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How to choose the right LED light source

How to choose the right LED light source for in ground light?

With the growing demand for energy saving and environmental protection, we are increasingly using LED lights for in ground light design. The LED market is currently is a mixture of fish and dragon, good and bad. Various manufacturers and businesses are pushing hard to promote their own products. Regarding this chaos, our view is better to let him send a test instead of listening.

Eurborn Co., Ltd will start the selection of LED of in ground light includes appearance, heat dissipation, light distribution, glare, installation, etc. Today, we will not talk about the parameters of lamps and lanterns, just talk about the light source. Will you really know how to choose a good LED light source? The main parameters of the light source are: current, power, luminous flux, luminous attenuation, light color and color rendering. Our focus today is to talk about the last two items, first briefly talk about the first four items.

First of all, we often say: "How many watts of light do I want?" This habit is to continue the previous traditional light source. Back then, the light source was only have several fixed wattages, basically you can only choose among those wattages, you can't adjust it freely, and the current LED today, the power supply is slightly changed, the power will be changed immediately! When the same LED light source of in ground light is driven with bigger current, the power will go up, but it will cause light decrease in efficiency and increase in light decay. Please see the picture below


Generally speaking, redundancy = waste. But it saves the working current of the LED. When the drive current reaches the maximum allowable rating under the circumstances, reducing the drive current by 1/3, the sacrificed luminous flux is very limited, but the benefits are huge:

Light attenuation is greatly reduced;

Life span is greatly extended;

Significantly improved reliability;

Higher power utilization;

Therefore, for a good LED light source of in ground light, the driving current should use about 70% of the maximum rated current.

In this case, the designer should directly request the luminous flux. As for what wattage to use, it should be decided by the manufacturer. This is to promote manufacturers to pursue efficiency and stability, instead of sacrificing efficiency and life by pushing up the wattage of the light source blindly.

The above mentioned includes these parameters: current, power, luminous flux, and luminous attenuation. There is a close relationship between them, and you should pay attention to them in use: Which one is what you really need?
Light color

In the era of traditional light sources, when it comes to color temperature, everyone only cares about the "yellow light and white light", not the problem of light color deviation. Anyway, the color temperature of the traditional light source is only that kind, just choose one, and generally it will not go too much wrong. In the LED era, we found that the light color of in ground light has many and any kind. Even the same batch of lamp beads may deviate to a lot of strangeness, many differences.

Everyone says LED is good, energy-saving and environmentally friendly. But there are really many companies that make LEDs rotten! The following is a large-scale project sent by a friends which purpose A real-life application of a famous domestic brand of LED lamps and lanterns, look at this light distribution, this color temperature consistency, this faint blue light….

In view of this chaos, a conscientious in ground LED lighting factory promised to customers: "Our lamps have a color temperature deviation within ±150K!" When the company is making product selection, the specifications indicate: "It requires deviation of the color temperature of the lamp beads is within ±150K"

This 150K is based on the conclusion of quoting traditional literature: "The color temperature deviation is within ±150K, which is difficult for the human eye to detect." They believe that if the color temperature is “within ±150K” which the inconsistencies can be avoided. In fact, it’s really not that simple.

As an example, in the aging room of this factory, I saw two groups of light bars with obviously different light colors. One group was normal warm white, and the other group was obviously biased. As shown in the figure, we could find the difference between the two light bars. One reddish and one greenish. According to the above statement, even the human eyes could tell the different of course the color temperature difference must be higher than 150K.


As you can tell, two light sources that look completely different to the human eye have a "correlated color temperature" difference of only 20K!

Isn't the conclusion that "color temperature deviation is within ±150K, it is difficult for the human eye to detect" wrong? Don't worry, please allow me explain slowly: Let me talk about the two concepts of color temperature vs (CT) correlated color temperature (CCT). We usually refer to the "color temperature" of the light source to in ground light, but in fact, we generally quote the "correlated color temperature" column on the test report. The definition of these two parameters in "Architectural Lighting Design Standard GB50034-2013"

Color Temperature

When the chromaticity of the light source is the same as that of a 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 K.

Correlated Color Temperature

When the chromaticity point of the light source of in ground light is not on the blackbody locus, and the chromaticity of the light source is closest to the chromaticity of a blackbody at a certain temperature, the absolute temperature of the blackbody is the correlated color temperature of the light source, referred to as correlated color temperature. The unit is K.


The latitude and longitude on the map indicate the location of the city, and the (x, y) coordinate value on the "color coordinate map" indicates the location of a certain light color. Look at the picture below, the position (0.1, 0.8) is pure green, and the position (07, 0.25) is pure red. The middle part is basically white light. This kind of "degree of whiteness" cannot be described in words, so there is the concept of "color temperature" The light emitted by the tungsten filament bulb at different temperatures is represented as a line on the color coordinate diagram, called "black body locus", abbreviated as BBL, also called "Planck curve". The color emitted by black body radiation, our eyes look like "normal white light." Once the color coordinate of the light source deviates from this curve, we think it has a "color cast".


