Photometric LEDs - Colorimetric Characterization and Related Test Standards and Certifications

one. Testing Standards and Laboratory Accreditation
 
1. Provisions of the ANSI C78.377 standard
The color temperature classification of "Energy Star" follows the classification technology of the color temperature of fluorescent lamps, but changes the 6 color temperature grades that define the color temperature of fluorescent lamps: 2700K, 3000K, 3500K, 4000K, 5000K and 6500K. The location is at the equivalent of the MacAdam ellipse or beyond. It is now stipulated that the four corner areas belong to the same color temperature level. As shown in Figure 1:

Figure 1 Quadrilateral of the 8 CCTs shown in the 1931 chromaticity diagram
The color coordinates specified by the eight grades are listed in Table 2 below:
 
Table 2: Coordinate values ​​of the four corners of the 8 quadrilaterals that define the color temperature of SSL products
 
2. The provisions of IES LM-79-08 standard
IES LM-79-08 "Approved Methods of Electrical and Photometric Measurement of Solid State Lighting Products" was first published in 2008. This standard gives guidelines for methods of measuring solid-state lighting products (SSL: solid-state lighting products).
The method given in the photometric measurement standard for lamps and light sources is that the light sources or lamps are measured separately. At present, the type of SSL products may be lamps or light sources, but the LED light sources in lamps cannot be separated from lamps and can be easily replaced like traditional light sources. These existing standards cannot be directly used for SSL products. SSL products need to use absolute photometers. rather than using the relative method.
Typical photometric information required for SSL products is total luminous flux (lm), luminous efficacy (lm/W), light intensity in one or more directions (cd), chromaticity coordinates, relative color temperature, and color rendering index. These parameters are determined by photometric measurements in this standard. The standard specifies procedures and precautions to be followed for reproducible measurements of total luminous flux, electrical power, light intensity distribution, and chromaticity under standard conditions.
(1) Scope of application
For LED-based SSL products with electronic controls and heat sink, only AC or DC drive. The covered product forms are lamps (appliances containing light sources), and integrated LED lamps.
This standard does not apply to SSL products that require external driver circuits or external heat sinks such as LED chips, LED inserts, or LED assemblies. This standard does not apply to appliances designed for SSL products that do not contain a light source, nor does it apply to determining product performance where inter-individual variability is to be considered.
(2) Test environment requirements
Ambient temperature: 25°C ± 1°C (measured within 1 meter from the SSL product and at the same height as the SSL product) Air flow: only air flow that does not affect electrical and photometric data
Power requirements: The AC power waveform should work under the sinusoidal AC voltage with the specified frequency (such as: 50Hz/60Hz) whose harmonic component sum does not exceed 3% of the fundamental wave; the voltage stability requires that the AC or DC power supply should be under load. stable within 0.2%.
Product stability: The stability time is generally set from 30min (small LED integrated lamp) to more than 2 hours (large SSL lamp); the stability condition is 30 minutes (or more) every 15 minutes, and the light output fluctuation is less than 0.5%.
Working orientation (installation position): The installation direction of the tested sample during testing and stabilization should be consistent with the direction of actual use.
Circuit requirements: An electrical parameter meter should be arranged to monitor the input voltage and input current of the sample under test, and the calibration uncertainty under AC voltage should not be greater than 0.2% (DC voltage conditions).
The calibration uncertainty of the AC power meter is not more than 0.5%.
 
