Fluorescents-5 Color of Light

This is the 5th post on fluorescent lighting, and covers short issues that can be examined fairly quickly.  Originally, we were to have 2 issues here, but the second will be a full post of its own.

Light Color

click for list of our general discussions (in PDF) on lighting and visual response

People have complained about fluorescent lighting for about as long as I can remember. The dissatisfaction grows from how the visible light is generated and how the eye interprets the light it receives.

Techniques have made certain fluorescent lighting choices quite acceptable, even very nice.  The final answer is that an FL lighting will never be able to precisely match incandescent, but has its own unique lighting shades.


Spectrum for Incandescent bulb is like blackbody at 2700 K

ILB lighting is generated by heating the filament.  The graph shows the color spectrum emitted by a hot object  (blackbody) at a temperature of 2700 K. As shown, incandescent spectra change smoothly across the wavelengths and are similar to to black body emission (with some wiggles due to the filament).

The emission for an object at 2700 K grows from nearly zero in deep blue (400 nm), brighter at red (650) and peaks at 1000 nm, in the near infrared..


Spectrum of a phosphor. Rated 3500 K, Dotted BB at 4300 K

FL tubes generate visible light when the UV from its mercury is converted to colors by its phosphor layer.  The phosphor layer has a color temperature rating of 3500 K. The spectrum for blackbody at 4300 K is shown by the dotted line.

Phosphors emit visible light in discrete color bands (“lines”).  For this phosphor, orange (about 540 nm) is the highest intensity with peaks of lower intensity at shorter wavelengths, down to violet (400 nm).  The amplitudes of the spikes -sort of-  follows the continuous amplitudes of a spectrum generated by a heated object, (see next paragraph), but this particular phosphor is missing in the red, violet is a bit too small, and there is little emission between dominant color peaks.  The brightest intensity for a matching heated object (blackbody) would be deep red.   It would give the feeling of warm light with reddish overtones.  Our FL lamp would be more green with very little warm red.

Color temperature refers to the smooth spectrum of an object in thermal equilibrium. For the fluorescents which emit in discrete lines, it is terribly misleading.

The only possible reason we can accept fluorescent lighting is that our eyes do not see discrete wavelengths to way our ears hear sound wavelengths.  (With sound, we normally talk about its frequency rather than the related wavelength.)  What we see as rich vivid colors is what we would hear if our hearing perceived only a single chord of the same 3 notes, varying only in the relative intensities of those notes (the 3 visual “notes” would be blue, green and red). This is discussed in the Fluorescents-1 discussion of color temperature.

Color evaluation under FL lights can be confusing.  Attaching a “Temperature Color” value means only that the amplitudes of the dominant spikes tend to follow the curve of a continuous blackbody spectrum.  The temperature of this FL spectrum is rated at 3500 K.  You could find another FL  bulb rated at 3500 K whose spikes are as close to the amplitude of the blackbody curve, but have may have very high red lines, miss the green, might have another line in the blue.  Your eye would see big differences in the two phosphor net colors. Color temperature numbers for a  fluorescent light are an imperfect rating.

Color Rendering Index (CRI)   ALL attempts to compare spiky fluorescent spectra with smooth heat-generated light must be faulty because FL spectra are not smooth!  The CRI is another way to compare FL bulbs to incandescents using a scale from 0 (poor) to 100 (exact).  It is a good idea to have house lamps with CRI between 90 and 100, for human comfort.  This is another flawed standard, but it is the only one accepted internationally, and it will be used until a better one has been developed.

Good color, as with tasty food, depends on what you are used to. Here is a different explanation.  We may be used to incandescent colors, but will have to accept the spiky spectra as real and change our tastes.  Laws throughout the world are mandating the move from presumably energy expensive incandescent lighting.    Learn to change what you can but accept and be happy with the inevitable.


click for a list our posts on illumination technology

Color labels for fluorescent lights is imperfect. Most people are more comfortable with red tints in the lighting used about the house.  Because of the difference between continuous spectra and spiky spectra,  there will never be a good comparison between visible light emitted by a heated object and visible light emitted by a phosphor screen.  They are fundamentally different processes.

Is one color better or worse than the other?  No, they cannot be compared, you just have to acclimate yourself to the new lighting.  Fluorescents are hard to match between brands because each one has its own phosphor screen compounding.  Too bad you cannot specify replacement lamps by their phosphor. The phosphor screen is the source of lumen depreciation that dims all fluorescents, as pointed out in the last post.

You get a draw if you use color to compare fluorescent and incandescent lights.  Next time, we will look at environmental concerns,  mercury and other heavy metals that can leach out of the tube and into the outside world.  That may help more.

Charles J. Armentrout, Ann Arbor
2011 Aug 14
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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