Short answer
Discharge lamps make light from excited gas atoms, which emit at a few fixed wavelengths rather than across the spectrum. Fluorescent tubes convert mercury's ultraviolet line with phosphors but keep visible mercury spikes; low-pressure sodium emits almost only the yellow D lines near 589 nm, so it renders no hue at all. The spikier the spectrum, the more colours depend on whether their reflectance happens to line up with it.
A fluorescent tube is a low-pressure mercury discharge. Its strongest emission is ultraviolet, at 253.7 nm, which the eye cannot see; a phosphor coating on the glass absorbs it and re-emits visible light. Mercury also emits directly in the visible, and those lines pass through the phosphor largely untouched: violet at 404.7 nm, blue at 435.8 nm, green at 546.1 nm and a yellow pair at about 577 and 579 nm. Every fluorescent spectrum is therefore a phosphor curve with narrow mercury spikes standing on it. The green line in particular sits close to the peak of daytime sensitivity, which helps efficacy and is one reason photographs under fluorescent light often carry a green cast.
Older 'cool white' tubes, represented by the CIE's FL2, used a single broad halophosphate phosphor: efficient, with a broad hump but weak red, so they rendered skin and red objects poorly. Tri-phosphor tubes, represented by FL11, use three narrow rare-earth phosphors placed near the blue, green and orange-red. They look brighter and render most everyday colours better, but their power is bunched into three peaks. On the CIE data sampled at 10 nm, FL11 puts about 60% of its power between 600 and 730 nm yet only around 5% beyond 640 nm — nearly all of it in a narrow band near 610 nm. A deep red that reflects mainly above 640 nm has little to work with, even though the lamp looks warm — whereas a red whose reflectance rises steeply near 610 nm is pushed into extra saturation, as the calculation on the R9 page shows.
Low-pressure sodium lamps, once common on roads, emit almost entirely in the sodium D lines at 588.995 and 589.592 nm. With effectively one wavelength, any surface can only reflect more or less of that one yellow, so every object is a shade of the same hue and colour discrimination vanishes: a red and a green car that happen to reflect equally at 589 nm look identical. High-pressure sodium raises the vapour pressure until the lines broaden into a band across yellow, orange and red; some hue returns, but blues and greens receive very little light and look dark. The site's night-time street lighting page follows how both are being replaced.
Two surfaces that match under daylight match because their reflectances integrate to the same three cone signals across a smooth spectrum. A line spectrum samples each reflectance at a few points instead. If one surface has a small bump or dip at 546 nm and the other does not, the green mercury line amplifies that difference into a visible mismatch. This is why colour-matching industries check samples under a fluorescent source as well as daylight and incandescent, and why the CIE's method for grading daylight simulators uses metameric pairs: a good simulator must not reveal differences that daylight would hide. It is also why a single index such as Ra says so little about line sources.
Linear fluorescent tubes remain in many offices, schools and older shops, and compact fluorescents in homes that have not yet switched to LED. Sodium is still found on older roads and in industrial yards. Anyone photographing, specifying or matching colour in those spaces is working under line spectra, and should expect camera white balance to leave a residual green or orange cast that a single correction cannot remove, because the error varies with each surface's reflectance rather than being a uniform tint.
| Element | Wavelength (nm, air) | Region | Role in lamps |
|---|---|---|---|
| Mercury | 253.7 | Ultraviolet | Main output of a fluorescent discharge; excites the phosphor |
| Mercury | 404.7 | Violet | Visible spike in fluorescent spectra |
| Mercury | 435.8 | Blue | Visible spike in fluorescent spectra |
| Mercury | 546.1 | Green | Strong visible spike near peak eye sensitivity |
| Mercury | 577.0 and 579.1 | Yellow | Weaker visible pair |
| Sodium | 589.0 and 589.6 | Yellow | Almost the entire output of low-pressure sodium |
Why: Office tri-phosphor fluorescent light samples the paint's reflectance at three peaks; home lighting does not.
Fix: Approve under daylight or a high-fidelity source as well, and check under the home's own lamps.
Why: The error from a line spectrum depends on each surface's reflectance, not a uniform cast.
Fix: Replace or supplement the light for colour-critical shots; correction software only approximates.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Neutral mercury's strongest lines include 253.7 nm in the ultraviolet and visible lines at 404.7, 435.8 and 546.1 nm, with a weaker yellow pair near 577 and 579 nm.
Source: NIST Handbook of Basic Atomic Spectroscopic Data: strong lines of neutral mercury
FactStrong evidence
The sodium D lines lie at 588.995 and 589.592 nm (air wavelengths), which is why low-pressure sodium light is effectively monochromatic yellow.
Source: NIST Handbook of Basic Atomic Spectroscopic Data: strong lines of neutral sodium
StandardStrong evidence
The CIE grades daylight simulators with a metamerism index computed from pairs of samples that match under the reference daylight, adding an ultraviolet-range index for fluorescent samples.
Source: CIE 051.2-1999 A Method for Assessing the Quality of Daylight Simulators for Colorimetry
Colourwise analysisModerate evidence
Averaged over 10 nm bands, the CIE tri-phosphor illuminant FL11 puts about a third of its 380–730 nm power between 600 and 730 nm, concentrated in its narrow red phosphor band near 610 nm, and only about 7% beyond 640 nm, against about 23% for daylight D65.
Based on: Calculated by Colourwise from the CIE FL11 and D65 tables, reduced to 10 nm band means.
Caveat: At 10 nm the narrow phosphor peaks are sampled coarsely, so band shares are approximate.
Source: CIE datasets (colour-matching functions, illuminants)
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.