3. IES LM-80 standard
The standard was officially announced in October 2008 by the Illuminating Society of North America (IESNA Approved Method for Lumen Maintenance Testing of LED LightSources), and it is the second published standard for SSL after LM-79-08 (electrical and photometric measurement of solid-state lighting products). Product testing standards. The purpose is to maintain a set of prescribed methods for measuring the luminous flux of LED light sources, so as to standardize the understanding and practice of the industry, and is another important standard for unifying the detection methods of LED product quality.
Suitable for measuring the maintenance of luminous flux in LED packages, arrays and modules. The device under test operates under controlled conditions powered by external auxiliary equipment to obtain the best data available for comparison.
ENERGY STAR SSL product certification requirements for L70 (time to 70% lumen maintenance) are, L70 for a claimed 25000h life LED, average lumen output for 6000h samples ≥91.8% of initial value; L70 for a claimed 35000h life LED, at 6000 hours The average lumen output of the sample is ≥94.1% of the initial value.
(1) Environmental and physical conditions
LED light sources are to be stored and tested in a relatively clean environment, the light source is clean and free of fingerprints, and the manufacturer's operating instructions are on it. For easy distinction, there should be a solid mark on the sample.
The test environment should not have excessive vibration and shock.
Reduce air convection to match light source activation and operation.
The working direction of the LED light source shall be in accordance with the manufacturer's regulations and shall not be affected by each other.
(2) Temperature and humidity
1) Test temperature:
Between photometric measurements the LED is to operate at the minimum of the three base temperatures Ts, using the same drive current. The three susceptor temperatures Ts shall be 55ºC and 85ºC, plus a temperature selected by the manufacturer. The base temperature and drive current selected by the manufacturer should represent expectations for the customer's application and should be within the recommended operating temperature range. The base temperature should be controlled to -2ºC during the life test, and the ambient air temperature should be kept at the base temperature -5ºC during the test.
2) The relative humidity of the test environment: less than 65% during the whole life test time.
(3) Electrical and thermal conditions
1) Voltage
The input voltage (rms) and frequency should meet the requirements of the driver. If DC is used, the ripple voltage should not exceed 2% of the DC output voltage.
The total harmonic content in the voltage waveform of the input power supply is less than 3% of the fundamental wave.
2) Current
During the life test, the current should be monitored and stabilized at ±3% of the rated rms value;
When measuring photometric properties, stabilize at ±0.5% of the rated rms value;
Test LEDs with currents recommended by the manufacturer and consistent with actual operation.
3) Auxiliary equipment
The external device complies with the manufacturer's specifications.
4) Base temperature
The base temperature is the temperature measured with a thermocouple at the point of the LED light source package as specified by the manufacturer. Thermocouple measurement systems shall comply with the relevant requirements of ASTM E230.
The base temperature Ts is always monitored during the lifetime measurement. The measuring point (thermocouple contact point) is located at the place specified by the manufacturer.
(4) Test measurement process
The test consists of a life test and a photometric (including chromaticity) test.
During the life test, the working time of each LED should be accurately recorded, and the timing should be stopped once the LED under test is damaged. The uncertainty of the chronograph is ±0.5%.
Test at the drive current specified by the manufacturer.
A spectroradiometer is recommended for measuring photometric and colorimetric parameters.
To provide the ambient temperature throughout the test in the test report, the lumens and color of the LED should be measured with the
It is first cooled to room temperature and then carried out at an ambient temperature of 25±2ºC.
(5) Test method for lumen maintenance of LED light source
At the specified ambient temperature, operate for at least 6000 hours, and collect data at least every 1000 hours.
10,000 hours of work is recommended for testing the prediction model.
LEDs are not like other light sources, so there is no need for a test cycle involving light-off, just at 100% rating.
It is best to simultaneously measure the color change during each interval of lumen maintenance.
 
two. Principles of Photometry and Colorimetry
 
1. Radiation and optical quantities and their units
Radiation: a physical quantity that purely describes electromagnetic radiation;
Optical Quantity: Measure visible light with visual perception.
 
(1) Radiation dose
Radiant Energy (Qe): Reflected, transmitted, and received energy in joules (J).
Radiant flux (￠e): The radiant energy per unit time, in watts (W).
Radiation (Me): The radiant flux (W/㎡) emitted by the radiation source per unit emission area.
Illumination (Ee): The radiant flux (W/㎡) received on the unit irradiated area of ​​the radiation irradiated surface.
Irradiance (Ie): The radiant flux (W/sr) emitted by a point radiation source per unit solid angle.
 
Radiance (Le): The radiance of the element area of ​​the radiation source in the θ direction is the radiant flux emitted by the unit projected area of ​​the radiation surface on the plane perpendicular to the direction within the unit solid angle (W/(sr·㎡). )).
 
