The first standard chosen was a source that was familiar and common to everybody: a candle. It was a specifically sized candle with specific material and molded in a specific way. The amount of light emitted from such a candle became the first unit of 'brightness'. It was called 'one candlepower'.

Imagine such a candle at the center of an darkened room, the energy of this candle is radiating equally in all directions. The farther the distance from its flame, the less light it appears to be shedding.
Light from a point source (the candle) radiates outward in all directions such that it uniformly illuminates the surface of an ever expanding sphere. As the radius of that sphere gets larger, the surface area grows even more and thus the energy from the candle is spread ever thinner.
This is the 'Inverse Square Law' (the radius of the sphere is R,
the surface of the sphere is 4x R²).

Luminous Flux (lumen)
The total light output from electric sources is expressed in lumens.
Lumens = {Lux * Area} / {Screen Gain}

Luminance (NIT, cd/m2)
The specific light that comes off the surface of a lighting device, whether off a filament, light bulb, lens, louver, tabletop, etc. Luminance varies with the direction and its gloss characteristics. Luminance is measured in candela per square foot.

Illuminance (lux)
The density of luminous flux on a surface, is measured in lux (one lumen per square meter) or footcandles (one lumen per square foot).

Candela
The unit of the intensity of light at the source equal to the amount of light from the flame of a candle in any direction.

Footcandle
The measure for the density of light as it reaches a surface. One footcandle equals one lumen per square foot. Footcandle is sensitive to the inverse square law and the angle at which the light reaches the surface.

Color Temperature
The color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (°K). The range of used color temperatures is about 2800°K to 6000°K.

Color Rendering Index (CRI)
The ability of a light source to produce color in objects. The CRI is expressed on a scale from 0-100, where 100 is the best in producing vibrant color in objects.

Luminous Efficiency
Describes the efficiency of a lamp and is is expressed as the ratio of the emitted luminous flux in lumen and the power used in watts.
The theoretically maximum value at 555 nm would be 683 lm/W. Today's lamps vary between 10 and 250 lm/W.

The speed of light in vacuum is 186,282 miles per second / nearly 300,000 kilometers per second. A light year is the distance light travels in one year at the speed of 186,282 miles per second, about 6 trillion miles.

A measurement of white color chromaticity as a result of a balanced mixture of red, green and blue. Higher color temperature gives bluish white (cool white) and lower color temperature gives reddish white (warm white).

For pigment colors, the primary colors are RED, YELLOW, and BLUE. These colors cannot be mixed by any other color. All other colors are combinations of these colors.
Secondary colors are ORANGE, GREEN, and PURPLE, these colors are the combinations of the primary colors: red and yellow = orange, yellow and blue = green, blue and red = purple.
Tertiary colors are YELLOW-ORANGE, RED-ORANGE, RED-PURPLE, BLUE-PURPLE, BLUE-GREEN, and YELLOW-GREEN. These are combinations of the secondary colors.

Complementary colors are the colors that sit opposite to each other on the color wheel. By using colors that are opposite, a color scheme with high contrast will be created: red and green, blue and orange.