Optimizing Light Distribution:  Analysis for Flat LED Downlights

To overcome the limitations of traditional LED downlights, such as high installation height, low luminous efficiency, uneven brightness, and severe glare, we designed a flat LED downlight that has a low installation height, high luminous efficiency, high uniformity, and low glare. Using ProE software, we created a 3D model of the flat LED downlight and used TracePrO software to simulate and analyze the impact of the angle between its side-reflecting and bottom on brightness uniformity and glare. Results from the BM-7A luminometer test demonstrated that the flat LED downlight achieved a brightness uniformity of 89.6%. The near-field distribution photometer GOl9O0 confirmed this result, and the flat LED downlight’s beam angle of 99.7° effectively reduced glare. Further testing using an integrating sphere optical test revealed the flat LED downlight efficiency at a color temperature of 3000K reached 106.67 lm/W. Our results demonstrate that the designed flat LED downlight is an effective solution to achieve high-quality lighting.


LED downlights are an energy-efficient, low-carbon, and long-lasting alternative to traditional downlights. They offer a sleek and attractive appearance, with no exposed light source or harsh glare, and provide a soft and uniform visual effect. However, currently, available LED downlights have a few drawbacks, such as heavy weight, low luminous efficiency, poor brightness uniformity and color rendering, and high installation height of over 20cm, making them unsuitable for home lighting. Additionally, some downlights can produce excessive glare, which can cause damage to the visual nervous system. To address these issues, Sinolumi presents a design for a flat LED downlight that is only 3cm in height, making it suitable for all lighting applications. The proposed downlight boasts high luminous efficiency, excellent uniformity, and low glare, providing a superior lighting experience.

Structural Design of a Flat LED Downlight

In this study, we focused on designing an 8-inch downlight. Downlights are categorized by size, with 4″, 6″, 8″, and 10″ being common. Typically, smaller downlights are easier to manage in terms of brightness, uniformity, and glare due to their compact size. However, the larger 8-inch downlight presents more of a challenge. To design this flat  LED downlight, we created a 3D model using ProE software, which is shown in Figure 1. Additionally, we examined the internal structure of the light, as seen in Figure 2. The main components include the lamp housing, aluminum PCB, SMD LED, reflector, and diffuser plate.

Figure1. Flat LED Downlight 3D View
Figure1. Flat LED Downlight Structure

Key Optical Parameters

Color temperature 

Correlated color temperature refers to the temperature of a blackbody radiator that emits light of color most similar to that of a given stimulus of the same brightness, expressed in units of Kelvin (K). Common correlated color temperatures for LED tube lights are typically 3000K, 4000K, and 6500K. Generally, LED light sources with lower color temperatures have lower luminous efficacy. Therefore, a color temperature of 3000K was chosen for this design to achieve a high display efficiency in a color temperature range where high efficiency is not easily attainable.

Kelvin Color Temperature Chart

CRI(color rendering index)

The ability to accurately reproduce an object’s original colors is known as color rendering and is typically measured using the color rendering index (CRI) of a light source. The higher the CRI value of a light source, the better it’s color rendering capabilities. Sunlight is typically considered to have a CRI of 100, and the CIE has established 15 color rendering indices, ranging from R1 (light gray-red) to R15 (yellow skin color). The average of the first eight indices is recorded as Ra, which characterizes the overall color rendering of a light source. R9 specifically measures the ability to accurately reproduce red colors, and a higher value indicates better red color reproduction. In order to ensure high color reproduction, both Ra and R9 values must be high. According to the CQC3128-2013 standard, the minimum initial CRI value for LED downlights is Ra≥80 and R9>0. Therefore, this design aims to achieve an initial CRI value of Ra≥80 and R9>0.

CRI(color rendering index) Chart

Beam Angle 

The beam angle of a flat LED downlight is primarily determined by an important indicator of visual comfort known as glare. Glare occurs when the distribution of luminance is inappropriate, resulting in extreme contrasts in brightness either in space or time, causing visual discomfort and reducing visibility. The normal range of visual attention for the human eye is 30 degrees above to 60 degrees below the horizontal plane. If the LED downlight produces strong and harsh light within this range, it will result in glare. To prevent glare, the angle of the downlight should not exceed 120 degrees. The design goal of this paper is to limit the light angle within 100 degrees, in order to ensure that it provides optimal visual comfort while also providing adequate lighting.

Beam Angle

Uniformity of the light source 

The LED downlight designed in this project is a surface light source, and the uniformity of brightness has an important impact on glare control. Brightness uniformity is defined as the ratio of the lowest brightness to the highest brightness on the output surface when the luminaire is operating normally. The lower the brightness uniformity, the more likely it is that some areas of the surface light source will be too dim or too bright, which can cause glare and also affect the aesthetics of the luminaire. Therefore, brightness uniformity is an important optical parameter for LED flat panel downlights.

Currently, many LED downlights on the market do not consider brightness uniformity as an important parameter. Through testing, most downlights on the market have a brightness uniformity of less than 60%, which greatly affects the lighting effect and aesthetics of the downlights. In this paper, the designed LED flat panel downlight has a brightness uniformity of over 80%, which greatly improves the lighting effect and aesthetics of the downlight.

