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Illumination in the spotlight

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technical & practice A recent seminar on external lighting was a testing, but rewarding, time for all concerned

'Darkness is good' may be a curious comment from a lighting company representative but Rob Mitchell, divisional manager for KIM Lighting, is unrepentant. 'We like shadows, ' he says, by which he means that, especially in a dimly lit environment, external lighting must contrast the contours of a building rather than simply 'illuminate' it.

I was at a seminar presented by Light Projects in London, an event more geared to educating young lighting engineers about the merits of appreciating architecture than in trying to convince me to brush up my physics. All the same, we had been ushered to our tables, replete with pens and papers, rulers, protractors and calculators. We were in for a long haul; not for the mathematically challenged.

Priming lighting engineers to take account of architectural features and, indeed, educating them to have the awareness to show them up to their best advantage, was an admirable theme to the day. Even though one of the conclusions was the ubiquitous call that lighting engineers should be brought in early in the design process to add practical expertise and make the architect aware of specialist issues, it was none the less relevant for that. By default, it was a useful prompt that architects should also be aware of related specialisms.

All too often, architects have an 'I can do that'mentality and proceed to set out landscaping and external lighting layouts with only a notional appreciation of their consequences.

Spotlighting at right angles, directly onto an elevation, for example, will inevitably flatten out the features of a building and lose a sense of depth.

Too high an intensity beam will glare at the point where the beam centre impacts, the concentrated aureole of which will make the rest of the building seem darker than it really is.

Maintaining the same scale lighting pole at varying distances from a building will create uninteresting uniformity.

On this latter point, Mitchell recommends higher poles further away from the building (within a landscaped car park, for example), with narrow cut-off optics to provide a more uniform spread of light with which to improve initial access visibility. The nearer to the building, the lower should be the pole; providing brighter light for higher illumination levels, definition and visual interest.

At close proximity to the building, bollards or floorlamps should be used in preference to poles as they help reduce the scale of the entrance way and do not block the view of the building.Wall lights can be added to enhance and delineate the routes and highlight particular features.

The issue of light trespass, namely light shining where it doesn't belong (light pollution), is an increasing concern and lighting design needs careful consideration and input from local authority environmental officers if you suspect that there is going to be an issue.

However, even with the most directional beams illuminating a building, nothing is going to stop light migration. Given that there are no standard regulatory texts on the subject, the general conclusion was that an external lighting scheme should try to do the best possible job at illumination while trying to work within whatever particular pollution constraints pertain at the time.

Colour render Buildings seen in daytime benefit from a non-static daylight source;

that is, the light incident upon it and around it is constantly changing its source point, has varying intensities and hues, and is variously direct or reflected.

When applying spotlighting to the same building, unless vast amounts of money are available, the light source is fixed and unchanging. Therefore, the most basic consideration must be to ascertain your most advantageous principal direction of view.

Did you know that a 16 year old needs 10 lux less light to complete a task than a 60 year old? Try arguing that one with the building control officer when you are designing a college lecture theatre or an old people's home.

Also, did you know that if you have a red brick building and you want to shine a blue light on it - for whatever reason - there is no lamp in existence that will succeed in colouring the brickwork blue?

Several slides were presented of gaudy pink buildings which, one can but hope, were not intended to be that colour, but were simply the result of assuming that paint colour logic would prevail in light.

Optical allusions Andy Baker, business development manager of KIM and the hard man of the presentational double act, took the floor for the calculation session.

Even though most lighting schemes are calculated using proprietary computer software packages, this back-to-basics session was intended to refresh our memories of the mathematical principles involved in in producing a lighting scheme.

Although he stressed that decorative lighting schemes should be about architectural or artistic interpretation, they still need to comply with rigorous 'calculation science'.

Light and shade The brightness of an object diminishes relative to the inverse of the square of the distance of the object from the illumination source, one of the methods used by astronomers to gauge the distance of stars. The inverse square law enables designers to accurately assess how bright an object will look when lit.

The cosine law deals with geometric considerations - that the apparent illumination of an object diminishes relative to the cosine of the incidence of the beam angle to the illuminated surface (namely, the steeper the angle of incline of the lighting the less bright the object will be).

Considering the photometric per1formance of lighting equipment, Baker displayed a wide range of hardware and demonstrated the use of accurate optical systems to achieve desired effects, stating that there was a 'gulf between optical systems available, some tending towards the 'Kodak Instamatic' and others more towards the 'Hasselblad Medium Format''.

As I left to take a headache tablet, the rest of the audience were preparing themselves for the fieldwork. The sample questions that they'd been labouring over all afternoon were actually modelled on real data from an existing building, and they were about to test their theories.

Even though I was extremely rusty on the maths, I thought that this was a genuinely novel presentation to practitioners, and all the more worthy for its attempt to make the 'students' aware of the bigger issues.

Content-led, cross-fertilisation opportunities such as this are few and far between. There should be more. In this way, unlike the passive content of most CPD seminars, we might all actually learn something.

Lighting Projects can be contacted on 020 7231 8282 SHEDDING LIGHT ON DEFINITIONS Luminance (L) How bright an object appears to be in comparison with others in a given direction, at a given point of a real or imaginary surface.Quotient of the luminous flux transmitted by an elementary beam passing through the given point and propagating in the solid angle containing the given direction, and the product of the solid angle, the area of a section of that beam containing the given point, and the angle between the normal to that section and the direction of the beam.

Unit: candela per square metre (cd/m 2)Illuminance (E) How much measurable light is hitting at a point on a surface.Quotient of luminous flux incident on a element of a surface containing the point, and the area of that element.

Unit: lux (lx) Luminous intensity (Iv, I) How much light comes out of a fitting and in what direction.Quotient of the luminous flux leaving the source, propagated in an element of solid angle containing the given direction, and the element of solid angle.

Unit: candela (cd) Peak intensity The beam centre of a light pattern, usually the brightest spot.The luminous intensity of a luminaire in the direction of the beam axis.

Beam angle The beam spread pattern which equates to 50 per cent of the peak intensity. The direction in the centre of the solid angle which is bounded by directions having luminous intensities of 50 per cent of the maximum intensities of a luminaire.

Field angle The beam spread pattern which equates to 10 per cent of the peak intensity. The direction in the centre of the solid angle which is bounded by directions having luminous intensities of 10 per cent of the maximum intensities of a luminaire.

Colour temperature A measure of how 'warm'or 'cold'a light source appears to be.Temperature of a black body that emits radiation of the same chromacity as the radiation considered Colour Rendering Index (Ra)

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