The architecture of light: architectural lighting design concepts and techniques. A textbook of procedures and practices for the architect, interior designer and lighting designer pdfdrive com

Color Rendering Indices (CRI) in the 60’s and 70’s are relatively incomplete and are unacceptable for making critical color decisions.
Figure 7.1 Color Rendering Index (CRI) of different light sources expressed from 0 to 100.
CRI values in the 80’s do a reasonable job of revealing colors
CRI values in the 90’s render colors very accurately and are appropriate for color critical environments.
CORRELATED COLOR TEMPERATURE
This is a method of describing the apparent color of a light source that is very nearly neutral. When a light source appears colored to our eye, it is due to an unbalanced spectral output. If a source emits little or no green light, it may appear reddish or “warm.” A source may also appear “warm” or reddish if it emits every color, but emits a higher proportion of red.
Color Temperatures are expressed in Degrees Kelvin or simply Kelvins (because The Kelvin scale is absolute, it needs no units).
The reason that we express this color appearance as a temperature is a
result of the experimenting that led to the scale. The color temperature scale is expressed as the colors that a black body radiator exhibits as it is heated to extreme temperatures. A black body radiator is akin to a fancy block of iron that won’t melt. As this fancy block of iron is heated to high temperatures, it begins to glow. The first color that the iron block will glow is a dull, deep red. If heated further, the same block of iron will begin to glow orange and then yellow.
Experiments show that if heated even higher, the color exhibited by this block of
iron will travel all the way through the color spectrum. Thus, the next stop is green, onward to blue. The path that this color transition makes is not linear, so
Figure 7.2 The range of color temperatures used to describe common electric light sources
the green happens to be very, very pale and is, for our purposes, considered a colorless neutral.
The glowing red color of the iron occurs at a temperature of about 1800
Kelvins. With this logic, when a light source exhibits this reddish color, rather than call it reddish, we describe the light source as having a Color Temperature of 1800 Kelvins. In this system, orange occurs at about 2500 Kelvins, followed by yellow at about 2800 Kelvins. Following the progression of temperature and the color spectrum, keeping in mind that we use the pale green as our neutral, the Color Temperature translations follow the table in Figure 7.2.
We most often use this system of color description for our engineered light sources like fluorescent lamps, Light Emitting Diodes, and High Intensity Discharge lamps.
These color associations are
only approximations and differ in meaning from source to source and even brand to brand. This leaves plenty of room for discrepancy, but the basics of Color Temperature to describe the slight color of engineered sources like fluorescent remains useful.