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Old 01-13-2012, 07:07 PM   #71
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Hey MJS that is an interesting test, but unfortunately that test cannot accurately examine the true Lumen output. I know you even said that you weren't sure about the correctness of a few things, but to reveal test results with many forms of guesstimate's and preliminary forms of information isn't really fair to us and everyone else. We have thoroughly tested and had professional outside sources test our lights as well. To truly get an accurate factual output of Lumens you would need a Goniophotometer. (An Optic test, designed to measure the intensity of light reflected from a surface at various angles) Please don't take this the wrong way I am not trying to be rude or take away from your hard work, but to accurately test these lights this is what you need.
I won't take it the wrong way. I am familiar with a Goniophotometer. Actually, the turntable test is really accurate and repeatable for a fixture with an even round field. Not so accurate when the beam is shaped into a different horizontal and vertical spread. And I am not trying to determine total lumen output but rather usable output. A hageon lamp with no housing around it could have high lumen output but all the light shining to the side and back would be wasted if you only wanted to see in one direction. But that's why you spend money on reflector and housing designs, no?

If you have had the fixtures tested than you should have all of the data necessary to prepare either an Iso-Illuminance curve or better yet a Cosine Candella Plot (Cartesian Curve).

Background info with credit to the author, T Pincu.


In 1957 the Illuminating Engineering Society established a standard for reporting photometric performance of incandescent filament lighting units used in theatre, film, and TV production. That became the defacto standard for reporting performance in performance lighting equipment.


Candelas and Candlepower are the same thing. Beam Candlepower is Candelas in a specific direction as in a field of light from a spotlight.

If you take a 2’ diameter sphere and put a one candela or candlepower source in the center of the sphere, it will generate 1 lumen or footcandle over every square foot of the interior surface of the sphere. A footcandle is a lumen per square foot.

A Little Technical Stuff

A steradian is a unit solid angle (think of a little pyramid) subtending (defining) an area on the surface of the interior of a sphere equal to the square of the sphere’s radius.


The formula for the area of a sphere is 4 π R2. The radius of the sphere is one foot. Therefore 4 x 3.14159 x 12 = 12.57 square feet. The one candela source will generate a total of 12.57 lumens on the surface of the sphere.

If the source is 10 Candelas, 10 lumens will be generated over each square foot of the sphere for a total of 125.7 lumens

If the source is 100 Candelas,100 lumens will be generated over each square foot of the sphere for a total of 1257 lumens

If the source is 1,000,000 Candelas, 1,000,000 lumens will be generated over each square foot of the sphere for a total of 12,570,000 lumens. The increasing amount of candelas is a function of the brightness and power of the source.


A measure of a spotlight’s ability to deliver footcandle values at any given distance to a point on a surface. Regardless of the distance candelas will always be the same for a given fixture.

Candelas = Foot Candles x Distance2

Footcandles = lumens/square foot.
A meter = 3.281 feet. A meter squared = 10.76 square feet.
Lux = lumens/square meter.
Dividing candelas by distance in meters squared = lux.
Dividing candelas by distance squared in feet = footcandles.
Lumen efficiency = output of a light source (lamp) or fixture.
Beam Lumens = fixture output measured to 50% of center intensity.
Field Lumens = fixture output measured to 10% of center intensity.

The field angle of a fixture is measured to 10% of center intensity for photometric reporting purposes .


Calculating fixture efficiency and understanding the IES Method

Surface area of a 1’ radius sphere formula - 4 π R2 = 4 x 3.14159 x 12 =12.57 sq. ft.

A one candela (or candle-power) source emitting 1 footcandle uniformly over every square foot of the inside of the 1’ radius sphere produces a lumen per square foot or 12.57 lumens.

By scaling up, a 10 candela source in the same sphere emits 10 lumens per square foot or 125.7 lumens, and a 100 candela source emits 1257 lumens.

By using this technique any directional source can be analyzed for lumen efficiency through a technique using ZONAL CONSTANTS.

What’s a zonal constant? It’s area in decimal square feet within a 1 foot radius sphere. Zonal constant was a term to describe the zone or area within a defined spherical space by the IES and the technique is built into most analytical devices’ software. The constants can be applied once candlepower is established.

Candelas x the zonal constant measured in the center of the zone = the lumens in the zone.

Once you understand how to apply this, you can take any spotlight and determine its efficiency by measuring the candelas in each of the zones, converting the candelas to lumens by multiplying by the zonal constant in the zone measured, and adding up the lumens to 50% and 10%. Those two numbers divided the published lumen output of the lamp used provide the lumen efficiency at 50% and 10% of the spotlight’s performance.

