BTS256-EF - mobile light meter for photopic and scotopic illuminance, EVE factor, luminous color, color rendering index, luminous spectrum and flicker.

The BTS256-EF is a high-quality light meter that is well suited for illuminance and luminous color measurements in all application areas thanks to its compact design, high-quality light sensor and precise cosine field of view function. Another special feature of the device is its ability to perform flicker measurements. 

BTS256-EF - mobile light meter for photopic and scotopic illuminance, EVE factor, luminous color, color rendering index, luminous spectrum and flicker

The BTS256-EF is a high-quality light meter that is well suited for illuminance and luminous color measurements in all application areas thanks to its compact design, high-quality light sensor and precise cosine field of view function.

BiTec light sensor for precise light measurements

One of the outstanding properties of this mobile light meter is its BiTec light sensor. This combines the characteristic properties of a silicon photodiode with those of a low-noise CMOS diode array. The BiTec sensor guarantees precise photometric and spectral-radiometric measurement values over a large dynamic range through mutual correction of the measurement signals of both sensors. 

Silicon photodiode detector, fast and linear

When taking into account the dynamic range, linearity and speed, silicon photodiodes have always been and are still the ultimate light detectors. A photometric corrected silicon photodiode is therefore incorporated in the BiTec light sensor of the BTS256-EF light meter. Its matching to the photometric V(λ) responsivity is improved by the diode array’s spectral measurement data.

Flicker measurement 

Flicker measurement entails identification of a possible AC component of the light signal with a 5kHz bandwidth. Besides the flicker frequency, other flicker parameters i.e. Flicker percent and flicker index are also displayed. The device additionally performs a Fast Fourier Transformation so as to display other frequency components in the signal. For more details, see the technical article on flicker.

Diode array detector for spectral measurement data

The CMOS diode array of the BiTec Light sensor provides precise measurement data required for the luminous spectrum. This data is then used for calculation of the color values and scotopic illuminance as well as for optimization of the photometric responsivity.

Optimized Signal to Noise Ratio 

The noise signal from diode array detectors has significant effect on the signal to noise ratio (SNR) and hence on the quality of the measurement signal as well. A remote controlled shutter in the BiTec light sensor enables online compensation of the dark signals that are dependent on the temperature and integration time. 

Pulse Width Modulated (PWM) LED lamps

The fast photodiode of the BiTec light sensor enables the BTS256-EF to automatically synchronize itself to the frequency of lamps operated in PWM mode.

Precise cosine corrected field of view

A cosine corrected field of view is absolutely necessary for illuminance measurement devices. For extended illuminance setups, the cosine matching has significant influence on the measurement uncertainty of the light meter. The BTS256-EF light meter is equipped with a diffuser window with a 20mm diameter whose cosine matching ensures an f2 ≤ 3% uncertainty which corresponds to the DIN 5036 requirements for the quality class B. Both detectors of the BiTec light sensor are centrally aligned behind the diffuser window and therefore have the same view angle.

DIN quality class A and B luxmeter

Through the cutting-edge Bi-Technology sensor concept, use of modern electronics, online dark signal adjustment and the possibility to perform temperature compensation, the device meets the conditions specified for two of the DIN 5036 quality classes.

  • In terms of the f2 uncertainty i.e. ≤ 3%, the device corresponds to the quality class B (DIN 5032 section 7).
  • In applications where limitation of the viewing angle is acceptable, the BTS256-EF, with its f1’, u, f3 and f4 values, corresponds to the quality class A (DIN 5032 section 7).

Diverse photometric and colorimetric measurands 

A much broader diversity of measurands as the one offered by conventional light meters is required for verification and testing of LED lamps. The BTS256-EF offers fourteen measurands and thereby meets all the requirements for a modern light meter:

  • Ep photopic illuminance
  • Es scotopic illuminance
  • Es/Ep ratio of night and daylight vision
  • EVE “Equivalent Visual Efficiency” factor
  • Ee irradiance
  • Eλ spectral irradiance
  • x, y CIE 1931 color coordinates
  • u‘, v‘ CIE 1976 color coordinates
  • CT color temperature
  • Δuv Deviation from the blackbody locus
  • λdom dominant wavelength
  • λp peak intensity wavelength
  • λ0,5 spectral half-width
  • Purity color purity
  • CRI Ra and R1 to R15 Color Rendering Index

Non-visual response measurands

In addition to conventional rods and cones, the retina has melanopsin-containing photosensitive ganglion cells. Vision scientists have demonstrated unequivocally that melanopic light reception affects brightness perception and that the conventional photopic measurements require a correction to maintain a proper correlation with perceived brightness. In cooperation with the researchers who originally determined this correlation factor, Gigahertz-Optik now includes this Melanopic/Photopic and Brightness Perception function in the BTS256-E series meters.

