Base
The H4 lamp uses either a P43t base or p45t base in accord with IEC 60061.
Power and output
Under ECE Regulation 37, which governs automotive filament lamps in most of the world, the H4 lamp's nominal rating is 55 watts at 12 volts, and its test rating is 68W (maximum) and 1550 ±15% lumens at 13.2V. It also contains provisions for 24-volt, 70-watt H4 lamps.
The United States government does not recognise ECE regulations, but rather applies its own regulations.
Despite the two different sets of specified voltages and test protocols, the lamps themselves are the same; with few exceptions, 12-volt H4 lamps are generally manufactured and tested to comply with both regulations.
Colour of light
Under ECE regulations, H4 lamps are required to emit white or selective yellow light. U.S. regulations require H4 lamps to emit white light. Under both ECE and U.S. specifications, the allowable range of white light is quite large; some H4 lamps have a slight blue or yellow tint to the glass yet still produce light legally acceptable under the requirement for white light.
