Motion sensor lights
Cordless night lights with a magnetic holder and USB charging, plug-in lights, wooden and round wall lights and head torches, sold singly and in sets of ten and twenty. A motion sensor cannot see through glass, because it reads wavelengths of eight to fourteen micrometres and ordinary glass absorbs them. The sensor on its own draws about sixty microamps, so what empties the battery is not the waiting but the lighting, and runtime is counted in the number of switch-ons. Warm white belongs by the bed and in the hallway, because suppression of melatonin secretion is strongest in the blue part of the spectrum.
Motion sensor lights
A light with a motion sensor comes on by itself when somebody walks past and goes off again after a while. Most of the category is cordless night lights with a magnetic holder, rechargeable over USB, with a battery of around 1200 milliamp hours and a head that turns eighty-eight or three hundred and sixty degrees. Alongside them there are plug-in lights, wooden and round wall lights, a light for under the toilet lid, and head torches. They are sold singly and in sets of ten and twenty, in cool, neutral and warm white around three thousand kelvin.
A motion sensor cannot see through glass, so behind a window or behind the glass door of a cabinet it does not work. It reads thermal radiation at a wavelength of roughly eight to fourteen micrometres, the kind human skin gives off, and ordinary glass absorbs exactly those wavelengths. A light standing indoors behind the window therefore will not come on for somebody passing outside. A glass display case needs a fitting with the sensor on the side of the glass where the movement happens.
The sensor does not measure heat but the difference between a person’s surface temperature and the background’s, so in hot weather its range drops. It sees the surface and not the inside, and skin sits at around thirty-two to thirty-four degrees, through clothing lower still. When the surroundings approach that temperature the difference nearly disappears and the sensor stops responding reliably, so on a baked south wall or in a loft it happens sooner than you would expect. In a hallway or a cupboard the temperature difference is steady and this problem does not arise.
The sensor on its own draws about sixty microamps, so on a battery of 1200 milliamp hours it would sit listening for over two years. What empties the battery is therefore not the waiting but the lighting. Runtime is counted in the number of switch-ons rather than in days, and a light in a busy hallway lasts a fraction of what it lasts in the pantry. Before reaching for more capacity, shorten the on-time or turn the sensor so that it does not trigger on every passer-by.
A night light is not meant to wake you, and that is exactly why its colour matters. Suppression of melatonin secretion is strongest around four hundred and sixty nanometres, that is in the blue part of the spectrum, of which cool white has considerably more than warm white. Warm white around three thousand kelvin belongs by the bed, in the hallway and by the toilet, while cool white suits the cupboard or the workshop.
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