The brightness-producing capability of a source

e is determined by its luminous flux, which measures the perceived power of light.

The human eye is not equally sensitive to all wavelengths of light. The sensitivity is maximum for a wavelength of $555 \text{ nm}$ (yellow-green region) and decreases for other wavelengths. Thus, the brightness depends on the emitted wavelength.

Additionally, for a specific wavelength, a source emitting more radiant energy per unit time (radiant power) will produce a greater visual sensation. Thus, the brightness also depends on the power of the source.

Therefore, the brightness-producing capability is dependent on both the emitted wavelength and its power.