LED lamp measurements

One of the newest additions to NIOM’s instrumentation is equipment for optical radiation measurements.

While its primary purpose is to monitor the emission from our LED curing lamps rapidly and accurately, the new equipment can perform spectral measurements of several other light sources as well. Output values can be chosen, conveniently, as spectral irradiance (W/cm² nm) or irradiance (W/cm²), since a spectrum of the lamp emission is always obtained. NIOM can offer spectral measurements of your curing lamps both for clinical and research use.

The equipment consists of a spectrometer connected to an integrating sphere via a fibre optic cable. The integrating sphere scatters the light when the light source is pointed at an opening in the sphere. The light is collected and transmitted through the fibre optic cable to the spectrometer. A spectrometer is an instrument that registers the energy of the light at each wavelength by creation of a spectrum over an array of photodiodes.

The spectrometer has a CCD detector (linear array, 2048 pixels) and is USB powered. These features ensure rapid sample speed (milliseconds) and data handling (milliseconds–minutes). Our system is configured for spectral irradiance measurements in the wavelength range 350–880 nm, but the software also provides other radiometric quantities, as well as photometric and colourimetric quantities based on plug-in applications.

The 15 centimetre diameter integrating sphere ensures all radiation emission to be collected inside. Generally, using a sphere will decrease positioning error. This particular sphere size is less sensitive to positioning of the LED than many smaller spheres sizes. More than 98% of the light in the specified wavelength range is reflected inside the PTFE-covered sphere, and a baffle prevents light from escaping from it.

The software is easy to operate, and files can be converted to different formats, such as Excel and ASCII. Graphs, also, can be saved in several formats (e.g. jpg, pdf).The spectrometer is portable and light-weight. The integrating sphere, also light-weight, is mounted in a stand for benchtop use. It requires more careful handling than the spectrometer, but can be moved easily.

We are in the process of getting acquainted with our new equipment, and have performed measurements on previously characterised light sources. The values obtained with the new equipment were well within the measurement error range of the double monochromator spectrometer with which the lamps were previously characterised.

The equipment calibration is traceable to National Institute of Standards and Technology (www.nist.gov) and, through collaboration with the Optics Laboratory at the NRPA (www.nrpa.no), to other calibration institutions (www.ptb.de; www.pmodwrc.ch).