  Correlated color temperature (CCT) is a semi-quantitative system that roughly
describes the spectra of lamps. This parameter gives the temperature (measured
in kelvins) of the black body that would show the hue more similar to that of
the light emitted by the lamp. Modern lamps for indoor and outdoor lighting
display many spectral energy distributions, most of them extremely different to
those of black bodies, what makes CCT to be far from a perfect descriptor from
the physical point of view. The spectral index system presented in this work
provides an accurate, objective, quantitative procedure to characterize the
spectral properties of lamps, with just a few numbers. The system is an
adaptation to lighting technology of the classical procedures of multi-band
astronomical photometry with wide and intermediate-band filters. We describe
the basic concepts and we apply the system to a representative set of lamps of
many kinds. The results lead to interesting, sometimes surprising conclusions.
The spectral index system is extremely easy to implement from the spectral data
that are routinely measured at laboratories. Thus, including this kind of
computations in the standard protocols for the certification of lamps will be
really straightforward, and will enrich the technical description of lighting
devices.
