Lighting quality refers to whether the lighting source satisfies indicators like visual functionality, visual comfort, safety, and visual aesthetics. Properly applying these quality indicators can give your lighting space a fresh feel, especially in the age of LED lighting where lighting quality is crucial. Using these indicators when purchasing LED light sources can yield better results with less effort. Below, we’ll explore the primary indicators of lighting quality.
1. Color Temperature
Color temperature defines the hue of white light, distinguishing between reddish or bluish tones. It’s expressed using absolute temperature units, measured in Kelvin (K). Typically, the color temperature range for indoor lighting is 2800K to 6500K.
White light, similar to sunlight, comprises various colors—primarily red, green, and blue. Sunlight is a mix of these colors, as demonstrated by passing light through a prism (right image).
To describe white light color, we use color temperature. White light with more blue components appears cooler (like winter sunlight), while light with more red tones looks warmer (such as morning or evening sunlight). Color temperature is the sole way to express white light hue.
Artificial light sources also blend various colors to create white light, described similarly using color temperature. Spectral analysis, a specialized method, is often used for physical analysis of white light. Testing requires professional instruments, as shown in the spectrum examples below.
From the figures, sunlight has a rich and complete spectrum, while artificial light sources like incandescent bulbs and LEDs offer better spectral distribution compared to high-pressure sodium lamps and fluorescent lamps. Fluorescent lamps contain harmful UV components, while incandescent lamps consume significant energy and will soon be phased out. LEDs will likely become the dominant lighting source.
Describing white light color sometimes involves classifications like warm white, natural white, daylight white, and cool white. However, this categorization is too broad for assessing lighting quality. Instead, color temperature should be used. Typically, the correlation between LED white color temperature and light color is depicted in the chart below (a brief description). The international standard color temperature distinction should follow the CIE1931 chromaticity diagram, as illustrated in the image.
Accurate color temperature values require professional instruments. To simplify understanding, we’ve created a visual guide to distinguish lighting products' color temperatures, as shown below:
2. Color Rendering
Color rendering describes how well a light source reveals the true colors of objects. It's represented by the color rendering index (Ra), ranging from 0 to 100. Higher Ra values indicate better color rendering, reducing color distortion of illuminated surfaces. Professional instruments are required to test the color rendering of light sources.
From the solar spectrum, we see sunlight has the richest spectrum and best color rendering. Artificial light sources always render colors worse than sunlight. Comparing the color of your palm or face under sunlight versus artificial light is the easiest way to assess color rendering. The closer the colors match under sunlight, the better the color rendering. If your palm looks gray or yellow, the color rendering is poor. If it looks reddish, it’s normal (see image below).
For LED light sources, Ra can be grouped into three levels: Ra < 69, 70 ≤ Ra < 80, and Ra ≥ 80. For high-quality indoor lighting, a light source with Ra > 80 should be used. The color rendering index and degree of color reduction can be referenced in the visual comparison below.
China adopts the CIE test standard. For LED sources, the general Ra doesn’t fully reflect their color rendering performance. CIE hasn’t provided new standards. For users seeking high color rendering, referring to the Planck curve in the CIE1931 chromaticity diagram (black line in the figure) is recommended. The closer the test coordinates are to the Planck curve, the better the color rendering ability.
3. Illuminance Value of the Light Source
Illuminance is the luminous flux a light source projects onto the surface of an object being illuminated. It indicates the brightness or darkness of the object's surface and is measured in lux (Lux). Higher illuminance values mean the object is brighter.
Illuminance is closely tied to the distance between the light source and the object. The farther the distance, the lower the illuminance. It also depends on the light distribution curve of the luminaire. Smaller light output angles result in higher illuminance, while larger angles lead to lower values. Special instruments are needed for accurate measurement.
From a photometric perspective, luminous flux is the key indicator. As a lighting product, it primarily reflects the brightness or darkness of the illuminated surface. Illuminance values describe the lighting effect more precisely. High or low indoor illuminance affects perceived brightness and human eye health. Thus, indoor illuminance must adhere to national standards (see Table 1). Recommended values shouldn’t be excessively high or low.
Table 1: Residential Building Lighting Illuminance Standards
4. Luminaire Light Distribution Curve
Indoor lighting effects depend on luminaire layout and light distribution curves. Good lighting results come from rational layouts and proper light distribution applications. Layouts and light distribution determine the visual function and comfort of indoor lighting, creating depth and layers in the space. Appropriate light distribution improves the overall lighting quality.
The purpose of a luminaire is to secure and protect the light source, as well as to decorate and beautify the environment. Another role is redistributing light output to match the luminaire's designed light output angle. This is called light distribution. The luminaire’s light distribution curve describes its light output pattern. Smaller light distribution angles make surfaces feel brighter. Luminaire light distribution curves are tested using specialized instruments. Common light distribution curves are shown below.
