usually defined in terms of number of photons. The energy content of a photon varies, depending on its wavelength (light colour). For a particular optical energy, almost one and a half as many red photons can be produced compared with blue. This means that often red light sources produce more efficient light photons than blue light sources. However the plant has different sensitivities for various colours of light, and that influences different light-sensitive activities as well. Using both efficient light sources for plants and effective light recipes is important to obtain the optimal results in plant production.
The only part of the global radiation spectrum which can be used by a plant for its photosynthesis is between 400-700 nm, what is called the PAR-light (Photosynthetically Active Radiation). The amount of photons that are present in the PAR region are called growth light (indicated in micromol (μmol)). So, around 45% of global radiation is PAR light. Photosynthesis is the basic process that leads to the growth of a plant and light is an essential part of it. This energy is used to form glucose from carbon dioxide gas (CO2) and water, which are taken up by leaves and roots.
A careful selection of the spectral content will be used in WP4 through the realisation of SSL modules adapted to the illumination of closed growth chambers for vegetable production and the selective lighting of fruits in order to improve their nutritional quality.
Light is the main driver for growth of plants. Plants capture light by their leaves and use this light in photosynthesis for producing sugars (assimilates) with which the plant can grow. Lamps are used in greenhouses to realize a constant high production rate and quality throughout the year for vegetables, cut flowers and pot plants. Most of the lighting is realized by high pressure sodium lamps (HPS) with a fixed spectrum. Light intensity can be controlled only by switching on and off of all lamps at low frequency. Usually, a time window per day is defined during which the lamps can be switched on and off.
During that time window the lamps are turned on when solar radiation is below a threshold value. Furthermore, these HPS lamps can only be used when they are above the crop at a distance of least one metre, as otherwise the light distribution is not uniform and some plant parts might get too hot. Recently, a few growers have started to grow plants under LED light, where they are so far using only blue and red LEDs. SSL lighting open many new possibilities to save energy and control crop production and product quality, which will lead to a real innovative systems in horticulture:
- In contrast to a fixed spectrum strategy, with a tuneable LED module the spectrum can be chosen and adapted dynamically to the continuously changing demands of the plants, in order to improve not only growth and development of the plants but also for improving the quality (taste, vitamin C).
- SSL modules and luminaires can be positioned in between the crop canopy, improving the distribution over the leaves and minimizing reflection losses.
- The lamps can be dynamically dimmed based upon rapid changes in the greenhouse climate.
- It is possible to monitor the efficiency of the light use by plants with chlorophyll fluorescence techniques combined with equations for plant physiological processes and closed-feedback control algorithms.
- This project is the first to deliver a proof of concept for dynamic monitoring and controlling both intensity and spectrum of lighting in horticulture through dedicated SSL modules for research and a final luminaire for demonstration.
Models simulating plant growth based on physiological processes are already available in the literature since some time ago. These models typically do not include the 3D structure of the plants and the 3D distribution of the different light spectra in a crop canopy.
In the last decade, the research field of functional structural plant models (FSPM) has emerged and is now very active. These models combine:
- Simulation of plant processes
- Simulation of the development of the plant architecture in 3D, and
- Ray tracing of spectral light beams
Progress beyond the State-of-the-art in light for high productivity horticulture
Our FSP models are at the fore front of scientific FSPM community. Here, we will apply these simulation models for estimating the light distribution within the crop canopy depending on spectrum and positioning of SSL devices and the consequences or growth of the plants.
The secondary optics of the final demonstration luminaires will be designed in accordance to the specifications given in WP4 to warrant an optimal plant growth.
It has been long recognized that light spectrum strongly affects plant functioning, as it affects many physiological plant processes. It is also known that plants show a circadian rhythm in their sensitivity to light. Here we will study not only the effects of steady state SSL spectrum, but also a daylight pattern variation in the spectra of the SSL modules.
Also, it is well known that healthy and tasty food contributes to an improved quality of life. Strong health claims associated with fresh horticultural products can only be made based on their Vitamin C content. In general, levels of Vitamin C in commercially grown products are too low to justify strong claims.
The effect that different spectral bands and intensities have on Vitamin C content of fruits will also be studied. Fruits and vegetables contain biologically active substances as well as mineral elements: these are important components for a healthy diet and may prevent many kinds of diseases. Several studies suggest that tomato consumption reduces the risk of chronic diseases such as cardiovascular disease, cancer and age-related macular degeneration. This protective action is typically attributed to the tomato antioxidant lycopene but tomatoes also contain other healthful components such as carotenoids, flavonoids and vitamin C.
The improvement of the nutritional quality of vegetables and fruits is possible by means of cost-effective biofortification strategies, which may further improve human diet and health. A review on tomato health components and their relationship with light intensity, quality and duration as well as temperature, has been done by Dorais, Ehret and Papadopoulos.
Red light has been reported to accumulate carotenoid in tomato . In contrast, the content of vitamin C increased with infrared light. Recently we found that LED light focused towards tomato fruits during their development on the plant, doubled their Vitamin C content.
