Friday, February 13, 2015

testing fluorescent light fixtures - the test jig

Continuation of work done last year, described in this article.

Here at I have eight dual four foot ceiling mounted fluorescent fixtures. I want to add more light in the center of the main shop, probably two more fixtures, so ten altogether. The purpose of this work is to compare a variety of alternatives to come up with safe, modern high efficiency shop lighting while re-using as much as possible from the existing fixtures.


(click any picture to enlarge)

Because I wanted to do a series of comparisons, and it is not very convenient with the fixtures mounted on the ceiling, I built a simple test jig from wood scraps to temporarily hold a fixture to be tested at a standard height above a table (about 38").

The feet of the jig also can support a fixture off the table when I want to remove the cover to attach a thermocouple to the ballast or other changes. Some of these fixtures have sharp edges and protruding screws out of the backside that might scratch or damage the table surface. The two shelf brackets support the fixture when it is in the raised position yet allow easy changeover between fixtures and access to the bulbs.

A light meter will be positioned on the table surface directly below the center of the fixture.


I used a number of instruments to measure the energy into the lamps and the light output. None of these tools are particularly special or expensive. Since I would be making a series of comparisons, the absolute accuracy was not as important as consistent results and I would achieve those results by making careful measurements in the same way (as much as possible) between the various alternatives.

The light meter I use is a generic, made in China, bought on Ebay. Mine measures in footcandles and I used it on the most sensitive scale (200 footcandles full scale). I have just looked at "light meters" on Ebay and meters of this style and class sell for about CAD$25-50. Mine gives stable and consistent results and for my purposes, to know whether one source is relatively bighter than another, the absolute accuracy is not that important.

One footcandle is equal to one lumen per square foot or approximately 10.764 lux.

To help get consistent results, 1) I started the tests with a fresh 9 volt battery, 2) I placed a masking tape cross-hair on the table top so I put the meter head in the same place each time if I had to move it and 3) I tested the light meter before each fixture under an LED table lamp nearby (not shown) in the same position under the same bulb and ensured that I got the same reading each time before beginning the test run.

To measure factors about the power input to the fixture, I am using a "Kill A Watt" meter from P3. I have an older version that I have used for a number of years. The current model of what I have is a P4400 but mine seems to be the same.

The Kill A Watt plugs into a power outlet (in this case a Woods extension cord) and itself has a socket on the front into which I plug the cord to the fixture I am testing. The Kill A Watt makes its measurements on the electricity which passes through it to the load, the fixture I want to test.

I use the Kill A Watt to read the voltage, the current (in Amps), the Power (in watts) and the apparent power (in VA). The built in timer is also handy for telling how long the test lamp has been ON. I make a series of measurements at 5, 10, 15, 20, 25 and 60 minutes and after each measurement, I set the Kill A Watt to show elapsed time to help prompt me for the next measurement.

To measure temperature, I use a two input Fluke model 52II. Since there is a significant change in gas tube (fluorescent) light output with the temperature of the gas, it is useful to know the room ambient temperature. When the fixture first comes on, light output is less than when the gas warms up. The gas warms due to the electricity passing through it. At low temperatures, such as early morning in the shop during winter, the older fluorescent fixtures may not light at all.

The Fluke 52II uses two thermocouples to sense temperature. One of the thermocouples is shown in the picture, the small bead at the end of the wire. One thermocouple was loosely attached to a wood beam across the top of the test jig about the same height as the fixture, but not touching it, just hanging in the air. The other sensor I used to measure the temperature of the ballast in the fixture.

I included the auto transformer so that I could vary the voltage. I didn't do this during the regular test runs but experimented separately with varying the voltage on only some of the fixtures to see how sensitive they were to reduced voltage. It turned out that they were not very sensitive (well regulated).

The Woods extension cord which the Kill A Watt is plugged into is itself plugged into the auto transformer. The auto transformer is plugged into a regular grounded wall outlet in the shop.

In case I have confused you, here is a block diagram.

I have one more objective that I have not fully figured out yet. I want some measure of the color temperature or spectra of the lamps. By now we are probably all familiar with the desirability of "warm white" light as opposed to "cool white". Without an expensive spectrophotometer, such as one of these, I must improvise.

I think that by laying a sheet of white paper under the fixture, where the light meter normally sits and taking a photo in RAW mode with my excellent Canon G15 camera that I should be able to make some judgements or measurements of the color temperature with Photoshop or some other program that can read the RAW image. In RAW mode, the file produced by the camera is an uncompressed, unaltered record of the data produced at each pixel without any compression or compensation (like white balance).

If you have any ideas about how to do this, please leave me a comment below (I moderate the comments to prevent spam so they appear after I approve them) or write to me directly at george at ffwdm dot com.

This is the test jig in operation. I have tested a number of fixtures already with different types of gas and LED tubes and a variety of ballasts. I am recording and graphing the data and will present the results in upcoming posts.

Thanks for your interest!

George Plhak
Lions Head, Ontario, Canada

shop lighting reading list
a parabolic workshop light
led household bulbs
exploring efficient workshop lighting alternatives
work light led retrofit
testing fluorescent light fixtures - the test jig (this article)
testing fluorescent light fixtures - the test method (video)
testing fluorescent fixtures - 40 watt
efficient workshop lighting 2
updated bench lighting

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