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Section 2: Light Sources

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Introduction

The light source (the lamp) is the device that actually converts electrical energy to visible light.

Light sources of interest in highway illumination design fall into two groups — incandescent lamps and electric discharge lamps. This section covers the various types of incandescent and electric discharge lamps.

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Incandescent Lamps

The following table describes of two types of incandescent lamps along with usage notes for each.

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Lamp Type

Description

Usage

Common Incandescent

Consist of a tungsten filament enclosed in a glass envelope (the bulb) attached to a metal base. The bulb is evacuated and an inert gas (argon or nitrogen) is intro-duced into the bulb for longer life and better efficiency. For wattages less than 40, the filament usually operates in a vacuum, without the inert gas. A voltage applied to the filament through the base causes a current to flow through the filament which causes it to incandesce.

Low initial cost, but low efficacy (lumens per watt) and short lamp life.

Tungsten Halogen (quartz iodide)

Has a halogen, such as iodine, introduced into the lamp envelope. This lengthens lamp life and improves efficiency.

Tungsten halogen lamps are not normally used in highway lighting.



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Electric Discharge Lamps

Electric discharge lamps produce light by the passage of an electric current through a vapor or gas instead of through a tungsten wire. The application of an electrical potential ionizes the gas, and permits current to flow between two electrodes located at opposite ends of the lamp. The electrons which comprise the current stream, or “arc discharge,” collide with the atoms of the gas or vapor, causing light to be emitted. Electric discharge sources have a negative resistance characteristic, and a transformer or ballast must be provided to limit the current.

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Electric Discharge Lamp Types

Electric discharge lamps come in five common types. The following table provides descriptions of each type along with usage notes.

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Lamp Type

Description

Usage Notes

Fluorescent

Tubular bulb containing mercury. The mercury arc operating at a low vapor pres-sure produces ultraviolet light. The inside of the bulb is coated with a phos-phor, and the ultraviolet light striking the phosphor causes visible light to be emitted.

Medium initial cost, long life, high efficacy (30 – 70 lm/w), but light varies with ambient tempera-ture. Linear light source more difficult to control.

Induction Fluorescent

Same as fluorescent, except not tubular.

High efficiency (75 lm/w). Extremely long 100,000 hour life.

Mercury Vapor (MV)

Mercury arc in quartz arc tube. Operates at higher pressure and temp-erature than fluorescent. Arc produces visible light and some ultraviolet light. Far-ultraviolet filtered out by glass envelope. Clear lamps and phosphor-coated lamps available.

Phosphor-coated lamp used for sign lighting. High initial cost, long life, high efficacy (30 – 65 lm/w)*. Smaller light source than fluorescent.

High Pressure Sodium (HPS)

Arc in ceramic tube containing sodium and other elements. Provides light primarily in yellow spectrum but other elements provide light in blue, green, orange and red to improve color rendition. Requires a starting aid to provide pulse to start arc stream.

High initial cost, long life, high efficacy (45–150 lm/w)*, small light source.

Low Pressure Sodium (LPS)

Arc in long tubular glass envelope containing sodium only. Light is mono-chromatic yellow with poor color rendering.

High initial cost, mod-erately long life, high ef-fica-cy (145 – 185 lm/w)*. Light source hard to control photo-metrically.

Metal Halide (MH)

Similar to mercury lamp but contains various metal halides in addition to mercury. Excellent color rendering.

High initial cost, mod-er-ately long life, high efficacy (75 – 125 lm/w)*.

*The theoretical maximum efficacy is 683 lm/w. (lm/w = lumens per watt.)



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High-Intensity Discharge Lamps

Of the electric discharge lamps, mercury vapor (MV), high pressure sodium (HPS), and low pressure sodium (LPS) are referred to as “high-intensity” discharge lamps. The following table compares their features.

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Lamp Wattage and Type

Life (in hrs.)

Initial Lumen Output

Input Watts*

250

MV

24,000+

11,200

285

400

MV

24,000+

21,000

449

1000

MV

24,000+

57,000

1067

150

HPS

24,000

15,000

184

200

HPS

24,000

22,000

246

250

HPS

24,000

27,500

296

400

HPS

24,000

50,000

478

1000

HPS

24,000

140,000

1200

55

LPS

18,000

8,000

83

90

LPS

18,000

13,500

131

135

LPS

18,000

22,500

197

180

LPS

18,000

33,000

247

*Ballast input watts. For MV and HPS it is the average initial watts from actual tests. For LPS it is the average of initial and end of lamp life.



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Best Choice

For normal roadway lighting (including high mast lighting) the high pressure sodium lamp is considered the best choice because of its efficiency, long lamp life, good color rendition, and small light source (which allows better photometric control and hence better utilization).

Induction fluorescent is suitable for low mounting heights and other special applications.

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Spectral Energy Distribution Comparison

compares the color (spectral energy) distribution of a 400-watt mercury vapor lamp (Warm Deluxe) and a 400-watt high pressure sodium lamp (Lumalux). Note that the mercury vapor lamp performs better at the blue end of the spectrum, while the high pressure sodium lamp’s light is concentrated in the yellow-orange range.

Comparison of 400-watt mercury vapor lamp
(Warm Deluxe) and a 400-watt high pressure sodium lamp (Lumalux)
color distribution. (click in image to see full-size image) Anchor: #i999380grtop

Figure 5-1. Comparison of 400-watt mercury vapor lamp (Warm Deluxe) and a 400-watt high pressure sodium lamp (Lumalux) color distribution.

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