Why Use a Spectrometer for Light Measurement

Spectroscopy, Spectrometers & Light Measurement

Spectroscopy is a huge subject, aiming to studying all possible interactions between light and matter. One fundamental tool of spectroscopy is the spectrometer, which on its own is simply an instrument that accurately measures the light which enters it.

The spectrometer achieves this by separating the light into its constituent wavelengths. Then, by measuring the amount of light at these wavelength, records the result as a spectrum. The element most commonly used inside a spectrometer to disperse the light into its component wavelengths is the diffraction grating. Ever looked at the bottom of a CD and noticed when you move it you can see colours of the rainbow? The surface of the CD is splitting the light into its component wavelengths. The CD is acting like a diffraction grating.

Light & Colour

The visible part of the spectrum is a critical feature of our everyday world. Our perception of colour is derived from the eye’s response to visible light of different wavelengths. The apparent colour of an object is due to the combined effect of the visible photons reaching our eyes from that object. This is true whether the object is emitting its own light or reflecting light from another source.

White light is a mixture of light wavelengths (colours). When white light strikes an object and is completely reflected, we see the same combination of colours and perceive the object to be white. Conversely, when all light striking an object is absorbed and none is reflected the object appears black. We perceive colour when some wavelengths of light are reflected (or transmitted in the case of a solution) more than others. The wide range and shades of colours that we see are simply variations in the visible light spectrum reaching the eye.

For accurate light measurement and evaluation of luminaires, a spectrometer is the best device to use, especially with advances in technology allowing for handheld micro spectrometers. A laboratory device that can fit in your pocket!

How is a spectrometer calibrated for accurate light measurements?

The advantage of having a digital instrument for measuring light is that it can be calibrated against known levels of signal so that it always returns accurate, reliable and consistent measurement. It does not suffer from the subjectivity of human perception. In this way, it becomes possible to build databases, understand product variability, compare installations, and many other approaches to contextualize. We do this by using a NIST-traceable (National Institute of Standards) light source, the gold standard, to calibrate the WaveGo.

Advantages over a lux meter

One of the main advantages of using a spectrometer instead of a lux meter is revealed in the name – a lux meter only measures lux, which as we know does not tell the full story. To put it in another context, it is like being able to roughly measure the calories of food but not knowing how much of that is made up by protein, fats, sugars. As stated above a spectrometer can calculate any light measurement unit you are interested in, CCT, CRI etc. In effect the Swiss army knife of light measurement!

Lux, CCT, CRI, TM30 etc. To characterise light you need to measure more than just one parameter

Secondly, the accuracy of measurements is a significant advantage of spectrometers over lux meters. As the spectrometer measures over the entire visible range of light, and in small intervals (the resolution), the data output is far more reliable than a single measurement taken by a lux meter. If you want confidence in the data then an accurate light measurement device, a spectrometer, is what you need.

In summary, by using a spectrometer rather than a lux meter you will have significantly more accurate and comprehensive data, allowing a far greater insight into the luminaire you are measuring.

Not all spectrometers are the same

As with all technology, one type of spectrometer will perform better than another. If you want a great phone, you might look at processor speed, RAM, camera megapixels, screen resolution etc. So, what are the main specifications to look out for when buying a spectrometer?

Resolution – This is one of the key factors in determining how accurate the data is, the lower the resolution the more measurements taken over the range and the more accurate the result.

Illuminance Range – The range of light intensities that can be accurately measured. If the range doesn’t go down to 50 lux then you won’t be able to measure low light levels with confidence.

Illuminance, CCT, x,y Accuracy – How accurate the measurements are, again the lower the percentage the better. Pay attention to the light intensity these accuracy levels are stated at however, the greater the lux the more accurate the result inherently (due to a greater number of photons hitting the detector). For example, 3% at 250 lux is more accurate than 3% at 1000 lux. Likewise, 3% at 1000 lux is not necessarily be better than 4% at 250 lux.

When it comes to spectroscopy, you have the confidence of over 25 years of Ocean Optics and Wave Illumination knowledge and experience in your hands.

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