# Tristimulus Values/Candle Power

## Tristimulus Values

Another system of color quantification defines colors in terms of tristimulus values. This is a method developed by the International Commission on Illumination (CIE), an organization dedicated to standardization in illumination and related areas. The tristimulus values are the percentages of each primary in an additive color mixture: X (red primary), Y (green primary), and Z (blue primary). Color coordinates are then calculated from the tristimulus values. Lower case x, y, and z are used to designate the color coordinates. The color coordinates represent relative percentages of each primary required to produce it.

x = X (X + Y + Z)
y = Y/(X + Y + Z)
l = x + y + Z

Any possible color can be designated by its x and y coordinate on the CIE diagram. If two colors with different color coordinates are mixed, a line connecting their points on the CIE diagram will indicate all the possible mixtures they can achieve.

### Candle Power

The next focus for quantifying a lighting design is on proper level of illumination. The Candela (or Candle) is the basic unit of light measure. The Candle is described as source’s luminous intensity, or the intensity of light in a given direction. An ordinary wax candle has a luminous intensity of one Candela in a horizontal direction.

The time rate flow of light is described as luminous flux. The standard unit of luminous flux is the Lumen (L). The L differs from the Candle in that it is a measure of light flux irrespective of direction. One L is the light flux transmitted on a one ft2 area where every point on the area is one foot away from a one Candela source.

Illumination is the density of luminous flux on a surface. Illumination is measured in units of footcandle (FC). An FC is the illumination at a point on a surface, which is one foot from perpendicular to a uniform point source of one Candle. Brightness is described as luminous intensity in a given direction per unity of projected area. A surface or an object has brightness by reason of light emitted, reflected or transmitted. Brightness is ordinarily independent of distance of observation. Symbolized by either B or B1, brightness is measured in Candle per square inch (C/in2) or footlambert (fl). Brightness can be expressed either as Candles per unit area, or L per unit area. A surface emitting or reflecting light in a given direction at the rate of one C/in2 of projected area has a brightness in that direction of one C/in2. A surface, which has a brightness equal to the uniform brightness of a perfectly diffusing surface emitting or reflecting one lumen per ft2, has a brightness of one fl. The fl is also the average brightness of any surface emitting or reflecting light at the rate of one L per ft2. A lambert is the brightness of a surface emitting or reflecting one lumen per square centimeter.

Initial Lumens is the measurement of the intensity of the lamp initially. Lamps do get dim over time and replacing them after 60% of the rated life is recommended for best performance.

A lumen is the unit that expresses the total quantity of light given of by a source, regardless of direction. A lumen is defined as the amount of light falling on a surface of one square foot, every point of which is one foot away from a source, (a section of the inside surface of a sphere) of one candlepower. A uniform source of one candlepower placed in a sphere emits 12.57 lumens.  