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EMD and the Technical University of Darmstadt are researching the colors of lightStage Image

EMD and the Technical University of Darmstadt are researching the colors of light

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LEDs simulate sunlight

The search for natural light

2013/4/16

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Light-emitting diodes (LEDs) are the light sources of the future. Special phosphor mixtures are now being used to modify the spectrum of LEDs in order to make them more similar to sunlight or candlelight. These mixtures are being tested by EMD in cooperation with the Technical University of Darmstadt.

  • Peter Bodrogi carries out research at the Lighting Technology department of the Technical University of Darmstadt
  • Peter Bodrogi carries out research at the Lighting Technology department of the Technical University of Darmstadt
    © Technical University of Darmstadt

    The English physicist Sir Isaac Newton discovered colors in light in 1666, when a glass prism he held up to the sun projected a rainbow-like band of color onto a wall. He then discovered he could use another prism to transform this spectrum back into white light, thereby establishing a physically tenable theory of color.

    “As engineers, we are followers of Newton.“

    Andreas Benker
    applications engineer

    These days, Peter Bodrogi and his colleagues at the Lighting Technology department at TU Darmstadt use an electronic spectroradiometer instead of a prism to study human perception of light.

    Light has many colors


    He points a camera-like device at one of three light boxes that contain plates on which red and green peppers, an orange, and a banana are arranged, illuminated by different light sources. He gestures toward a display screen on his measuring device. “The spectral distribution of this light is not even — you can see a strong blue maximum on the left end of it,” Bodrogi says. This blue light is generated by an LED unit, one of many that are undergoing practical testing at EMD. These tests are being conducted because a physical analysis of the spectrum is not alone sufficient for determining what people perceive as being “natural” light.

  • Phosphor researcher Andreas Benker equips the test boxes with the corresponding light modules before the start of a test runEnlarge
  • Phosphor researcher Andreas Benker equips the test boxes with the corresponding light modules before the start of a test run
    © EMD/Eva Speith

    This brings us to Newton’s antipode, Johann Wolfgang von Goethe, the great German poet whose own theory of colors comes close to the broad range of feelings triggered in people when they are exposed to different colors.

    A large portion of Goethe’s Theory of Colors from 1810 focuses on the psychology of color. “As engineers, we are followers of Newton, but our experiments are also grounded in the ideas formulated by Goethe,” says Benker.

    Phosphors produce natural light


    Benker, an applications engineer, is developing techniques for producing the most natural light possible with LEDs. In many ways, light-emitting diodes offer a highly attractive alternative to incandescent bulbs and fluorescent tubes. For example, they efficiently generate a high level of brightness on a very small surface, something designers, architects, and environmentally conscious consumers appreciate.

    There is a downside, however — namely, the fact that most of the light produced by an LED lies within a specific spectral region defined by the physics of its semiconductor and how the semiconductor is doped. As a result, LEDs can only emit light in a spectrum perceived as natural if they are combined with a phosphor. For example, a blue LED with its short-wave light can excite a yellow phosphor to emit yellow light, and the mixture of the blue and yellow will produce white light.

    How an LED works

     
     

     

  • LED chip: The basic element of an LED is a chip whose semiconductor crystals emit blue light as soon as a current flows through them. However, blue light is undesirable; in most cases, white light is requiredEnlarge
  • Lighting material: In order to generate white light, a silicon mixture containing yellow lighting material is filled into the chip. The blue light emitted by the chip induces the lighting material to emit yellow lightEnlarge
  • White light: However, the lighting material does not transform the blue light completely into yellow light. Some of the blue light passes unchanged through the layer of lighting material so that the blue and yellow spectral colors mix to yield white lightEnlarge
  • Additive color mixing: The principle of luminescence conversion (blue light + yellow light = white light) is the technologically ideal method for generating white light with an LED. In order to generate “warm” white light, additional red or orange lighting materials must be addedEnlarge
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    Things are not that simple, however, because humans are not fond of cold white light but instead prefer various lighting moods — i.e. a yellowish light that corresponds to the naturally changing daylight toward the evening, which triggers the release of certain hormones in parallel with the changing light.

    Nevertheless, it is still not possible to fully predict what will happen when theories from a lab are actually tested on human senses. In other words, the effect a certain color will have on people can only be determined through practical comparisons.

    “That is exactly what we do here,” says Benker, referring to the tests he and TU Darmstadt scientists are conducting with groups of up to 20 people on the physiological effect of LEDs combined with different phosphorescent materials.

    Inside the test boxes, different objects are illuminated by various lights, causing the quality of the light to vary

    Inside the test boxes, different objects are illuminated by various lights, causing the quality of the light to vary

    © EMD/Eva Speith

    Just a few microgram are often enough

    2013/4/16

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    EMD is researching and developing these phosphors for completely different types of LED applications, ranging from interior and exterior lighting systems for residential buildings to light sources for flatscreen panels and the automotive sector, and lighting for display presentations of fruit or jewelry. The amount of phosphor used in each LED ranges from a few microgram to several milligrams. The rule of thumb here is that one kilogram of phosphorescent material is needed for roughly one million LEDs.

    EMD’s extensive testing systematically assesses the quality of color during various phases of development. During the tests, subjects carry out highly complex comparisons between the objectively recorded images from the light box and their individual notions of what a tasty banana, a ripe pepper, or a juicy orange should look like. Such notions themselves are often not physically objective — for example, if the subjects have internalized the lighting arrangements they have seen in supermarkets for making meat or vegetables seem more colorful than they actually are.

    Gut feeling


    Still, it is exactly these subjective factors that researchers are trying to analyze. That is why Bodrogi instructs the test subjects “not to think too much; go with your gut feeling.” This type of instinctive reaction is also important when subjects are asked not to evaluate the individual objects in a light box but rather the harmony of colors, the brightness, or the tone of what is actually a white background.

    The studies have shown that the perception of color does not always involve the same type of nerve-stimuli processing. Instead, the brain interprets color differently depending on the situation, the object in question, and its own experience. As a result, humans are also the final arbiters when it comes to the development of phosphors.

     
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