The H4 bulb is commonly used for both low beam and high beam headlights in vehicles. However, in the context of identifying which specific H4 bulb is designated for the low beam, it is the H4L bulb that is typically used for low beam illumination. This distinction is important because the low beam is designed to provide sufficient light for driving conditions without blinding oncoming traffic. The H4L bulb is engineered to emit light at a lower intensity, which is crucial for safety and legal compliance.

The reason for this distinction lies in the design of the bulb and its filament. According to data from the Society of Automotive Engineers (SAE), the H4L bulb has a filament that is positioned differently within the bulb compared to the standard H4 bulb. This positioning results in a more focused beam pattern that is suitable for low beam use. The SAE standards specify that low beams should have a beam pattern that is lower and wider, which helps to minimize glare for other drivers.

Additionally, the National Highway Traffic Safety Administration (NHTSA) in the United States mandates specific lighting requirements for vehicles, including the use of low and high beams for different driving conditions. The H4L bulb meets these requirements by providing a low beam that is compliant with safety regulations. The NHTSA's guidelines are based on extensive research and testing to ensure that vehicle lighting does not pose a risk to public safety.

In summary, the H4L bulb is the low beam variant of the H4 bulb, designed to meet safety standards and provide a focused beam pattern suitable for low beam illumination. This distinction is crucial for ensuring that vehicles comply with legal requirements and maintain safe driving conditions.