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Evaluating the flicker of LED lighting systems requires an extremely high level of precision due to the rapid response of LEDs to changes in the electrical supply. Such evaluations are done by measuring the amplitude and frequency of any modulation of  the light (flicker measurement using the BTS256-EF). The pulse width modulation implemented in many LED drivers makes it necessary to also consider higher frequencies in flicker evaluation.

BTS256-EF – Highquality, handheld meter with flicker evaluation function

In addition to the complete photometric and colorimetric capabilities of the BTS256-E unit, the BTS256-EF also performs light flicker measurement.  This makes the BTS256-EF the perfect spectral light meter for comprehensive measurements of lighting systems. With the BTS256-EF, you can easily measure both lighting and flicker properties. This is made possible by the fast Si photodiode within its BiTec sensor and its associated high bandwidth amplifier. The device can be used to measure flicker frequencies in the range between 0.25 Hz and 5 kHz. It displays flicker index, percentage and frequency as well as min and max illuminance.  The BTS256-EF also determines the FFT for clear illustration of other frequency components and their percentage share.

Calibration of the BTS256-EF

One essential quality feature of photometric devices is their precise and traceable calibration. The BTS256-EF is calibrated by Gigahertz-Optik’s calibration laboratory that was accredited by DAkkS (D-K-15047-01-00) for the spectral responsivity and spectral irradiance according to ISO/IEC 17025. The calibration also included the corresponding accessory components. Every device is delivered with its respective calibration certificate.

 Options for the BTS256-EF light meter

-       Software development kit for integration of the device in the user’s own software

BTS256-EF for complex measurements in lighting technology, including flicker measurement

1) BTS256-E 2) Precision cosine diffuser 3) BiTec sensor with Si photodiode, CMOS diode array spectrometer and shutter 4) Photometric Si-photodiode with fast amplifier 5) Microprocessor 6) Display 7) Control Buttons 8) USB 2.0 interface 9) Light incident

BTS256-EF light meter with precise cosine field of view function

Display of the measurement signals and the flicker frequency

Display showing the illuminance and flicker parameters

Display showing frequency components determined through Fast Fourier Transformation

Display showing the scotopic and photopic illuminance and the S/P ratio as per IES TM-24-13

The measurement parameters protocol is saved together with a measurement data file

S-BTS256 user software with modular setup desktop

The graphical display module can be zoomed to full screen

Short description

Spectroradiometer for the illuminance (photopic, scotopic, melanopic), spectrum, light color, color rendering index, and flicker

Main features

Mobile measurement device, BiTec sensor with a V-lambda photodiode and low stray light CMOS spectroradiometer with a 10 nm optical bandwidth and additional optical bandwidth correction (CIE214), remote-controlled offset shutter, precise cosine-corrected field of view, data  logger, automatic PWM synchronization, flicker measurement

Range of measurement

1 lx to 199,000 lx, 380 nm to 750 nm, flicker frequency between 0.25 Hz and 5 kHz

Typical applications

Precise spectral light meter for the lighting industry


Factory calibration, traceable to international standards


class B DIN 5032 part 7 or AA according to JIS C 1609-1:2006
class A DIN 5032 part 7 for f1’, u, f3 and f4 or general precision class according to JIS C 1609-1:2006

Light sensor

Bi-Technology sensor with a photometric broadband detector and a array spectrometer. Integrated aperture for automatic dark signal adjustment.

Input optic

Diffuser window with 20mm diameter, cosine corrected field of view, f2 Error ≤ 3%


Spectral responsivity with fine CIE photometric matching. Online correction of the photometric matching through spectral measurement data (spectral missmatch factor correction)


Using as a handheld device without software, limited by internal storage:

Measurement Time (Sensor) Measurement Time (Flicker) Sampling Rate Upper Cut-Off Frequency Lower Cut-Off Frequency Frequency uncertainty at acceptable S/N ratio FFT frequency resolution
50ms 41.0 ms 20µs 5kHz 60Hz 1%+-0.5Hz 25Hz
100ms 81.9 ms 40µs 5kHz 30Hz 1%+-0.5Hz 12,5Hz
200ms 163.8 ms 80µs 2,5kHz 15Hz 1%+-0.5Hz 6,3Hz
500ms 327.7 ms 160µs 1,2kHz 8Hz 1%+-0.5Hz 3,2Hz
1000ms 655.4 ms 320µs 0,6kHz 4Hz 1%+-0.5Hz 1,6Hz
3000ms 2620 ms 1280µs 150Hz 1Hz 1%+-0.1Hz 0,4Hz
6000ms 5240 ms 2560µs 75Hz 0,5Hz 1%+-0.05Hz 0,2Hz
12000ms 10486 ms 5120µs 33Hz 0,25Hz 1%+-0.02Hz 0,1Hz

