LUMENS, LUX & WATTS. WHAT DOES IT ALL MEAN?
LED Light Bars and Lumens
Lumens (lm) are considered to be the measure of the total amount of light visible from a beam.
Lumens tell us how bright. Certainly, the lumen rating plays a part in determining the brightness of the light, but should never be the single deciding factor when choosing which LED light bar you should purchase. The reason we say this is, the ANSI standard for measuring lumens is in an integrated sphere, in other words, the lumen output is measured at close proximity to the light source.
In driving light terms, Lumen output is not the best indicator of the performance of any particular LED light bar or driving light, given we are more concerned about our driving lights performance at 20 to 300m. As an example, you could have two light bars side by side with equal lumen ratings, but different reflector or lens configurations with hugely varying light output at say 150m. In short, the delivered light (lux) is the more appropriate rating. Two different lights bar both with 10,000 Lumens can have very different lux reading at 100m. We have seen Lux measurement of similar spec bars vary by 50% or more at a mere 10m, despite having identical on paper Lumen output. If we remove all of the optics or reflectors from any given light bar, the delivered lumens count doesn't change (in fact it is likely to go up), but needless to say, the beam performance is affected drastically.
Aside from the fact that most Lumen numbers we encounter are totally fictitious, there are many other important metrics that should be considered.
MORE WATTS = MORE LIGHT, RIGHT? WRONG.
Have you ever wondered why the cheapest LED driving lights seem to have the highest “Watts”, and the premium gear produced by the most respected manufacturers have far less in comparison?
In brief terms, Watts is a measure of the amount of energy consumed by any given luminaries. Thanks to older incandescent light bulbs, people are used to looking at Watts to determine light output. With this older style technology, it is correct to assume that 100 Watt light bulb is likely to be brighter when compared to a 60 Watt bulb. This same logic, however, cannot be applied to LED driving lights or LEDs in general. When it comes to LED, it is ok to think about Watts as a measure of input power, but it’s never ok to think of Watts as a measure of output. There is no relationship between Watts & Lumens and no relationship between Watts & Lux when it comes to LED Driving Lights or LED Light Bars once the LED are placed inside of a circuit.
Two different LED Lightbars with different LED emitters can consume an equal number of Watts but differ widely in light Lumen output (aka Efficacy). For example, one 3W LED may have an efficiency of 128Lm/W where another may only have 65Lm/W, therefore, deciding which LED light bar or driving light to purchase based on Watt is very misleading.
LEDs very existence is low power, high output. Big power consumption and big Watts go against the very existence of LEDs. All the high end cutting edge LEDs being released by the top tier manufacturers are consuming less and less current/Watts but produce even higher Lumens. In 2006 CREE's best LED on offer at the time was producing 131 Lumens Per Watt (LPW) - in 2014 their best emitter was producing 303 LPW.
The comparison shot below drives this home perfectly.
DRIVING LIGHTS & LUX - A very important metric, but not the end all be all.
Lux is defined as being the measure of light intensity, as perceived by the human eye. It is the measure of light at a given distance on a surface. Driving light manufacturers have pushed aside Lumens in favour of Lux (Lx) and is 2016 latest buzz word in the driving light arena. Lux distance data is definitely an important metric, but is misleading if considered in isolation. A laser pointer could theoretically have a peak beam distance of 5km at 1 lux, but i think we would all agree that a pair of laser pointers will make for an awful driving light.
When driving light manufacturers carry out photometric testing to obtain isolux data, the goniophotometer used to test this metric only measures the peak intensity at the centre of the entire beam, which is great, but what about the rest of the beam? If 5 different driving lights all have 1 lux at 500m, which one do you buy? To obtain remarkable isolux numbers it is simply a matter of focusing down the beam, but beam focusing comes at the expense of light coverage. The very best driving lights are the ones which strike the best possible balance between brightness (Lm) and beam throw (Lx). Some of the best of the best driving light manufacturers refuse to quote lux distance numbers on their flagship products because taken in isolation is misleading in particular when customer's are comparing Lux data between separate manufacturers. The best driving light isn't necessarily the one that achieves the longest beam throw. The best driving lights in our view are the ones that have a well-formed usable overall beam shape, and most importantly, the one which is best suited to the customer's application. In this regard, even the customer geographical location plays an important role. The nature of the roads in far north Queensland demand a different beam shapes to that which is found in Victoria.
We hope this blog entry has answered some of your questions, but as always if you still need some advice, please feel free to give us a call on (03) 8360 9261, or if you are in Victoria, pop in to our warehouse 7 Hammer Court, Hoppers Crossing VIC 3029.