A sphere photometer is a fast and effective device for measuring the luminous flux and efficiency of LED products, involving measurements of luminous flux in LED products, product testing, and quality control departments. It is currently the main equipment for detecting LED luminous flux and efficiency. How to reasonably utilize the sphere photometer to obtain accurate luminous flux is a widely concerned issue. This paper aims to explore the technical key points of measuring the total luminous flux of LEDs using a sphere photometer.
Principle of the Sphere Photometer
A spherical photometer is a type of...Score ballPhotometers for measuring the total luminous flux of light sources, with the basic structure shown in Figure 1. Place the light source S in...ScoreballInternally, the light probe D is attached to the probe hole on the surface of the sphere, which is located at 4π degrees from the center or 2π degrees near the wall. The hole is situated on the wall and houses a scatterer. Detector D is a photometric probe, with its output current converted to voltage via an I/V converter, which is then displayed on the reader R. If probe D serves as the fiber-optic collection surface, it collects light and transmits it through a fiber-optic connection to a spectroradiometer. The total luminous flux is calculated by utilizing the spectral luminous efficiency function based on the measured spectral power in the visible region. Two methods are combined: using both the photometric probe and the spectroradiometer simultaneously, where the photometric probe measures the flux, and the spectroradiometer measures the spectral power distribution.
1a、Schematic Diagram of Spherical Photometer (Photometric Detection System)
1b、Illustration of a Spherical Luminance Meter (with Spectroradiometer as the detection system)
The layer inside the ball is coated with white diffusing material, which requires excellent diffusing properties and no spectral selectivity. The total luminous flux emitted by the light source S is φ, the reflectance is ρ, and the radius of the ball is R. Based onScore ballTheory: The luminous intensity produced by Light Source S on the surface of the sphereFor a certain integral sphere, R and ρ are fixed. Therefore, the above formula indicates that due to multiple diffuse reflections from the inner wall of the sphere, the indirect illuminance values at each point on the inner wall of the sphere are equal and proportional to the total luminous flux of the light source. In Figure 1, the role of the baffle B is to prevent the light source S from directly illuminating the detector D. If the baffle B is removed from inside the integral sphere, the light from the source would directly照射 to the probe D, at which point the illuminance at the probe D's position depends on the luminous intensity of the source in that direction. The illuminance values at different points on the inner wall of the sphere are not equal and do not proportionally correspond to the total luminous flux of the source.
By using Formula (1), the total luminous flux of the measured lamp can be measured by comparing the luminous flux of the lamp signal with the standard lamp signal.
Rtest is the photoelectric reading of the standard lamp, Rstd is the photoelectric reading of the standard lamp, φ std is the total luminous flux value of the standard lamp, and C is the luminous flux constant.
LED Features
The LED tube is a new generation of energy-saving light source, featuring high efficiency, long lifespan, rich colors, and a wide dynamic adjustment range, making it a high-tech product that contributes to energy and emission reduction. Generally speaking, compared to traditional incandescent bulbs, LED lights have the following characteristics:
      （1）The chromaticity spectrum varies greatly. The classic white light LED is achieved by exciting blue light with yellow-green phosphors to mix and produce white.Typical Luminescent Spectra of LEDs and Incandescent LampsThis feature requires mismatch corrections to the spectral photopic efficiency function during the measurement with the spherical photometer.
      （2）Inhomogeneous spatial luminescence. Individual LEDs exhibit strong directional properties, with a spatial luminous intensity distribution curve that significantly differs from that of incandescent bulbs. This characteristic necessitates the inclusion of the integrating sphere's spatial response function during measurements with a spherical photometer.
      （3）Node temperature has a significant impact. Since the LED's light-emitting chip relies on the temperature of the PN junction, the LED's luminous flux is affected by ambient temperature, heat dissipation conditions, and preheating time. Only about 5 minutes of incandescent light can achieve thermal equilibrium, and it is not sensitive to surrounding temperatures. This characteristic requires thorough preheating for measuring the LED luminous flux and maintaining the specified ignition posture and environmental temperature.
Key Technical Points for LED Total Luminous Flux Measurement

（1）The standard light quantity values used are reliable, and the verification or calibration certificates are within their validity period.

The standard incandescent bulbs currently in use in our country are the BDT and BDP models. These bulbs are stable and reliable, capable of accurately measuring flux values. Additionally, the high-color temperature standard light sources, which employ a distributed temperature standard, cover the visible light spectrum and effectively calibrate the radiant spectrum in spherical photometers.

（2）A well-equipped lab environment and sufficient preheating time enhance the repeatability of LED measurements.
Due to the sensitivity of LEDs to environmental temperatures, clean and temperature-controlled laboratory conditions are required. During routine LED light source testing, to achieve thermal equilibrium, the preheating time for LED bulbs and other light sources should be around 5 minutes, with an ideal duration of 0.5 to 1 hour. After full aging, the repeatability of the LED light sources can reach above 0.5%.

（3）The equipment boasts excellent performance.
To achieve the ideal state of the integrating sphere in practical use, its diameter should be as large as possible while meeting the requirements for photometric detection sensitivity. The internal coating material must be clean and uniform, and its reflectivity must comply with the requirements of CIE 84-1989, "The measurement of luminous flux." The internal brackets and fixtures must not obstruct the light on the lamp and should be coated with a diffused reflection material. The baffle position should be appropriate, ensuring that the measured light source does not directly illuminate the detector when taking the area. The photodetector performs well. If a photometric probe corrected with V(λ) is used, f1' must be sufficiently small (laboratory grade); if the photometric detector is a spectroradiometer, it should have low stray light, small wavelength position error, a large dynamic range, and good linearity.
(4) Spectral efficiency function mismatch correction.
During photometric probe testing, there is a discrepancy between the actual response curve and the ideal spectral photovisual efficiency function V(λ). Additionally, the diffuse layer inside the integrating sphere and the frosted glass on the window do not reflect the spectrum with an ideal flat curve. These factors cause the response curve of the spherical photometer to deviate from V(λ), leading to mismatch errors in V(λ). The mismatch can be estimated through the following calculations and mismatch correction measurements.
In the midst of...Ptest(λ) represents the relative light power distribution of the measured light source, Pstd(λ) denotes the relative spectral power distribution of the standard lamp, and S(λ) is the spectral diffuse reflectance of the inner wall of the integrating sphere. For instance, when calibrating a spherical photometer with BDP, the correction factor is 1.01; for measuring a blue LED lamp, the coefficient can reach 1.10 or higher.
(5) Correction of the Spherical Space Response Function
When there is a significant difference in the normalized luminous intensity spatial distribution curve between LED and standard lamps during testing, special attention must be paid to this correction factor. Due to the uneven internal coating and the presence of internal components (such as a shutter screen) in the spherical photometer, the illuminance values produced by detectors in different areas of the inner wall of the integrating sphere are not equal, i.e., the spatial response distribution function (Spatial Response Function) of the integrating sphere.nse distribution function, SRDF)：
K(θ, φ) represents the intensity of a narrow beam of light projected onto the inner wall of an integrating sphere at the center position (θ, φ) within the sphere, after multiple diffuse reflections, as measured by the detector. K(0,0) is typically considered as the unit value of 1.
The spatial response distribution function correction coefficient for the integral ball can be expressed as:
Itest (θ, φ) and Istd (θ, φ) represent the normalized spatial distribution curves of the light intensity for the measured LED and the standard lamp, respectively. From Equation (5), it is known that if the internal spatial response of the integrating sphere is significantly uneven (such as yellowing of some coatings, accumulation of dust at the bottom, a large baffle area, etc.), and there is a significant difference in the spatial distribution curves of light intensity between the measured LED lamp and the standard lamp. For instance, if a BDP is used to calibrate a spherical photometer, and then measure an LED spotlight, the correction coefficient can reach 1.10.
(6) Self-absorption correction
When the LED and standard lamp have significantly different physical dimensions or states, the absorption correction coefficient should be calculated. Illuminate a stable auxiliary light at an appropriate position within the sphere (usually on the sphere wall) to block its light from reaching the window and the lamp under test. Place the standard lamp tube in the normal position of the installed light source. When the detector reads Astd, remove the standard lamp and place the LED under test in the same position. The detector reads Atest, and the calculation formula is:
      （7The Benchmark Light utilizes LED value transfer to avoid the aforementioned corrections, simplifying and ensuring the reliability of the measurement process.
If the total luminous flux, normalized luminous intensity spatial distribution curve, and luminous spectrum of the light source and standard light source are relatively close, then the above three correction coefficients are all close to 1 and can be neglected. It is recommended to use a stable luminous diode transmission standard lamp and directly apply formula (2) to calculate the luminous flux.
Summary
The spherical photometer meets all the hardware requirements, and as long as the operator maintains the cleanliness of the integrating sphere and ensures that the optical characteristics of the measured LED lamp are close to those of the standard lamp (or LED value transfer reference lamp), the total luminous flux measurement results for the LED lamps are completely controllable for error. When there are significant differences in the spectrum, spatial distribution of light intensity, or appearance between the measured lamp and the standard lamp (or LED transfer reference lamp), corresponding corrections should be made to reduce the measurement uncertainty.