The BTS256-EF displays:

  • E (Melanopic / Photopic) display title
  • Ep – This is Photopic Illuminance (lx)
  • Em / Ep – This is Melanopic Irradiance (mW/m2) / Photopic Illuminance (lx) = mW/lm
  • Em’ / E– This is Melanopic Illuminance (lx) / Photopic Illuminance (lx) = unit-less ratio
  • B0.32 – This is Brightness Perception (lx) – other user defined constants may be input through menu setting

Display modes for standard measurements

The BTS256-EF Bi-Tec sensor luxmeter has several display modes where the necessary measurands for the common photometric measurements are incorporated. The cursor buttons can be used to switch between the displays.

Customized display modes

The BTS256-EF enables users to individually configure and save display modes. The required measurands can hereby be selected.

Info display for measurement parameters

An incorporated Info Display shows all the relevant measurement parameters.

Easy operation and handling

Three control buttons and the diligently structured menu navigation make the device easy and safe to operate the device. For instance, in the standard measurement mode, the user does not have to manually set the measurement parameters since these are automatically and best adjusted to match the measurement conditions. In the expert mode, the user has access to all measurement parameters. 

Many useful extra functions

  • If the user wants to distance himself from the device’s field of view, he/she can set a time-delayed measurement.
  • The display can automatically be dimmed during the measurement.
  • An acoustic signal can be set to mark the end of a measurement.
  • A layout created using a PC with the support points for the single measurements can be adopted in the device for measurement of the illuminance distribution. The measurement points can be progressively processed and saved.
  • Fast data logger measurement mode where time-clocked readings of the photodiode can be recorded at a maximum measurement rate of up to 100ms.
  • A second data logger measurement mode that enables time-clocked recording of all measurands including the spectral measurement data.
  • Date and time in real-time can also be set.

Tripod mount

The measurement device has a tripod stand socket on the bottom side.

Protective cap with chain

The protective cap for the diffuser window is attached onto the device.

Option WiFi

With this option the device can be controlled by the S-BTS256 per WiFi.

Use without PC

The BTS256-EF has all the necessary functions enabling free operation of the device without having to depend on a computer. Furthermore, the rechargeable batteries have a capacity of more than 8 operation hours. The USB power adapter enables recharging of the device without a PC.

Use with PC

The BTS256-EF additionally has a USB 2.0 interface for use with a PC. This enables both data exchange and battery charging.

User software 

The S-BTS256 software included in the device’s price provides all the necessary functions for the measurements, measurement data display and data transfer. The cutting-edge, flexible desktop concept of the software offers the user an individual constellation of the required measurement values. This can be a full screen filled with lux measurement values or a matrix with both numerical and graphical fields. Each desktop constellation can easily be saved for future use.

Software Development Kit 

Gigahertz-Optik offers programmers the S-SDK-BTS256 Software Development Tool. This can be used with LabView from National Instruments, .NET from Microsoft and C/C++. The SDKs simplify integration of the BTS256-E in an internally developed software.

Standard display showing the signal progression and signal frequency

Standard display showing the illuminance and flicker parameters

Standard display showing frequency components determined through Fast Fourier Transformation

Light meter with precise cosine field of view function

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


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%

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

Integral Detector

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



≤3% (f1' a*(sz(λ)) respectively F*(sz(λ)) corrected)

Max. measurable illuminance

≥199,999 lx

Noise equivalent illuminance

≤ 0.01lx



Measurement Time

(0.1 - 6000) ms

spectral irradiance sensitivity

Spektrale Empfindlichkeit BTS256 E

f2 uncertainty

f2 uncertainty 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
GB-GD-360-RB40 Goniometer for the measurement of 2π sources.

Features: Measurement of the luminous and radiant intensity distribution as well as luminous flux and radiant power from compact spot lamps and light-emitting diodes. Measurement distance 100 mm to 2000 mm. Remote control. Optional Lightmeters, user software, etc.
S-SDK-BTS256 Software Development Kit for BTS256 variants
S-BTS256 Application software for BTS256 variants
Article-Nr Modell Description Notice
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 CD, 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

15298108 S-BTS256

User software for BTS256 and variants.

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