# What is The Limit Light Effect of LED

Last year, a company’s LED luminous efficacy made a major breakthrough, reaching 303lm/W. Various experts have also begun to make bold predictions for the next step, 400lm/W, 500lm/W, 1000lm/W…

But do you really understand luminous flux? Today we come to understand what is the maximum limit light effect of light? How is electricity converted into light? Different light sources have different principles. But that’s about it in general.

Therefore, everyone is familiar with the formula for calculating the luminous efficiency of a light bulb, that is, the luminous flux divided by the total power. So, what is the limit of this luminous efficiency? I’m afraid not everyone knows this.

## What Does Electricity Become in A Light Bulb?

First, let’s recall the law of conservation of energy. It means that energy must be conserved during energy conversion. This is a natural law that cannot be violated. Take LED light bulbs as an example. If we use all the electric power, when the LED device is working to convert the electric power P, it will produce two kinds. One is used to emit light (optical power PV), and the other is used to generate heat (thermal power PH).

Let’s take an example. The picture below is a photoelectric test report of a COB device:

The radiant flux Φe marked in red in the report is the optical power. From the perspective of energy conservation, the LED consumes 31.21W of electric power, 11.97W (accounting for 38.36%) becomes optical power, 19.24W (accounting for 61.64%) is converted into thermal power, and emits 3922 lm of light.

If thermal power is not considered (pretend to consume only 11.97W of electricity), the ultimate luminous efficacy or luminous efficacy (LER) that this device can achieve is:

## What is Luminous Efficiency (LER)?

Above we mentioned a professional term, optical performance (LER), but what is it? In a hurry, ask me.

Now suppose we invent an ideal LED. It generates no heat and converts 100% of electrical power P into optical power PV. At this time, the light effect is the theoretical maximum. This value also has a scientific name called luminous efficacy of radiation (LER for short).

The theoretical calculation formula of LER is as follows:

The Km in the formula is called the maximum luminous efficacy, which is equal to 683 lm/W. It means that the human eye is most sensitive to yellow-green light (555nm), and the optical power of 1W is equivalent to the luminous flux of 683 lm. When the shape of the spectrum is determined, the luminous efficacy (LER) is also determined. Give another example.

In the picture above, we can see that the luminous efficacy (LER) of the halogen lamp is only 130 lm/W. That is to say, the theoretical maximum luminous efficacy of the halogen lamp is 130lm/W. The main reason is that the spectrum of halogen lamps contains a lot of red light. From the visual function V(λ) (shown by the black dotted line), it can be seen. Human eyes are not sensitive to these red lights, resulting in a relatively low luminous efficacy (LER).

And LED 1 removes more red light. Most of the spectrum is yellow-green light to which the human eye is sensitive. Therefore, the luminous efficacy (LER) can reach 330 lm/W. However, the CRI is only 80 due to more red light.

LED 2 contains more red light, with a display index of 97. The luminous efficacy (LER) can also reach 260 lm/W. Therefore, LER means that you have many kinds of light in your hand, which is converted into a unified luminous flux according to the V(λ) visibility function. The most sensitive light to the human eye is yellow-green light at 555nm. Its luminous efficacy (LER) is 683 lm/W. This is the theoretical maximum light effect. It can also be seen from Table 2 that the luminous efficacy (LER) of blue light at 455nm is only 32.8 lm/W.

## Light Effect And Light Color Quality

In 2013, American Po-Chieh Hung et al. published an article in the IEEE journal <JOURNAL OF DISPLAY TECHNOLOGY>. In the article, he simulated and calculated the limit light effects of various CRIs and color temperatures, and organized them into a chart, as shown in the figure below.

We can see from the figure that the theoretical maximum luminous efficiency of white light is only 480lm/W. And at this time, the CRI is only 30, and the color rendering effect is extremely poor.

In order to improve the finger display, the spectrum needs to contain more blue light and red light that the human eye is not sensitive to. Light effect and light color quality, just in one bulb. Because of this, when choosing a light source in lighting design, there is often no best, only the most suitable.

Now, do you understand why the light effect of white light cannot reach 1000 lm/W? Also welcome to contact ADNLITE, we are a Chinese manufacturer of LED LANDSCAPE lamps, Solar Street light, Led street lights with a history of more than 10 years.