By Software controlled use the above analysis or the following high resolution analysis is possible:

Measurement Time (Sensor) Measurement Time (Flicker) Sampling Rate Upper Cut-Off Frequency Lower Cut-Off Frequency Frequency uncertainty at acceptable S/N ratio

FFT frequency resolution

5 ms


3932 ms



3932 ms

20 µs 5 kHz

2.5/meas. time

1 % ± 0.5 Hz

1/meas. time

3932 ms


12000 ms

3932 ms 20 µs 5 kHz 0.6 Hz 1 % ± 0.5 Hz 0.25 Hz
Calibration uncertainty

Illuminance +/-2.2%

Spectral Detector

CMOS diode-array

Wavelength Range

(380 - 750) nm

Optical Bandwidth

10 nm

Data Resolution

1 nm

Integration Time

(5.2 - 30000) ms


Automatic aperture for dark signal measurements with the same integration time as that of light measurements. Aperture delay = 100ms.

Typ. Measurement Time

199,999 lx ≤ 5ms (white light)

100 lx ≤ 1s (white light)

Color measurement range spectral

(1- 199,999) lx


Scotopic measurement range spectral (1 - 199,999) lx
Calibration uncertainty of scotopic Illuminance +/-2.2%

Peak wavelength

+/- 1nm

Dominant wavelength

+/- 1nm

Δx, Δy reproducibility

+/- 0.0001 (Standard illuminant type A)

+/- 0.0002 (LED)

Δx, Δy uncertainty

+/- 0.002 (Standard illuminant type A)

+/- 0.005 (typ. LED)

CCT Measurement range

(1700 - 17000) K


+/- 50K (standard illuminant type A)

+/- 4% (depending on the LED spectrum)

CRI (color rendering index)

Ra and R1 to R15

Stray Light

6E-4 (Blue LED)

6E-4 (Green LED)

6E-4 (Red LED)

1E-3 (White LED)

Calibration uncertainty

Spectral irradiance

(380 - 399) nm:  +/- 7 %
(400 - 750) nm:  +/- 4 %

Spectral irradiance responsivity (380 - 750) nm

Integral Detector

≤6% (uncorrected)

≤3% (f1' a*(sz(λ)) respectively F*(sz(λ)) corrected by spectral data, done automatically by BTS technology)

Max. measurable illuminance

≥199,999 lx

Noise equivalent illuminance

≤ 0.01lx



Measurement Time

(0.1 - 6000) ms

spectral irradiance responsivity

responsivity BTS256 E

f2 uncertainty

f2 BTS256 E


16Bit, 25ns instruction cycle time

Charging voltage

5VDC, 450mA per USB


USB 2.0 (Type B USB)

Option WiFi: WiFi 2,4 GHz (external antenna, range > 100m)


Operation: -10°C bis +30°C

Storage: -10°C bis +50°C


Splashproof IP54

Transport case

333mm x 280mm x 70mm, 650g


159mm x 85mm x 45mm (Length x Width x Height)


500 g

Type Description File-Type Download
Datasheet BTS256-EF PDF
Produktname Product Image Description Show product
S-SDK-BTS256 Software Development Kit for BTS256 variants.
Article-Nr Modell Description Request / Compare
15298440 BTS256-EF

BTS256-EF, user manual (D or E), S-BTS256 user software CD, USB cable for PC operation and battery charging using the PC, USB power adapter (EU, USA or GB), BHO-17 hard-top casing

15298441 BTS256-EF WiFi

BTS256-EF WiFi, user manual (D or E), WiFi antenna, S-BTS256 user software, USB cable for PC operation and battery charging using the PC, USB power adapter (EU, USA or GB), BHO-17 hard-top casing

15300751 K-BTS256-E-I

Recalibration of the BTS256-EF. Including calibration certificate.

15298218 S-SDK-BTS256

Software Development Kit; Software and users guide on CD

Further Information

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