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Talking Photometry: The Torch with the Power of a Million Candles

We're not complete geeks here at Photometric Testing. We do get some time off, and we recently attended a performance by Rhod Gilbert who recounts discovering a torch with the power of a million candles. All for £3.99 - a must-have, we say!

Candle    Rhod Gilbert is an amazingly funny stand-up comedian. Rhod is gifted at telling anecdotes - the one about the torch with the power of a million candles caught our attention - obviously. First, let's be pedantic. Its not the "power" in question (which is a measure of the electrical power consumption), its the luminous intensity, or candle power that we're really interested in. And what is luminous intensity measured in? Candelas, of course!

Rhod recounts how he stopped at a petrol filling station and discovered the torch that would change his life - the torch with the power of a million candles. The joke is that a torch with the power of 999,999 candles is not sufficient, only a torch with 1,000,000 candelas will do. Watch a video of Rhod describing this amazing torch on YouTube by clicking on this link (beware adult language).

Photometrically, we would like to make the following, slightly more serious point. When describing the output of a lamp or luminaire, it can be tempting to quote whichever parameter achieves the highest value. Kind of like, "my fitting has got more lumens than yours". Or, "my lamp emits higher candelas than yours". This can sometimes be unhelpful and only serves to confuse the consumer.

Remember that luminous flux (measured in lumens) is the total amount of light emitted by a light source in all directions, scaled spectrally (photopically) to how the human eye would perceive the light. We routinely measure the total flux of lamps and LEDs using an integrating sphere. For directional lamps and spotlights, it has been normal to measure the peak luminous intensity using a lux meter. Luminous intensity is the measure of how many lumens of luminous flux are emitted in a given direction per unit solid angle. Solid angle is measured in units called steradians, where a cone of 90° is equivalent to half a steradian. Thus, one lumen per steradian equals one candela. For a given number of lumens, the narrower the beam angle from the light source, the higher the number of candelas.

So, a high number of candelas may sound exciting - and for a torch, this may mean a bright, laser-like beam. But for lighting up your room? That's where luminous intensity and candelas fall down.

To fully describe the output of a luminaire, we make a goniophotometric test. This involves making a measurement of the luminous intensity from the fitting over all angles. We then compile this data into a standard photometric file, a data file that can be used in lighting design sofwtare to predict the number and positioning of luminaires required to achieve the desired illuminance levels in the lighting scheme. Photometric Testing provides an accurate, cost effective and responsive service for clients who require photometric data in industry-standard .ies and .ldt formats.

In a more recent development related to directional lamps such as GU10 and MR16, the EU has started to require the reporting of "useful lumens". According to European Commission regulation EC/244/2009, a directional lamp is defined as one having at least 80% of its total light output in a cone of pi steradians (120°). What matters for directional lamps is not the total light output but 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. Photometric Testing is also able to provide measurements of useful lumens. Read more about useful lumens here.