EGEE 102
Energy Conservation for Environmental Protection

How Much Lighting is Needed


How much light is needed in a room depends on the task(s) being performed (contrast, requirements, space, size, etc.). There are three different types of task-oriented lighting: Ambient, Task, and Accent. The light requirement also depends on the ages of the occupants and the importance of speed and accuracy of the task.

  • Ambient lighting is general purpose lighting—an example is the lighting used in hallways for safety and security. An illumination of 30–50 fc is generally the maximum that one needs for this purpose.
  • Task lighting is lighting that is designed for specific tasks. Reading and writing are the most light-intensive tasks and require about 50 fc at home. Tasks like cooking, sewing, or repairing a wrist watch require more -- about 200–300 fc. However, the area with this level of illumination will be small. Increasing the light everywhere is not required and is a waste of energy.
  • Accent lighting is the lighting that is provided to highlight certain objects or areas, for example, the use of floodlights to highlight a painting or a statue. Accent lighting also illuminates walls, so they blend more closely with naturally bright areas like ceilings and windows. Accent lighting can be high intensity or subtle.

How Much Light is needed?

Now we are going to examine the question, "How much lighting we need?" The answer depends on what kind of task we are doing.

a. Task being performed; Ambient, Task, or Accent

Say if you are just walking in the hallway; you need a little bit of light to see each other and to recognize what is there. That lighting is called ambient lighting, or general purpose lighting. Here's an example where I am sitting here in this conference room and trying to take notes or reading a book, and this type of lighting, the type of lighting that I need here, is called task lighting. I am doing the specific task of reading a book. Or you can take another example of cooking. You know, you are cooking something, and you need to know exactly whether it is done or not; you need lighting there. That is again, task oriented lighting.

So, it depends on the task that you are doing. There are three types usually: first is the ambient, the second one is task lighting, and the third one is accent lighting. The idea there is you want to highlight certain aspects; maybe a painting, maybe a statue that you own or any other thing, a map. You know, whatever you want to highlight. Instead of highlighting entire room lighting, you can highlight that particular piece with some special lighting, and that type of lighting is called accent lighting.

b. Age of the occupants

How much light we need also depends on the age of the group, the age of the people that are in there, in the room. Usually, older people need higher amounts of light than younger people.

c. Importance of speed and accuracy

We also must consider the speed and accuracy with which we want to do certain things. Say, for example, a person is repairing a small wristwatch. Obviously they need more lighting, especially if they want to repair very fast. So speed and accuracy with which we want to do a job also dictate how much light we want.

Color Rendering Index

Lamps are assigned a color temperature (according to the Kelvin temperature scale) based on their "coolness" or "warmness." The human eye perceives colors as cool if they are at the blue-green end of the color spectrum, and warm if they are at the red end of the spectrum.

Instructions: Click "play" to see examples of the light sources that temperatures represent. (Note: The video has no audio.)

Color Rendering Index
Click here for a text description.

Color Rendering Index

Based on the previous description of color temperature, here are some examples of light sources that a color temperature (measured in Kelvin) might represent.

  • 10,000° K - Clear blue sky
  • 7,000° K - Overcast sky
  • 5,500° K - The Sun at noon
  • 4,000° K - Fluorescent light
  • 3,000° K - Halogen bulb
  • 2,800° K - Light bulb
  • 1,900° K - Candle light
  • 1,000° K - Sunrise

The ability to see colors properly is another aspect of lighting quality. Objects' colors appear to be different under different types of light. The color rendering index (CRI) scale is used to compare the effect of a light source on the color appearance of its surroundings. A scale of 0 to 100 defines the CRI. A higher CRI means better color rendering, or less color shift.

Instructions: Move the drag button in the center of the picture below to see the difference between low CRI and high CRI.

Factors Affecting the Number of Lamps Required

Instructions: Click on the hot spots below to determine the factors that affect the number of lamps required: