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Technical environment

Global standard 220V-240V/50Hz-60Hz
Standard for USA/Canada 120V/60Hz, 277V/60Hz
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Color rendering of LEDs

Perfect color rendering  of LEDs with different-colored material samples

Color rendering generally refers to the ability of a light source to render colors as "true to nature" as possible. Color rendering is a property of light sources. It depends on the spectral composition of the light emitted by the respective light source and not on the light color (color temperature) itself. Common methods of evaluating color rendering are the CRI and the TM-30 method.

Overview on the topic LED color rendering

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What is the color spectrum of an LED?

Light sources such as incandescent lamps or the sun which emit light due to high temperatures have a spectrum that includes all areas of the visible spectrum. For this reason, the color rendition is very good.
LED color rendering

(1) LED with very good colour rendering, (2) LED with good colour rendering

The spectrum of white LEDs is less uniform because of light generation by semiconductors and phosphors. It drops significantly at the edges of the visible area, with stronger components and gaps in between. The spectrum of common white LEDs is made up of the narrow component of a blue LED and the broader component produced by phosphors. LEDs with better phosphors and a more uniform spectrum show better color rendering properties.

Spectrum 1 depicts an LED with good color rendering, and spectrum 2 an LED with very good color rendering.

What does the color rendering index CRI mean with LEDs?

The color rendering index CRI, also called Ra in German-speaking countries, is the most common method of representing the quality of color rendering in a measurable way and as objectively as possible.

To determine the LED color rendering index CRI,the light source to be evaluated is compared with a reference light source (either an incandescent lamp or the standardized illuminant D65 for "medium daylight", depending on the light color). The comparison is made for test colors R1 to R8 (exclusively pastel colors), and the arithmetic mean of the color rendering of these 8 test colors gives the general color rendering index CRI.

The maximum achievable CRI value is 100. A CRI value higher than 80 is considered "good", and values above 90 are classified as "very good".
R1 - R14 reference colors for CRI method

R1 - R14 reference colors for CRI method

A weakness of the general color rendering index CRI is the assessment of color rendering over only 8 pastel colors with no consideration of saturated hues. Red hues in particular are relevant in many applications (e.g. fashion, cosmetics, food) and are not considered in the CRI. In the procedure for determining the color rendering index, 6 additional test colors have therefore been defined for which color rendering can additionally be determined and specified for more detailed assessment. Saturated red is test color 9, and the color rendering for red is therefore known as "R9" and is often given as a supplement due to its high relevance for applications. With white LEDs, values of R9 > 50 are already considered very good, and values of > 90 are excellent.

Which CRI values apply to indoor spaces?

The CRI value should be at least 80 in indoor areas containing people. The European standard DIN EN 12464-1 for the lighting of indoor workplaces specifies minimum requirements for different work areas and visual tasks. The more important the correct evaluation of colors is for the respective activity, the higher are the requirements for color rendering – maximum requirements are found, for example, in the health sector.

What does TM-30 mean with the color rendering of LEDs?

As an alternative to the CRI method, the American standard TM-30-20, harmonized with CIE 227, defines the values Rf and Rg. These refer to the similarity of a test light source to a reference spectrum in terms of color fidelity (Rf - fidelity) and saturation (Rg - gamut). The reference light source is a standard CIE-D light source corresponding to daylight.

Rf is comparable with CRI and the calculation method and maximum value of 100 are identical. By considering 99 instead of 8 reference colors, lower values often result with Rf compared to CRI.

Rg specifies the color range, i.e. the size of the displayable color space. A light source with precise, true-to-nature color rendering has an Rg value of 100; both smaller and larger values are possible.

Rg values above 100 are an indication that color hues are particularly emphasized and rendered more strongly than those produced by the reference light source.
TM-30 color vector graphic

TM-30 color vector graphic

The Rf and Rg values are usually shown in a diagram. However, this alone does not provide definitive information about the color rendering quality of a lamp. This is only possible with the color vector graphic that displays the size and direction of color shift for selected test colors compared to a reference light source.

If the determined curve is located outside the circle valid for the reference, colors in this area are rendered with oversaturation. If it lies within the circle, the corresponding colors are rendered with a lower saturation.

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