The requirements for a spectrophotometer light source include:
a) bright across a wide wavelength range;
b) stable over time;
c) long service life; and
d) low cost.
"a) Bright across a wide wavelength range" demands both a high degree of brightness and uniform brightness across the measurement wavelength range (uniform brightness distribution).
"A high degree of brightness" is a necessary requirement to obtain photometric values with a high S/N ratio. However, as increasing the brightness of the light source generally results in a reduction in condition "c) long service life," the balance between the two must be considered.
As the sensitivity characteristics of the detector are also relevant during actual measurements, a "uniform brightness distribution" is an important condition for acquiring photometric values with a uniform S/N ratio within the measurement wavelength range.