What is the Color Rendering Index (CRI)?
The Color Rendering Index (CRI) measures a light source's capability to accurately reproduce the colors of illuminated objects. Though the definition seems straightforward, it encompasses several aspects. Let's break it down into three essential parts:
I: CRI as a Score with a Maximum of 100
CRI is a quantitative score that evaluates a light source's ability to render colors. Like test scores, a higher CRI value represents better performance, with 100 being the ideal. Scores of 90 and above are considered excellent, while those below 80 are typically deemed poor. This metric provides a convenient, single-number evaluation.
II: CRI Applies to Artificial, White Light Sources
Light sources are classified as either artificial or natural. When considering color quality, we focus on artificial forms, such as LED and fluorescent lamps, in comparison to daylight or sunlight—a natural light source. CRI specifically measures the color accuracy of artificial white light.
III: CRI Measures Reflected Colors Under Artificial Lighting
Color perception under natural light, like sunlight, results from the spectrum's interaction with objects. An object's color is determined by the colors it reflects. Artificial light sources, like LEDs, aim to replicate daylight's color rendering. CRI quantifies how closely an artificial light reproduces the colors of objects compared to natural daylight.
Understanding CRI's Invisibility and Measurement
CRI is not evident by simply observing the light's color. Instead, it becomes apparent when shining the light on various objects with differing colors. The CRI measurement process involves algorithmic calculations based on spectral measurements, comparing the light source's reflected colors to those under natural daylight of the same color temperature.
Measuring CRI
The CRI measurement involves:
1.Determining the light source's color temperature.
2.Virtually shining the light on 15 test color samples (TCS).
3.Comparing the reflected colors to those under natural daylight of the same color temperature.
Calculating the "R" score for each TCS.
4.Averaging the R values to obtain the CRI score.
For color temperatures below 5000K, Planckian radiation spectrum (e.g., incandescent, halogen) is used for comparison.
Common CRI Values and Acceptability
A CRI of 80 is the general baseline for indoor and commercial lighting. Applications requiring high color accuracy, such as hospitals or textile factories, often aim for 90 CRI and above. Comparing individual R values, particularly R9, can help when selecting lights with CRI above 90.