Naturally there are pros and cons to
each of the schemes. The advantage to
using a blue LED and a yellow phosphor
is its high theoretical efficacy, which is
attractive for the creation of a cheap, bright
white-light source. However, this benefit
comes at the expense of a lower value
for the colour-rendering index (CRI; see
Box 1 for definition), which is typically so
low that such devices are undesirable for
indoor use. Ultraviolet LEDs with phosphor
mixtures provide a better CRI value and
are suitable for indoor applications but at
the expense of poorer efficacy. To control
white light dynamically, the third approach,
a combination of three (or more) LEDs
of different wavelengths is attractive,
and may lead to higher efficacies than
the ultraviolet–phosphor LEDs, but will
generally be the most expensive option until
further advances are made. The historic
development of luminous efficacy for
the most common white-light sources is
shown in Fig. 3.
Naturally there are pros and cons to
each of the schemes. The advantage to
using a blue LED and a yellow phosphor
is its high theoretical efficacy, which is
attractive for the creation of a cheap, bright
white-light source. However, this benefit
comes at the expense of a lower value
for the colour-rendering index (CRI; see
Box 1 for definition), which is typically so
low that such devices are undesirable for
indoor use. Ultraviolet LEDs with phosphor
mixtures provide a better CRI value and
are suitable for indoor applications but at
the expense of poorer efficacy. To control
white light dynamically, the third approach,
a combination of three (or more) LEDs
of different wavelengths is attractive,
and may lead to higher efficacies than
the ultraviolet–phosphor LEDs, but will
generally be the most expensive option until
further advances are made. The historic
development of luminous efficacy for
the most common white-light sources is
shown in Fig. 3.
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