Feature Lighting & Lighting Controls


A guide to lighting principals O


ne constant humanity has had throughout


its history is the


Sun. In an evolutionary context the Sun is the only light source that matters and is the starting point for understanding how we, as humans, interact with light. In later articles we will look at such familiar terms as CCT or colour temperature, colour render- ing, glare ratings and luminance limits - all of which can be related to how the eye has evolved to work with sunlight. First we will take a brief look at the var- ious ways of producing light and the main features of those sources.


Fundamentals


Light is a form of energy. A light source can be thought of as a system that changes one form of energy into another - a light bulb turns electrical energy into light, a match turns chem- ical energy into light. Every example here is a situation where some part of the system uses one form of energy and converts it to photons of light.


Incandescence


These are all examples of things that are so hot they glow - incandescence. The Sun is heated by nuclear fusion, candle flames are heated by the reaction of wax burning in oxygen, and the tungsten fil- aments in incandescent light bulbs are heated by the electrical current passing through them. All are clearly very dif- ferent types of system, but all can be grouped together as the photons of light they emit rely purely on temperature.


Flourescence


Fluorescent lamps, in their jungle of dif- ferent shapes, sizes, colours and techni- cal designations, all work in much the same way. An electrical discharge (think lightning) passes through the gasses within the tube - this causes the gases to emit ultraviolet (UV) radiation and it is primarily mercury that is responsible for this. UV has a party


trick, and that is to make certain materi- als glow (fluoresce) - highlighter pens, whiter-than-white washing machine detergents, gaudy 80s t-shirts, nightclub interior decoration, and the white dust of phosphors that coat the inside of fluo- rescent lamps, all come into their own when UV is around.


An LED (light emitting diode) is, in essence, a tiny ‘chip’ of semiconductor with wires connected to it. When an electrical current passes the right way through the semiconductor layer, the electrons within it are shuffled around, moving from one part of the chip to another. A result of this movement is that the electrons emit photons of light as they fall to the place they’d rather be. For a given LED chip, this ‘fall’ always results in the same type of photon being released - that is to say any given LED can only be one colour - and white is not a colour. The answer lies in our old friend fluorescence - this time we pick as efficient an LED/phosphor package as possible, which happens to be deep blue LEDs and yellow looking phosphors.


Sodium discharge lamps Squeezing in with a section of its own is sodium. Generating light by making electrons repeatedly ‘fall’ to lower energy states, sodium is chosen as the eye is very sensitive to the photons it generates - this technically makes it very efficient. The fact remains that it is orangy-yellow and not really suit- able for general lighting.


Colour mixing


Incandescent sources are a mixture of all possible colours - this is occasion-


S6


Above: the glowing filament in an


MR16 incandescent lamp


Below: the white dust of phosphors that coat the inside of fluorescent lamps come into their own when UV is around


ally presented to us, in spectacular form, as a rainbow. Clearly it is possi- ble to see a picture of a rainbow on the TV, but it is fairly common knowl- edge that display screens only use red, green and blue (RGB) pixels. We know the Sun gives us white light, we know a TV screen can look white, but the eye is not optimised for RGB and when we start taking shortcuts and missing out the rest of the colours in the rainbow, we start to get some dis- concerting effects.


In the first in a short series of articles (to be continued online at www.connectingindustry.com) Oliver Buchan, technical manager for Luxonic Lighting, takes a look at the technical background to some of the more familiar terms that crop up when talking about selecting and specifying lighting


Left: an LED is, in essence, a tiny ‘chip’ of


semiconductor with wires connected to it


This, curiously enough, is where the next article will pick up from. Join us next time to take a quick tour of concepts such as colour rendering, colour temperature (CCT) - no, they’re not the same thing - SDCM and MacAdam Ellipse - yes, they are the same thing.


Luxonic Lighting www.luxonic.co.uk T: 01256 375 623


Enter 223 FEBRUARY 2014 Lighting & Lighting Controls


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