 
(2) Optical quantity
Corresponding to the amount of radiation, there are the following optical quantities:
Luminous flux (lm), light output (lm/㎡), illuminance (lx), luminous intensity (cd), brightness (cd/㎡).
Luminous intensity is one of the seven basic quantities of the International System of Units. The luminous intensity is 1cd.
 
2. The relationship between optical quantity and radiation quantity
1) Spectral light efficiency function
The relationship between the optical quantity and the radiation quantity depends on the visual properties of the human eye. The sensitivity of the human eye to different wavelengths of light
Long function: the peak value of the photopic spectral light efficiency function V(λ) is λ=5.55×10～7m; the peak value of the scotopic spectral light efficiency function V′(λ) is λ=5.07×10～7m.
2) The relationship between optical quantity and radiation quantity
In a small wavelength range:
Under photopic conditions: d￠v(λ)=Km V(λ)￠e(λ)dλ
Under the condition of scotopic vision: d￠v(λ)=(λ)￠e(λ)dλ
Km=683lm/w, Km′=1755lm/w.
Over the entire visible spectrum:
Bright Vision:
￠v=KmV(λ)￠e(λ)dλ
Dark Vision:
￠v=(λ)￠e(λ)dλ
 
3. Basic knowledge of colorimetry
Black body: An object that can absorb all incident electromagnetic waves without reflection or transmission.
Color temperature: When the color emitted by the illuminant is the same as the color emitted by the black body (reference illuminator) when heated to a certain temperature, the temperature of the black body is called the temperature of the illuminant.
Color temperature, referred to as color temperature, in K.
Correlated color temperature: refers to the temperature of the black body radiator that is most similar to the color of the stimulus with the same luminance.
Color rendering: refers to the objective effect of the light emitted by the light source on the object and the degree of display of the true color of the object. It is an important indicator for evaluating the lighting source.
Color rendering index: Compare the similarity of the target color between the test light source and the reference light source at the same color temperature.
Special Color Rendering Index (Ri): The degree of sign of color rendering compared to 15 standard illuminants.
General color rendering index (Ra): The average value of the degree of color rendering sign compared to the first 8 standard illuminants. which is.
 
CIE chromaticity calculation method:
CIE 1931 Standard Chromaticity System
 
 
 
three. Luminous flux and energy efficiency measurements
The total luminous flux of SSL products should be measured using an integrating sphere system or a goniophotometer. Which method to choose depends on the quantity to be measured (color, light intensity distribution), the size of the SSL product and other requirements.
The integrating sphere system is suitable for measuring the total luminous flux and color of integrated LED lamps and relatively small size LED lamps. The advantage of the integrating sphere is that the measurement is fast and does not require a darkroom. The goniophotometer provides light intensity distribution and total luminous flux measurement methods, it can measure the total luminous flux of relatively large SSL products, and of course can also be used to measure the total luminous flux of relatively large SSL products.
Small size SSL products. Air movement may affect the measurement of temperature sensitive SSL products and care should be taken when using the goniophotometer method. In addition, compared with integrating spheres, measurement with a goniophotometer is time-consuming.
 
1. Integrating sphere method
Integrating sphere methods include integrating spheres with spectroradiometers and integrating spheres with photometers (sphere-photometer systems).
Integrating spheres with photometers (sphere-photometer systems) are integrating sphere systems that utilize a photometer as the probe of the integrating sphere, due to potentially large spectral errors when measuring the luminous flux of SSL products (if no mismatch correction is used). Matching errors, and the amount of color requires the use of a separate measuring device, so although this method is acceptable it is rarely recommended.
 
