Industry Knowledge

What They Don’t Want You to Know About Lumen Output

13/10/2015

A common misconception when specifying lighting is that the greater the light output, or the higher the lumens, the better the lighting solution will be. In reality, it’s not that simple - too much light can actually have a negative effect on the quality of output.

This article will focus on the truth behind light output; the advantages of LED lighting over traditional technology and differences between LED fittings to consider.

 

The Importance of Targeting the Light

Traditional luminaires often provide a greater total lumen output than LED, but that is because they need to. In a traditional discharge luminaire, a lot of the light is lost in the reflector and in a discriminate spread of the light, so a large percentage of the available lumens are wasted.  In an LED luminaire, by more accurately targeting the light it allows a more effective use of the lumen output; providing a superior light output from a fraction of the power. As an example, SPARTAN Floodlights use revolutionary Vario technology; a choice of  8 different interchangeable beam patterns, both circular and elliptical, from a narrow 10° spot to a much wider 120° beam; this ensures the light is targeted exactly where it is needed and not wasted by illuminating areas beyond the target area. In contrast, the physical size of a traditional discharge lamp dictates the size of the luminaire body and its reflector system, restricting the lamps effectiveness when compared to a superior LED fitting

The ability of LED luminaires to accommodate lens technology such as Vario makes for the best use of the available light output and ultimately allows significant energy savings to be made; a traditional 400W floodlight can typically be replaced by a 136W SPARTAN LED Flood.

It is not the total lumens that is important, but 'usable lumens'.

 

The ‘Lumens per Watt’ Myth

A common misconception when evaluating a lighting solution is the importance of lumens per watt. While LED scores favourably on this comparison in relation to more power-thirsty traditional discharge technology, specifying lighting based on these criteria alone is restrictive. While it indicates significant energy savings, it does not indicate a superior quality of light. As an example, when an LED passes through an optic, a lens or a reflector, some of the total lumens are lost. The result is that the lumens per watt drops, so the simple ‘win’ at lumens per watt is to produce raw LED luminaires without additional optics. The diligent manufacturer uses optics to focus and shape the beam of light to suit each application and to maximise effective lumens.

Lumens per watt are also a particularly bad measurement for linear fittings where the luminaire is typically mounted at low levels, close to the eye. In order to achieve higher lumens per watt, some manufacturers resort to using higher power LEDs; while this will generate a greater overall lumen output, it will often have a negative impact on the uniformity ratio of the light and cause increased glare issues, which are important considerations when developing lighting designs with strict parameters. Raytec took the decision to use a higher quantity of medium power LEDs on all linear fittings to produce a better quality of light for the application and reduce glare, rather than focus purely on “lumens per watt”.

It is not the “lumens per watt” that is important; it is the quality of light.

 

Professional Lighting Designs are Vital

Until recently professional lighting designs would rarely be requested by end-users because, to over-illuminate a given area would often go un-noticed.  The requirements of an installation would often be to provide light fittings that would illuminate a space adequately, with no consideration given to the light distribution, the colour rendition of the lamp or its power consumption. Now, there is greater emphasis on the quality of output to meet certain parameters and regulations to ensure that most, if not all of the light, is put exactly where required. Overspill light often has to be quantified too – and overspill light is energy wastage.

Using lighting design software is a useful way to demonstrate the effects and drawbacks of using a higher power LED fitting.  We will look at two comparisons between SPARTAN Linear WL168 and the issues which can arise as a result of using a higher power LED fitting

Raytec offer a free of charge professional lighting design service to all our customers.

Glare

The CIE (International Commission on Illumination) provide recommendations on a maximum UGR (unified glare rating) to ensure lighting solutions are suitable and safe for their application. Essentially, the lower the UGR the less “glare” and that means better conditions for the human eye. The CIE has classified the maximum allowed UGR for different environments as per the table below (source: http://www.cie.co.at/); 

 

By using more point sources (a larger number of medium power LEDs rather than fewer high power LEDs), the standard SPARTAN Linear delivers lower glare ratings than  typical LED illuminators making SPARTAN Linear more suitable for working environments and “easier on the eye”.  SPARTAN Linear can also be supplied with an opaque lens to further improve the UGR for any of the applications listed in the table above. 