Luminaire light distribution relates to the luminaire's lighting function. Track spotlights and ceiling lights typically use 15° or 30° distributions, downlights use 30° or 60°, bulbs and ceiling lights use 120°, and panel lights use 60° to 90° distributions.
5. Luminous Efficiency of the Light Source
The brightness of a light source is described by luminous flux, measured in lumens (lm). Higher luminous flux indicates brighter light. The ratio of the light source’s luminous flux to its power consumption is called luminous efficiency, measured in lm/W (lumens per watt).
Luminous efficiency is a critical indicator of light source quality. Higher efficiency means the light source is more energy-efficient. LED light sources have a luminous efficiency of about 90–130 lm/W, while energy-saving lamps range from 48–80 lm/W. Incandescent lamps have an efficiency of 9–12 lm/W, and poorly made LED sources only reach 60–80 lm/W. Higher efficiency and better quality go hand in hand.
6. Luminaire Efficiency
Indoor lighting rarely relies on standalone light sources. Typically, they’re mounted in luminaires. Once installed, the luminaire’s light output is lower than that of the individual light source. The ratio of these outputs is called luminaire efficiency. Higher efficiency signifies better luminaire quality, higher energy-saving potential, and is a key measure of luminaire quality. Comparing efficiencies indirectly evaluates luminaire quality.
The relationship between the light source’s luminous efficiency, luminaire efficiency, and luminaire illuminance is that the luminaire’s luminous flux output is proportional to its efficiency. Luminaire illuminance is proportional to the light source’s luminous efficiency, and both depend on the luminaire’s light distribution curve. High illuminance doesn’t necessarily mean high luminous flux. Even with a highly efficient light source, placing it in a low-efficiency luminaire reduces the luminaire’s lumen output significantly. Higher luminous efficiency of the light source and higher luminaire efficiency mean better energy-saving indicators. These are vital indicators for high-quality lighting products.
7. Glare
Glare refers to discomfort caused by excessive light intensity from a light source. When a light source is glaring, it often has issues with glare.
On city streets at night, when cars with high beams approach, the glare we experience is glare. As shown, indoor lighting also suffers from glare. Glare makes people uncomfortable and can even cause temporary blindness. It impacts children and the elderly most, affecting lighting quality—a problem worth addressing.
Glare and indoor illuminance and energy-saving indicators are interdependent. Brighter light sources tend to cause glare, balancing these factors requires careful consideration. Higher luminous flux and illuminance increase glare. Smaller light distribution angles also raise glare, while lower brightness decreases it but lowers energy-saving metrics. Controlling glare focuses on visual comfort. For homes with children and elderly, glare control is especially important.
8. Flicker
Flicker is a phenomenon where the brightness of a light source changes over time. Long exposure to flickering light sources causes visual fatigue. The maximum flicker period of a light source is 0.02 seconds, while the human eye retains images for 0.04 seconds.
If the flicker period is faster than the human eye’s retention time, the eye doesn’t notice the flicker, though the visual cells do, causing fatigue. Higher flicker frequencies reduce visual fatigue. Low-frequency flicker harms eye health and lighting quality.
Flicker isn’t easily visible to the naked eye. Here’s an easy way to detect it: use a camera’s video mode to aim at the light source, adjusting the distance until stripes appear on the screen. Bright and dark stripes indicate flicker. Wider stripes suggest high flicker, while faint or no stripes mean low flicker. Not all phones can detect flicker, so testing with multiple devices is advisable.
9. Safety of Lighting Equipment
Lighting equipment safety includes electric shock risks, leakage hazards, high-temperature burns, explosions, installation reliability, safety markings, and environmental suitability.
Equipment safety is regulated by national standards. Observing product appearance, certification marks, driver power supply quality, and provided data helps assess reliability. A simple method is observing pricing: overly expensive or very cheap products warrant caution.
To evaluate pricing, calculate cost per watt. Products costing below average need scrutiny. Measuring the actual active power of the light source is essential, as many LED sources claim the same power as energy-saving lamps but are falsely inflated.
10. Energy-Saving Indicators of Lighting Equipment
The ultimate goal of lighting is visual beauty. Enjoying this beauty often requires prolonged use, but high energy consumption burdens users financially and psychologically, diminishing lighting quality. Hence, we’ve included energy-saving indicators in our quality criteria.
Energy-saving indicators relate to:
1) Light source luminous efficiency.
2) Luminaire efficiency.
3) Reasonable illumination design and space illumination values.
4) Driver power supply efficiency.
5) Heat dissipation performance of LED sources.
We emphasize driver power supply efficiency and LED heat dissipation. For LEDs, higher driver efficiency increases light source efficiency, making the light source more energy-efficient. Both power efficiency and power factor are distinct concepts. High values indicate good driver quality.
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