These are the terms that come up when you compare bright lights, SAD lamps and artificial skylights. Each has a number where one exists, and a link to the guide that goes further.
Brightness
Lumen
The total visible light a source gives out, the SI unit of luminous flux. A 60 W-equivalent household bulb gives about 800 lumens, a typical flush ceiling light about 1,450, and Sunday Light 34,500. See the brightest light fittings you can put in a home.
Lux
Light arriving on a surface: one lux is one lumen per square metre. Most homes sit at 100-300 lux and a well-lit office at about 500, the level EN 12464-1 sets for reading and writing. See how many lumens a room needs to feel like daylight.
Inverse square law
Light from a small source falls off with the square of the distance: twice as far away, a quarter of the light. It's why a typical SAD lamp gives 10,000 lux only at 10-40 cm. See the brightest light for a dark flat.
Luminous efficacy
Light out per watt in, in lumens per watt. Warehouse high bays reach about 155 lm/W at CRI 70+. High-CRI LEDs give less: a CRI 95 panel gets 4,000 lumens from 40 W, or 100 lm/W.
Lumen method
The standard way to estimate a room's average lux: lumens × utilisation factor × maintenance factor ÷ floor area in m². With typical values of 0.5 and 0.8, about 40% of a fitting's rated lumens become average lux at desk height. The worked examples are in how many lumens a room needs.
Colour and light quality
Colour temperature (kelvin)
The colour of white light, given as the temperature in kelvin (K) of a glowing black body of the same colour. Household warm white is 2700-3000K; standard daylight, D65, is 6500K. Sunday Light runs from 2650K to 6000K. See how to light a north-facing room.
CRI (colour rendering index)
How faithfully a light shows colours compared with a reference light of the same colour temperature, scored out of 100. The headline figure, Ra, averages eight test colours, R1 to R8 (CIE 13.3). EU rules set 80 as the minimum for general lighting; Sunday Light is 97+. See high-CRI ceiling lights.
R9
The CRI score for saturated red, which isn't part of the Ra average. One LED Waveform Lighting measured scored Ra 79 and R9 -1.4. Good high-CRI bulbs and panels publish R9 of 80-90 or more.
TM-30
A newer colour-rendering method from the Illuminating Engineering Society that scores 99 colour samples instead of eight. It reports Rf for colour fidelity and Rg for saturation (US Department of Energy).
Tunable white
A light whose colour temperature you can change, from warm to cool. Sunday Light runs cool 6000K in the morning and warmer, down to 2650K, through the afternoon, on a schedule you set once in the app.
Full spectrum
A marketing term with no technical definition. The Lighting Research Center found each manufacturer making up its own definition. Compare CRI and R9 instead.
Flicker
Rapid, repeated change in a light's output. EU ecodesign rules cap flicker from mains LED light sources at a Pst LM of 1.0 at full load. Sunday Light is flicker-free.
The body clock
Melanopsin and ipRGCs
Melanopsin is a light-sensitive pigment in a small set of retinal cells, the intrinsically photosensitive retinal ganglion cells (ipRGCs), most sensitive at around 490 nm, blue-cyan light. They are the main route by which light sets the body clock. In 1999 Professor Russell Foster's group showed that mice with no rods or cones still set their clocks by light. He advises Sunday Light. See health.
Melanopic EDI
Melanopic equivalent daylight illuminance: the lux of standard daylight that would stimulate melanopsin as much as the light you have. A 2022 expert consensus recommends at least 250 by day, at most 10 in the three hours before bed and at most 1 while asleep (Brown et al., PLOS Biology).
Circadian rhythm
The roughly 24-hour cycle of physical, mental and behavioural change, run by a master clock in the brain's suprachiasmatic nucleus. Light and dark have the biggest influence on it. See the benefits of morning light.
Phase response curve
How far light shifts your body clock, depending on when you see it. Light before the low point of your core body temperature, in the evening and night, delays the clock; light after it, in the early morning, advances it (Khalsa et al., 2003). See using light to speed up jet lag recovery.
SAD (seasonal affective disorder)
In the NHS's words, "a type of depression that usually happens in the winter". See light therapy for seasonal affective disorder.
Bright light therapy
Sitting in bright light, usually 10,000 lux for 20-30 minutes each morning. In the Can-SAD trial, 30 minutes of 10,000 lux each morning matched the antidepressant fluoxetine, with 67% of patients responding to each (Lam et al., 2006). See how bright light therapy works.
Skylights and daylight
Rayleigh scattering
Scattering of light by particles much smaller than its wavelength. It rises with the inverse fourth power of wavelength, so blue light scatters about 4.3 times more than red, which is why the sky is blue and the setting sun orange. Sunday Light's sheet scatters the bluer light and reflects the warmer light in the same way. See how it works.
Artificial skylight
A light that recreates a sky overhead, in look and in brightness. CoeLux's 45 HC gives 4,100 lumens and is recessed into a ceiling void of about 1.2 m. Sunday Light, a sunlight ceiling light, hangs like a chandelier and gives 34,500. See the four kinds of artificial skylight.
Sky panel
A flat LED panel or ceiling light made to look like a patch of sky, usually on a daylight schedule. Output runs from 1,400 lumens for one Nanoleaf Skylight panel to 7,000 for Govee's Sky Ceiling Light. See artificial skylights compared.
Daylight factor
The daylight inside a room as a percentage of the daylight outside at the same moment, with direct sun left out. UK guidance in BS 8206-2 set minimum averages of 2% for kitchens, 1.5% for living rooms and 1% for bedrooms. See basement apartment lighting that feels like daylight.
