CIE Standard Illuminants are “theoretical spectral power distribution (SPD) data” defined by the International Commission on Illumination (CIE). They are not actual physical light sources, but rather standardized data providing a unified benchmark for color measurement and optical testing. They cover various types corresponding to different natural or artificial lighting scenarios. For instance, Standard Illuminants A, B, and C were introduced in 1931, representing tungsten filament lamps (incandescent lamps), direct sunlight, and average daylight, respectively; the D series (1967) represents variations of daylight; Standard Illuminant E represents an equal-energy illuminant; the F series (2004) corresponds to different fluorescent lamp spectra; and the LED series (2018) is an independent standard system developed by the CIE specifically for LED light sources.
Note: CIE standard illuminants are abstract optical radiators with specified spectral power distributions, whereas standard light sources are actual physical light sources that can realize these spectral distributions.

Standard Illuminant A
Intended to represent typical gas-filled tungsten filament lamp (incandescent lamp) ambient lighting, with a relative color temperature of 2856 K. Its spectral distribution is a continuous spectrum that monotonically decreases from long wavelengths (red) to short wavelengths (violet), exhibiting a distinct warm tone tendency.

Standard Illuminant B
Intended to represent direct sunlight environments, with a correlated color temperature of approximately 4874 K. Its spectral distribution lies between the low color temperature of A and the high color temperature of C. Although it once served as a benchmark for noon sunlight, it has now been largely replaced by the more accurate D series illuminants in modern colorimetry applications.

Standard Illuminant C
Intended to represent average daylight without ultraviolet components (which can be reproduced by a standard light source using Illuminant A with specific filters), with a correlated color temperature of approximately 6774 K. Although gradually replaced by the D series in modern industry, it is still used in evaluating certain traditional materials without fluorescent properties.

Figure 1: Relative spectral power distribution of Standard Illuminants A, B, and C

(Image source: https://en.wikipedia.org/wiki/Standard_illuminant#Illuminant_series_LED)

Standard Illuminant D
Intended to represent typical daylight including the ultraviolet band. Its spectral distribution is based on actual daylight measurement data, covering a broad range from 300 nm to 830 nm. It is currently the most widely used benchmark illuminant in the global color industry for evaluating the true colors of objects under natural light.
Typical illuminants:

• D65 (CCT 6504 K): Represents average full-sky daylight, with a correlated color temperature of approximately 6504 K. The spectral center of gravity shifts towards short wavelengths (blue light), with abundant blue and ultraviolet energy. Visually, it presents a neutral to cool characteristic and is the most universally used daylight simulation benchmark worldwide.
  
• D55 (CCT 5503 K): Represents noon sunlight, with a correlated color temperature of approximately 5503 K. Its spectral distribution tends to be balanced between long and short wavelengths. Compared to D65, its blue light proportion is reduced while red and yellow light energy is enhanced, resulting in a more neutral to warm visual perception, meeting the evaluation needs of the printing and graphic arts industry for paper media.
  
• D75 (CCT 7504 K): Represents bluish north sky light, with a correlated color temperature of approximately 7504 K. Its spectral energy is highly concentrated in the short wavelength region.
  

Standard Illuminant F
Intended to represent various fluorescent lighting environments. Its spectral characteristics consist of discontinuous line emission peaks produced by rare-earth phosphors superimposed on a continuous spectrum. This series includes 12 standard spectra (F1-F12) and is mainly used to evaluate the color appearance of products under artificial light sources in commercial or office environments.
Typical illuminants:

• F2 (CCT 4230 K): Represents typical cool white fluorescent (CWF) lamps, the most common standard lighting benchmark in office environments, with a spectral distribution showing a distinct neutral to cool tendency.
  
• F7 (CCT 6500 K): Represents broad-band daylight fluorescent lamps, intended to simulate daylight color temperature, often used in industrial inspection environments with high color rendering requirements.
  
• F11 (CCT 4000 K): Represents typical tri-phosphor fluorescent lamps (TL84), achieving high luminous efficacy through three narrow-band emission peaks. It is a standard comparison light source commonly used in modern global retail stores.
  
• F12 (CCT 3000 K): Represents typical warm white tri-phosphor fluorescent lamps (U30), characterized by narrow-band emission peaks and a soft, warm light color, often used to create a cozy and comfortable lighting environment in commercial retail, dining, or residential settings.
  

Standard Illuminant E
Standard Illuminant E is an idealized equal-energy radiator whose spectral radiant exitance (denoted as $M_{e,\lambda}$) is constant across the entire visible spectrum (380–780 nm). In the CIE XYZ color space, its chromaticity coordinates are $(1/3, 1/3)$. It is not a black body and does not possess a color temperature itself, but it can be approximated by a D series illuminant with a correlated color temperature of 5455 K.

CIE 1931 Chromaticity Diagram: Chromaticity coordinate distribution of typical standard illuminants

(Image source: https://en.wikipedia.org/wiki/Standard_illuminant#/media/File:Planckian-locus-whitepoints-crop.png)
Standard Illuminant E is located below the Planckian locus (black-body locus), and its correlated color temperature (CCT) is roughly the same as that of D55.

LED Illuminant Series
With the increasing popularity of LED lighting, the CIE officially released a series of standard illuminants specifically for LED light sources in 2018 (CIE 15:2018). This series is designed based on typical LED spectra from different technological routes and mainly includes the following categories:

• LED-B series: Simulates the spectrum of blue-pumped phosphor LEDs, covering common color temperatures from warm white to cool white.
  
• LED-V series: Simulates the spectrum of violet-pumped phosphor LEDs, with slightly higher ultraviolet content, suitable for color evaluation of special materials.
  
• LED-R series: Simulates the spectrum of LEDs directly synthesized from red, green, and blue chips, exhibiting distinct discrete peaks in the spectrum.