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Wednesday, November 25, 2020

What is Colour Rendering Index (CRI / Ra) ?

The color rendering ability of a light source to an object is called color rendering, which is compared with the appearance of the object under the same color temperature or reference light source ( incandescent lamp or painting light). The spectral content emitted by the light determines the light color of the light source, but the same light color can be composed of many, a few or even only two monochromatic light waves, and the color rendering of each color is also very different. Light sources of the same light color have different spectral compositions, and light sources with a wider spectral composition are more likely to provide better color rendering quality.

When there is little or lack of the main wave reflected by the object under the reference light source in the light source spectrum , the color will produce obvious color shift. The greater the degree of chromatic aberration, the worse the color rendering of the light source. The color rendering index coefficient (Kau fman) is still a common method to define the color rendering of light sources.

Faithful color rendering

A light source with a high color rendering index (Ra) must be used to correctly express the original color of the substance, and its value is close to 100, and the color rendering is the best.

Effect color

To clearly emphasize a specific color, and express the beauty of life, you can use additive colors to enhance the color rendering effect. A low color temperature light source, can be more vivid red; color temperature light source is irradiated with medium, having a cool feeling blue; high color temperature light source irradiating the object with a cold feeling.

Take the average value of the color rendering index of the first 8 common colors and record it as Ra to characterize the color rendering of this light source. The color rendering index is represented by Ra. The larger the Ra value, the better the color rendering of the light source.Take the average value of the color rendering index of 15 common colors and record it as Re.

The color rendering index has 15 colors and 15 color names:

R1, light grayish red;

R2, dark grayish yellow;

R3: saturated yellow-green;

R4, medium yellow-green;

R5, light blue-green;

R6, light blue;

R7, lavender blue;

R8, light red purple;

R9, saturated red;

R10, saturated yellow;

R11, saturated green;

R12, saturated blue;

R13, Caucasian skin color;

R14, leaf green;

R15, yellow skin color.

Relationship with color rendering

When there is little or lack of the main wave reflected by the object under the reference light source in the light source spectrum, the color will have obvious chromatic aberration. The greater the degree of chromatic aberration, the worse the color rendering of the light source. The color rendering index coefficient (Kau fman) is still a common method to define the color rendering of a light source.

The color rendering index of sunlight is defined as 100, and the color rendering index of incandescent lamps is very close to sunlight, so it is regarded as an ideal reference light source. This system uses 8 standard color samples with medium chroma to test, compare the deviation degree of the 8 colors under the test light source and the same color temperature benchmark, to measure the color rendering index of the light source, and take the average deviation value Ra20-Ra100 , Ra100 is the highest, the greater the average color difference, the lower the Ra value. Light sources below 20 are generally not suitable for general use.

Index (Ra) grade color general applications

90-100 1A Excellent place where precise color contrast is required

80—89 1B / Places that require correct color judgment

60—79 2 Places that generally require medium color rendering

40—59 3 / Places with low requirements for color rendering and small color difference

20—39 4 Poor places with no specific requirements for color rendering

The theoretical color rendering index of incandescent lamps is 100, but there are many kinds of incandescent lamps in real life and their applications are different, so their Ra values are not completely the same, it can only be said to be close to 100, which is the lamp with the best color rendering.

The color rendering index values of specific lamps can be seen in the table below.

Light source color rendering index Ra

Incandescent lamp 97

Daylight fluorescent lamp 80—94

White fluorescent lamp 75-85

Warm white fluorescent lamp 80-90

Tungsten halogen lamp 95—99

High pressure mercury lamp 22—51

High pressure sodium lamp 20-30

Metal halide lamp 60-65

Sodium thallium indium lamp 60-65

Dysprosium lamp above 85

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