How to Read Photometrics – Photometrics 101 (Part 2)

Posted by:     Tags:  , , , , , , , , , , , ,     Posted date:  October 18, 2011  |  No comment

October 18, 2011

Photometrics 101

Photometric information is generally provided by a manufacturer with each fixture to assist in the lighting layout.

Efficiency: Compares how much light is emitted from a luminaire given its wattage consumption; however, efficiency does not indicate the quality of light or visual comfort of a luminaire.

Efficiency % =  Lamp Lumens / Total Wattage Used

Unit LPW: Indicates total system efficacy; however, unit LPW provides no indication of the light’s direction or quality. Some luminaires with a lower LPW may produce a more desirable lighting effect than a luminaire with a higher LPW.

LPW = (Lamp Lumens x Luminaire Efficiency) / Luminaire Watts

Candelas: Polar candela diagrams graphically illustrate the light intensity at specific directions from nadir. Intensity is on the vertical axis, and radial lines indicate elevation angles at 30,° 60° and 90° from nadir. For symmetrical downlights, only one half of a plane of data is shown; for wall washers, both wall and downlight data are shown. Consult specification sheets for full diagrams of non-rotationally symmetric units.


Luminance: Average luminance numerically indicates the brightness of the aperture. Luminance is provided for various viewing directions, the 0° direction being where the wall wash or accent light is aimed. To convert the preferred cd/m2
measure to the older footlamberts, multiply by 3.462. Typically, luminance reveals more about the quality and comfort of a lighting system than illuminance alone.

Cone of Light: Useful tools for rapid lighting comparisons and calculations, cones of light calculate initial footcandle levels for a single unit based upon point calculation techniques. Beam diameters are rounded to the nearest half-foot.

Downlight: These cones of light provide single-unit performance with no inter-reflections from surfaces. Data listed is for mounting height, footcandle values at nadir, and resulting beam diameter. Please note:
1. Mounting heights are from the fixture plane to the illuminated work plane (task).
2. Footcandle values are at nadir (0°).
3. Beam diameter is defined as 50% maximum footcandle values. This allows rapid spacing of units for uniform illumination, allowing overlapping of 50% levels.
4. Maximum footcandle values may not always occur at nadir; batwing distributions produce maximum values surrounding nadir.
5. Proration factors are provided for other reflector finishes.



Beam Aiming: Beam aiming diagrams allow a designer to easily select the proper distance from a wall to locate a luminaire and get the center beam of the lamp where desired. For lighting art objects on a wall, the 30° aiming is preferred. At this angle, 1/3 of thebeam’s length will be above the CB point, and 2/3 will be below it. Thus, if a painting is three feet tall, plan for the CB to be aimed 1 foot below the top of the painting. For increased modeling of three-dimensional objects, two lights are typically used, a key light and fill light. Both are aimed at least 30° elevation and are located 45° off axis.

Accent: Patterns of light from adjustable accent luminaires are dependent upon the lamp type, wattage, lamp tilt and location of illuminated plane. Single-unit performance data is provided for horizontal and vertical planes, with the lamp tilted at either 0°, 30°, or 45° aiming. Please note:
1. Aiming angle is measured from nadir.
2. D is distance from the floor or wall.
3. Footcandle values are maximum values.
4. Effective Visual Beam (EVB) is determined by 50% of the maximum footcandle level.
5. Beam length and width based upon the EVB.
6. CB is the distance at which center beam of the lamp occurs either from the ceiling or nadir.

Wall Wash Data:
Asymmetric wall wash distributions are provided with two types of performance charts. A single-unit performance chart plots the illuminance levels at one-foot increments along and down a wall. Multiple-unit performance charts report the performance of the middle units computed from a four unit layout. Illuminance values are plotted centerline of unit and centered between units.
1. Illuminance values are cosine-corrected initial values.
2. No room surface inter-reflections contribute to illuminance values.
3. Changing unit spacing will affect the illumination level.

New FC = (Existing Spacing x Average Table FC Level) / New Spacing

Light Loss Factors: Many variables affect the illumination level after installation. Two of the greatest factors are Lamp Lumen Depreciation (LLD) and Luminaire Dirt Depreciation (LDD). Additional light loss factors affect the illumination level over time. It is the designer’s responsibility to understand and apply the appropriate factors. Lamp lumen depreciation accounts for the reduction in lumen output that all lamps experience as they age. Typical factors are listed in the following chart.

Luminaire dirt depreciation accounts for the reduction in light output of a fixture
due to accumulation of dirt on the surfaces of the fixture. The three factors that
determine the luminaire dirt depreciation are the optical distribution, cleanliness
of the environment and its cleaning cycle.

Effects of Lower Operating Voltage on Lumen Output: Any incandescent (including halogen) lamp operating at less than its designed voltage will produce less lumens and a lower color temperature than stated. This effect can be of great magnitude with low voltage lamps. The formulas below will create the multipliers to apply to photometric calculations and predictably plan the light levels of a given space. The table summarizes the more common voltages experienced. Formula to correct output for lower secondary voltage.

(Actual Xfr Output/Lamp Voltage)3.6 = % of published output e.g. (10.6 / 12.0)3.6 = 64%

Note: If using Excel simply enter =POWER((10.6/12.0),3.6)

Formula to correct color temp for lower secondary voltage:

(Volts Running / Volts Rated).42 = % of rated color temp e.g. (10.6 / 12.0).42 = .95%

Note: If using Excel simply enter =POWER((10.6/12.0),.42)

For in-depth information about the contents of the How to Read Photometrics – Photometric 101 (Part 2), engage one of Alcon Lighting’s designers by posting your question in the Ultimate Energy Efficient Lighting forum:

About the author

Alcon Lighting was created by professionals with experience in energy efficient lighting sales, manufacturing, distribution, and design. By keeping current with the latest trends in lighting and interior design, Alcon Lighting strives to carry products that will be the focus of your warehouse, office and home’s décor.

Wanna say something?