When you are comparing spotlight performance with another, make sure you are comparing beam lumens with the other spotlight and field lumens with the other spotlight. You can’t compare beam and field lumens.

The illustrations that follow describe the angular zones and their constants. All this is to illustrate the method behind the calculations.

Notice how the smallest zone in the 1 foot radius sphere, - 0 to 10 degrees compares with the 40 to 50 degree area. In the center of the field regardless of the candelas produced the lumens produced are significantly less that the lumens in the 40 degree area because the area of the zone is so small.

The candelas x the zonal constant or area in square feet determines the lumens in each zone. Those lumens added together tell you the lumen output of the spotlight in the beam or field. For measuring purposes only one half of the sphere is used.

A simple method for measuring a fixture’s output. The light is mounted on a turntable calibrated in degrees and rotated about its center. The distance is constant for each reading. Ideally the square root of 1000 or 31.6 foot throw distance will permit you to calculate candelas quickly – footcandles times 1000 = candelas or beam candlepower for each reading. The formula is FC x D2 = Candelas

Candlepower times the zonal constant or area in square feet = the lumens in that zone. The lumens in each zone added up to 10% of center intensity = the field lumens of the fixtures. Lumens added up to 50% of center intensity = beam lumens of the fixtures. Beam and field lumens divided by the published lumens of the lamp = fixture efficiency in beam and field.

This illustration shows what the various zones types look like depending on which one is used.

Zonal constants are areas in square feet based on the 1 foot radius sphere. But they can be applied at any distance and angle. 1 degree zones might be used to measure the lumen output of a 5 degree fixture. 2 degree zones might be used for a 10 or 20 degree unit. 5 degree zones for a 40 or 50 degree unit.

As long as you define the area being illuminated by the zonal constants you can extract the lumens in that area

Turntable solution:
Voltage - Must be at rated voltage of the lamp
Lamp Condition – Should be new
Fixture Condition – Should be new and properly aligned to provide
cosine distribution if adjustable

Test Distances

Square root of 1000 = 31.62 feet. Footcandle levels read at this distance x 1000 provides candela levels instantly. If fixture is too large or it’s field angle too narrow, longer test distances must be used.

So the turntable test is not a good test for fixtures with shaped beams (as opposed to a round beam). That's why I was going to retest using a different method. The new test to be done using the Dubinovsky method. The test set-up is straight-forward with data being recorded, and calculations being made, in an Excel spreadsheet.

On a piece of board approximately 4 feetin diameter, mark three circles with diameters 1’, 2’ and 3’ concentrically revolving around a common center point. Then, divide the circles with lines at 45 degrees (see Fig. 1).

Fig. 1

Draw small targets upon each point that the lines intersect the circles. Then draw one target in the center point. Mount the board on a stand so that the center point of the circle is at the same height as the center of the light fixture to be measured (see Fig. 2).

Fig. 2

For Spot fixtures, or a hard-edge beam, point the light fixture output at the board and adjust the distance of the fixture from the board so that the outer edge of the beam is the same diameter as the outside circle of the target on the board. Measure the distance of the front of the fixture output lens from the board and record on the spreadsheet. Record the light level readings at each of the 25 points and record into the spreadsheet.

For Wash fixtures, or a soft-edge beam, point light fixture output at the board and adjust the distance of the fixture from the board so that the illumination at the outer edge is 10% of the center point illumination. Measure the distance of the front of the fixture output lens from the board and record on the spreadsheet. Record the light level readings at each of the 25 points and record into the spreadsheet.
When all the values have been entered into the document, the total lumen output will be calculated and displayed along with the beam angle.

The test stand for this is easy, I just need to sit down and build the Excel spreadsheet.

PS: I know the BD light is bright but so are lots of other lights. My point is that what customers need is objective data free of the "my light's brighter than yours" marketing hype. Ideally independent testing or mfg testing to a standard that could be applied across the industry. Then a potential customer can compare fixtures based on real data to determine what suits their needs. BWOE, the Squadron LED is listed at 44 watts, the 8" racelight is approx 46 watts (12v x 3.8 amp). So if power consumption is a non-factor then does the 8" light give more usable light? But maybe size or mounting is a concern and the LED is the better choice? Different needs for different folks.
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MJS screwed with this post 01-13-2012 at 08:02 PM
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