sion, rare-earth phosphors, producing CRIs
of 70 to 78.RE-80lamps use a thick coat of
the narrow-emission, rare-earth phosphors,
producing CRIs of 80 to 86.
Quad-phosphor RE-90 lamps do not
use the narrow-emission phosphors; they
contain four wider-emission phosphors that
produce CRIs of 95 at 3000 K and 98 at
5000 K (color plates 22 and 23). RE-90
rare-earth lamps with CRIs of 95 to 98 are
the highest color-rendering fluorescent
lamps available.
High-Intensity Discharge (HID)
Sources
As with fluorescent lamps, high-intensity dis-
charge (HID) lamps produce a discontinuous
spectrum. The different metals in the arc of
the various HID sources yield different color-
rendering abilities.
If you were to throw salt on a barbecue,
the sodium chloride would make the flames
appear yellow. Similarly, the sodium in high-
pressure sodium lamps makes their color
appear yellow. If you were to throw mercury
on a barbecue, although this is not recom-
mended because it would cause mercury
poisoning, the mercury would make the
flames appear blue. Similarly, the mercury in
mercury vapor lamps makes their color
appear blue.
High-pressure sodium lamps produce
predominantly yellow light at 2100 K, which
creates a shift in how we perceive almost all
observed colors. Reds, greens, blues, and
violets are muted (color plate 24). High-pres-
sure sodium lamps have CRIs of 21 and 22.
By further increasing the gas pressure
inside the lamp,white high-pressure sodium
lamps produce incandescent-like color at
2700 K with good color-rendering properties
and a CRI of 85 (color plate 25).
Low-pressure sodiumlamps emit all vis-
ible energy at 589 nm, which means that
they render only materials that reflect light at
that wavelength. All other colors appear gray
(color plate 26). Low-pressure sodium
lamps are assigned a CRI of 0.
The clear mercury vapor lamp produces
a cool, “white” light of predominantly blue
and green energy. The lack of energy at the
warm (red) end of the spectrum results in
poor color rendering; people appear ghastly.
Clear mercury vapor lamps exhibit particu-
larly poor rendering of red. Rendering of
other colors is fair, but blues appear purplish
(color plate 27). Clear mercury vapor lamps
have CRIs between 15 and 20.
Applying a phosphor coating to the
inside surface of a mercury lamp’s outer
bulb slightly improves the color-rendering
properties, but also reduces its efficacy. The
phosphors convert invisible ultraviolet
energy into visible light (color plate 28).
Phosphor-coated mercury vapor lamps have
CRIs of 45 to 50.
Metal halide lamps are similar in con-
struction to mercury vapor lamps, except
that various metal halides have been added.
These halides add missing wavelengths that
improve the mercury lamp’s spectral distri-
bution, yielding a more uniform spectrum
and better color rendering, but reds are
slightly muted (color plate 29). The addition
of phosphor coatings on the inside of the
bulb provides diffusion and some additional
color improvement (color plate 30). A broad
range of color-rendering quality exists in the
different metal halide lamps; many experi-
ence lamp-to-lamp color inconsistency and
color shift over the lamp life. The majority of
metal halide lamps have CRIs of 65 to 70.
Newceramic metal halidelamps com-
bine the ceramic arc tube technology of
high-pressure sodium lamps with existing
metal halide chemistry. The ceramic arc
tube minimizes color variation between
lamps and limits color shift during lamp life.COLOR