General and special lighting
Lighting covers a diversity of applications from general purpose illumination to special uses within the automotive and aviation industries as well as horticulture and many other niche areas. Gigahertz-Optik GmbH produces an extensive range of light measuring instruments for both LED and traditional lighting technologies. Some typical application examples using Gigahertz-Optik GmbH products follow.
Measurement of solid-state lighting for indoor work places
LED technologies, collectively referred to as solid-state lighting (SSL), have revolutionised the field of general lighting creating exciting possibilities, but also bringing measurement challenges for manufacturers and installers alike. The development and manufacture of LED devices, LED lamps, and LED luminaires all require extensive testing and qualification. For this purpose, high-quality light measurement systems for spectral characterization are used, which provide precise measurement data. However, the planning and verification of SSL schemes also benefit from spectral and color measurement capabilities not offered by the lux meter products traditionally used for such purposes.
The European standard, EN 12464-1 , defines lighting requirements for indoor work areas.
The standard includes the following measurement criteria based on the intended use of the lighting:
- the minimum required average illuminance per task (referred to as “maintained illuminance”);
- the minimum required color rendering (specified as a minimum CIE Ra value).
The spectral distribution of SSL can be very different to that of traditional lighting technologies resulting in potentially significant errors in illuminance measurements made with filter-based lux meters. Determining the color rendering index requires the measurement of the spectral power distribution of the light. Therefore, both measurement requirements are best satisfied by light meters with full spectral measurement capability such as the MSC15 which eliminate the spectral mismatch errors of lux meters and yield full colorimetric data. The BTS256-EF light meter, for example, also permits the assessment of light flicker and stroboscopic effects.
 EN 12464-1:2011 Light and lighting. Lighting of work places. Indoor work places.Read less
Measurement of LED grow lights used in horticulture
LED lighting offers horticulturalists many potential benefits such as increased crop yields, improved product quality, and control of particular plant characteristics as well as the normal solid-state lighting benefits of reduced energy and maintenance costs. LED lighting can supplement natural sunlight within greenhouses to extend fruit, vegetable and flower growing seasons, particularly during the shorter days of winter.
LEDs allow control of the amount and spectral composition of light which can be used to govern a plant’s growth rate, shape and flowering. LEDs can also be positioned much closer to foliage than traditional lighting technologies, such as high pressure sodium lamps, as they do not radiate heat.
Therefore, energy efficient LED grow lights are a key enabling technology for a new form of horticulture, known as vertical farms, where crops are grown in vertically stacked layers in the absence of any natural daylight typically. Vertical farms are expected to play an important role in feeding the rapidly growing population of our increasingly urbanized world.
The horticultural world is very active researching spectral and intensity mixes, often referred to as "lighting recipes", to optimise crop growth and yield with LED lighting. Development and use of these "lighting recipes" in modern horticulture requires spectral measurement data on the pure and mixed colours produced by LED grow lights. This requires a new generation of sensor to replace the traditional PAR sensor, a quantum device that provides no spectral information. The MSC15 is a convenient, low-cost spectral light meter, ideal for routine measurements of LED lighting within horticulture. The additional functions of the BTS256-EF lend itself to more demanding horticultural research tasks.
In 1972, K. McCree  demonstrated that photosynthetic response correlates with the amount of photons that reach a plant rather than the energy of the light. See Technical Article – Measurement of PAR for an in-depth explanation. Photosynthetically Active Radiation, PAR, is just a descriptive term for radiation within the 400-700nm wavelength range (CIE Publication 106) . The commonly used quantitative PAR terms are:
- Photosynthetic Photon Flux (PPF): measurement of the total number of photons emitted by a light source each second within PAR wavelength range. Measured in μmol/s. Analogous to ‘lumens’ for visible light.
- Photosynthetic Photon Flux Density (PPFD): measurement of the total number of photons within PAR wavelength range that reach a surface each second measured over a one square meter area. Measured in μmol/m2/s. Analogous to ‘lux’ for visible light.
