In the old days there was only one type of light bulb, incandescent bulbsand the only chose you had was what wattage. {Today|Now| there are lots of different lighting technologies to select from. I want to discuss the reasons for choosing 1 bulb over the next. Lighting design can make or break a design so selecting the right bulb is crucial. If you are a restaurant designer you want to make the food look good. If you are a retail designer you want to make the merchandise look good. You get the point. There are 4 properties that I want to focus on to give a good foundation in what to look for when selecting a light.

• Wattage. This is the most common and straight forward measurement used, but it is also commonly misused. Wattage is simply a measurement of the amount of electricity used by a bulb, but it is often used as a proxy for the amount of light a bulb produces. This worked out fine back when all lights were incandescent, but now that we have new technology we need to be accurate about what wattage means. Electricity is purchased by kilowatt-hours (1000 watts per hour written 'Kwh') so you can simple take the bulb wattage times electric rate (Say 10 cents a Kwh) and get how much you would pay to run that bulb for 1000 hours. A 60 watt bulb cost $6 to run for 1000 hours. (60x$0.1=$6)
• Lumens. The real measurement of light output is Lumens not Wattage and it is abbreviated 'lm'. For the record the technical definitions is 'a unit of luminous flux, equal to the luminous flux emitted in a unit solid angle by a point source of one candle intensity.' So what that means is a candle is 1 lumen. If you want to understand the efficiency of any given light divide the lumens by the wattage to get lumens per watt. A traditional 60 watt incandescent light produces about 500 lumens or 8.3 lumens per watt. While a 20 watt CFL produces about 960 lumens or 48 lumens per watt, much better.
• Color Temperature. This gets a little more complicated, but in design terms it is probably the most important. Color temperature is the measurement of the lights color stated in units of absolute temperature, kelvin (K). The color of the light has a great impact on the look and mood of a space. This is what you need to focus on in the examples above. (Does this light make my food look good?) Lower temperatures are more yellow and oddly referrer to as warm light. While higher temperature are more blue and are called cool lights. The clash of design and science I suppose. Traditional incandescent lights are around 2,700K and are visibility yellow and the first generation fluorescent lights were 4,000K and blue-green in color.
• Color Rendering Index (CRI). This is almost as important as Temperature, to design. The technical definition for the record is 'Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant'. What this means is how well can you make out the color of something with any given light. Despite the definition the scale is nice and simple 100 is perfect and 0 is terrible. Anything above 80 is acceptable you can get this with most any modern fluorescent, halogen or high end ceramic metal halide light. If you are lighting up artwork 90 is a good starting place. Cheap regular Metal Halide lights can be in the 50 to 70 range which is fine in a warehouse but not a retail application. Although I can point out a lot of stores that do use Metal Halide lights and suffer for it.

I hope this helps to reduce the confusion about light selection.