LightCalc Splash Banner hover“ title=
Michael De Luca, CKD, ASID, NCIDQ
Lighting Design Testimonials
Lighting Design Support
LightCalc Contact
Lighting Plan
Lighting Plan
Lighting Plan
Lighting Plan

Beam Angle

Any directional lamp emits light energy in the shape of a cone. As you might expect, the candlepower intensity is the greatest at the center of the cone and it diminishes the closer it gets to the edge of the cone; eventually, in theory, to zero. The usable portion of the cone is defined at the point where the candlepower falls to 50% of the candlepower at the center. Our eyes perceive this portion of the lamps cone as one intensity of light even though, at its edges, the intensity has dropped to half. This portion of the total cone of light is termed the Beam Angle.

Beam Angle

It is the beam angle we are concerned with when it comes to spacing one directional lamp to another.

The Beam Angle is found in the lamp manufacturers' lamp specifications.

Candlepower (Cp)

The unit for measuring the quantity or intensity of light energy emitted by a directional lamp (i.e., narrow spot, spot, flood, wide flood, etc.).

Candlepower is expressed in candelas. Confusion is caused for some when one manufacturer lists a lamps candlepower as "Center-Beam CandlePower" while another notes it as "Mean Candlepower" while still another uses "Candela" as the heading for the same information in their lamp specifications. Any of these listings refer to the lamp candlepower and are considered to be the same.

Candlepower is used in the Inverse Square Law calculation.

Some texts indicate that 1 candela = 12.57 lumens; do not try to convert lumens to candelas using this formula. Results will be inaccurate.

Color Temperature

The color of the lamp itself as compared to the color of a black reference substance when heated to various temperatures, Kelvin, and the effect the lamp color has on the color of an object being illuminated by it.

  • Cool lamps range from: 3,600k to 6,500k
  • Neutral lamps range from: 3,000k to 3,600k
  • Warm lamps range from: 2,700k to 3,000k
  • Fluorescent lamps are available in Cool, neutral, & warm.
  • Halogen lamps are considered neutral.
  • Incandescent lamps are considered warm.

A properly selected lamp will have appropriate color temperature and provide a good CRI (color rendition index).

Lamps must reproduce the colors in the room properly. Without a lengthy discussion as to how it all works, just remember the following:

For task, accent, and art lighting use halogen lamps. PAR halogen or MR16 lamps (all of which are halogen) do an excellent job. For art, MR16 lamps are preferred since they throw their heat back and away from the art.

For general lighting, use PAR halogen lamps or compact fluorescent. Compact fluorescent lamps should be about 3500 kelvin (they range from 2700 - 6500 kelvin) to approximate the color of halogen lighting which may be used elsewhere in the room. Doing this ensures that all like colored objects will look the same.

These pictures are intended to show the quality of colors illuminated by various lamps.

Represented within the limits of modern monitors.

Color Temperature In Lighting Design

When using other types of fluorescent, remember to use lamps that are over 80 CRI (color rendition index) AND between 3000 - 3500 kelvin. All compact fluorescent lamps are over 80 CRI. CRI rates the lamps ability to reproduce an objects color accurately. Lamps falling outside these recommendations, in my opinion, destroy color!

CU Coefficient of Utilization

The efficiency in which the luminaire (combination of fixture, fixture trim, and specific lamp) directs lumens to the workplane expressed as a percentage. This CU percentage is listed in a table produced by the fixture manufacturer and is used in the Lumen Method to determine the number of fixtures required to achieve a given footcandle level in a space. The CU takes into account the shape, height, and color of the room. To select a number from a CU table, the ceiling reflectance, wall reflectance, floor reflectance, and the room cavity ratio (RCR) must be determined.

CRI - Color Rendition Index

A scale from 1 to 100 indicating a lamps ability to render an objects color accurately. 100 is the best rating. For interior design work, only lamps over 80 should be used.

  • Fluorescent lamps are available in 50, 60, 70, 80, & 90 CRI.
  • Incandescent and incandescent halogen lamps are 99 CRI.

A properly selected lamp will have good color rendering properties and be of an appropriate color temperature.

