8.6 Types of Lighting: High-intensity Discharge

High-Intensity Discharge (HID) Lamps

High-intensity discharge (HID) lamps are a type of electrical gas-discharge lamp that produces light by creating an electric arc between two tungsten electrodes housed inside a transparent quartz or ceramic arc tube. The arc tube is filled with gas (typically xenon or argon) and metal salts.

How HID Lamps Work

HID lamps operate similarly to fluorescent lamps in that both generate light using an electric arc between electrodes. However, there are key differences:

• Arc length: HID lamps have a much shorter, more compact arc
• Light output: HID lamps produce significantly more light, heat, and pressure within the arc tube
• Efficiency: HID lamps are generally more efficient than fluorescent lamps, producing more lumens per watt

When the lamp is turned on, the ballast provides a high-voltage pulse to initiate the arc. As the lamp operates, the metal salts inside the arc tube vaporize and become part of the plasma, producing intense light.

Types of HID Lamps

Below are the main HID lamp types, listed in increasing order of efficacy (lumens per watt):

1. Mercury Vapor (35–65 lm/W)
   • The oldest HID technology
   • Bluish-green light with poor color rendering
   • Common in older streetlights and industrial facilities
   • Being phased out in many areas due to low efficiency
2. Metal Halide (75–100 lm/W)
   • Excellent color rendering and bright white light
   • Widely used in stadiums, gymnasiums, retail spaces, and parking lots
   • Contains rare earth metals that produce a full spectrum of light
3. High-Pressure Sodium (HPS) (85–150 lm/W)
   • Golden-yellow light
   • Very efficient with long lamp life (up to 24,000 hours)
   • Common in street lighting, warehouses, and outdoor security lighting
   • Poor color rendering (makes colors appear brownish or gray)

Note: Low-pressure sodium (LPS) lamps are sometimes mentioned alongside HID lamps due to their high efficiency (100–200 lm/W), but they are technically a different category of discharge lamp. LPS lamps produce monochromatic yellow light and are primarily used in areas where color recognition is not important, such as highway lighting.

Ballasts and Operation

Like fluorescent lamps, HID lamps require ballasts to:

• Regulate current flow through the lamp
• Provide the high voltage needed to start the arc
• Prevent the lamp from drawing excessive current once operating

Important operational characteristics:

• Warm-up time: HID lamps take 3–5 minutes to reach full brightness when first turned on because the ballast needs time to establish the arc and vaporize the metal salts
• Restrike time: If an HID lamp is turned off while hot, it must cool down (5–15 minutes) before it can restart. This is a significant limitation for applications requiring instant on/off capability.
• Position sensitivity: Some HID lamps must be operated in specific orientations (base up, base down, or horizontal) for optimal performance and lifespan

Advantages and Disadvantages

Advantages:

• High luminous efficacy (more light per watt than incandescent or halogen)
• Long service life (10,000–24,000 hours)
• Compact size relative to light output
• Good for high-ceiling and outdoor applications

Disadvantages:

• Long warm-up and restrike times
• Require ballasts (adding cost and complexity)
• Contain mercury (environmental hazard)
• Light output degrades over time
• Color rendering varies by type (poor for HPS, good for metal halide)
• Being replaced by LED technology in many applications

Modern Context

While HID lamps were once the standard for high-bay industrial lighting, street lighting, and large-area illumination, they are increasingly being replaced by LED lighting, which offers:

• Instant on/off with no restrike time
• Higher efficiency
• Better color rendering
• Longer lifespan
• No mercury content
• Lower maintenance costs

Text description of the HID Light Bulb image.

The image shows four illustrations of different types of light bulbs, each labeled with its name. From left to right, the first bulb is labeled "Mercury Vapor," and it features a teardrop shape with a transparent enclosure and internal components visible. The second bulb is labeled "Metal Halide," similar in shape to the first with slight variations in the internal structure. Third is the "High-Pressure Sodium" bulb, which is more elongated with a central amber-colored tube inside. The last bulb is labeled "Xenon," which has a slim, cylindrical design with a more complex base. Each bulb has a distinctive internal configuration reflecting its function.

Credit: © Pennsylvania State University. (2026). CoPilot.