There is strong scientific evidence of the potential of light for fostering the production of nutraceutical components, although all previous studies have used a selection of fixed light spectral components.
Through WP4, we plan to demonstrate that dynamical changes in the light spectrum and direction are crucial and determine the rate of generation of health promoting substances. The spectral requirements and temporal patterns will be implemented in a final SSL luminaire specifically designed for horticulture applications in the demonstration WP5.
Improving the nutritional quality of fresh vegetables and fruits also represents an effective strategy for improving their marketing. An increasing proportion of consumers are in fact willing to pay more for products, which are perceived as providing nutritional and/or health benefits. To satisfy this consumer demand, the application of technical innovations that allow optimisation of nutritional quality has become a major target for the project.
Before the advent of the LED technology, the most typical light sources in museum lighting were halogen lamps and metal halide lamps. One of the greatest benefits of halogen lamps is its high colour rendering index (CRI), but they emit much energy in the infrared part of the spectrum. This translates into a significant thermal load for the artworks. They have a low correlated colour temperature and emit relatively low power at short wavelengths of visible spectrum (400 nm- 500 nm), thus do not render colours as were seen by the artist when originally painted outdoors.
Luminous efficacy of halogen lamps is typically in the order of 20 lm/W, hence their energy consumption is huge. Also, because of their short lifetime (1000- 3000 hours) these light sources need to be changed frequently.
Metal halide lamps have slightly lower colour rendering index (80-90), and luminous efficacy of 70-105 lm/W. Their 10.000-20.000 hours of lifetime means 6-7 years lifetime in case of museums. Due to their considerable UV power output (160-700 μW/lm) UV cut-off filters become mandatory, which decreases luminous efficacy.
At present time, fixed-spectrum LEDs are being already applied in the field of museum lighting. The greatest advantage of LED light sources is that the radiation in UV and IR part of the spectrum is negligible compared to that of other light sources with broadband spectrum. The CRI of white phosphor LEDs are typically between 70 and 95, their luminous efficacy is – even in case of the less efficient warm white LEDs – at least 120-150 lm/W, and the lifetime is typically about 50.000 hours. By extending the white phosphor LED with one or more narrow-band colour LEDs in the luminaire, a tuneable LED luminaire can be built, which can be adjusted to optimize the colour appearance of artwork.
The work of restorators needs improved luminance levels at a spot with excellent colour rendering. In order to be able to fulfil these requirements, restorators are currently using halogen lamps for their work. Halogen light sources with typical powers of 1000-2000W are usually closely placed to the restorator’s working environment, who usually complain about the raise in temperature promoted by the radiated heat.
In addition, restorators are also struggling with the fact that current light sources lack spectral power at short wavelengths of the visible spectrum, which makes it impossible to distinguish among different blue hues.
Progress beyond the state-of-the-art in artwork illumination
A fixed-spectrum LED module intended for artwork illumination can be designed for the preservation of a particular artwork as a consequence of reduced ultraviolet (UV) and infrared (IR) components in the emission spectra. The absence of IR means that the LED light source does not produce heat directed toward the surface of the painting or fresco.
Therefore, light sources can be placed closer to the artwork to be illuminated, and excellent illuminance values can be provided by consuming less power. At the same time, the absence of UV prevents degradation of potentially sensitive materials.
However, many different materials are used for artwork, and each responds differently when exposed to different wavelengths of light (not only in the UV and IR part of emitted spectra, but also in the visible part).
Indeed, an international team of scientists used synchrotron X-rays to better understand why some bright yellow colours in Vincent Van Gogh’s paintings with time are turning brown, while others do not. The research focus was on chrome yellow, favoured by Van Gogh to depict sunshine and light, several types of which were found to be very sensitive to darkening under green and blue light.
As a result of this research, stable and unstable forms of chrome yellow paint were identified in the artworks of 18th century painters, and museum curators are advised to tailor their lighting systems.
In HI-LED, we focus on the spectral responsivity of dominant pigments that have been used in different European art periods. An ideal museum lighting system with optimized spectrum should address art preservation and visitors’ acceptance issues as well.
In the evaluation of colour quality, different metrics are typically used:
- Colour quality metrics are based on different criteria. The CIE Colour Rendering Index (CRI) was developed using Munsell colour samples, a very special type of inks for printing.
- NIST CQS was designed to show typical errors, when some three-band LED light sources were used.
- nCRI tries to correct this by designing artificial reflectance spectra that will prohibit light source manufacturers to play with the light source spectra, providing very special lamp spectra of high efficacy and good colour rendering only for the CIE or CQS test samples. These samples are not representative of museum objects.
Our investigation approach is led by Prof. Schanda’s group at the University of Pannonia and concentrates on a colour rendering type of test method that is based on reflection spectra of pigments used during the Centuries by artists, which will validate the goodness of several types of spectral designs for artwork illumination.
A special contribution will be to check how far the given pigments are light fast, and in case of reduced light fastness to see whether with specially tailored light source spectra the good colour rendering and high stability can be obtained, taking special precautions to preserve the artefacts.