Nursery lighting levels and blackout dimensions

A nursery has to do two things that no other room asks for: deliver a usable amount of light at three in the morning without waking anyone, and deliver almost none at seven. Both are arithmetic problems with specific answers, and both are usually solved by buying the wrong component. The rest of kids room and nursery dimensions is about clearances. This page is about quantities of light, and about apertures measured in millimetres.

The datum: which surface the lux is measured at#

Illuminance is lumens per square metre landing on a surface, so a lux figure without a named surface is meaningless. Three surfaces matter in a nursery and they are at very different heights.

• The floor, at 0 mm, which is what a general lighting calculation gives you and what most published room figures refer to.
• The changing surface, at 36 to 39 in (91 to 99 cm), which is a task plane and needs its own figure. Heights are set out in changing table height, depth and reach.
• The cot mattress, at 10 to 24 in (25 to 61 cm) depending on the base setting, which is where the sleeping figure has to be low.

A ceiling fitting at 96 in (244 cm) delivers roughly 40 per cent more illuminance to a 39 in changing table than to the floor, purely from the inverse square relationship over the shorter distance, so a room designed to 150 lux at floor level is already near 200 lux at the changing surface. Working the relationships in both directions is covered in lumens, lux and watts converted.

Lux targets in a nursery, by task#

The 300 lux for dressing is the IES level for grooming tasks and is the one published figure in the table that transfers directly. The night figures at the top are our own recommendation rather than a standard, and they are set by what the eye needs rather than by any code: a dark-adapted adult can see a face clearly at 1 lux and can see enough to position a bottle at 5.

The spread across the table is a factor of 600 between the dimmest and the brightest row. No single fitting covers that, which is the whole reason a nursery needs at least three separately switched sources, on the principle set out in how to plan layered lighting.

Getting to 5 lux: the lumen arithmetic#

Take a 12 x 10 ft (3.66 x 3.05 m) nursery, which is 120 sq ft or 11.1 m2.

Illuminance is total delivered lumens divided by area, and not every lamp lumen lands on the working plane. In a domestic room with light walls, the utilisation factor for a ceiling fitting is around 0.5. So:

lamp lumens = target lux x area / utilisation factor

For a 5 lux night target: 5 x 11.1 / 0.5 = 111 lumens. For a 150 lux daytime target: 150 x 11.1 / 0.5 = 3,330 lumens, which a single fitting can supply but which should be split across a ceiling source and a lamp. The same calculation for any room is done by the room lighting lumen calculator, and the room-by-room targets behind it are in how many lumens a room needs.

The 111 lumen figure is the interesting one, because almost no domestic lamp is made that small. An 800 lumen lamp, the 60 W equivalent, has to be dimmed to 14 per cent of its output. A 1,521 lumen lamp has to reach 7 per cent. A three-lamp pendant totalling 2,400 lumens has to reach 4.6 per cent.

Dimming: the bottom of the range is the specification#

Dimmer performance is quoted as a minimum dim level, expressed as a percentage of full output. Most mains-dimmable LED lamps bottom out at 10 per cent. Better drivers reach 1 per cent, and dedicated 0.1 per cent drivers exist but are rare domestically.

Read the fourth column against the 5 lux target and a design rule falls out that is worth stating as a formula:

max installed lumens = target lux x area / (utilisation factor x min dim level)

For 5 lux in an 11.1 m2 room on a 10 per cent driver, that is 5 x 11.1 / (0.5 x 0.10) = 1,110 lumens. So the ceiling circuit in a nursery of this size should carry no more than about 1,100 lumens if you want the same switch to serve the night feed. Anything more and you need either a 1 per cent driver or a second, separately switched low-output source.

There is a perceptual footnote worth knowing. Perceived brightness follows roughly the square root of measured illuminance, so a fitting dimmed to 25 per cent of its output looks about half as bright. That is why a dimmer feels as though it has stopped working long before it has, and why people conclude the room is dark enough at 30 lux when the meter says otherwise.

Colour temperature and the night light#

Use 2,700 K for the room's ordinary light, which is the domestic default and the right choice for skin tones, and specify CRI 90 or better at the changing surface where you are judging colour on a small person. The reasoning behind both figures is in colour temperature and CRI at home.

For the night source, go warmer and much dimmer: 1,800 to 2,200 K and 1 to 5 lumens total. Two dimensions govern where it goes.

• Height. Mount it below 24 in (61 cm), which puts it under the cot mattress at its highest setting so the source itself is never in the baby's line of sight. A plug-in at socket height, 12 to 18 in (30 to 46 cm), does this automatically. Socket heights by room are in socket and outlet heights.
• Direction. Aim it at a wall or a skirting rather than into the room. A 1 lumen source viewed directly is a glare source in a dark-adapted eye, whatever its output.

A dimmer or a two-way switch at the door saves the whole scheme, because the alternative is crossing a dark room to reach the lamp. Switch positions and the distance from the door opening are in light switch heights and placement.

Blackout: where the light actually comes in#

Overlap and channel dimensions#

The reveal measurement in the seventh row is the one to take first, because it decides the whole approach. A window with 100 mm (4 in) of flat plaster reveal each side can take channels inside the opening and look tidy. A window with 20 mm of reveal, or a frame that fills the opening, cannot, and the answer is an outside mount with the overlaps in the first three rows. The mounting depths that each blind type needs are in blind and shade sizing: inside versus outside mount.

Curtains, and what a pole costs you#

A blackout curtain can match a channelled blind, but only if three details are right, and a curtain pole makes all three difficult.

• The head. Fabric hanging from rings on a pole leaves the full width open above the heading. A track with a pelmet or a boxed head closes it. Rod and track heights, and the reasoning about where to mount them, are in curtain length and hanging height.
• The ends. A track needs a return to the wall at each end, bringing the fabric back to the plaster so there is no line of sight past the leading edge. A pole cannot do this, and the 3 in (76 mm) gap between the end of a pole curtain and the wall is 0.006 m2 over a 2 m drop, about 48 lumens a side.
• The centre. Overlap the two leading edges by 4 in (100 mm) minimum. An overlap arm on the track holds it; without one the pair meets edge to edge and parts in any draught.

Fullness affects opacity too. At 2x fullness a blackout lining is doubled over in the folds, so pinholes and needle holes matter less than they do on a flat blind. The fullness multipliers and the stack-back they produce are in curtain width, fullness and stack-back.

Where the cot, the chair and the changing table actually sit relative to the window governs which of these matters most, and that layout is worked through in nursery layout and clearances. Terms such as reveal, return, stack-back and utilisation factor are defined with their dimensions in the glossary of interior dimensions.