At Raytec we know that controlling glare is vital. If glare is avoided then the luminaire can be used for any application without causing discomfort or fatigue.

 

Uniformity

Good lighting practice ensures that the spread of light is as uniform as possible. High intensity areas of light ‘Hot Spots’, interspersed with darker patches, make it difficult for the human eye to adjust to different light levels in an area. It is often better to have a few lower output light sources illuminating an area than a single powerful luminaire that risks severe hot spots, glare and dark patches or shadows. Just think about how many luminaires are used to light sports stadiums? That is because the TV broadcasters demand even illumination for the best possible pictures.

As well as reducing glare, SPARTANs configuration of using a greater number of medium power LEDs in Linear fittings also provides a greater uniformity of illuminance, even when compared to other LED solutions. The example lighting design below design shows that SPARTAN provides a uniformity rating (Emin/Eav) of 0.86; a more even, balanced output than the 0.75 rating of the fitting using a fewer number of higher power LEDs.

The more uniform the light then the better the quality of light. 

 

 After looking at the example design, it demonstrates that although the higher power LED fitting has a higher total lumen output, this is detrimental to the quality of the light. In contrast, SPARTAN Linear gives a slightly lower, but adequate lux level, performs better on glare and uniformity, and 9 times out of 10 would be the better solution in an environment where people are working.

It is the quantity and quality of the light that together define the performance of a lighting solution.

Spartan Linear LED solution                         High Power LED solution

+ Less Glare                                            - Worse Glare

+ Better uniformity                                      - Worse Uniformity

+ Suitable for most environments                       - Suitable only for industrial work coarse

-  Lower total lumens                                    - Higher total lumens

 

Comparing Manufacturers

Understanding differences in the specification of luminaires from different manufacturers should also be considered when specifying lighting, as differences do occur. Here are some the things to look out for;

Lumen output - there is a large degree of uncertainty when comparing lumen output figures between manufacturers.  Many quote the lumen output at source, i.e. the data received directly from the LED supplier, rather than the actual lumen output from the luminaire. Consideration often isn’t given to the light transmission of attached lenses or visors; even a high quality clear glass lens that appears completely transparent can have a light loss factor of approx. 10% which goes unstated by many unscrupulous manufacturers.  Reflector material can lose up to 30% of its specular reflectivity if not formed correctly around the lamp preventing light from leaving the luminaire.

Power Consumption - another misquoted figure is the power consumption of a light fitting, which should include the full circuit watts including the control gear.  For example a 400w high pressure sodium floodlight with wire wound ballast, ignitor and power factor correction capacitor will consume an extra 45w.

LED Assembly - It is likewise important to know how the LED is mounted onto the circuit boards and more crucially how they are cooled. LED’s are solid state components so not as sensitive as lamps that contain filaments or arc-tubes that are often subjected to vibration, impact or voltage fluctuations which shorten their lifespan. LEDs, if thermally managed well, will survive many 1000’s of hours when in operation

 

In summary, a fittings total lumen output is often not the best way to judge it’s suitability to an application. It can be easy and unscientific to up lumen output to win a specification battle at the expense of the performance and quality of the light. While greater output levels may be achieved from traditional luminaires, if only a fraction of this is targeted where needed, then lumen output and consumption are being wasted. Similarly, the temptation for manufacturers to drive LEDs too high can cause problems with glare and uniformity of light.

At Raytec we always design products with the end-user in mind. Our SPARTAN floodlights use high power LEDs with a unique VARIO lens system to shape the light and distribute it where needed. However, SPARTAN Linear, uses medium power LEDs which produce a marginally lower output but can significantly improve the overall quality of the final solution in down-lighting applications.

It’s not about the number of lumens. The performance of a luminaire is controlled by two main factors, quantity in the form of ‘usable lumens’ and quality in the form of beam shapes and colour performance; this is where Raytec excels.

 

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