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Talking Photometry: When is a Lumen not very Useful?

A high number of lumens is always good, but it's where you point them that really matters. Let's talk about the concept of "useful lumens" and how it applies to directional lamps. For spotlights (for example, GU10 or MR16 lamps), quoting just total lumens or peak candelas is not sufficient to describe the light output of the lamp or fitting, hence a new metric has been devised called the useful lumen.

Useful Lumens   We are often asked whether a spotlight should be specified according to the number of lumens or candelas it produces. It is convenient to quote the number of lumens from an incandescent light bulb or individual LED, and specifing the peak luminous intensity in candelas from a spotlight can lead to an unfeasibly high number (see: "the torch with the power of a million candles"). Neither metric (luminous flux or luminous intensity) provides a truly meaningful description of the performance of directional lamps (spotlights). Instead, a new metric has been developed called useful lumens (or useful luminous flux). But what is a useful lumen?

There is a definition of directional versus non-directional lamps given in European Commission regulation EC/244/2009 (published March 2009). A directional lamp is defined as one having at least 80% of its total light output in a cone of pi steradians (120°). A non-directional lamp is one that is not directional per the previous definition! All reflectorised lamps are generally directional. What matters for directional lamps is not the total light output (luminous flux, in lumens) but where the light is shining. The more light that is coupled forwards into the 120° cone, the better. Light emitted outside of this cone is generally considered to be wasted ("spill"). So in order to create a more meaningful metric, EC/244/2009 defines something called useful lumens, this being the flux contained in the 120° cone. This new EU approach promotes lamps which have good photometric efficiency based on the proportion of light emitted into the 120° cone.

As an aside, traditional directional lamps have been based upon tungsten halogen sources with integral reflectors (like the GU10). As we move to more energy efficient sources, lamp manufacturers have developed replacement spotlights using compact fluorescents and LEDs. The problem with CFLs is that they are physically large and the light cannot easily be collected and delivered into the desired 120° cone with reasonably sized reflectors. LEDs are much better in this regard, and comparing the number of useful lumens that different lamp types produce will have the effect of promoting LEDs above reflectorised CFLs in terms of energy efficiency.

Until now, we have tested lamps for total flux (lumens) using an integrating sphere, directional lamps for peak intensity (candelas) using a simple lux meter and luminaires for their variation in luminous intensity with angle using a goniophotometer (to generate so-called "photometric data"). With the introduction of the new useful lumens metric, we must now start to also measure the luminous intensity at each angle (in candelas) from a directional lamp over a 2pi steradian (180°) cone using a goniophotometer. We then integrate the angular intensity values over a 120° cone, first to prove that the desired 80% fractional flux is emiited within this cone and to yield the total number of lumens in that cone (the “useful lumens”).

To be clear, a simple integrating sphere measurement will only yield the total lumens emitted over all angles. There is no way of calculating the “useful lumens” in a defined cone from a sphere measurement. Instead you have to perform a measurement of the luminous intensity at each angle using a goniophotometer, something which Photometric Testing will be only too pleased to help you with. To fully satisfy the new EU requirements, we can measure useful lumens as well as colour rendering, power factor and luminous efficacy.