Spectral glasses enable a pragmatic approach to evaluate the colour rendering of light. This technique could help architects, interior and lighting designers to analyse quickly how light reproduces colours on objects. The film by Thomas Schielke covers vivid impressions of our architectural and urban environment with a variety of light sources ranging from sunlight to modern LEDs. The images reveal a fascinating look beyond white light to search for inherent light colours.

Discovering light colours with spectral glasses

The foil in the spectral glasses breaks the light into its different visible electromagnetic waves. Each wavelength of the incoming light spectrum is directed by a diffraction grating into a different direction. This transmissive optic creates a continuous rainbow of colours depending on the light source. The effect might visually resemble a prism, but it differs significantly. Depending on the grating multiple diffracted orders appear and lead to the characteristic multiple images of the light source.

Photolithography processes facilitate the construction of the grating based on a holographic interference model. In addition, the width of the rainbow also denotes the brilliance of a light source: Wide rainbow patterns derive from a more diffuse light source whereas a narrow rainbow implies a brilliant one like LED or a low-voltage halogen lamp.

Continuous spectrum versus band spectrum

The visible spectrum of the human eye spans from 380nm to 780 nm: From violet, via blue, green, yellow, and orange to red. A light source, which contains a continuous spectrum – like incandescent lamps – includes all shades between the main colours and therefore guarantees an excellent colour rendition. This attribute is very important in museum or retail lighting to ensure a perfect appearance of coloured art work or textiles. In contrast to incandescent lamps, fluorescent lamps generate a band spectrum.

Certain wavelengths form a peak whereas the colours in between generate a lower radiation and thus reduce the colour rendering quality. Test colour samples are used to compare a specific light source to a reference source in order to define the colour rendering index (CRI). For an approximate evaluation the spectral glasses could be used as an easy to use tool. From a continuous rainbow spectrum one could deduce a high CRI with an excellent lighting quality, whereas bands of colours would indicate a lower colour rendering quality.

Efficient light quality with LEDs

Even if incandescent lamps reveal an outstanding colour rendition they show a low luminous efficacy due to their additional infrared radiation. This light source requires much more energy than modern light sources like LEDs which mainly have a spectrum in the visible range. The luminous efficacy of LEDs at the moment is about five times higher than a conventional incandescent lamp. The technology of blue LEDs with a yellow phosphor coating enables a high colour rendition and offers a valuable step to improve sustainable lighting.

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Beyond the visible rainbow: UV and IR radiation

Below the visible range of violet, the smaller wavelengths start with ultra-violet radiation which cause colours to fade, for example in old paintings lit by sunlight. The infrared rays beyond the visible red emit thermal radiation. Incandescent lamps especially have the disadvantage of providing more infrared radiation than visible light, which leads to a very low luminous efficacy and requires bigger cooling systems in buildings. However, LEDs provide an attractive alternative as their luminous efficacy is much higher because they do not emit infrared radiation.