Luminous Flux Measurement of Compact Fluorescent Lamps (CFL)  

Compact Fluorescent Lamps are an effective alternative to incandescent lamps offering higher efficiency and longer life time. The light measurement quantities to qualify the CFL in R&D, production control, quality control and incoming inspection are luminous flux, color temperature, spectral flux distribution and switch on-time. The best cost and time effective measurement instrumentation for these applications are integrating spheres equipped with light and color meter for the photometric measurement quantity luminous flux and luminous color temperature. This technical note explains how to measure the luminous flux and luminous color of compact fluorescent lamps using the Gigahertz-Optik BTS256-LED light analyzer combined with a large diameter integrating sphere.        

One objective of this tech note was to compare the luminous flux measurement of a compact fluorescent lamp with measured data supplied by an other test laboratory. The lamp was specified as 7W at 230V / 50Hz at room temperature. The measurement conditions were specified with the lamp operating with the lamp socket in ‘up’ position at 25°C ambient temperature. Operating lamp voltage is 230V. Burn-in time before measurement 8 hours .

In support this tech note actual measurements were performed using Gigahertz-Optik’s sales support demonstration equipment to fulfill a second objective of this note which is the proper measurement technique and system properties. The following Gigahertz-Optik equipment was used:

Integrating Sphere:
Based on the size and intensity of the light source device under test (DUT) a 500mm / 12 inch diameter integrating sphere was selected. The hinge frame design of the integrating sphere enables one hemisphere to open/close so that the lamp can be assembled to the E27 type lamp socket.
The lamp socket post is height adjustable to allow the lamp bulb to be positioned in the center of the sphere. Also, the lamp socket is designed with four-separate-contact technology where two contacts are connected to the power supply and the other two contacts to a voltmeter. The very high internal resistance of the voltmeter prohibits a voltage drop at the two contacts and leads used for voltage measurement enabling a very precise measurement of the lamp operating voltage.

An auxiliary lamp mounted onto the integrating sphere supports the compensation of the measurement error effected by the self-absorption effect of the DUT itself. A baffle placed in front of the detector port blocks a 100mm diameter spherical space in the sphere center, which dictates the maximum allowable size of the DUT as well.  

Light Meter:
Gigahertz-Optik’s BTS256-LED light analyzer offers state-ofthe-art light measurement technology for accurate light measurements. Initially designed for simple and accurate measurement of total flux and relevant luminous color data of LEDs the BTS256-LED is also adaptable for the measurement of other type light sources including CFL. This is due in particular to its powerful bi-technology sensor that includes a
photometric photodiode and diode array spectrometer to ensure accurate measurement values independent of the light source emission spectrum. The light analyzer mounts onto the sphere detector port by means of its bayonet type adapter. It is calibrated on the sphere to form a luminous flux measurement instrument. Calibration is performed in Gigahertz-Optik’s calibration laboratory for light measurement quantities against calibration standards traceable to national and international metrology laboratories.     

Electronics:
Devices included a constant AC voltage power supply for precise voltage adjustment and stable operating voltage during the measurement. The voltage was set to the measured value at the lamp socket to avoid uncertainty caused by any voltage drop at the power leads and contacts.    

Constant DC current power supply to power the auxiliary lamp.
Voltmeter with very high internal resistance to measure the lamp operating voltage.
Temperature meter to monitor the temperature inside the integration sphere with the temperature probe located behind the detector baffle.
Laptop PC running Windows 7 and Gigahertz-Optik SBTS256-LED user software.

Measurement Steps:

Substitution Correction:
To compensate the error effected due to self-absorption of the DUT placed inside the sphere a substitution measurement was performed before the actual measurement.
First the auxiliary lamp flux was measured with and without the DUT with lamp holder inside the sphere. For this particular lamp a -8% substitution error was determined. As a result the actual luminous flux measurement was corrected by this factor. Note that the substitution measurement routine as well as the correction of the measurement value is fully supported in the S-BTS256-LED software.

Burn-in Measurement:
For a better understanding of the burn-in characteristics of compact fluorescent lamps the BTS256-LED light analyzer was set to data logger mode for subsequent measurements. For a faster sampling rate and to reduce the amount of measured data the light analyzer’s diode array part of its bitech sensor was de-activated.
Initially before measurement the internal sphere temperature was 25°C. The temperature at the lamp socket reached +60°C after 20min. The sphere temperature increased 1.5 to 2°C during the measurement period.
The test CFL luminous flux reached the 60% point (185.90 lm) of its maximum flux (309.84 lm) after 12.5s (measured using the switch-on mode of the S-BTS256-LED software). The luminous flux stabilized after 30 minutes at a level of 60% of its peak intensity.    

The spectral flux was measured after a burn-in time of 8 hours. Three measurements were made 15 minutes apart to check lamp stability as well as the repeatability of the measurement device. The measured value of peak intensity at 612 nm were:

• 0.02904 W
• 0.029038 W
• 0.029038 W

Luminous Flux Measurement:
The actual luminous flux measurement is done using the light analyzer’s photometric detector. This is one of the unique features of the BTS256-LED tester’s bi-tech sensor which combines a diode array with a photometric detector. Measuring light sources with narrowband spectra and/or spikes (e.g. LEDs and CFLs) an integral photometric detector without Pixel GAPs and much better linearity than CCD or CMOS detectors guarantees accurate luminous flux measurements. To compensate for any spectral mismatch error of the photometric detector to the CIE V(λ) photometric standard vision function the on-line measured spectral measurement values of the bi-tech sensor’s diode array spectrometer are used to improve the photometric detector accuracy.

The luminous flux was measured after a burn in time of 8 hours. Three measurements were made 15 minutes apart to check lamp stability as well as the repeatability of the measurement device. The measured luminous flux values were:

• 305.48 lm
• 305.48 lm
• 305.48 lm

Measurement of Luminous Color Data:
The DUT relevant luminous color values are calculated using the spectral measurement data supplied by the bi-tech sensor. Three measurements were made 15 minutes apart to check lamp stability as well as the repeatability of the measurement device.    

1. Correlated Color Temperature:

2732K, 2732K, 2722K

2. Color Rendering Index:

Ra	80,39	Ra	80,387	Ra	80,387
R1	95,17	R1	95,166	R1	95,166
R2	94,00	R2	93,997	R2	93,997
R3	56,58	R3	56,583	R3	56,583
R4	87,41	R4	87,41	R4	87,41
R5	84,41	R5	84,414	R5	84,414
R6	82,08	R6	82,083	R6	82,083
R7	85,35	R7	85,35	R7	85,35
R8	58,09	R8	58,092	R8	58,092
R9	-18,19	R9	-18,187	R9	-18,187
R10	51,53	R10	51,525	R10	51,525
R11	75,34	R11	75,344	R11	75,344
R12	45,52	R12	45,524	R12	45,524
R13	98,04	R13	98,035	R13	98,035
R14	69,75	R14	69,75	R14	69,75

Lamp Orientation Dependent Luminous Flux:
For a better understanding of the influence of lamp orientation (twelve, six and three o’clock) to the luminous flux during operation the flux was measured with the DUT in the three different orientations.

Software Support
All measurements and documentation were generated using the S-BTS256-LED software supplied with the BTS256-LED tester. The software allows the user to select and display his desired measurement criteria and screen set-up tailored to his individual application. In the top example display numerical information combined with spectral plot and CIE 1936 color coordinate plot is shown. The bottom example is set-up for numerical information combined with larger size spectral plot. The S-SDK-BTS256-LED software development kit with DLL for C and C++ programming and Labview Vi is available for those customers interested in integrating the BTS256-LED into their own software.