Media room lighting levels, bias light and step lights

Lighting a media room is the only interior lighting problem where the design target for most of the evening is close to zero. That inverts the usual method. Instead of asking how much light a room needs, you ask how little it can be given while people still find a seat, and then you ask where the light that is left ends up.

Measure at the screen, not at the floor#

Every other room is specified by illuminance on a horizontal working plane. A media room is not, because the surface that matters is vertical and it is the screen. The datum is lux measured at the centre of the screen with the meter facing out into the room, because that is the light that lands on the picture and lifts its blacks.

Take the reading with everything else off, one source at a time. A downlight that reads 300 lux on the floor beneath it may add only 2 lux at the screen if it is behind the seating and properly baffled, while a small uplight on the front wall can add 15 lux at the screen while barely registering anywhere else.

Those are five states and they want five circuits, or at least five scenes on a control system. The switch positions and heights that go with them are covered in light switch heights and placement, and the general method of building a room's light in layers is in how to plan layered lighting.

Where the light in a dark room actually goes#

Treat the room as a crude integrating sphere. A 100 in 16:9 screen is 87.2 x 49.0 in, which is 29.7 sq ft (2.76 m2). Run it at the SMPTE reference luminance of 16 ft-L (55 cd per square metre) and the screen is throwing 16 x 29.7, about 475 lumens, back into the room.

That light bounces. The average illuminance it produces is the flux times the average reflectance, divided by the total surface area times one minus the reflectance. For a 15 x 18 x 8 ft room the surfaces total 1,068 sq ft (99 m2).

A projector advertised at 100,000 to 1 measures 13 to 1 in a white room. Nothing in the electronics can recover that, because the loss happens after the light has left the lens.

The bias light, sized properly#

A bias light is a strip behind a direct-view screen that lifts the surround out of black so the eye stops over-adapting. The video engineering convention, from the broadcast viewing recommendations in ITU-R BT.500 and long-standing SMPTE practice, is a surround at about 10 per cent of peak white, at D65.

Work it forward. A television calibrated to 100 to 120 cd per square metre of peak white wants a surround at 10 to 12 cd per square metre. For a matte wall, illuminance equals luminance times pi divided by reflectance, so a mid-grey wall at 0.5 needs 3.14 x 11 / 0.5, which is about 70 lux. Spread over the 3 sq m of wall that reads as the surround, that is roughly 200 lumens landing on the wall and, allowing for spill, a strip rated at 200 to 400 lm in total.

Three placement dimensions matter, and all three are small:

• Set the strip in 2 to 4 in (50 to 100 mm) from every edge of the panel, or it draws a bright outline instead of a soft field.
• Stand the panel 3 to 6 in (75 to 150 mm) off the wall. Closer and the light has no room to spread. Wall mounts that hold a screen at 1.5 in (38 mm) make bias lighting nearly impossible.
• Keep the wall neutral for at least 12 in (30 cm) all round the panel. A coloured wall tints the reference white the eye adapts to, and the picture takes the complementary cast.

Specify 6500 K and CRI 90 or better. It will be the only 6500 K source in the house, which is the point: everything else should sit at 2700 to 3000 K, for the reasons set out in colour temperature and CRI at home.

None of this applies to a projection room. There the screen wall is the surround and it wants to be as dark as the rest, so a bias light behind a projection screen is simply veiling light with a marketing name.

Step lights, aisles and the egress figure#

Once the general lighting is off, the only thing standing between a guest and a riser is a handful of very small fixtures. The number worth borrowing comes from public buildings: NFPA 101 and the IBC require a means of egress to be illuminated to an average of 1 foot-candle (10.8 lux) at floor level. A house is not obliged to meet it, and it is still the right target for a dark room with a change of level in it.

That target is easy to hit. A step light throwing 15 lumens onto about 4 sq ft of floor delivers nearly 4 fc (40 lux) locally, four times the egress figure, while adding well under 1 lux at the screen once the carpet has absorbed most of it. Small numbers are enough because the eye is dark-adapted.

Shielding matters more than output. Any fixture below the seated eye line at 42 in (107 cm) is potentially in view of somebody in the back row, so specify a full downward cut-off. Check the geometry rather than trusting the photograph: a light at 14 in (36 cm) mounted 12 ft (3.66 m) from an eye at 44 in (112 cm) is seen along a line 12 degrees above horizontal, so anything that blocks emission above the horizontal is invisible from the seat.

Riser heights come first, because they decide whether a marker is needed at all. A riser of 7 in (178 mm) matches a normal stair and reads as one. A 12 in (30 cm) riser, which is what a second row of seating usually needs for sightlines, does not, and it must be marked along its full length. The riser arithmetic is in theatre seating rows and risers and the general rise and run limits in stair dimensions still apply where the change of level is treated as a step.

Keep the aisle itself at 36 to 44 in (91 to 112 cm) clear, and light both edges rather than the middle: the edges are what people are trying to find.

Lumens when the lights are up#

A dark room is expensive to light. Illuminance depends on how much of the emitted light eventually reaches the plane you care about, and in a room with a 0.2 ceiling and 0.2 walls the coefficient of utilisation for recessed downlights falls to roughly 0.40, against 0.65 in a white room.

For 270 sq ft (25 m2) at 15 fc for cleaning, 4,050 lm has to arrive. Divide by 0.40 and you need about 10,000 installed lumens, which is twelve 800 lm downlights, against 6,200 lm in a pale room of the same size. Sizing this against your own room is what the room lighting lumen calculator is for, and the underlying method is in how many lumens a room needs. If the relationship between lumens, lux and old wattages is still doing your head in, lumens, lux and watts converted sets it out.

Place them where they cannot see the screen. Keep the first row of downlights at least 36 to 48 in (91 to 122 cm) back from the screen wall, use deep-regressed trims with a 40 to 50 degree cut-off and black baffles, and lay the rows out using the spacing criterion method in recessed lighting spacing and placement. A wall washer aimed at the screen wall is the single worst fixture you can install in the room.