Introduction
Pro-Lite serves customers working in lighting and illumination with a complete solution to measuring the brightness, colour, colour rendering and spatial distribution of lighting products in the architectural, automotive, avionics and many other sectors. This application note reviews the tools of the trade used in lighting metrology.
The Tools of the Trade
Light is all around us (as The Troggs nearly sang), but to verify that the light level and colour are up to scratch, we rely upon specialist photometric and colorimetric test equipment. In this case study, I’ll review the tools of the trade used in light metrology, in both laboratory and field environments.
Measuring Illuminance
Figure 1: T-10A Illuminance Photometer (Konica Minolta)
Let’s start with the simple lux meter, more correctly known as an illuminance photometer. This is used to measure the amount of light that’s illuminating a surface and is about as simple as light metrology gets. You can purchase a “lux meter” app for your smart phone for under £1, but to be brutally honest, you’d be better off investing your money in a packet of crisps. The lux meter app is simply fancy software that exploits the camera sensor in your phone. Unfortunately, your camera sensor neither matches the spectral sensitivity of the human eye (the “photopic response”) nor is it equipped with the cosine diffuser necessary to correctly scale off-axis illumination. So while a modern smart phone is many things, a lux meter it is not.
Good quality illuminance meters tend to cost in the many hundreds of Pounds and are characterised by a photopically filtered sensor fitted with a good quality cosine diffuser. An example of the state-of-the-art is the Konica Minolta T-10A, stocked by Pro-Lite (Figure 1). Just remember that illuminance varies with the distance from the light source (this is called the “inverse squared” relationship) and you should be careful to stand behind the lux meter, especially when wearing light coloured clothing that can reflect stray light into the photometer. How strange it is that we photometrists like to ply our trade in the dark!
A variation on the theme is the illuminance colorimeter, but whereas the simple lux meter just records illuminance in lux, the colorimeter deploys three sensors that are spectrally matched to the tristimulus response of the human eye in order to measure chromaticity and correlated colour temperature. This is important if your “human centric” lighting needs to be tuned to cool white to keep you awake, or warm white to help send you to sleep. Step forward the CL-200A colorimeter, also from Konica Minolta (Figure 2).
Figure 2: CL-200A Illuminance Colorimeter (Konica Minolta)
Goniophotometry
From hand held light meters to what is most definitely not even remotely portable – the goniophotometer. When planning a lighting installation in a shop or office, designers will exploit so-called “photometric files”. These are machine readable datafiles that contain the candela luminous intensity values over all angles from a luminaire, and are usually assigned either the .ies or .ldt file extensions. These invaluable pieces of information are created using a piece of laboratory equipment called a goniophotometer. The light source under test is mounted on a 2-axis motorised motion platform and our old friend the lux meter records the illuminance over a range of angles. The photometric file so produced can be opened in lighting design software and the performance of the luminaire modelled in different sized rooms. Interestingly, the software will even take into account the reflectance of the floor, walls and ceiling. Hint – for the best performance, avoid black carpet and paint! Pro-Lite supplies the professional-grade goniophotometers made by SSL Resource in Finland (Figure 3).
Figure 3: Goniophotometers (SSL Resource)
Sphere Photometry
We cannot neglect to mention the good old integrating sphere. First invented in 1894 by German engineer Richard Ulbricht, but based on a theory published in 1892 by English scientist W E Sumpner, the integrating sphere couples an illuminance photometer to a hollow sphere painted matte white. Together, these allow you to integrate and measure the total luminous flux (lumens) of a lamp placed within the sphere. Pro-Lite supplies the integrating sphere photometers made by Labsphere (Figure 4).
Figure 4: Integrating Spheres (Labsphere)
Spectroradiometry
Returning to a more recent technology, the humble filter photometer or tristimulus colorimeter can be replaced by a spectroradiometer, and this adds the ability to measure the colour rendering of a light source and also to measure the radiometric performance of UV and horticultural lighting products. Although more expensive, a spectroradiometer can be relied upon for more accurate readings and some models – notably the Specbos from German firm JETI – can be used to measure both the illuminance from lighting but also the luminance from a display (Figure 5).
Figure 5: Spectroradiometers (JETI)
Luminance Meters
Our tour of light measurement equipment ends with what for many is the future of photometry – the imaging photometer or colorimeter. This is a sensitive CCD or CMOS based camera adapted to the photopic or colorimetric spectral response of the human eye and which records 2D luminance, illuminance or colorimetric images. As such, it is the direct machine analogue of the human vision system and is proving popular in all areas of light metrology, from evaluating the beam pattern of automobile headlamps to checking that your new smart phone displays the right white point and doesn’t suffer from any dead pixels or other blemishes. Pro-Lite supplies the imaging photometers made by Canadian firm Westboro Photonics (Figure 6).
Figure 6: Imaging Photometers and Colorimeters (Westboro Photonics)
Further Information
Pro-Lite provides a solution for almost any application in measuring the colour and brightness of light sources and displays. From a simple, inexpensive lux meter, to the world’s most advanced imaging photometers, we have a light meter that you can rely upon to give you accurate, repeatable data. We supply reference standards and targets as well as calibration light sources to calibrate your instrument. Our repertoire includes instruments that measure the amount of light (radiometers, photometers, spectroradiometers), the colour of light (colorimeters), the angular distribution of light (goniophotometers) as well as the 2D spatial distribution of light (imaging photometers & colorimeters).
Pro-Lite also specialises in supplying instruments that measure the optical properties of materials, including reflectance, transmittance, optical flatness, solar absorptance, emittance, and BRDF/BTDF scatter. Last but by no means least, we supply calibration light sources, solar simulators and uniform light sources for testing image sensors, remote sensing systems and cameras.
