What is Kelvin?
The kelvin is often used in the measure of the colour temperature of light sources.
Colour temperature is a characteristic of visible light that has important applications in lighting, photography, publishing, manufacturing, and other fields. The colour temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Colour temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K. Colour temperatures around 6500K are called daylight (stark white) and those around 5000k are called cool colours (natural white), while lower colour temperatures (2700-3000K) are called warm colours (warm yellow).
Deciding what kelvin to use in your home or office can be a personal choice. Most people prefer day light colours in kitchens and office work areas as this gives the appearance of a brighter white. Many people like to add warmth to their living areas by using a warmer colour. If your areas are separated, you can mix these colours. For those who are undecided, choosing a neutral (cool white colour) is the best option.
What is a Lumen?
The lumen is the luminous flux, a measure of the power of light perceived by the human eye. Luminous flux differs from radiant flux in that luminous flux measurements (such as lumens) are intended to reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements (such as watts) indicate the total power of light emitted. The lumen is defined in relation to the candela as: 1 lm = 1 cd·sr.
The lumen can be thought of casually as a measure of the total 'amount' of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.
The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2.
A single fluorescent light fixture that produces a luminous flux of 12000 lumens might light a residential kitchen with an illuminance of 500 lux. Lighting a larger area to the same illuminance requires a proportionately greater number of lumens.
Lamps used for lighting are commonly labelled with their light output in lumens.
What is a Lux?
Not to be confused with a lumen, the lux is the unit of illuminance and luminous emittance measuring luminous power per area.
It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception.