What is D65 and what is it used for?
What is D65 and what is it used for?What is white?
A seemingly simple question can have many answers, depending on who you ask. And for many applications, the exact definition can have significant consequences.
One such definition is called Illuminant D65. In this article, we'll go through the exact specification and how you can use this to make smart decisions about white lighting purchases.
What is D65 used for?
D65 is used extensively in industries that require a simulation of natural daylight, such as digital art, film, photography, colorimetry and other visually demanding applications.
The driving force behind the idea of D65 is to ensure that a single standard for lighting can be used across products, manufacturers and industries.
Even when the subject in question does not have a white color (e.g. red paint), D65 is still used as the starting point to measure the reflected color. Why? Just like with CRI in lamps, different shades or spectral compositions of white light can produce different appearances.
This is why a D65 light source is crucial for accurate color perception and evaluation.
Unlike correlated color temperature (CCT), for which the same CCT value can have different green/magenta hues, D65 unambiguously defines an absolute spectrum and color point. (More on this below).
Note that for graphic arts and printing industry applications, a similar but different standard called D50 is used.
D65 for Bias Lighting
D65 is a very common standard used for monitor and display calibration. In short, this means that when a monitor attempts to display a pure white point (e.g. RGB values of 255/255/255) the color emitted by the display matches exactly the D65 color point.
When performing color-critical tasks on a monitor or display, ambient lighting conditions can alter our perception of the colors on the screen. While these may be fun "optical illusions" for everyday people, this phenomenon can be extremely detrimental to producing or evaluating color-critical work on a computer screen.
In the graphic above, squares A and B are the exact same shade of grey! (Yes, it's hard to believe, but it's true.) Due to the color of the surrounding squares, our visual system perceives B to be "white" and A to be "black" despite them being the same shade.
Similarly, ambient lighting can play a significant role in one's perception of a monitor display, and this makes having D65 calibrated ambient lighting just as important.
See more on Waveform Lighting's D65 Bias Lighting solutions.
Defining D65 as a spectral power distribution
D65 is a particular embodiment of the D series of illuminants defined by the CIE, and has a color temperature of approximately 6500K.
The D series illuminants attempt to simulate the spectral power distribution of natural daylight. It includes the spectral influences of both direct sunlight as well as the diffused blue sky on a clear day.
D65 roughly corresponds to the color temperature of the sky on a clear day around noon.
First and foremost, D65 is defined by a particular spectral power distribution that is generated by a D series daylight simulator.
As you can see, it is generally broad and covers the entire visible spectrum. Because it is a daylight simulation, the spectrum is not completely smooth and is influenced by various absorption and diffusion that occurs in Earth's atmosphere.
D65 as chromaticity coordinates
In addition to its spectral definition, D65 can also be defined by its chromaticity coordinates in the CIE 1931 xy plane.
The D65 coordinates are (0.31271, 0.32902).
Which definition is better - D65 as a spectrum or D65 as coordinate points?
Answering this question requires an understanding of what the spectral power distribution tells us versus what chromaticity points tell us.
The spectral power distribution is akin to a light source's metaphorical DNA - it is the fundamental data about a light from which color temperature, CRI and other photometrics are calculated from. Chromaticity coordinates are also derived from the spectral power distribution.
As a result, comparing a light source to the D65 spectrum can tell us a great deal more than comparing it to just the D65 chromaticity coordinates.
In general, for applications where D65 light is used to shine onto an object, the spectral definition is very important. In other words, the light source should come as close to the D65 spectrum as possible.
This is because the appearance of an object's color is dependent on the colors that are reflected off of that object. If the wavelengths that make up the illumination source are far off from that of the D65 spectrum, the reflected spectrum will also likely to be different.
Applications where this would be the case include indoor lighting for color inspection and photography.
In reality, however, it can be much more convenient to compare chromaticity coordinates, and comparing spectral power distributions and quantifying similarity can be challenging. In such cases, using the D65 chromaticity coordinates in conjunction with a CRI evaluation would be a good choice.
D65 is not the same as 6500K!
It is important to understand what the first "C" in CCT stands for - correlated color temperature. A single CCT value, including 6500K, can have many green/magenta hue variations that all correlate to a color temperature of 6500K.
Most manufacturers will have an internal tolerance on the amount of green/magenta hue permissible, but another problem arises when determining the target color point itself.
ANSI separately developed its standard for 6500K using the xy chromaticity coordinates of (0.313, 0.337), and most manufacturers will strive to hit this color point as their target color point. How far off is this from the D65 xy chromaticity coordinates of (0.31271, 0.32902)?
The answer - quite different!
Graphically, we can see that the ANSI 6500K center point lies far above the D65 point, as well as the daylight locus.
In terms of MacAdam ellipses / SDCM, ANSI 6500K and D65 are approximately 3.5 steps away from each other, a certainly noticeable color difference.
Duv values of ANSI 6500K vs D65 also differ significantly - 0.0071 vs 0.0032.
Qualitatively, this means that ANSI 6500K will give you a significantly greener hue, while D65 most accurately matches daylight.
What this means is that, when looking for D65 light sources, simply confirming a CCT of 6500K is not enough - you must confirm that the chromaticity coordinates match D65.
In fact, many manufacturers will consider a D65 light source to be inferior due to its distance from the ANSI standard.
D65 with 99 CRI
We've developed a versatile lighting solution for applications needing to approximate D65 as closely as possible. The D65 color point ensures a crisp, pure daylight white, and furthermore, our 99 CRI rating provides unmatched color accuracy and reproduction. Our ABSOLUTE SERIES LED module is available for purchase in both D65 as well as D50 variants.
Contact us to learn more.
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