Color rendering index (CRI) is a measure of the level of match between a light source and the true color of an object under natural light. This parameter allows users to orientate themselves in determining the quality and characteristics of accurate reproduction of the hues and colors of objects using an artificial source.
One of the features of the human eye is the change in shades and color picture when the tone of light radiation directed at an object changes. Under one shade of lighting, the subject looks natural, while the light flux of another shade gives the subject incorrect colors and an unnatural look. Color temperature is one of the main factors affecting the propagation of certain light sources.
Peculiarities
The parameters of the lamp should be as close as possible to natural lighting, this can achieve the most naturalistic look of the environment and the luminous flux acceptable to the human eye. In order to more accurately determine the range of light, a relative value is used, called the color rendering index.
Light is a rather complex and capacious concept. Its main characteristic in terms of color separation is its spectral composition. Roughly speaking, in order for a yellow object to look yellow, it must reflect only the waves of this frequency, absorbing the rest. When working with Ra standards, the quality of the lighting of the subject matters. The reason is in one of the properties of the human eye: the higher the quality of illumination, the more subtle shades the eye can distinguish.
The term CRI first appeared in circulation in the 60s of the twentieth century, it was developed to compare lamps with a continuous spectrum. Later, light sources appeared with other characteristics. Today, CRI has a numerical value from 0 to 100, which characterizes the ratio of artificial light to the true one obtained in natural light. In SI, this value is referred to as CRI. Translated from English, this abbreviation stands for color rendering index, the same parameter is called Ra.
Important! CRI is the color rendering index while Ra is its value.
Quality assessment and index determination
The reference value of Ra is taken from sunlight, its value is taken as 100. It should be noted that in the process of development it has been repeatedly changed and improved. So, until 1974, the method for determining color rendition consisted of checking 8 reference true colors, in comparison with those obtained when working with the tested source. After 74 years, 6 additional shades were added to the main ones. At present, the method for determining Ra consists in calculating the color shifts of 14 reference color patterns with true sunlight (100 units) or a completely black incandescent body, which, as you know, absorbs all frequencies, this value is taken as 0.
The measurement is made as follows:
- The light source, the color rendering of which is to be determined, is directed to the color templates. Equipment specially designed for this purpose determines the color spectrum of the reference plate under the test lamp;
- True light sources are directed to the standard, the characteristics of the lighting are recorded using special equipment;
- With the help of a computer and developed software, the difference between the luminous flux and the lamp level in comparison with the reference true light is calculated;
- The arithmetic mean of the CRI is calculated, which will be considered a parameter of this light source.
The disadvantage of this method is the limitation of the definition of color rendering. The method works only with continuous spectral light sources, the coefficient of which is more than 90Ra. At lower values, this color rendering index is inaccurate in the definition, since a situation may arise when several lamps with the same Ra value will have a different shade of light and illuminate objects in different ways, having different color temperatures.
Despite new developments, it was not possible to get rid of this shortcoming, for this reason it is the temperature that is currently the main characteristic in determining the color rendering of light sources. It should be noted that the main direction in the development of light sources is currently white LEDs, but even in this case, the determination of the index by this method is not accurate due to the small presence of the red frequency in their radiation. At the same time, the color rendering of white LEDs is significantly higher than indicated by the control parameters determined by the method indicated above.
Interesting. The basic CRI tones have the following designations: R1 - withered rose, R2 - greenish yellow with a bias towards brown, mustard color, R3 - one of the shades of green, lime green, R4 - light emerald green, R5 - turquoise, R6 - bright heavenly blue hue, R7 - purple, called purple asters, R8 - lilac.
CRI problems, search for new standards
Despite the previously manifested shortcomings of the CRI technique, an official decision on them appeared only in 2007, it was in this year that the international commission of illumination determined the insufficiency of this index for determining the parameters of white LEDs. A new measurement technique was already developed by 2010. The CQS, or color quality scale, is based on 15 basic saturated color standards. But, despite the external similarity of the name of the index and the presence of reference samples, this measurement technique is of a completely different type. There is no such strong influence of mono shades in the CQS system. So, for example, the influence of red is not so pronounced. This allows this calculation system to fit more closely into the color spectrum of light acceptable to humans.
But despite this, the CQS color rendering index is imperfect, in it, as in the earlier CRI system, there is completely no factor for correcting the hue of light depending on its hue and saturation. The human eye is a rather complex device, and it has not yet been possible to fully convey its capabilities for assessing light radiation to the standard. In 2015, a new standard, TM-30-15, was introduced, unlike the earlier ones, this time 99 color templates were used, including not only reference samples, but also various kinds of objects. When calculating, parameters such as saturation and tint of the luminous flux are used. But to date, this index has not become widespread due to the complexity of the measurement and its inconsistency with a number of parameters of other lighting sources, except for LED lamps.
Modernity and selection by light characteristics
As mentioned above, the newly emerging standards CQS and TM-30-15, despite their accuracy, have not received widespread acceptance. Basically, the obsolete CRI is still used to determine the value of the correct rendering of color shades. For this reason, when selecting light sources, first of all, it is necessary to pay attention to CRI lamps above 90 Ra, since they are the ones that are closest to the true light. Smaller values can only be used for industrial and domestic purposes. A value of less than 70 Ra should be used only when installing street lighting, as well as in emergency and low-demand networks.
For residential and office premises, it is ideal to use lamps that have passed the TM-30-15 certification - this is the standard that most fully reflects the true color rendition.
