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Comparing Apples and Oranges |
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Written by Steve Heising
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Jan 13, 2010 at 02:43 PM |
An Engineer for the City of San Diego setup a bench test to compare the 5550K 93 CRI Sunwave T8 Plus lamps to the 5000K 82 CRI Phillips 25 watt T8 lamps. The preliminary results are summarized in San Diego Lamp Test Report.PDF
I am surprised that the #2 and #3 samples were not running exactly the same wattage. #2 should have a higher photopic number compared to the #3 Sunwave. I suspect that #2 needs to burn in longer and that the wattage will rise to match #3 But the Light output will also increase. When I have tested #2 and #3 lamps side by side with the same ballast, they draw the same wattage. I have also tested the Glow Lux 5000K T8 Plus lamp and as it shares the same lamp tube and cathode design, runs at the same 24 +/- watts per lamp on a NEMA Premium Sylvania QHE Normal Ballast. The Philips XEW Alto lamp will run lower, but lower means less light output. At some point with a low ballast factor ballast each 25 watt lamp runs at 17 watts. In this case, you could run one Sunwave lamp at 34 watts and have the same light.
The angle of the test board and the position of the light meter needs to be the same in each test. the position from the wall is also important.
From the data presented, dividing the Scotopic by the Photopic gives the S/P Ratio.
Phillips XEW lamp 5000K 82 CRI S/P ratio is 1.68
5000K AEP NEC HGX 5000K 86 CRI S/P ratio is 1.79
Sunwave 5550K 93 CRI S/P ratio is 2.1
Philips XEW lamp (90 / 4) is running at 22.50 Watts per lamp
AEP NEC HGX lamp (95 / 4) is running at 23.75 Watts per lamp
Sunwave 5550K 93 CRI FS lamp (99.5 / 4) is running at 24.88 Watts per lamp.
- more -
Let the new lamps burn in for awhile (testing labs use 100 hours) for more meaningful and accurate results.
Burn in new lamps and ballasts first. Cycle them on and off a few times. Then let the lamps warm up for at least 20 minutes before testing. Then turn everything off swap the test lamps to the other fixtures. Then turn it all back on wait 20 minutes and measure again. If there are 3 sets of test fixtures, swap lamps again and test. Then compare the average readings for the Sunwave lamps to the average readings for the Philips lamps. Is there any difference in the measurements between the 3 sets of test fixtures? Testing in each fixture helps to cancel out any variation in ballasts, fixtures, kill-a-watt meters and light meter positioning. Its not a big enough sample of lamps and fixtures for statistical significance, but your results for this test will be more reliable and more accurate.
Another test is to install lamps them in a couple of fixtures and measure light at the work surface directly under the fixtures.
Light drops off with the square of the distance and the distance is typically 8 or 9 foot ceiling less desk height of 2.5 feet or 5.5 to 6.5 feet. You don't get watts, but you do get foot candles on the desk top. In this way you can double check that IES minimums are met and you will have tried it in more than one fixture. The IES minimimums don't take a scotopic correction factor into account, but the minimums have been droping as we change from lighting for predominately paper tasks to lighting now primarily for computer tasks. Saving energy, saving carbon and improving the health or "greening" our buildings is becoming more and more important.
If something doesn't meet the minimums, than at least you can state what the minimums are and explain why using a scotopic correction factor may in fact be ok for saving 15% or more energy, with no compromise to visual acquity and comfort or performance. There may even be performance enhancements if biological and ergonomic effects of blue daylight light wavelengths are realized. There are numerous studies that suggest that natural daylight and better artificial daylight are related to health and wellness. Many have commented that it is easier to see without their glasses or that they feel less tired at the end of the day.
Repeat the test by swapping lamps into the other fixtures. In this way the operating temperature will be correct as the ballasts lamps and reflectors are all in their correct operating postions. This is important as heat rises and often some heat is trapped in the fixture raising the operating temperature. (and the light output) This is another reason why I like to wait for at least 20 minutes for everything to heat up to operating temperatures)
Testing the lamps in the actual fixtures where they are to be used provides additional useful information.
One can measure the foot candles before, then replace lamps with burned in test lamps. Let everything warm up for 20 minutes and walk around with a light meter. You will also want to look at color photos and spreadsheets with a lot of fine print and just see what you see.
The amount of light that is reflected in one direction (typically down to the worksurface) off of a bare bulb 2 lamp strip can vary a lot depending on the reflectance of the wall surfaces and other light sources in the space. Proximity to a light colored wall can effect the results. Proximity to a window will affect the results. The reflectance and efficiency of the fixture is important in directing or aiming the light. A bare bulb 2 lamp strip with no reflector sends out light in a lot of different directions. Measure the light at the work surface and you get a reading of "X footcandles." The light that exits horizontally from the lamp needs to bounce back off a wall or ceiling in order to be measured as part of X. Add a specular (shiny metallic 95% or even a clean white 89%) reflector to the fixture, and much of the light that was directed upward and horizontally from the top and both sides of the tubes is now directed downward. Now the light meter reads 2(X) to 3(X) footcandles on the work surface.
Fixture efficiency is never 100% as some light never makes it out of a given fixture. For the 2 lamp strip, the light that refects off the other lamp and off the ballast cover back into the lamp does not get out of the fixture and is lost. It's generally nicest to have both direct light (as in task ambient) and indirect light as in light that bounces off of walls and ceilings so that there is no glare in modern computer tasks driven workplaces. 30 foot candles is a lot when the computer monitor is putting out another 10 foot candles. Light off the ceiling makes the room larger. Light bouncing off the walls evens out light vs dark contrasts in the space.