The following is an introduction to the integrating sphere system of a spectroradiometer.
(1) The size should be large enough
The size of the integrating sphere should be large enough to avoid additional temperature rise from the light source under test and to reduce measurement errors caused by self-absorption of the baffle and the SSL product under test.
Compact light: balls ≥ 1m.
Larger lamps (eg: 4-inch linear fluorescent lamps and HID lamps): 1.5m or more balls.
Measuring 500W or more power source: 2m or more ball.
Within a 4∏ sphere, the total surface area of ​​the SSL product shall not exceed 2% of the total area of ​​the sphere wall. For example, in a 2m sphere, a sphere is less than 30mm in diameter.
The longitudinal dimension of the wire product should be less than 2/3 of the diameter of the ball.
Within a 2∏ ball, the diameter of the opening where the SSL product is installed should be less than 1/3 the diameter of the ball.
(2) An auxiliary light shall be installed
The auxiliary lamp is used to measure the self-absorption of the light source under test to obtain a self-absorption factor α(λ).
(3) The reflectivity of the inner coating of the spherical wall is 90%-98%
(4) Standard lamp
The standard lamp for measuring full-spectrum radiant flux is usually a broad-spectrum quartz tungsten halogen incandescent lamp to calibrate the full visible region of the spectroradiometer. For the 2Π sphere, only standard lamps with front light distribution are required. For 4Π spheres, standard lamps with omnidirectional light distribution are usually used, but standard lamps with front light distribution can also be used. It should be noted that different firing positions will change the light output of the incandescent lamp.
(5) The measuring principle device (integrating sphere plus spectroradiometer) must be calibrated against a standard calibrated to the total spectral radiant flux. Since the integrating sphere is included in this calibration system, it is not necessary to know the spectral flux of the sphere. By comparing with the reference standard ΦREF(λ), the total spectral radiant flux ΦTEST (λ) of the SSL product under test is obtained:

where: yTEST (λ) and yREF (λ) are the spectroradiometer readings of the SSL product under test and reference standard.
α(λ) is the self-absorption factor,
yaux,TEST(λ) is the reading of the auxiliary lamp by the spectroradiometer when the lamp under test is not working and the auxiliary lamp is working, yaux,REF(λ) is the reading of the spectroradiometer on the auxiliary lamp when the standard lamp is not working and the auxiliary lamp is working .
From the measured total spectral radiant flux ΦTEST (λ) (W/nm), the total luminous flux ΦTEST [lm] is:

Wherein Km=683lm/W
So you get SSL

The luminous flux of the product.
2. Spectrophotometer
Spectrophotometers are often used to measure the distribution of light intensity so that the total luminous flux can be obtained.
The goniophotometer should be a structure that can keep the ignition position of the lamp unchanged. Therefore, only C-type goniophotometers are acceptable. The C-type goniophotometer consists of a moving probe and a moving mirror. It should be noted that mirror-type goniophotometers have an easily polarized detection system due to the slight polarization of the mirror itself. Sensitivity to polarized light can lead to large errors when measuring the total luminous flux of SSL products that emit polarized light. When measuring such SSL products, a spectrophotometer without a mirror is recommended. For this purpose, some goniophotometers have the option to mount a photometric head directly on the swivel arm. Care should be taken to adjust to avoid reflection of light from the spectrophotometer mechanics or other surfaces, including the SSL product itself, onto the photometer. Rotation of the positioning device should minimize thermal balance disturbance to the SSL product.
 
 
(1) Measuring principle of total luminous flux
Measure the light intensity distribution I (θ, ¢) of the light source, and the total luminous flux can be obtained by the following formula

If the optical head is calibrated to measure illuminance E(θ, ¢),

where r is the rotation radius of the reference plane of the optical head. Measuring light intensity requires a sufficiently long photometric radius r.
 
 
3. Luminous efficacy
The luminous efficacy (lm/W) of the SSL product is obtained from the quotient of the measured total luminous flux ΦTEST (lm) of the tested SSL product and the measured input electric power PTEST (W), as shown in the following formula:

 
4. Problems in LED luminous flux testing
(1) The main problem:
The spatial light distribution of LEDs varies greatly;
Spectral distribution is different from standard lamps;
The effect of heat dissipation.
(2) Precise measurement requirements:
Avoid the central beam from irradiating singular areas such as spherical seams;
High reflectance coating: 0.90~0.98;
Full spectrum measurement to avoid spectral response matching errors of photometric probes;
TEC thermostatic control (device temperature control).