Why do led tape light with the same color temperature look different?

In the era of traditional light source, when it comes to color temperature, we only care about “yellow light and white light”, and don’t care much about the problem of light color deviation. Anyway, the color temperature of the traditional light source is a few kinds, choose a line, generally will not run away from where. To LED era, we found: LED light color of any kind. Even if it is the same batch of beads, it is possible to deviation to strange, red, green and lively

LED is said to be good, energy saving and environmental protection. But you know what? The LED do rotten enterprises really many! The following is the application of LED lamps of a famous domestic brand in a large project sent by V friends. Look at the light distribution, the consistency of color temperature, and the faint blue light…

In view of this chaos, a conscience led bar light factory promised to customers: “our lamps, color temperature deviation within ±150K!” , there are also design companies in the product selection, the specification book indicates: “lamp bead color temperature deviation within ±150K”.

This 150K is based on the traditional literature conclusion: “color temperature deviation within ±150K, the human eye is difficult to detect”. They think: the provision of the color temperature “±150K within”, you can avoid the occurrence of red green things. Well, it’s really not that simple…

For example, in the aging room of this plant, I saw two sets of light bars with distinct light colors, one normal warm white, the other distinctly green. As shown in the picture, can you find the difference between the two lights? One red and one green, yes or no? According to the above statement, the human eye can see the obvious difference, the color temperature gap must be higher than 150K?

The an took out from the bag “artifact” — miniature spectrum tester lighting passport, respectively measured to them once. The result is this…

See yet? Human eyes look completely different from two light sources, “correlation color temperature” only 20K difference! Do you really think… The color on the right is 20K warmer, and you can see it?

Is the conclusion that “color temperature deviation within ±150K, the human eye is difficult to detect” wrong? Let me explain: Color temperature (CT) vs. relative color temperature (CCT). We usually say the light source “color temperature”, in fact, generally refer to the “relevant color temperature” column on the test report. The definition of these two parameters in Architectural Lighting Design Standard GB50034-2013:

Color Temperature see Colour Temperature

When the color of the light source is the same as that of the black body at a certain temperature, the absolute temperature of the black body is the color temperature of the light source. Also known as “chroma”. The unit is kelvin.

Correlated Colour Temperature

When the color point of the light source is not on the black body track, and the color of the light source is closest to that of the black body at a certain temperature, the absolute temperature of the black body is the relative color temperature of the light source, referred to as the correlation color temperature. The unit is kelvin.

To explain what is “relevant”, take an easy to understand chestnut. Suppose there is only one railway in our country: the Beijing-Guangzhou line. In my early years I was poor and had to travel by train. Wife makes a phone call to check guard: “where are you now?” I happened to be in Zhengzhou, replied: I am in Zhengzhou. Tomorrow she asked again, I said: wuhan now. Therefore, we used to use Beijing, Zhengzhou, Wuhan, Guangzhou to mark their location.

Suppose that one day I am rich enough to buy a car and go on a road trip, leaving the Beijing-Guangzhou line and arriving in Xi ‘an. The wife has no culture, only know the Beijing-Guangzhou line, do not know where Xi ‘an. I had to find a place closest to the Beijing-Guangzhou line and tell her I was in Zhengzhou. You say I nonsense: “Xi ‘an from Zhengzhou hot yao far, you also mean to say?” But, yeah! I said “relevant location”! If you are in Nanjing, you can say: “I am in Wuhan!”

The above is purely a joke to illustrate the relationship between “position” and “related position”. Let’s go back to the color temperature.

The latitude and longitude on a map indicate the location of a city, and the (x, y) coordinates on a color map indicate the location of a light color. In the figure below, (0.1, 0.8) is pure green, and (0.7, 0.25) is pure red. The middle part is basically a relatively white light, and this “white degree” can not be described in words, hence the concept of “color temperature”.

The light emitted by a tungsten bulb at different temperatures is represented on a color map as a line called a “black body track,” abbreviated BBL, or “Planck curve.” Blackbody radiation is the color of light that our eyes look at as “normal white light.” Once the color coordinates of the light source deviate from this curve, we consider it “biased.”

Our earliest tungsten light bulbs, no matter how they were made, would only fall on this line of warm and cold white light (the thick black line in the picture), and we call the color of light at different points on this line “color temperature.” Nowadays, with advanced technology, the color of white light produced by us does not necessarily fall on this line. We can only find a “nearest” point and read the color temperature of this point, which is called its “correlation color temperature”. Now you know? Even if the CCT of two sources is exactly the same, the color of the light can be quite different, not to mention the deviation of ±150K. Just like the above “Xi ‘an vs Zhengzhou”…

Zoom in on the “isotherm” at 3000K:

What? You still don’t get it? There’s nothing I can do about it… In a word, believe Xugong: “it is not enough to say only the color temperature, even if everyone is 3000K, there will be red or green.” Check gang said in Zhengzhou, he may be in Xi ‘an, ha ha ~

That…… What should we look at?

Let’s learn a new metric: SDCM.

Still using the chestnuts above, the “correlation temperature” of the two sets of bars is only 20K different! It’s almost identical. But in fact, they are distinctly different colors of light. What’s the problem?

Here’s the truth: Let’s take a look at their SDCM:

The image on the left shows the warm white 3265K on the left. Note the yellow dot to the right of the green ellipse, which is the position of the light source on the chromaticity diagram. The right picture is green on the right, and his position is out of the red ellipse.

Look not to understand? We still use chestnuts from the beijing-Guangzhou line last time.

Last time I was in Xi ‘an, I told my wife I was in Zhengzhou. Later she was smart, learn to read maps, know that in addition to the Beijing-Guangzhou line, there is a very broad territory. Draw several circles on the map: yellow, blue, green, red, respectively, 50, 100, 150, 200 kilometers from “downtown Zhengzhou”. Regulations: in the future, in addition to the city, report the distance from the city center, more than 200 kilometers, can not say “in Zhengzhou.”

This “distance circle”, used in chromaticity coordinates, is the concept of the SDCM.

Let’s take a look at the position of the two light sources on the chromaticity chart above. Their nearest values to the black body curve are 3265K and 3282K respectively. It looks like they are only 20K apart, but in fact they are much farther apart ~ as far as Xi ‘an is from Zhengzhou ~

There is no 3200K line in the test software, but only 3500K. Let’s draw a circle of 3200K by ourselves:

The yellow, blue, green and red circles represent 1, 3, 5 and 7 steps from the “perfect light color” respectively.

What does this “step” mean? Why not perfect circle but ellipse? … If you must understand, please enter “140803” to see the article “Please don’t misinterpret the McAdam Ellipse”.

In fact, it doesn’t matter if you don’t understand, as long as you remember: when the difference of light and color is within 5 steps, the human eye basically can’t distinguish, that’s enough. The new NATIONAL standard also stipulates: “the color tolerance of the same light source should not be greater than 5 SDCM”.

Let’s see: the point below is within 5 steps of the “perfect” light color, which we consider to be a more beautiful light color. And the top spot, has run out of 7 steps to go, the human eye can clearly detect his partial color.

We will use SDCM to evaluate light color, so how do we measure this parameter? I suggest you bring a spectrometer with you… No kidding, portable spectrometers! For outdoor lighting, the accuracy of light color is particularly important. Red and green are ugly. So, we want to polish the eyes, their own “cloud know light artifact”, to catch unqualified products.

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