The Sunwave light is more like daylight in color. The light is transparent to daylight designs. 5000K 82 CRI is good, but 5550K 93CRI is better. We have used it with photosensor controls as built in to Axis Daylight Harvesting Ballasts so that the fixtures near windows or skylights automatically adjust down when there is sufficient natural light automatically saving even more electricity.
Its particularly pleasing to bounce white light off of a white ceiling. One can then supplement with CFL or LED task lighting that is at least 4100 to 5000K. Remove all 3500K lamps in the test areas as possible. It's often nice to solicit volunteers for a prototype retrofit. In this way people actually get to experience working under the light for a few weeks. Prototyping can yeild big dividends in confirming the validy of the plan, and in catching and correcting problems in the proposed implementation and in tuning light levels for a final installation.
Presenting an average of the Scotopic and Photopic readings does not yield useful information.
A higher Scotopic reading means that there is more light in the scotopic region which is light seen by the rods and is related to pupil response and visual acquity. The eye sees the light, sees more blue in the light and overall the light is perceived to be brighter. For the Sunwave, a calculation for a relative brightness factor or Visually Effective (or Scotopically Corrected) Lumens results in a 1.8 factor for visual brightness. That is it is 1.8 times as bright as the same lamp in a 3500K warm white color.
So, initial lumens depreciate maybe 5% at 40% of expected life. Initial lumens is operation at 32 watts. The light output is reduced if they are operating at less than rated wattage which is controlled by the Ballast factor. A ballast factor of 1.0 would run the lamp at 32 watts. A 0.88 ballast factor reduces light output from the rated lumens by 22%. A low 0,77 ballast factor reduces it by 33%. Light output is reduced if the systems operates at temperature colder than optimum. Lamps and fixtures get dirty, plastic lenses get yellow and so on. Maintenance is deferred. Lamps turn pink when their mercury is exhausted. All the things that collectively work to reduce light output are called Light Loss Factors.
Measuring the CRI (with the solar light meter) would be useful information.
(I believe it can do this.)
I would be curious to find out if the 82 CRI for the Philips and the 86 CRI for the NEC HGX (aka Super ECO T8 950 Plus), and the 93 CRI for the is Sunwave T8 955 Plus lamps are conservative, accurate +/- some %, or exaggerated. I see a lot of difference between an 82 CRI lamp and a 93 CRI lamp. There is one Phillips lamp listed with a 98 CRI. F32 T8 TL950. lt requires a rapid start preheat ballast. It is rated for 20,000 hours (not 24,000) and it only produces 2000 initial lumens. I don't see any other >90 CRI >5000K commercial lamps listed in the Philips catalog. There are some T12's listed for consumer use with 92 CRI 5000K, but T12's are going away.
Ambient and lamp system operating temperature are important considerations.
Note the temperature in the lamp test area, and then again close to the lamps after they have warmed up for 20 minutes. The T8 plus lamps like to run at 25 degrees C (77 degrees F). This is room temperature. The 25-watt Philips lamps like to run at 30 degrees C which is (86 degrees F) which is too hot for me and would necessitate AC. These temperatures are from a graph on page 95 of the 2009 Philips Lighting Catalog. T5 lamps are popular of late. We have T5's with Sunwave phosphors, however, these lamps also like to operate at 35 degrees C which is 95 degrees F. It may be 95 up at the ceiling, but that is hot. Cool them down with AC and the light levels drop off significantly,
It would be useful to know the Average Age of the City's existing ballasts.
Ballasts can last 15 to 20 years. If ballasts are 8 to 10 years old, does it make any sense to scrap them only part way through their useful life. From a Life Cycle Cost perspective considering the embodied energy in the existing ballasts, the cost and carbon embodied in the new ballasts with shipping, and the labor to swap them out, using a more efficient lamp, with a longer life, with high output, high CRI, Spectrally Enhanced lamp makes a lot of sense. It's brighter to the eye, but uses less energy to achieve comparable brightness. User acceptance studies and economics validation studies have been done in California by DOE EERE FEMP and the University of California.
Are there areas that are past due for a general relamping? Have you noticed any pink lamps? Are some lamps burned out or twisted out by the occupants. The business case is easiest to make where some past due maintenance can be avoided. One needs to change out fully depreciated and dirty lamps, wipe down the fixtures and replace bad ballasts anyway so one can calculate the ROI on the incremental investment in more efficient lamps. Avoided maintenance costs are often overlooked in calculating return on investment. If it is true as the Lamp Engineering Report Concludes, that the Sunwave Lamps will last longer on average a three or four year relamping rotation might be replaced with a four or five year cycle which would significantly impact life cycle costs.
One should note that most 2 lamp ballasts are also rated to overdrive 1 lamp. In this way a 2-lamp fixture running at 58 watts in a hallway or corridor could become a 1 lamp Sunwave running at 30 - 32 watts. Sometimes one can simply remove a lamp. but clipping and capping one leg on an existing ballast and removing the tombstones prevents snap back. A three lamp ballast can often be used to over drive 2 lamps slightly for increased light output over a 2 lamp ballast from the same ballast family. When one lamp burns out, the remaining 2 are over driven (run hotter and brighter) to compensate for the missing lamp. the two remaining lamps now work harder and burn out faster and ultimately the burned out lamps get replaced. The most notible exception are the GE Ultramax ballasts. A four lamp ballast can drive 1 to 4 lamps without overdriving the remaining lamps.
For fixtures that are wired for Bi level switching, the 2 lamp ballast can be used to drive one lamp at 30 watts. the single lamp ballast can drive the other one at 24 watts, and you end up with a 3 way 24/30/54 watt system using only 2 lamps.
Are the ballasts in the test fixtures representative of the ballasts in the rest of the fixtures or are they new?
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Last Updated ( Jan 14, 2010 at 03:23 PM )
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