- Day Light Integral (DLI): cumulative measurement of the total number of photons within PAR wavelength range that reach a surface during 24 hour period, measured over a one square meter area. Measured in mol/m2/d.
Besides optimising photosynthetic response, LED lighting offers many possibilities to exploit the fact that plants have additional photoreceptors responsive to UV and far-red radiation that impact plant development. This has prompted the American Society of Agricultural and Biological Engineers, ANSI/ASABE S640 , to introduce new metrics such as:
- Ultraviolet Photon Flux Density (PFDuv), 280 nm to 400 nm
- Far-Red Photon Flux Density (PFDfr), 700 nm to 800 nm
 Ref 1 McCree, K.J., 1972. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agric. Meteorol. 9: 191-216.
 Definition PAR metric (CIE Publication 106, Section 8, 1993Read less
Light measurement requirements for Human Centric Lighting
Contemporary scientific research has shown that in addition to the cones and rods that provide us with color and night-time vision respectively, our retinas also have non-image forming photoreceptors, called intrinsically photosensitive retinal ganglion cells (ipRGCs), which play a major role in entraining our circadian rhythms. Of particular interest is the spectral responsivity of ipRGCs which contain a photopigment called melanopsin. Whereas our cones and rods have spectral responsivities defined by the V(λ) and V’(λ) functions for photopic and scotopic vision, ipRGCs have their own ‘melanopic’ spectral responsivity with peak sensitivity in the blue spectral region, around 480nm.
Modern insights into human chronobiology combined with the possibilities of SSL lighting offer many new opportunities to improve health and wellbeing through appropriate lighting, most commonly referred to as ‘human centric lighting’ (HCL), but other terms such ‘circadian lighting’, ‘biodynamic lighting’ or ‘biologically-effective lighting’ are also regularly used.
In its basic form, HCL typically attempts to control the Correlated Color Temperature (CCT) of lighting in a way that emulates natural daylight. More sophisticated techniques involve the measurement of additional metrics such as melanopic illuminance (CIE TN 003:2015)  or Circadian Stimulus (LRC Rensselaer) . The accurate measurement of all of these metrics requires spectral characterization of the lighting.
The measurement of ‘equivalent melanopic lux’ (EML or melanopic illuminance), as required by the WELL Building Standard  for example, and ‘melanopic daylight equivalent illuminance’ are standard features of all our spectral light meters, including the low cost MSC15 meter.
HCL is not only concerned with our ipRGC response to the amount and timing of ‘blue light’ received. Rod and cone photoreceptors also contribute to our circadian phototransduction. Knowledge of other wavelengths may also prove useful to HCL designers. For example, ‘red light’ has been shown to increase alertness without suppressing melatonin. For research purposes, the CIE now recommends (CIE TN 003:2015)  reporting all of the 5 α-opic equivalent illuminances for s-cones, m-cones, l-cones, rods and ipRGCs. Accordingly, the BTS256-EF spectral light meter and the BTS2048-VL laboratory spectroradiometer systems report the following metrics:
|Ev,mel||melanopic daylight equivalent illuminance||lx|
Flicker measurements of SSL products and installations
In the past, the introduction of electronic ballasts for fluorescent lamps largely eliminated concerns over light flicker caused by lamps and luminaires themselves. However, the widespread introduction of solid-state lighting (SSL) has, once again, made light flicker a topic of particular interest. This results from the fact that the LED light follows very quickly and proportionally the current that flows through the LED.
The variation in light output over time from a light source can have both visual and non-visual detrimental effects on the observer. These effects are collectively referred to as ‘Temporal Light Artefacts’ (TLAs).
CIE TN 006:2016  identifies three types of visually perceptible TLAs:
- Flicker: perception of visual unsteadiness induced by a light stimulus the luminance or spectral distribution of which fluctuates with time, for a static observer in a static environment.
