What Is the Color Rendering Index?
The color rendering index (CRI) is a metric that considers how accurately a color is rendered under lighting conditions as compared to its rendering under natural light. Another way to think of it that may be simpler to understand is that CRI is a measurement of the ability of light to “reveal” colors and impact how they’re perceived.
When it comes to architectural glass, a CRI measurement will indicate how differently the color of objects behind glass will be rendered as compared to objects under natural daylight. Architects thinking about CRI are likely to ask themselves, “If I put this type of glass in my window opening, what effect will it have on the color of the interior view of the outside?”
Generally, CRI values in the glass industry fall into certain ranges:
| 90-100 | Excellent color rendering |
| 60-85 | Fair, though colors may appear somewhat distorted |
| 55 and below | Poor |
Does CRI Measure Lightness and Darkness?
No – CRI is not a measurement of how light or dark an insulating glass unit (IGU) is, nor is the CRI value affected by how light or dark a piece of glass is.
CRI measurements are focused on hue and saturation. For this reason, glass products that are tinted, hued or colored are more likely to feature a lower CRI since the color of the glass influences how views through it are perceived. The more neutral your glass selection is when it comes to color, the higher the CRI value will be.
On the other hand, neutral-tinted glasses such as grey will not have as big an effect on CRI because the glazing of the glass itself does not significantly change how colors are viewed through the glass, even if the tint of the glass is quite dark.
What Effects Do Different Substrates and Coatings Have on CRI?
Low-iron glass substrates offer some of the highest CRI values across the glass industry thanks to the ultra-clear nature of the product. With significantly less green tint than regular clear glass, low-iron substrates can offer CRI values higher than 95, which is often even higher than clear glass. While clear glass offers great CRI performance as well, its green characteristics can have an impact in IGUs when multiple lites are used.
Low-emissivity coatings can impact CRI, though the degree to which these coatings affect CRI values is dependent on the color profile of the coating. The less neutral the color, the lower the CRI value will be.
However, due to the transparent nature of glass, sometimes the difference in CRI between various substrates, coatings and tints can be very small. In fact, some tinted glasses can even offer color rendering performance similar to low-iron clear glass and low-emissivity coated glasses.
What Should I Consider About CRI When Selecting Glass?
First, it’s important to note that a high CRI value isn’t everything. While CRI is an important metric to consider when it comes to making glass selections, it is not always necessary to sacrifice elements of your design intent to maintain a high CRI. Colored glasses do carry a lower CRI value, but if the intent is to feature color, a lower value is usually acceptable.
However, if vibrant colors are not a design priority, consider specifying low-iron glass substrates that improve clarity in an IGU as well as neutral-tinted glasses to improve color rendering.
For additional information about designing with Vitro Architectural Glass products, please visit vitroglazings.com.
For any other glass questions, please contact Vitro Glass or call 1-855-VTRO-GLS (1-855-887-6457).
Updated on April 21, 2023
