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FS2-Series Flashlight Sphere Spectroradiometers

Labsphere FS2 Flashlight Spectroradiometer

Accurate lumen measurements of flashlights & torches

The Labsphere FS2 series integrating sphere spectroradiometers are specialist light measurement instruments that have been purpose-designed for accurately measuring the forward (2π) flux from flashlights (torches) and other narrow beam angle light sources. The FS2 series are available with integrating spheres of 25 (10-inches), 50 (20-inches) and 103cm (40-inches) diameter. A CDS-series spectrometer controlled from LightMtrX software provides for ease-of-use, instant acquisitions and powerful analysis.

Purpose Designed for Narrow Beam Angle Sources

Each FS2 spectroradiometer employs a specially modified Labsphere LMS-series interior access integrating sphere that supports measurements of 4π flux of samples mounted within the sphere as well as 2π (forward) flux of samples placed at the external source port on the wall of he sphere. The external source port has been enlarged compared with the standard LMS spheres: 50mm for the FS2-1060 (10-inch sphere); 100mm for the FS2-2060 (20-inch sphere); and 178mm for the FS2-4060 (40-inch sphere). A convenient support structure is attached to the sphere frame to hold the device under test at the enlarged external source port, while modified baffles within the sphere compensate for the enlarged port and provide effective screening of the sample from the detector port on the sphere. Each sphere is supplied with a number of interchangeable port reducers that minimise the open port area.

Accurate Flux Measurements Regardless of the Lamp Type

The FS2 employs a compact, high speed CDS-600 CCD array spectrometer that provides accurate luminous flux and colour temperature measurements regardless of whether the flashlight employs LEDs or more traditional lamps (halogen, xenon or krypton). Unlike an ordinary filter photometer, the CDS-600 measures the spectral radiant flux from 350-850nm and applies the CIE standard photopic observer function in software. The result is accurate lumen readings regardless of the type of lamp used in the device under test.

Instantaneous Lumen and CCT Data

The CDS-600 CCD array spectrometer not only promotes measurement accuracy, it also provides instantaneous spectral acquisitions. Unlike tradition scanning spectroradiometers that record the spectral power one wavelength at a time, the CSD-600 records a complete spectrum (350-850nm) in an instant – the measurement time varies with the power of the device under test, but the typical scan time is under one second. LightMtrX software takes care of the photometric and colorimetric computations, providing lumens, chromaticity and colour temperature data in a flash.

Forward Flux Standard

In accordance with the recommendations given in IES LM 79-08 for measurements of the 2π flux from solid state lighting, each FS2 system is supplied with a standard of forward spectral flux. In the past, integrating sphere photometers and spectroradiometers were calibrated by placing a standard lamp in the centre of the sphere. The device under test was then either placed in the centre of the sphere for total flux (4π) measurements, or at the external source port for forward flux (2π) measurements. This approach does suffer from an error which arises from the possibility of a narrow beam angle sample mounted outside the sphere illuminating an area of sphere wall that exhibits a slightly lower localised reflectance. Integrating sphere theory tells us that small differences in sphere wall reflectance can result in a much larger change in the average sphere wall radiance; localised reflectance differences tend to be insignificant provided that the device under test produces a homogeneous, wide angle emission. The trouble with flashlights and other samples that produce a beam of light is that the consistency of reflectance of the small patch of sphere wall that is first illuminated becomes much more critical.

To minimise the effects of even a small, localised sphere wall reflectance non-uniformity, Labsphere developed its FFS-100 series of forward flux standards. These mount onto the external source port of the sphere and emit a torch-like beam of light that better matches the beam geometry of flashlights. This in turn minimises calibration errors compared with the use of a centrally mounted standard lamp.

LightMtrX Software for System Control

Each FS2 spectroradiometer system is controlled from LightMtrX software. Labsphere's LightMtrX is quite simply the easiest to use software yet released for performing spectroradiometric, photometric and colorimetric measurements of light sources. LightMtrX features user-defined "methods", an auxiliary correction capability and the ability to perform electrical and optical testing in conjunction with a Keithley 2400 series SourceMeter.

Auxiliary Correction for Accurate Measurements

Integrating sphere spectroradiometers are calibrated by substitution. The lamp standard of forward flux is placed at the external source port on the wall of the sphere and the spectroradiometer responsivity determined. The measured responsivity takes account not just of the light emitted from the standard lamp, but also the light reflected back into the sphere upon reillumination. When a flashlight sample is placed at the same port, the degree of reflectance may well be different which results in the measuired flux from the sample being over recorded.

There is, however, a very simple solution to this error. This is called auxiliary correction and involves the use of a low power, uncalibrated "auxiliary" lamp mounted on the wall of the sphere. This lamp is used to guage the relative absorption of the standard lamp compared with the device under test and to yield a correction factor that can be applied to subsequent readings of the luminous flux for the particular type of sample under test. All of the FS2 flashlight spectroradiometer systems are supplied as standard with an auxiliary correction lamp and companion power supply that can be used to fully correct for self absorption errors.

Also Measure Intensity & Illuminance

In addition to measuring luminous flux, you can add the capability of measuring luminous intensity and illuminance to your FS2 spectroradiometer system. Optional sensor heads can be attached to the optical fibre feed to the spectrometer that provide for alternative types of photometric measurements. The E-1000 irradiance sensor is an integrating sphere-based head with a near-perfect cosine response that is used for measurements of the spectral irradiance of light sources. The I-1000 & I-2000 intensity sensor heads provide for measurements of the spectral radiant intensity of individual LEDs in accordance with CIE publication number 127, condition A & B.

Specifications & Ordering Information

FS2 Flashlight Spectroradiometers




Product Code




Integrating Sphere Dia.

10" (0.25m)

20" (0.50m)

40" (1.02m)

External Source Port Dia.

2" (51mm)

4" (102mm)

7" (178mm)

Max. Lamp Power (W)








   Dynamic Range (lumens)1

0.56 ... 12,500

2.25 ... 50,000

9.0 ... 200,000

   Spectral Range (nm)2

350 ... 850

350 ... 850

350 ... 850

   Spectral Resolution (nm)

< 2

< 2

< 2

Max. Internal Sample Size      

   2D (cm)

5 x 5

10 x 10

18 x 18

System Configuration       

Integrating Sphere3








Forward Flux Standard




Port Frame Reducer4




Stabilised Power Supply




Auxiliary Correction Lamp




Lamp Switch Box 




Port Reducers5




Sample Holder




Options, Upgrades & Accessories



Required Options      




Other Options, Upgrades & Accessories      

   Temperature Probe




   Temp. Probe with Monitor




1. For a CIE standard illuminant A source.
2. The spectral range of the CDS-600 is 200-850nm However, the working range of the integrating sphere
    spectroradiometer system is set by the spectral flux standard lamp which is calibrated from 350-1050nm.
3. The LMS-series integrating spheres are modified when deployed in an FS2 syste. The external source port
    is enlarged and internal baffles modified to compensate.
4. The port frame reducer allows the FFS fowrd flux standard to be held at the external source port on the
    LMS-series integrating sphere.
5. Interchangeable port reducers attach to the external source port on the LMS-series integrating spheres to
    reduce the open port area.

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