- Stroboscopic Effect: change in motion perception induced by a light stimulus the luminance or spectral distribution of which fluctuates with time, for a static observer in a non-static environment.
- Phantom Array Effect (Ghosting): change in perceived shape or spatial positions of objects, induced by a light stimulus the luminance or spectral distribution of which fluctuates with time, for a non-static observer in a static environment.
Non‐visual TLAs are reported to have various physiological and psychological effects such as migraines, epileptic seizures, autistic behaviour, vertigo, etc. A comprehensive review of neurophysiological effects is presented in IEEE 1789:2015 .
TLAs can be caused by the internal drive electronics of LED lamps and luminaires as well as any associated control gear such as dimmer circuits. Additionally, TLAs can result from fluctuations and transients in the mains AC supply voltage. Existing building and lighting standards, such as the indoor work places standard, EN 12461-1 , recommend the avoidance of light flicker and stroboscopic effects. It warns that 'stroboscopic effects can lead to dangerous situations by changing the perceived motion of rotating or reciprocating machinery', but does not provide any metric or limit. The recommendations within IEEE 1789-2015 are now considered somewhat contentious (NEMA 77) . The US ENERGY STAR program  requires testing with specified dimmer circuits and particular requirements are given within California's Title 24: 2016 .
At the time of writing, there is no regulation within Europe regarding the testing and labelling of SSL products with regard to TLAs, although a new EN standard based on the recommendations of CIE TN 008:2017  is expected. See Technical Article - Flicker measurement using a BTS measurement device for a comprehensive description of the various metrics specified within existing publications for the measurement of TLAs. Two simple metrics are often referenced:
- Modulation Depth (MD) or Flicker percent – ratio of the difference and sum of the maximum and minimum light levels expressed as a percentage;
- Flicker Index (FI) - ratio of the areas above and below the average light levels.
However, these metrics do not distinguish between flicker and stroboscopic effect and do not account for the effect of frequency-dependent sensitivity or the wave shape of the light output. More sophisticated metrics are increasingly preferred:
- Short-term flicker severity, Pst (CIE TN006:2016)  assessment of perceived light flicker for frequencies up to 80Hz.
- Stroboscopic Visibility Measure, SVM (CIE TN006:2016)  considers effects on appearance of moving and rotating objects when illuminated with light modulation up to 2kHz.
- ASSIST Flicker Perception Metric, Mp (ASSIST Vol 11, Iss 3)  describes an objective method to assess the visual perception of flicker observed.
All of the above metrics (MD, FI, Pst, SVM and Mp) are measured and reported by the BTS256-EF Spectral Light and Flicker meter and BTS2048-VL based spectroradiometer systems. To assist SSL manufacturers with EMC Directive compliance (2004/108/EC), a turnkey Flicker Test System is available incorporating an objective flicker light meter in accordance with IEC TR 61547-1:2017  for testing:
- the intrinsic performance of all lighting equipment with stabilised AC mains supply voltage;
- the immunity performance of lighting equipment against fluctuations in AC mains supply voltage.
The term ‘flicker’ is also used in connection with voltage fluctuations and resulting flicker on public mains-voltage systems caused by equipment connected to the network. IEC 61000‐3‐3: 2017 is concerned with the limitation of such voltage fluctuations produced by equipment. LED lamp luminaires ≤ 600W are “deemed to comply”.
 EN 12464-1:2011 Light and lighting. Lighting of work places. Indoor work places.
 NEMA 77: 2017. Standard for temporal light artifacts: Test methods and guidance for acceptance criteria
 CEC Title 24: 2016. Reference appendices: Appendix JA10 – Test Method for Measuring Flicker of Lighting Systems and Reporting Requirements.
 IEC TR 61547‐1:2015. Technical Report: Equipment for general lighting purposes – EMC immunity requirements – Part 1: An objective voltage fluctuation immunity test method.Read less