Optical Radiation


The Optical Radiation Metrology Lab is responsible for the development, maintenance and dissemination of Singapore’s national measurement standards for radiometry, photometry, spectrophotometry and fiber optics. It also aims to advance measurement technologies related to the measurement of optical radiation, characterisation of light sources and optical detectors, and measurement of optical properties of materials in the ultraviolet, visible and infrared spectral regions.

With these capabilities, the lab is able to provide traceable calibration and measurement services to the industries such as solar energy related industry and aerospace maintenance companies as well as other users. It also offers technical consultancy and training.

Primary Standard For Optical Power

The primary standard for optical power is a cryogenic radiometer operating at an absolute temperature of ~13 K, based on thermal equivalence between optical and electrical heating.Combined with a group of intensity stabilised lasers, it is capable of measuring laser power with very low uncertainty (~ 0.01%) in the spectral range of 350 nm – 800 nm.

This facility has also been used to calibrate the spectral responsivity of various photo detectors and provide traceability to NMC's photometric and radiometric measurements.

 

 
Cryogenic radiometer


Intensity-stabilised laser facility

Photometry

The photometry lab in the optical radiation metrology (ORM) keeps measurement standards and equipment for the measurement of luminous flux, luminous intensity and colour temperature of light sources and calibration of illuminance, luminance meters.


The SI base unit of luminous intensity, the candela, has been realised at ORM since 2003. Three temperature stabilised trap-photometers with precision apertures and V(l)correction filters were constructed and fully characterised. The relative expanded uncertainty at a confidence level of approximately 95% (k=2) of the luminous responsivity of these trap-photometers is better than ±0.52%.



Integrating sphere for luminous flux measurement


8 - m optical bench for lux meter calibration

Primary Calibration Facility Of Reference Solar Cells

The new facility being developed by NMC is based on the most advanced technique - measurement of differential spectral responsivity of solar cells with direct traceability to SI radiometric unit.

The system is capable of calibrating primary reference solar cells with measurement uncertainty < 1%, which will enable Singapore to become a world class regional centre for solar PV cell calibration, testing and characterisation.

The project is funded by A*STAR under the renewable energy programme.


Differential spectral responsivity measurement for reference solar cell


Primary calibration facility of reference solar cells


Solar cells

Reference Spectrophotometer

The Reference Spectrophotometer developed at NMC is the reference standard for the spectral transmittance measurement. Spectral transmittance is a basic quantity in spectrophotometry.

The Reference Spectrophotometer is a single beam geometry instrument. The collimation of the parallel beam is 0.013 radian and the solid angle of collection of the detector system is 0.004 steradian. The fully characterised reference spectrophotometer establishes the national scale of spectral regular transmittance over the wavelength range of 350 nm to 1100 nm. Its measurement uncertainty is estimated to be 0.000052 transmittance units (0.052 % relative) for neutral density filters with nominal transmittance of 0.1 transmittance units.


Reference spectrophotometer


Schematic diagram of reference spectrophotometer

UV Broadband Radiometry Standard

Accurate measurement of UV radiation is critical for many industrial, medical, environmental, and other applications.

However, due to the lack of internationally recognised standard for the characterisation and calibration of this type of radiometers, significant discrepancies are common when radiometers from different manufacturers are used to measure the same UV radiation.

To meet the demands of the industry, NMC has been actively researching and developing methods for the characterisation and calibration of broadband UV radiometers.



UV radiometer calibration facility


UV radiometers


Fibre Optic Standards

    The calibration facility of fibre optical power meter has been developed at NMC using tunable diode laser sources as well as a reference standard - InGaAs photodiode.

    The facility for calibration of optical spectrum analyser has also been developed using tunable laser source and a wavelength meter which is traceable to a laser wavelength primary standard - iodine stabilised He-Ne laser.

    The calibration service for the loss (attenuation) and length (distance) of optical time domain reflectometer (OTDR) is available by using a group of reference standards maintained at NMC.



    Calibration facility for fibre optic power meter

    Primary Standard for Optical Radiation Source

    The primary standard for Optical Radiation Source is a variable high temperature (up to 3500 K) black body (VTBB), providing spectral radiance and irradiance reference values in the wavelength range of 250 nm – 2500 nm.

    A multi-wavelength filter radiometer (MWFR) containing 24 band pass filters with centre wavelengths from 250 nm to 1600 nm, a silicon trap detector and an InGaAs trap detector traceable to cryogenic radiometer, is used to directly measure the spectral irradiance of the VTBB at these wavelength points, and determine the best fit of Plank’s formula to obtain the continuous spectral reference values of the VTBB.

    The primary standard is used to calibrate transfer standard lamps (with measurement uncertainty of 1% - 2% depending on wavelength) to provide traceability to NMC's photometric and radiometric measurements. 


    High temperature black body (VTBB)


    Multi-wavelength filter radiometer (MWFR)

    Metrology for LED Lighting

    Light-emitting diode, commonly known as LED, is a semiconductor-based light source. It can now be found in many applications, such as LED lamps, LED light strips, LED TV and LED indicators.


    To ensure the reliability and quality of LED lightings, several performance parameters of LED have to be measured. Generally, LEDs are tested on their light output intensity, flux, efficiency/efficacy, brightness, colour, colour rendering index etc. The technologies for high power LEDs for lighting applications are still evolving.

     

    New LED lighting products are getting brighter and brighter. Some of them are so bright that people apparently feel uncomfortable when looking at them directly. The safety concern of LED lightings has been raised and should be tested if in doubt. The quantities needed to be measured for LED lighting safety in terms of optical radiation are spectral irradiance and spectral radiance.

     

    Over the years, NMC has established some LED metrology facilities; they are the CIE Averaged LED intensity measurement facility, LED intensity distribution facility and LED total/forward flux measurement facility.