Efficacy is the ratio of lumens (light output) to watts (electric power
consumed). A luminaire with a higher efficacy is more efficient at
converting electrical power to light. For example, an efficacy of 100
lumens per watt means that for every watt of power consumed, the light
output is 100 lumens.
The CRI (Colour Rendering Index) is a scale out of 100 which
quantitatively measures the ability of a given light source to
accurately render colour when compared to an ideal reference source,
such as the sun. If a light source has a CRI of 100, then colours should
look the same under that source as they would appear when lit by
natural sunlight. Visually, the higher the CRI the better, however the
trade-off is that a higher CRI will result in a lower efficacy. Generally a CRI above 80 is considered acceptable for most lighting applications.
Different light sources emit different coloured light. Some lights may
appear blue or cool, while others may appear yellow or warm. These
colour differences can be expressed numerically on the Kelvin (K) scale.
For example, a 3000K light will appear yellow and warm, whereas a 5000K
light will appear cool and blue.
The colour temperature that is best for you is dependent upon personal
preference and application. A warm light (e.g 3000K) would typically be suitable for
spaces such as bedrooms and living rooms, as the warm light creates ambience and a
cosy feel. A cooler light (e.g 4000K) would typically be more suited to kitchens,
laundries, offices, and spaces where tasks are performed.