Distance (D)

For down-lighitng, it is the measurement from the lamp to the workplane.


The ratio of lumens produced by a lamp to the watts consumed expressed as lumens per watt (LPW). The higher the lumens per watt, the more efficient the lamp.


The housing or assembly that holds the lamp and/or trim.

Fluorescent Phosphors

The color of fluorescent lamps is created by mineral phosphors in powder form which coat the inside of the lamp tube. The chemical make-up of these phosphors determines the lamps CRI, its color temperature, and how much light the lamp produces. There are four types of phosphor coatings:

Traditional Halophosphors are inexpensive coatings which usually provide the entire spectrum of light. But, there is a trade-off between color rendering and lumen output. Poor color rendering lamps such as "warm white" and "cool white" have high lumen output. Good color rendering lamps such as "warm white deluxe" and "cool white deluxe" have low lumen output.

Prime Color or Tri-Phosphors are very expensive coatings with good color rendering and high lumen output. Lamps of this type are produced under the trademark, Ultralume.

Double-Coat lamps Have a coat of halo-phosphor and a coat of tri-phosphor. Double-coat lamps which have a thick tri-phosphor coat are fairly expensive but have very good color rendering properties. They are known by the trademarks SPX, Designer 800 series, etc.. Double-coat lamps with a thin tri-phosphor coat are much less expensive, but still have full light output and reasonably good color rendering. These lamps are known by the trademarks SP, SPEC, Designer, and others.

Rare Earth Phosphors Have a thin and thick coat of rare earth phosphors and are just becoming available. The CRI for these lamps will be 70, 80, and 90 and in a variety of color temperatures.

FootCandle (Fc)

The unit of measurement indicating the light present on a surface or workplane. Footcandle levels are determined by the IES (Illuminating Engineering Society) to provide an adequate amount of light to a visual task whether it be walking through a space or studying a manuscript. Tasks requiring more visual acuity require a higher footcandle level.


Note: One footcandle is equal to the amount of light provided by an ordinary wax candle on a spherical surface with an area equal to one square foot one foot away from the flame.

To convert Fc to Lux, use the formula:

Lux = Fc x 10.76

To convert Lux to Fc, use the formula:

Fc = Lux x .0929

If artwork is to be incorporated into the space it must receive 5 times the amount of light as the surrounding space. This gives the art the perception of being highlighted or accented.

See Chapter 1, Lighting Concepts, for more information on footcandles and Chapter 4, Lighting Measurements, for details on how to determine footCandles for a room and how they are used in the Inverse Square Law and Lumen Method calculations.


The amount of light reflected off a surface. This reflected light adds to the overall illumination The degree of reflectance is greatly dependent on the color in the space. White reflects approximately 80% of the light while black reflects only 4%.

Inverse Square Law

For directional lamps (i.e., narrow spot, spot, flood, wide flood, etc.), a formula used to determine the relationship between the candlepower (Cp) of the lamp, the distance (D) the lamp is from the surface to be illuminated, and the footCandle (Fc) level produced on that surface. If any two of the three elements are known, the third can be determined.

Inverse Square Law

Example: an island counter is 3' high in a kitchen with an 8' ceiling height. Recessed cans are to be used with a lamp rated at 1150 candlepower. The distance from the lamp to the counter surface is approximately 5'. By squaring the distance (D2) between the counter and the lamp (5' x 5' = 25'), then dividing it into the lamps rated candlepower (1150), the footcandle level at the counter surface is found to be 46.


A man-made light source. Also known as a tube or bulb.


The visible portion of the electromagnetic spectrum extending from 380 nanometers (ultra-violet end) to 770 nanometers (infra-red end). White light is made up of three primary colors. The three primary colors of light are red, blue, and green. The primary pigment colors are red, blue, and yellow.

Line Voltage

120 volts. Lamps that operate at 120 volts are considered line voltage lamps.

Low Voltage

Lamps that operate at a lower voltage than line voltage are termed low voltage lamps. These lamps require a transformer to reduce line voltage to usually 12 or 24 volts.


The unit of measurement used to indicate the quantity or intensity of any lamp. Think of it in terms of the lamps raw power. Lumens are measured at the lamp.

