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What is an equivalent output?

At Interglo, we specialise in the value management of project lighting packages. I discussed our approach to this in my previous article “Finding the sweet spot in lighting”. 


While we see “or approved equivalent” as an opportunity, we also take it as a responsibility to make sure we do our research to ensure the products we propose are truly an equivalent product.


In this article, I want to specifically look at the term “output”. “What output”, or what “brightness” are questions we get asked often.

Unfortunately, in the age of LED Lighting this is not always a simple answer.
Historically, it was much easier. In the case of fluorescent fittings, you had either standard output, or high output tubes. The lumen output of the tube did vary from brand to brand, but fundamentally, if you were looking at a 2x28W fitting, you knew what you were starting with. Each fitting then had a Light Output Ratio, which could easily be compared between fittings to establish output.


With LED lighting, we now see the information presented in many different ways, and sometimes it is difficult to determine in a specification what is actually being specified. This is especially the case in linear profiles, where the information is often provided on a per meter basis.


The most common description we see is watts per meter. Presented by itself, this is almost useless in the absence of some lumen data. This is largely due to the fact that not all LED’s are created equal. This can be seen by looking at two extremes (in terms of performance, and price).
Using an example of a Chinese 24V DC flexible tape, you would commonly see a lumen efficacy of somewhere between 80-100 lumens per watt. Working at 20W/M, this would give a raw lumens of between 1600, and 2000 lumens per meter.


The other extreme would be something like the Tridonic LLE Advanced range. This product is also in a linear form factor, but is on a rigid PCB, running at a constant current, rather than constant voltage. This product can achieve up to 180 lumens per watt, depending on the operating current.
For example, the Tridonic LLE 24x560mm 2400lm 840 HV ADV5 at 250mA would be 21.7W and give approximately 3700LM/m.
As can be seen from this comparison, if we see a description of 20W/m with no supporting information, we could be looking at a range between 1600, and 3700 Lumens/m, which makes it very difficult to recommend an equivalent product.


When we are given lumen information, it can be helpful, but it is still not always clear. Sticking with the 20W/m comparison, I have seen the lumens per meter for a linear profile with an opal diffuser given as 3700lm/m. While this could be true of the LED source, it is not possible for this to be the fitting lumens per meter. I would expect, depending on the diffuser, for there to between 30 – 50% loss due to the opacity of the diffuser. To get 3700lm/m out of the fitting, you would be looking at between 5200 and 7400 lumens from the light source. This would equate to between 260 and 370 lumens per watt which is not achievable at this time.


The lumen data always needs to state whether it is the fitting lumens or source lumens to make this clear. Obviously getting the photometric data for the proposed fitting will clear this up, but this is often not made available. 


Knowing these variables is critical in assessing whether the performance of a fitting is comparable to another. If in doubt, send it through. Interglo specialises in the selection of cost effective, and viable alternatives where value management is required, or lead times need to be managed.

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