Last month we talked about the new measure for light – lumens – and how many lumens you would need to properly illuminate a room.  This month we are going to shed some more light on light.
First, to make sure you are buying the right light bulbs, all light bulb packages must state on the front:

  • Light output in lumens, which if you remember from last time is the actual amount of ambient light coming from the bulb.  For example, a 40-watt incandescent bulb will product about 500 lumens of light.
  • Energy used in watts
  • Life hours, which is the rated average life of the bulb as determined by engineering testing and probability analysis.  For example, that 40-watt, 500-lumen bulb typically provides 1,000 life hours.

The back of the bulb must have a label with the following information:

  • Light output in lumens (again)
  • Estimated yearly energy cost
  • Life expectancy, generally expressed in months or years, based on certain usage assumptions which may or may not be valid.
  • Light appearance.  This is expressed as a correlated color temperature (or CCT), and is the light’s appearance it terms of perceived warmth or coolness.  CCT is expressed as a temperature measured on the Kelvin temperature scale.  A low CCT (4,000 Kelvin and lower) indicates a warm light; a higher CCT (4,000 Kelvin and higher) indicates a cool light (yes, that is counterintuitive).  Cool lights will tend to look red; hot lights will look blue.
  • Energy used in watts (again)
  • Whether the light bulb contains mercury

Lumens and whether the bulb contains mercury must also be indicated on the bulb.

Two other terms you should be familiar with:

  • Lumens per watt/efficacy.  This measures the efficacy of the light.  It is simply the total light output (lumens) divided by the total power input (watts).  Again, both lumens and watts can be found on the front of the light bulb package.  The higher the efficacy, the less the bulb will cost to use.  Think of it like the cost per ounce measurements you might see in the grocery store.
  • Color accuracy/color rendering index.  This measure, also known as the CRI, describes the effect of a light source on the color of the objects being illuminated.  CRI is expressed on a scale of 1-100, with 100 being the maximum.  The higher the CRI the better.  CRIs in the 90-100 range are considered the best.  Incandescent bulbs are generally 100 CRI, while CFL bulbs are generally 82-88 CRI and LED bulbs 65-85 CRI.