LED lamp performance eight must-test items

Luminous intensity refers to the amount of light emitted in a specific direction. Because LEDs emit highly concentrated light, the inverse square law is not applicable at close distances. The CIE127 standard outlines two measurement conditions: Condition A (far field) and Condition B (near field), both using a detector area of 1 cm². Typically, Condition B is used for measuring luminous intensity due to its practicality in most applications.

Luminous Flux and Luminous Efficacy Testing

Luminous flux represents the total amount of visible light emitted by a light source. Two primary methods are used for its measurement:

(1) Integration Method: In this approach, a standard lamp and the test lamp are sequentially turned on inside an integrating sphere. Their readings from a photodetector—Es for the standard lamp and ED for the test lamp—are recorded. Given that the standard lamp's flux Φs is known, the test lamp’s flux ΦD can be calculated as ΦD = ED × Φs / Es. While this method is straightforward, it is sensitive to color temperature differences between the lamps, leading to potential inaccuracies.

(2) Spectroscopic Method: This method calculates luminous flux based on the spectral power distribution P(λ). A monochromator is used to measure the spectrum of the standard lamp from 380 nm to 780 nm within the integrating sphere. The same procedure is repeated for the test lamp, and the luminous flux is then compared. This method offers higher precision, especially for complex light sources.

Luminous efficacy is the ratio of the luminous flux to the electrical power consumed. For LEDs, this is typically measured under constant current conditions to ensure consistent performance evaluation.

Spectral Property Analysis

Spectral characteristics of LEDs include spectral power distribution, chromaticity coordinates, color temperature, and color rendering index (CRI).

The spectral power distribution describes how the light source emits different wavelengths with varying intensities. It is measured using a spectrophotometer or monochromator, comparing the test source with a standard lamp.

Chromaticity coordinates represent the color of the light on a coordinate system, usually the XYZ system. These values help define the color appearance of the light source.

Color temperature indicates the perceived color of the light, matching that of a blackbody radiator at a given temperature. It plays a crucial role in lighting design and affects the visual comfort of users.

Color rendering index (CRI) measures how well a light source reveals the true colors of objects. The general CRI (Ra) is the average of eight color samples, and improving it is a key focus in LED development.

Light Intensity Distribution Test

The relationship between light intensity and spatial angle defines the light intensity distribution. This is often represented as a closed curve, which is measured using an automatic distribution photometer due to the large number of data points involved.

Temperature Effects on LED Optical Properties

Temperature significantly impacts the optical performance of LEDs. Experiments show that changes in temperature can alter the emission spectrum and color coordinates, affecting overall light quality and efficiency.

Surface Brightness Measurement

Brightness in a specific direction is defined as the luminous intensity per unit area. Surface brightness is measured using devices such as surface brightness meters and aiming brightness meters, which have separate paths for aiming and measurement.

Electrical Parameter Testing of LED Lamps

Key electrical parameters include forward and reverse voltages and currents, which determine whether the LED operates correctly. There are two main testing approaches: measuring voltage at a fixed current or current at a fixed voltage.

(1) Forward Voltage: A forward current is applied, and the resulting voltage drop across the LED is measured. This is done using a DC voltmeter. Since LEDs have low resistance when conducting, an ammeter is connected externally for better accuracy.

(2) Reverse Current: A reverse voltage is applied, and the resulting current is measured. Due to high reverse resistance, the ammeter is connected internally for accurate readings.

Thermal Characteristics Testing of LED Luminaires

Thermal properties, such as thermal resistance and junction temperature, play a vital role in determining the performance and lifespan of LEDs. Thermal resistance refers to the resistance to heat flow between the PN junction and the housing, while junction temperature reflects the actual temperature of the semiconductor.

Common methods for measuring these include infrared imaging, spectroscopy, and electrical parameter analysis. The electrical parameter method is widely used, relying on the linear relationship between forward voltage and junction temperature. By measuring voltage changes at different temperatures, the junction temperature can be accurately determined.

Intern Editor: Liang Jieying