Words and Images by: Collyn Rivers N8054

This shows the difference between lumens and lux, and why an LED’s focused beam aids efficiency

Reader feedback usefully highlights issues clear to some, but not all. One such issue that relates to LED lighting reads: “I need good size LED lights for my motorhome … at least 50 watts each. Stores are trying to sell me 5-7 watt LEDs. They claim they are just as bright. How can they be if they are only 5-10 watts?”

That letter only seems odd if you stayed awake during basic physics.

Originally, light globes were sold using watts as an indicator of their light. Watts, in that sense, is/was totally misused but globes were bought and sold that way for 80 or more years.

A watt, however, is not a measure of light. It refers to the rate at which work is done, or required to do such work. Few need to know the next bit (but its omission curiously disturbs those who do already).

A watt is based on a unit called the joule and at the rate of one joule a second. One joule is the amount of energy that will heat 0.001 litre of water by about 0.25°C.

Confusion reigns if the term watts is used to indicate light output, etc. This is partly because nothing in this universe except my wife (who reads The Wanderer) is 100 percent efficient. There is always some loss when converting one form of energy (e.g. electricity) into another form – such as light. Or even charging lithium-ion batteries.

Incandescent globes are very hot fires that produce tiny amounts of light. A 100 watt globe gobbles 90-95 watts just producing heat. That fraction left over is visible light. The 100 watts is thus the electricity drawn, not the light emitted. This is fine for Inuit seeking warmth while reading Eskimo Weekly, but not for people generally. Watts is an incorrect but realistic indicator, however, for an electric bar fire as the electricity drawn by direct heating elements is not that much more than the heat generated.

LED EFFICIENCY

Halogen globes were the ultimate minilight- producing electric fires. Running at 700°C, a 50 watt halogen produced much the same light as a 100 watt incandescent.

LEDs do not work the same way. The earliest commercial globes were  about four times as efficient as the best incandescents. The top LEDs are now 20 times so. This renders wattage to not even provide a rough indication of light output: not least as it changes constantly.

LUMENS

The now used lumen is a measure of the total visible light from the light source (luminous flux). Lumens per watt thus indicates an LED’s efficiency. The most efficient currently on sale (late 2014) is Cree’s XP-L. It produces over 200 lumens per watt – 1226 lumens and 3.45 mm square – some 20 times that of the best incandescent.

FLUX AND LUX

Unless reflected, incandescent lights produce a ‘ball’ of light of which only a small part falls on any given area. The brightness is related to the distance from the light source – and aided by that reflected.

Currently, most LEDs produce defined cones of light – of 15-120 degrees and direct all the output (lumens) accordingly. The illumination level is known as lux:  i.e. it is the amount of light falling on a defined surface. Much like directing a hose rather than spraying water all around, lumens and lux are related but are not the same.

In many uses (e.g. cooking, reading, car headlamps) this hugely aids effi ciency. As light is delivered only where needed less of it is required than if sprayed around.

Good LEDs currently produce 100- 150 lumens/watt. For good background lighting you need about one watt per square metre. A six-metre motorhome (about 24 square metres) needs fi ve by 5 watt wide-beam LEDs, plus individually switched tighter beam LEDs for cooking and reading (e.g. Philips 7 watt units).

LEDs are not particularly voltage conscious. They don’t flicker when the fridge cycles on and off. They draw a fraction of whatever was there before. As long as the existing wiring is better than wet string it’s almost overkill.

COLOUR TEMPERATURE

A relic of thin wire getting very hot, LED ‘colour’ is expressed in degrees Kelvin. Warm white (like that of an incandescent) is about 3100 degrees K. It’s good for background lighting. A whiter 4000  degrees K suits kitchens and bathrooms, and for reading. Harsh ‘caravan or hospital white’ is about 5000 degrees K. Some like it – I don’t.

INSTALLING

LEDs with MR 16 pins directly replace existing 35 and 50 watt halogen globe assemblies. Some lack mechanical locking, however. They may fall out off-road. The GU 10 LED globes have heavier locking pins. These globes are usually 12 volt,  and supplied with a tiny 230-12 volt converter. Only buy LEDs initially from specialised lighting stores. There are bargains on eBay – but also a huge amount of junk.

This article is copyright 2014 Caravan and Motorhome Books. The above and similar issues relating to all aspects of caravan and motorhome electrics (and solar) are covered in my books – available directly from CMCA NHQ or visit mywebsite at www.caravanandmotorhomebooks.com 

Category: Technology
Written: Thu 01 Jan 2015
Printed: January, 2015
Published By: