Three ways to light a lecture theatre

Lecture theatres come in a range of sizes and can be flat or raked. The first requirement is that the audience can clearly see the lecturer and display area. Remember that the seating at the back may be 20 metres from the lectern or screen. Both horizontal and vertical (people’s faces) illumination are important. The lecturer needs to be able to see the audience clearly, too.

The recommended lighting level in BS EN 12464 is 500 lx, more than adequate for taking notes. If a projector is used, lights will need to be dimmed, but remember you still have to provide enough light for the audience. It is worthwhile being able to switch the display and audience area lighting separately.

If you are using wall washers, you should ensure that they dim with the audience lighting to avoid distracting brightness at the periphery of your audience’s vision. If you are using downlights, it is better to use wide angle ones if the ceiling is low.

The location of the luminaires is important. With direct/indirect luminaires and a sloping ceiling, you must ensure that viewers towards the back cannot see the ‘top’ of the luminaires. Recessed luminaires can cause a similar problem for lecturers, who might have a direct view of a glaring light source. Consider using baffles or louvres to control the light at high angles. There is some excellent guidance on lecture theatres in SLL Guide LG05.

Our lecture theatre is about 15 x 15m with a 6m-high ceiling.

This is a direct/indirect solution with some pros and some cons. The obvious downside is that you must take special care when determining how high the fittings should be suspended – you don’t want them to obstruct the line of sight. Always check the suspension height when you have a sloping ceiling. I have also been told that a purely indirect solution can lead to torpor in students, but that might just have been the lectures.Relamping can be an problem unless you have raising and lowering gear. LEDs would delay, but not prevent, this.

On the plus side, direct/indirect luminaires are extremely efficient and this solution has the lowest installed load of the three options. It works best where you have very high ceilings. The space is light and airy – some downlight-only schemes can make the ceiling appear dark.

TECH SPEC

Luminaires 32 suspended direct/indirect LED

Optical control Micro-prismatic panel

Average horizontal illuminance 484 lx

Electrical load 1.6kW

Pros Gives a sense of space

Cons Watch those sight lines

This is a very common solution and, traditionally, used AR111 or PAR lamps. They created a bit of contrast and could be dimmed to zero with simple control gear. Nowadays, we would use LEDs as the light source. I have used 35W units that emit 3,000 lm. If you are using LED retrofits, check compatibility with the dimming system. Watch the light distribution. If it is too wide, it can cause glare, but you should be able to achieve decent uniformity. A narrow distribution would seem to be the answer, but you can end up with unnatural modelling on people’s faces. Uniformity is a bit unpopular nowadays, but try writing in a dim lecture theatre when your notepad is beyond the edge of the beam. The big advantage of gimbals is that you can direct the light in a particular direction such as to illuminate the lectern or flipboard.

TECH SPEC

Luminaires 30 twin LED spotlights

Optical control Specular reflector

Average horizontal illuminance 546 lx

Electrical load 2.2kW

Pros Flexible and can create drama

Cons Tungsten ones are too inefficient

A plain uninterrupted ceiling except for four thin lines of light looks by far the best aesthetically. These are T5 with a frosted lens. I have compromised the LOR by using a very narrow lens, just 90 mm wide. You could use LEDs instead but the light output would still be comparatively low. There’s an old photometric law that says the size of the mouth opening determines how much light can be emitted – regardless of the efficiency of the source. What’s nice about the scheme is that it is glare free and unobtrusive. There is nothing to obscure sight lines or distract from the display area. Placing T5 lamps end to end can lead to dark areas between the luminaires, but some manufacturers can achieve continuous lines of light with T5. A lot of lecture theatres have voids above the ceiling from which you can re-lamp luminaires from above.

TECH SPEC

Luminaires 45 T5 in recessed aluminium extrusion

Optical control Frosted lens

Average horizontal illuminance 514 lx

Electrical load 3.5kW

Pros Best looking

Cons Heaviest load but could be improved with a wider diffuser

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