Characteristics of Vapor Discharge Lamps
Vapor Discharge Lamp Technologies
Low-Pressure Mercury Vapor Lamps (Fluorescent)
Low-pressure mercury vapor lamps, commonly known as fluorescent lamps, primarily emit ultraviolet (UV) radiation, predominantly at a wavelength of 253.7nm. To make this radiation useful for lighting, the inside walls of the tube are coated with fluorescent powders (phosphors) that convert UV radiation into visible light.
Two main types of phosphors are used:
- Powders producing a continuous spectrum.
- Tri-phosphor powders emitting a three-band spectrum (primary colors) which, when combined, produce white light with good color performance without significantly penalizing efficiency.
Key characteristics:
- Typically consist of a standard diameter cylindrical tube sealed at both ends with caps containing two contacts for the electrodes.
- Do not have an outer bulb.
- The discharge tube is filled with mercury vapor and a low-pressure inert gas (like argon) to initiate and control the electron discharge.
- Require a ballast and a starter (primer) for operation.
- Average lifetime: 5,000 to 7,000 hours.
- Performance depends on the specific phosphors used.
High-Pressure Mercury Vapor Lamps
As the pressure of the mercury vapor inside the discharge tube increases, the characteristic UV radiation of low-pressure lamps becomes less significant compared to emissions in the visible spectrum. Under these high-pressure conditions, the light emitted is primarily blue-green and lacks red radiation.
To address this and improve the lamp’s chromatic characteristics, fluorescent substances that emit in the red region of the spectrum are often added to the outer bulb coating. This results in better color balance.
Key characteristics:
- Improved color compared to uncoated high-pressure mercury lamps.
- Average lifetime: Approximately 8,000 hours.
Blended Light Lamps
Blended light lamps are a combination of a high-pressure mercury vapor discharge tube and an incandescent tungsten filament, housed within an outer bulb often coated with phosphors. The resulting light output is a superposition of:
- The mercury discharge spectrum.
- The continuous spectrum from the incandescent filament.
- Red radiation from the phosphor coating.
Key characteristics:
- Improved color rendering compared to standard high-pressure mercury lamps.
- Lamp duration is limited by the life of the incandescent filament.
- The filament also acts as a current stabilizer, eliminating the need for an external ballast.
Metal Halide Lamps
Adding metal halides (iodides of various metals) to the mercury vapor discharge tube significantly improves the color rendering capabilities compared to standard mercury vapor lamps. Each added substance introduces new lines to the emission spectrum, resulting in a fuller, more balanced light output.
Key characteristics:
- Excellent chromatic performance and color reproduction.
- Require a start-up period (approximately 10 minutes) for the discharge to stabilize.
- Need a special ignition device due to high starting voltages.
Low-Pressure Sodium Vapor Lamps
The discharge in low-pressure sodium (LPS) lamps produces nearly monochromatic radiation, consisting mainly of two very close spectral lines in the yellow region (around 589nm). This wavelength is very close to the peak sensitivity of the human eye, resulting in very high luminous efficacy.
Key characteristics:
- Highest efficiency among common lamp types.
- Poor color rendering due to the monochromatic light output. Colors appear yellow or grey.
- The discharge tube is often U-shaped to reduce heat loss and overall lamp size.
- Long lifetime.
- Commonly used for street lighting, security lighting, and decorative purposes where color rendering is not critical.
High-Pressure Sodium Vapor Lamps
High-pressure sodium (HPS) lamps operate with sodium vapor at a higher pressure and temperature. This broadens the spectral emission, covering almost the entire visible spectrum, although still concentrated in the yellow-orange region. This provides a pleasant golden-white light.
Key characteristics:
- Better color rendering and ability to reproduce colors compared to low-pressure sodium lamps.
- Good efficiency (though lower than LPS).
- Average life: Around 20,000 hours, typically limited by lumen depreciation, discharge tube failure, or an increase in operating voltage.
- Widely used for street, industrial, and security lighting.
Lamp Color Properties
Color Appearance (CCT)
The color appearance of a lamp is determined by its Correlated Color Temperature (CCT), measured in Kelvin (K). It defines the perceived hue of the light source. Three main grades are typically recognized:
- Cool Light: Bluish-white appearance (High CCT, e.g., >5000K).
- Neutral Light: White appearance (Mid-range CCT, e.g., 4000K-5000K).
- Warm Light: Reddish-white appearance (Low CCT, e.g., <3500K).
Color Rendering (CRI)
The color rendering describes how well a light source reveals the true colors of objects compared to a reference light source (like daylight or an incandescent lamp). It is measured by the Color Rendering Index (CRI), on a scale typically from 0 to 100.
- A higher CRI number indicates better color reproduction.
- Achieving a very high CRI often involves sacrificing some luminous efficacy (efficiency) and may increase power consumption.
The final perception of color is influenced by both the lamp’s color appearance (CCT) and its color rendering capability (CRI).