Luminous Intensity

in simple terms is a measure of the amount of light (in lumens) emitted from a light source as a function of angle. The SI unit for luminous intensity is candela (i.e.

MacAdam Ellipse

This refers to the region on a chromaticity diagram in which all the colours are indistinguishable, to the average human eye.

Milli-Candela (mcd)

One thousandth of a candela.

Multi-Chip LED

A single LED package containing two or more LED die. The different separate die can be different colours or can be illuminated independently or together.

Near Field

This is a notional measure of the distance from a light source.  It occurs when the observation plane is much closer to the source than 10x the largest transverse dimension within the source.

Near Field & Far Field

These are notional measures of the distance from a light source.

Optic

Either a lens or group of lenses

Peak Wavelength (λpk)

Peak Wavelength is defined as the single wavelength of saturated colour at the peak of the radiated spectrum.

Photometer

An instrument for measuring a property of light, especially luminous intensity or flux. A photometer uses special coloured filters to replicate the response of the human eye.

Photometry

Photometry is the science of the measurement of visible light, in terms of its perceived brightness to the human eye.

Photosynthetic Active Radiation (PAR)

This is the spectrum range (400nm - 700nm) that plants use for photosynthesis and corresponds roughly with the light visible to the human eye.

Photosynthetic Photon Efficacy (PPE)

µmol/J

Efficiency of a light source for converting electrical energy into PAR light (in μmol/J).