Three ways to light a library

Libraries aren’t what they used to be: my nearby public library is now a ‘resource centre’. This may upset the fogeys, but it does reflect the change inside. You must consider what people will be doing other than just looking for a book.

Libraries are now full of computer screens. These are almost certainly positive polarity (that means black lettering on a white background). Some might be of the older negative polarity variety. In either case, you must minimise the viewing intensity (strictly speaking, the luminance in cd/m2) of the luminaires where they might be reflected in the screens. There are recommended limits in BS EN 12464 and the Code for Lighting.

People do a lot of reading in libraries. Remember that older people require higher levels of illumination than younger ones. Depending on who uses the library, you will be designing to 300 or 500 lx.

Of course, libraries still have plenty of bookshelves and you need to light their vertical surfaces. The recommendation in the SLL Guide Lighting for Education, is 200 lx on the shelving near ground level.

The difficulties involved in lighting supermarket aisles are almost exactly the same as those for library shelves.

One solution is to fit luminaires to the tops of the displays. Another is to run rows of luminaires parallel to the aisles. This requires quite a specific geometry to the reflectors and mounting height to avoid wasting light. Another is the ‘cross-aisle’ solution which is much more forgiving of layout and geometry. The disadvantage is that you lose light on the tops of the shelving.

Our library has a 3.4m ceiling height and the area you can see in the rendering is about 15m wide by 14m to the back wall.

This scheme is simplicity itself – it just uses one type of ceiling-mounted luminaire. The unit is a twin lamp T5 surface mount with a gull-wing upper reflector. There is no direct view of the lamp and a fair amount of light is directed up to the ceiling.

The interesting aspect of this design is that the line of luminaires is at right-angles to the bookshelves. In supermarket terms, this is known as a cross-aisle design. Of course, some light is lost on the top of the shelves but this may not be significant. The solution works best when the shelves are comparatively shallow in relation to the space between them.

The lighting is uniform across the space, so the layout of the shelves, tables, computers and so on can be altered easily.

For this scheme to work well, you must choose a luminaire with good quality optics – not all matt white reflectors are the same. Similarly, you must ensure that not too much light is lost inside the luminaire – that is, it should have a high light output ratio.

 

Tech spec

  • Luminaires Twin 28W T5 surface-mount
  • Optical control Microprism panel plus matt white reflectors
  • Arrangement In rows, cross-aisle
  • Average horizontal illuminance on desk 301 lx. Average vertical illuminance on bookshelves, 168 lx
  • Electrical load 6.3W/m2
  • Pros Inexpensive
  • Cons Changing the fluorescent lamps

Here, again, we have used twin T5 units. The big difference is that they are suspended direct/indirect units. This obviously directs a lot more light to the ceiling and makes the space more airy. The extra optical efficiency means the installed load is slightly lower.

We have positioned the luminaires centrally over the bookshelf aisles. For this to work properly, you have to consider the width of the downward beam. If it is too wide, the upper shelves will be too bright and not enough illumination will reach the book spines closest to the floor. If the beam is too narrow, you might find that the floor is lit brilliantly but the upper half of the shelving is comparatively dark. Done well, this is an efficient way to light shelving from top to bottom. This gives more than twice the vertical illuminance on the shelves than option A.

One extra advantage of this scheme is that any reflections in the computer screens will only show the ends of the luminaires and these are comparatively low luminance. The reflections are unlikely to be distracting.

 

Tech spec

  • Luminaires Twin 28W T5 direct/indirect
  • Optical control Clear top cover with small louvres on underside
  • Arrangement As shown
  • Average horizontal illuminance on desk 317 lx. Average vertical illuminance on bookshelves, 430 lx
  • Electrical load 5.4W/m2
  • Pros Light and airy
  • Cons You need a high ceiling to make this work properly

This makes more of a visual statement. It’s the kind of thing you would expect to see in a modern library. We have opal pendants, 1m in diameter, containing about 70W of LEDs each. Also, there is local lighting on the central tables and the computer area. Finally, there are some dedicated shelf-lighting units. These are attached to the top shelf on short outreach arms. A properly designed reflector ensures good, even illumination from top to bottom.

This solution ensures the highest illumination on the shelving and task areas but also has the lowest overall electrical load.

Obviously, the initial capital cost would be substantially higher than that of the other schemes.

 

Tech spec

  • Luminaires Various
  • Optical control Various
  • Arrangement As shown
  • Average horizontal illuminance on desk 311 lx. Average vertical illuminance on bookshelves, 518 lx
  • Electrical load 4.3W/m2
  • Pros Bold appearance and lowest running costs
  • Cons Highest capital cost

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