Our earliest tungsten light bulb, no matter how it is made, its light color can only fall on this line that represents cold and warm white light (the thick black line in the picture). We call the light color at different positions on this line “Color temperature". Now that the technology is advanced, the white light we made, the color of the light falls on this line. We can only find a "nearest" point, read the color temperature of this point, and call it his "correlated color temperature." Now you know? Don’t say the deviation is ±150K. Even if the two light sources are exactly the same CCT, the light color may be quite different.

What Zoom in on the 3000K "isotherm": 


LED light source of in ground light, is not enough to just say that the color temperature is not enough. Even if everyone is 3000K, there will be red or greenish colors." Here is a new indicator: SDCM.

Still using the above example, these two sets of light bars, their "correlated color temperature" only differs by 20K! It can be said to be almost identical. But in fact, they are obviously different light colors. Where is the problem?


However, the truth is: let’s take a look at their SDCM diagram


The picture above is the warm white 3265K on the left. Please pay attention to the small yellow dot on the right of the green ellipse, which is the position of the light source on the chromaticity diagram. The picture below is greenish on the right, and his position has gone outside the red oval. Let’s take a look at the positions of the two light sources on the chromaticity diagram in the above example. Their closest values ​​to the black body curve are 3265K and 3282K, which seem to differ by only 20K, but in fact their distance is far away~.


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

The four circles of yellow, blue, green and red respectively represent 1, 3, 5, and 7 "steps" from the "perfect light color". Remember: when the difference in light color is within 5 steps, the human eye can't distinguish it basically, that's enough. The new national standard also stipulates: "The color tolerance of using similar light sources should not be greater than 5 SDCM."

Let's see: The following point is within 5 steps of the "perfect" light color. We think it is a more beautiful light color. As for the point above, 7 steps have been taken, and the human eye can clearly see his color cast.

We will use SDCM to evaluate light color, so how to measure this parameter? It is recommended that you bring a spectrometer with you, no joke, a portable spectrometer! For in ground light, the accuracy of light color is particularly important, because reddish and greenish colors are ugly.

And next is Color Renderingndex.

In ground light that requires high color rendering index is the lighting of buildings, such as wall washers used for building surface lighting and floodlights used for in ground light. Low color rendering index will seriously damage the beauty of the illuminated building or landscape.

For indoor applications, the importance of color rendering index is especially reflected in residential, retail stores, and hotel lighting and other occasions. For the office environment, the color rendering characteristics are not so important, because the office lighting is designed to provide the best lighting for the execution of the work, not for aesthetics.

Color rendering is an important aspect of evaluating the quality of lighting. Color Renderingndex is an important method to evaluate the color rendering of light sources. It is an important parameter to measure the color characteristics of artificial light sources. It is widely used to evaluate artificial lighting sources. Product effects under different Ra:

Generally speaking, the higher the color rendering index, the better the color rendering of the light source and the stronger the ability to restore the color of the object. But this is only "usually speaking". Is this really the case? Is it absolutely reliable to use color rendering index to evaluate the color reproduction power of a light source? Under what circumstances will there be exceptions?

In order to clarify these issues, we must first understand what the color rendering index is and how it is derived. CIE has well stipulated a set of methods for evaluating the color rendering of light sources. It uses 14 test color samples, tested with standard light sources to obtain a series of spectral brightness values, and stipulates that its color rendering index is 100. The color rendering index of the evaluated light source is scored against the standard light source according to a set of calculation methods. The 14 experimental color samples are as follows:


Among them, No. 1-8 is used for the evaluation of general color rendering index Ra, and 8 representative hues with medium saturation are selected. In addition to the eight standard color samples used to calculate the general color rendering index, CIE also provides six standard color samples for calculating the color rendering index of special colors for the selection of certain special color rendering properties of the light source, respectively, saturated Higher degrees of red, yellow, green, blue, European and American skin color and leaf green (No. 9-14). My country's light source color rendering index calculation method also adds R15, a color sample representing the skin tone of Asian women.

Here comes the problem: usually what we call the color rendering index value Ra is obtained based on the color rendering of 8 standard color samples by the light source. The 8 color samples have medium chroma and lightness, and they are all unsaturated colors. It is a good result to measure the color rendering of a light source with continuous spectrum and a wide frequency band, but it will cause problems for evaluating the light source with steep waveform and narrow frequency band.

The color rendering index Ra is high, is the color rendering must be good?
For example: We have tested 2 in ground light, see the following two pictures, the first row of each picture is the performance of the standard light source on various color samples, and the second row is the performance of the tested LED light source on various color samples.

The color rendering index of these two LED light sources of in ground light, calculated according to the standard test method, is:

The upper one has Ra=80 and the lower one has Ra=67. Surprise? The root reason? Actually, I have already talked about it above.

For any method, there may be places where it is not applicable. So, if it is specific to the space with very strict color requirements, what method should we use to judge whether a certain light source is suitable for use? My method may be a bit stupid: look at the light source spectrum.

The following is the spectral distribution of several typical light sources, namely daylight (Ra100), incandescent lamp (Ra100), fluorescent lamp (Ra80), a certain brand of LED (Ra93), metal halide lamp (Ra90).

Post time: Jan-27-2021