Color Precision

Color Precision

If X/Y coordinates and Color Quadrants are the most precise way to describe color, why do we use CCT at all? 

Until there were LEDs, it was rare to deviate from that black-body line. All incandescent filaments glow like a black body. Because of that, color temperature was a very accurate way to describe incandescent bulbs. 

When LEDs came around, even though there was serious variation in the color spectrum deviating from that blackbody line, there was no global standard established for what "Cool White" or "Warm White" precisely meant as a specific quadrant. We still only have broad "White" quadrants, which are a range of possible colors. 

Some industries defined their own quadrants - for example, we have a more specific "Selective Yellow" color defined from SAE. But unfortunately, there's no color quadrant that applies to descriptive colors for consumers. 

ANSI did define color quadrants for acceptable color temperatures, which are widely used by chip manufacturers. However, they didn't decide what each quadrant was actually called, leaving that up to lighting manufacturers. 

ANSI Quadrants:

So, "Cool White" and "Warm White" remain as descriptive terms only, without a standard established. And at this point, it's probably too late - CCT is well-known by consumers, even if it's not precise, because many manufacturers do not follow the ANSI quadrants.

How precise do we need to be?

At some point, two white lights do look the same. Right? That's what David MacAdam decided to research. He had people look at different lights and indicate when one looked like a different color. The result of his work gave us the MacAdam Ellipse. 

A MacAdam ellipse is shown as an oval shape on the chromaticity chart. If two lights shine with a color that's in the same ellipse, a human cannot perceive the difference:

The problem with color matching?

Humans are very good at noting differences in color. That original MacAdam ellipse is described as "1-step." Again, that's the region where there is no discernable difference in color. If you look at the image above, there are several ellipses shown that are progressively larger. This image shows 2-step, 4-step, and 7-step ellipses, which refers to the total standard deviations away from that original 1-step ellipse. 

We don't have a 1-step ellipse shown on most graphs, because it is impossible for LED manufacturers to even produce LEDs that are that close. Most LEDs will be sold in a 5-step region, or if you want to pay a very high price, you could purchase in a 3-step region. This still doesn't mean they will all perfectly match to the human eye, but it will be very, very close.

The McAdam ellipse scale is also known now as the SDCM Scale, which is a more straightforward description - Standard Deviation of Color Matching. 

In the end, it's all related. For example, ANSI actually generally followed a 7-step deviation when they created their CCT quadrants. Note the 7-step ellipse inside each quadrant:

So, what can we do about it?

No one has defined a quadrant for "Cool White" headlights. There's SAE White, but that's a big range, from cool to warm. So you could have two lights that both meet the legal requirements, but they look different to consumers. How can we keep our colors consistent? 

Diode will be working to establish its own color quadrants, to make sure our LEDs are consistently the same white color, or selective yellow, etc. These will basically just be "tighter" than the existing SAE white or selective yellow, so we can keep our products consistent.

We may have to deviate from this from time to time - for example, when we want to match a factory LED that looks a bit different. However, we should try to keep our own white LEDs as consistent as possible.

This isn't a new concept - in addition to specifying a McAdams ellipse, LED manufacturers also sort their own LEDs into small little quadrants, called "binning." Here is one graph provided by a manufacturer. As you can see, they show CCT lines, but it's all plotted on the same chromaticity chart, to be highly specific. When we buy LEDs, we might specify that we want the "SN" bin. Each LED manufacturer defines their own binning quadrants.

Note: this is hard!

It's very difficult to get LEDs to shine in a specific color. There are variations in manufacturing to deal with. As we've talked about, manufacturers don't even sell 1-step color matching LEDs. From there, the same LED will shine in a different color depending on the temperature, power level, and many other factors. Our LEDs will shift color very slightly as the light heats up. And then, color can shift based on the reflectors and optics. If you look at factory vehicles, you'll often note very small inconsistencies between headlights and fog lights, for example. The business of color matching LEDs gets complicated and difficult, very quickly.