There are a number of outdoor lighting options available to homeowners, and each has its one sets of advantages & disadvantages. Before picking one type of light over the other, lets examine what lighting experts & illuminescent engineers have to say.
MERCURY VAPOR LAMPS
NOTE #1: The oldest member of the HID (High Intensity Discharge) family, mercury vapor lamps work by arcing electricity through mercury vapor. While more efficient than incandescent, halogen and fluorescent lamps, mercury vapor lamps have the least efficient lumen-to-watt ratio of the entire HID family. This, combined with an improper color spectrum for horticultural applications, makes mercury vapor lamps a poor choice for a grow light or greenhouse applications.
NOTE #2: Standing under a mercury lamp's gentle radiance, we can see all the important information. We see the road-surface, we see the pavement and a clear delineation 'twixt the two. We see a mature brick wall covered in creepers, we see verdant trees and hedges. We see all this with our mesopic night-time vision, our eyes having adjusted to the conditions. We see a dusky-peach stain in the near-distance. We see someone's moving house.
NOTE #3: When new, the 175 watt mercury vapor lamp puts out about 8000 lumens (a lumen is a measure of the amount of light produced by a lamp), declining in light output with time. The mean lumen output (after some years of use) is about 7000 lumens. That amount of light output compares approximately to a 400 watt incandescent lamp, a 100 watt metal halide lamp, a 70 or 100 watt high pressure sodium lamp, or a 55 watt low pressure sodium lamp.
NOTE #4: Mercury vapor lamps are available in sizes ranging from 50 to 1,000 watts, offer low initial cost, long life, and color stability. However, they are approaching obsolescence, now that other HID light sources with better efficacy, more accurate color rendering, and improved lumen maintenance are becoming available at lower cost, especially in larger lamp sizes. In smaller sizes, mercury vapor lamps can be replaced with screw-in compact fluorescent lamps that do the same job at less than half the wattage, while providing better color.
HIGH-PRESSURE SODIUM LAMP
NOTE #1: High-pressure sodium lamps operate by igniting sodium, mercury and xenon gases within a sealed ceramic arc tube. Sodium lamps emit light energy in the yellow/red/orange regions of the spectrum; the red spectrum stimulates flowering and fruit production. Many indoor gardeners switch to sodium lamps when it is time to induce flowering or fruiting of their plants.
NOTE #2: The poor color-rendering of this High-Pressure Sodium lamp means that instead of easy-on-the-eye natural contrasts, all an aid to night-time visibility, we are presented with a scene of sodium splurge. Detail is subsumed by reflected glare and the direct glare from the fitting itself. This light is many times brighter than the mercury-lamp, and yet our actual ability to see is reduced. The glare creates an area of bold shadow directly ahead of the lumiere, which means that a piece of pavement just twenty yards away is rendered blind. Our eyes start adjusting to the multifold increase in light-level. We squint against the sudden glare.
NOTE #3: These HID bulbs contain a sodium gas and produce a yellow/orange light. They are very bright, and last up to 24,000 hours.
NOTE #4: Sodium lamps vary more widely than other HID lamps in their efficacy and color quality. For example, the light from some sodium sources is predominantly emitted in the longer wavelengths, between yellow and red. Light emitted in these wavelengths is less effective than light emitted at lower wavelengths because the human eye is less sensitive to photons in that region of the spectrum.
On the positive side, sodium lamps use no phosphors. Also, their low mercury content creates very little ultraviolet output, although some high-pressure sodium (HPS) lamps are coated to reduce glare and to widen light distribution.
Color rendering index (CRI) is a scale for describing the effect of a light source on the color appearance of objects being illuminated. A value of 100 is the maximum possible CRI. HPS lamps are available in three basic grades: Low grade lamps with a CRI of about 21, which are typically used in outdoor lighting; general purpose indoor lamps with CRI of about 60; and "white" lamps with a CRI of 80 or more.
METAL HALIDE LIGHTS
NOTE #1: Adding certain iodides of metals to the basic mercury vapor lamp produces the metal halide lamp which offers very high efficacy and high-quality light. Those characteristics make metal halide lamps a very effective tool for energy efficient lighting, but there are some drawbacks:
- Start-up typically takes 3 to 5 minutes, and restarting after a shutdown or power interruption takes 10 to 20 minutes. Lamps over 400 watts are available with instant restrike capability, but they require special ballasts.
- Metal halide units produce high levels of UV radiation that must be shielded by glass in the lamp or fixture.
- The smallest metal halide sources produce 2,500 lumens from a small 32-watt package--nearly as much light as a 4-foot fluorescent. This high intensity limits the range of application for metal halide lamps and requires the use of sophisticated fixtures.
Some of these problems have been overcome by new metal halide systems using what is known as pulse-start technology. In these systems, a brief, high-power pulse is used to ignite the lamps. Although pulse-start lamps can only be started with pulse-start ballasts, it is possible to use standard lamps with pulse-start ballasts. Pulse-start lamps are available in 50- to 450-watt models.
The benefits offered by the pulse-start systems include superior efficacy; more rapid, reliable starting, even in cold weather; less electrode damage with each start (which extends lamp life); better lumen maintenance; and faster restrike times (5 to 7 minutes, compared to 10 to 20 minutes for standard metal halide systems).
NOTE #2: In recent years, Metal halide lamp (MH) streetlights have illuminated the roadways and parking lots. Metal halide has long become popular in business installations, as it can be found in warehouses, schools, hospitals and office buildings. Unlike the old mercury lights, metal halide casts a true white light. It is not nearly as popular as its sodium or mercury counterparts, as it's newer and less efficient than sodium.
NOTE #3: Metal halide lights have also been used for retrofitting. Virtually all fixtures that are converted to metal halide have previously been lit with high-pressure sodium (HPS). Examples of retrofitted fixtures for metal halide use include the Thomas & Betts Model 25 and Model 327, as well as full-cutoff versions of GE's M400. MH lamps suffer color shift as they age though this has been improving. Actual life expectancy is about 10-12K hours on average. There has also been a noted issue with the lamps "exploding or shattering" during a failure. High cost and low life hours has kept them from becoming popular municipal lighting sources even though they have a much improved CRI around 85.
COMPACT FLORESCENT LIGHTING
NOTE #1: Compact fluorescent lamp/lighting (of CFLs) has been used more frequently as time has improved the quality of these lamps. These lamps have been used on municipal walkways and street lighting though they are still rare at this time. Improvements in reliability still need to be made.
NOTE #2: Some issues with them are high heat build up in the self contained ballast, low life/burnout due to frequent cycling (on/off) of the lamp, and the problem where most fluorescent sources become dimmer in cold weather (or fail to start at all). CFL efficiency is high and CRI is excellent around 85. CFL produces a color temperature around 3K with its light being "soft white" around that color temp. Higher color temps are available.
Ogeezer comment: I hope this helps in deciding which outdoor light is best for your needs or applications.