Uniformity of lighting

Luminous efficacy 

Luminous efficiency is the ratio of luminous flux to the power consumed, the industry is generally divided into light source luminous efficiency and luminous efficiency of lamps. Light source luminous efficiency, is the luminous flux from the light source and the ratio of the electrical power consumed. The luminous efficacy of the luminaire refers to the ratio of the initial total luminous flux emitted by the luminaire to the power consumed by the luminaire under the claimed conditions of use of the luminaire, also known as the luminous efficacy of the luminaire. Both luminous efficacy reflects the efficiency of converting electrical energy into light energy and is an indicator of the energy-saving characteristics of lighting products. The luminous efficacy referred to in this article is the luminous efficacy of lamps. Most of the LED downlights on the market today have a luminous efficacy of 30 lm/W ~ 80 m/W. The flat LED downlight designed in this project has a luminous efficacy of 100 lm/w at a color temperature of 3000K.

What is luminous

Optical design and simulation

LED PCBA design 

To improve the light efficiency and uniformity of LED downlights, while meeting the color rendering index requirements, SMD3528 LEDs with a color temperature of 3000K and a CRI of 83 were chosen as the light source. In order to ensure good heat dissipation, the LEDs were mounted on an aluminum PCB. The LED PCBA designed using ProE software is shown in Figure 3. It is designed for an 8-inch 20W flat LED downlight, and the LEDs are evenly distributed on the aluminum PCB to improve the brightness uniformity of the light-emitting surface.

Simulation parameter settings 

The parameters that affect the optical simulation results of the LED downlight mainly include the distribution of the LED light source, the reflectivity of the aluminum PCB, the reflectivity of the reflective sheet, and the transmittance of the diffuser. Here are the specific parameter values:

Figure 3: LED PCBA
  1. The distribution of the LED light source is shown in Figure 3.
  2. The reflectivity of the aluminum substrate is 80%.
  3. The reflectivity of the reflective film is 97%.
  4. The transmittance of the diffuser is 82%.
  1. The distribution of the LED light source is shown in Figure 3.
  2. The reflectivity of the aluminum substrate is 80%.
  3. The reflectivity of the reflective film is 97%.
  4. The transmittance of the diffuser is 82%.

It is important to note that these parameters can significantly impact the performance of the lighting effect, particularly in terms of light efficiency and uniformity. Therefore, it is necessary to carefully consider and optimize these parameters during the design and simulation process to ensure optimal performance of the flat LED downlight.

Simulation results 

The brightness uniformity of the LED flat downlight’s output surface is influenced by the angle between its side reflection and bottom (i.e., θ in Figure 2). Simulation results are presented for θ values of 113 degrees and 103 degrees. The established downlight model was introduced into Tracepro for light tracing, and the simulation results are shown in Figure 4 and Figure 5 for θ values of 113 degrees and 103 degrees, respectively. The brightness uniformity (the ratio of minimum to maximum brightness) of the flat LED downlight is about 62% for θ = 113 degrees and 86% for θ = 103 degrees. Hence, the angle between the side reflection and the bottom of the downlight plays an important role in determining brightness uniformity. By selecting the appropriate angle, the brightness uniformity can be improved by more than 20%, as indicated by the simulation results.

Figure 4
Figure 5
Top View of Flat LED Panel Downlight
Back View of Flat LED Downlight

Experimental measurement verification

 Based on the simulation results it is evident that the flat LED downlight has better brightness uniformity when the angle between the side reflection and the bottom is 103 degrees. This simulation result is presented in Figure 6, which displays both the front and back views of the light.

Brightness uniformity test 

The tested power of the LED flat downlight is 18.88W. Nine test points were selected using the BM-7A luminance meter, and their distribution is presented in Figure 7. The test results, displayed in Table 1, reveal that the brightness of the LED flat downlight’s edge is lower than that of the center, with a minimum value of 28319 cd/m2 and a maximum value of 3157 cd/m2, respectively. Consequently, the brightness uniformity, which is the ratio of the minimum value to the maximum value, is 89.6%. These results demonstrate that the flat LED downlight designed by Sinolumi exhibits high brightness uniformity, consistent with the optical simulation results, with an error of just 3.6%.

Figure 7 Test points the distribution
Test PointMeasured Brightness (cd/m2)Brightness UniformitySimulation Error (%)
Table 1

Beam angle test 

Using the GO1900 Far-field Goniophotometer, the Flat LED downlight was tested with a standard distribution photometric laboratory (with a black background) at a distance of 9.0m. The sample had a color temperature of 3000K and a color rendering index of 80. The test results, shown in Figure 8, indicate that the beam angle of the downlight is 100.9 degrees, which is within the expected design range and has a good anti-glare effect.

Other optical parameters testing 

The test results for the LED flat downlight are illustrated in Figure 9. According to these results, the downlight exhibits a color temperature of Tc=2973K, a color rendering index of Ra=83.7, and a luminous flux Φ=2151 lm. This efficacy is at least 25 lm/W higher than that of the current market for recessed downlights. Importantly, these test results align perfectly with the design parameters for the fat LED downlight.

Figure 8 Test result of beam angle
Figure 9 Test results of CCT, CRI, and luminous efficacy


The results of the tests conducted are consistent with the parameters of the downlight that was designed. This study aimed to design a LED flat downlight with high luminous efficacy and low glare. The structure of the downlight was modeled using ProE, and light tracing simulations were conducted using Tracepro. The effect of the angle between the side reflection and the bottom of the downlight on its brightness uniformity and glare was analyzed. Sinolumi flat LED downlight was manufactured based on the simulation results, and the test results indicated a brightness uniformity of 89.6%, which is much better than traditional LED downlights. As LED products are becoming more widely used in indoor lighting, this design of high luminous efficiency and low glare LED flat downlight will have practical applications in various settings.

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