14 CFR § 121.325 - Instruments and equipment for operations under IFR or over-the-top.
No person may operate an airplane under IFR or over-the-top conditions under this part unless it is equipped with the following instruments and equipment, in addition to those required by §§ 121.305 through 121.321 and 121.803:
(a) An airspeed indicating system with heated pitot tube or equivalent means for preventing malfunctioning due to icing.
(b) A sensitive altimeter.
(c) Instrument lights providing enough light to make each required instrument, switch, or similar instrument, easily readable and so installed that the direct rays are shielded from the flight crewmembers' eyes and that no objectionable reflections are visible to them, and a means of controlling the intensity of illumination unless it is shown that nondimming instrument lights are satisfactory.