How to Read a Foot-Candle Graph

Choosing the proper lighting and light fixtures for your home, business or office can be a daunting task. What seems like easy work comes down to interpreting foot-candle graphs, or photometric charts, and determining if the light coverage is sufficient for your intended use. Whether you are choosing path, area or deck lighting, it can be as easy as counting squares on the foot-candle graph and measuring the proper distance with a tape measure to ensure light is where it needs to be.

Photometric charts with beam spread in degrees

Instructions

- 1
  Lay out the photometric charts. You may have just one, but an interior designer or lighting specialist could provide you with two or three, with different degrees of coverage.
- 2
  Using a fine-tipped point, like that on a compass or pencil, count the number of squares that fall within the beam depicted on the chart. Refrain from counting any that fall outside of 50 percent as they won't count for foot-candling.
- 3
  Write down the number of squares counted for each of your charts. This number is how many foot-candles the specific watt bulb covers at a particular degree of coverage. This will vary, depending on the type of bulb (spot versus flood, for example), so make certain to note the bulb type on the chart.
- 4
  Search through charts of various lamps, based on foot-candle needs from the photometric chart. If you don't have any lamp charts, local lighting stores can provide you with them. General Electric also provides useful software that can be used for non-GE products.

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