Typical lighting facts label
Light Output/Lumens
Measures light output. The higher the number, the more light is emitted.
Watts
Measures energy required to light the product.
Lumens per Watt/Efficacy
Measures efficiency. The higher the number, the more efficient the product
Color Rendering Index
(CRI)
Measures color accuracy
Correlated Color
Temperature (CCT)
Measures light color.
“Cool” colors have higher
CCT; “Warm” colors have lower CCT.
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Han-Ping David Shieh

Sources with different CCT
However, the same CCT may produce different color appearances
CCT=6184K
Different appearances
CCT=6184K
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Different Spectra
Ref: Konica Minolta, “What is the
Colour Rendering Index?”
Han-Ping David Shieh

Definition of CRI:
•   Quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source
•   Value of CRI differs between 0 and 100. The value 100 means no color rendition difference between observed light bulb and reference illuminant.
Source
Incandescent/halogen light bulb
Cool white LED
Cool while linear fluorescent
Clear mercury-vapor
Low pressure sodium
CCT(K)
3200
6000
4200
6410
1800
CRI
100
~70
~62
~17
~5
CRI=62 CRI=90
CRI=90 CRI=70 CRI=50
Ref: G. Wyszecki, et al, Color Science: Concepts and Methods, Quantitative Data and Formulae .
Ref: OLLA Project Report, “Color Rendering Index (CRI) of State-of-the-Art Broadband Emitters”
Advanced Display Optics Lab
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Han-Ping David Shieh

Workflow of CRI measurement
Color appearance under test light source
8~15 specified test color samples
1 2 3 4
Color appearance under standard reference illuminant
5 6 7 8
General CRI (R a
)
9 10 11 12
<5000K
CCT of test source
13 14 15
Special use
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Color difference Δ E (in
CIE 1964 (U*V*W*) color space)
CRI=mean of
(100-4.6
Δ E)
>5000K
Standard blackbody
illuminant A
Selection of reference illuminant
Standard daylight illuminant D65
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Spectroradiometer CS-2000 from Konica Minolta
Handysize Smartphone Spectrometer from Allied Scientific Pro
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Han-Ping David Shieh

Sources with high CRI: High requirement of color reproduction
Museum lighting Retail lighting Indoor lighting
Source with relatively low CRI: Concerning high luminance rather than color reproduction
Street lighting
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Vehicle lighting (Special requirement of red light but not
Workshop lighting
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Ref: OSRAM Opto Semiconductors, “Light and
Color Method of Achieving High CRI with LEDs”

•   General CRI (R a
) uses only eight test color samples, which are all unsaturated. Appearance of more colors may not be reflected by R a
.
•   The assessment is totally based on appearance similarity to that of reference illuminant, which can assess incandescence and fluorescent sources well, but may be unsuitable for narrowband spectral emission, like SSL source.
CRI (R a
) is only 27
Visually appealing in fact
Spectrum of RGB LED
•   CRI remains the international standard for assessing color rendering capacity, however, new assessment are being developed, like Color
Quality Scale (CQS), CRI/Gaumut Area Index (GAI) …
Advanced Display Optics Lab
The 15 saturated color samples used by CQS
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