Lumens are used in the Lumen Method calculation to determine the number of fixtures necessary and their spacing to maintain a given footcandle level as prescribed by the IES (Illuminating Engineering Society).

Some texts indicate that 1 candela = 12.57 lumens; do not try to convert lumens to candelas using this formula. Results will be inaccurate.

Lumen Method

A series of formulas used to determine the number of luminaires necessary to illuminate a space and the spacing needed to maintain a given footcandle level. A CU table from the manufacturer is a prerequisite in performing these calculations.


A complete lighting unit; an assemblage made up of the fixture, trim, and the lamp.


The metric unit of measurement indicating the light present on a surface or workplane. One might call lux the metric footcandle. It differs in that instead of using a sphere with a radius of one foot it uses one meter. The amount of light falling on one square meter of the sphere is referred to as one lux.

Europeans manufacture many lighting products that are used in the states. Rather than learn the entire metric system of lighting measurements, do the calculations found in this book using footcandles, convert the results to lux using the factor below, then select the fixture or lamp in its metric equivalent.


Note: One lux is equal to the amount of light provided by an ordinary wax candle on a spherical surface with an area equal to one square meter one meter away from the flame.

To convert Lux to Fc, use the formula:

Fc = Lux x .0929

To convert Fc to Lux, use the formula:

Lux = Fc x 10.76


Zero angle of light perpendicular to the lamps face and bisecting the beam angle. If a lamp were placed in a typical recessed can in a level ceiling, nadir would be a vertical line perpendicular to the floor, ceiling, and lamp face.


The measurement of quantities associated with light.

RCR - Room Cavity Ratio

A number indicating the room proportions calculated from the length, width and height. The result is a number (1 to 10) that is used to select a value from a CU table.


Reflectance is the amount of light which reflects off an object. This quantity of light can be measured and is expressed as footlamberts. It isn't terribly important for the interior designer to know the technical definition of footlamberts; it is important, however to understand that the amount of light reflected off objects in a room adds to the overall illumination and must be taken into account when determining the footcandle requirement for the space. The color of an object determines to a large extent the amount of light reflecting off the object. See Chapter 4, Lighting Measurements, for more information.

Reflectance Table


The measurement between centers of luminaires.

Spacing should be at least equal to the beam diameter at the workplane to evenly maintain the footcandle level throughout a space. By decreasing this dimension the footcandles are increased; by increasing the spacing the footcandles are reduced.

Spacing Ratio

Often given by fixture manufacturer. Multiply the spacing ratio by the distance (in feet) from the fixture to the workplane.

Spill Light

Outside the beam angle is an area of "spill" light which extends from the beam angle out to a point in the lamps cone of light where the candlepower drops to 10% of the candlepower at the center of the cone.

Spill light is viable light, though not enough to consider for lamp spacing, it is enough to provide adequate illumination to wall cabinet fronts in a kitchen, for example. It is also useful as a "buffer zone" when ideal spacing can't be achieved due to existing ceiling joists locations, etc.


See Footcandle and Lux.


The amount of energy consumed by the lamp. Do not select a lamp based solely on wattage. This is a very common mistake. Wattage has nothing to do with the amount of light produced by a lamp.

A lamp should be chosen based on its candlepower or lumen rating which indicates the light energy intensity of the lamp. Only then should wattage be considered to find the most economical lamp among those lamps powerful enough to provide the proper footcandle level on the workplane.


In any space there is an actual or implied height at which an activity takes place. At this height an imaginary plane which cuts through the entire room is assumed; the workplane. It is on this workplane that the footcandle measurement is taken. The IES (Illuminating Engineering Society) has determined appropriate footcandle levels for the workplane in every room of the home.

For a kitchen, the countertop is the workplane at 3 feet above the floor, the bath can be anywhere from 2 1/2 to 3 feet, a dining room or desk is 2 1/2 feet. When there is not a specific surface upon which an activity will take place, such as in a family or living room, a 2 1/2 foot workplane height is assumed. Whether you are calculating general or task illumination, the light level is determined in footcandles measured at the workplane height.