Fluorescents-3: Not as bright as you think

Compact fluorescent lights (CFLs) are mandated for the US market starting in January 2012.   These bulbs certainly have good press;  they are reputed to do wonderful things, nice green attributes.   Here, we discuss probably the most serious problem with the CFL mandate:   how brightness wears out for CFL bulbs.

Fluorescent lights lose brightness.

For her garden, my wife plants seeds in our basement. She uses fluorescent tubes and the seedlings usually grow really well.  The problem she noticed was that every 3rd or 4th year the seedlings would have real problems – they developed symptoms of light deprivation, though bathed in light.  They died.

That is how I learned that each four foot lamp had to be replaced every 3 years.  I notice that the post-change light is much brighter than the pre-change.  Let’s see: 10 hours/day from February to May, 15 weeks × 7  days × 10 hours × 3 years.  Initially, these tubes were bright enough to keep the plants alive and healthy.  They lost the brightness (their initial lumen value dropped to unusable levels) after no more than  3500 hours.   The tubes were rated for 12 000 hours.

Misconception #1 — Fluorescents are good for their rated lifetime. Turns out, lifetime means the time from first turn on until 50% have broken due to mechanical or electrical faulures.  That is, before their “rated lifetime,” half will have failed.  It certainly does not mean things will live under them for that time.  They get dimmer and dimmer as they age.

Lamp Lumen Depreciation (LLD )    All lamps dim as they age.


LLD for incandescent lapms

Incandescents  lose intensity because the tungsten filament boils away and plates the inner surface of the bulb, becoming a light barrier.  Data source here.  Tungsten halide bulbs stop this.  Fill the bulb with inert gas such as xenon (Xe) and add small concentration of iodine (I) or bromine (Br). The gas stops the tungsten wall coating and redeposits the atoms back onto the filament, and adds to lifetime.

Tungsten halide bulbs show very little lumen depreciation!  Both types of incandescent bulbs are usable up to mechanical failure.


LLD for CFL bulbs

Fluorescents  lose intensity mostly because of the chemical changes in the phosphor coating on the inside walls. The absorption of the Hg ultraviolet emission and re-emission of visible light is due to the molecular structure of the coating, which will become disrupted with addition of stimulus energy.  (I personally ran into this when I developed special imaging equipment.)  Data for CFL bulbs are proprietary and hard to find; this is “typical” data published by the U.S. DOE. Click to read how phosphors work

Long straight FL tubes used in business have LLDs  are better than this figure for CFL bulbs.  But recall that my seedling lamps used FL tubes, not CFL bulbs and had become unacceptable after 3500 hours.  The ones that with the 2012 mandate are the CFLs with the unpleasant lumen depreciation.   I suspect the the problems are related to the narrow bore of the CFL tube.


Dust causes additional lumen loss

Dirt Accumulation 
All lamps dim as dirt builds up.

Dust settling on the bulb will cause dimming in addition to LLD effects.

This graph is from data, but the actual light dimming for your bulb will depend on the dust/dirt in the ambient air.  Multiply the LLD for your bulb  by the dimming due to dirt (0.84 after 48 months of not being cleaned).  This is the total light loss you will experience.

Ambient Temperature Effects 
CFL brightness declines at high and low temperatures.


Lumens and ambient temp

All FL bulbs have their maximum brightness at a specific temperature.  The curve of Lumens vs Temperature varies a lot from type to type; some bulbs are designed to run at cold temperatures, some at high temperatures.

The fall-off with temperature is pretty rapid.  Suppose the bulb is brightest at 25º C (77º F) as in the graph.  At 50º F (10º C), the brightness will be about 20 % less than the optimal temperature.  This will be true for brand new bulbs and ones near the end of their emission lifetime.

Cold tubes that operate below freezing (0º C) usually peak at or below 20º C and and are below rated intensity at 30º C.  All bulbs I have been able to find might work at very cold temperatures, but are well below peak emission and not at all bright.  There are tubes that are bright above 40º C (about 100º F) and these will be unsatisfactory at cold temperatures, may not even turn on.

These cold ambient and hot ambient tubes use special electronic tricks to make the light work, usually inductive pulsing or rapid switching, both of which will shorten the bulb mechanical lifetime from that quoted for room temperature bulbs.

In the future, we must have many different replacement stocks for the lights.  One pile for room temperature CFLs, another for Attic CFLs that work at high and low temperatures,  and yet another stock of outside CFLs that have to work in very cold temperatures.  Double this because in many climates, your outdoor fluorescents must be stocked in separate winter and summer groupings.  I predict that this change-out will become a pain in the neck.

A side effect of this temperature dependence is the orientation of the bulb with regard to its base.  The electronic ballast is in the base and degrades or misfunctions  at high temperatures, so you must have all the stockpiles above divided into at least 2 more categories:  bulbs put into that are screwed upright into normal lamps, and bulbs that are screwed into ceiling lights that have their bases above the light or horizontal and are in sconces.  There will be a lot of money tied into stock to keep the house/apartment running.  In the U.S., this puts a new meaning to CFLs as a “green” issue.


click for a list of LastTechAge posts on illumination technology

We must convert to CFLs by this coming January.  This is a done deal.  We will probably find we pay a bit more for power (around the house) but the real issue will be failure due to localized heating about the bulb, heating outside specified ranges. A second frustration will be the incessant dimming of each bulb used.  Did you count up the many categories  you will have to consider when you select bulbs for the different the locations around you?

Incandescents are the clear winner in the lumen depreciation face-off.  Tungsten-halogen bulbs have very little brightness fall and lifetimes close to CFL bulbs.  They also work well in almost all environmental conditions.

CFLs are easily hyped, will they be comfortable to live with?  Or have we discarded an important technology in favor of a Bumper-Sticker Green one?

Charles J. Armentrout, Ann Arbor
2011 Aug 9
This is listed under     Technology    …thread   Technology > Fluorescents
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About LastTechAge

I am a physicist with years of work in fusion labs, industry labs, and teaching (physics and math). I have watched the tech scene, watched societal trends and am alarmed. My interest is to help us all improve or maintain that which we worked so hard to achieve.
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