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, or example, the use of flood lights 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.
View the 2:38 video below to find out more about the three types of lighting.
Click here to for a transcript of How Much Light is needed?.
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 wrist watch. 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: Move your cursor up and down the color scale to see examples of the light sources that temperatures represent.
Click here for a text description of Color Rendering Index.
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 your mouse over the picture below and watch to see the difference between low CRI and high CRI.
Factors Affecting the Number of Lamps Required
Instructions: Place your cursor over the numbers below to determine the factors that affect the number of lamps required:
Click here to open a text description of the Factors Affecting the Number of Lamps Required for a Room.
Factors Affecting the Number of Lamps Required for a Room
- Fixture efficiency
Certain fixtures reflect more light than others. Fixtures that are not highly reflective may absorb some light, resulting in less light reaching the user.
- Lamp lumen output
The efficiency of a bulb increases with wattage. For example, a 40 watt bulb produces 505 lumens, where as a 100 watt bulb (2.5 times the 40 watts) produces 1750 lumens (4.32 times the 505 lumens).
- The reflectance of surrounding surfaces
Bright colors or reflective surfaces painted with glossy texture finishes will appear brighter than a surface with flat finish paint.
- The effects of light losses from lamp lumen depreciation and dirt accumulation
As lamps age, or when dirt accumulates on the bulb surface, the lumen output from the light bulb decreases. Therefore, newer light bulbs produce more light than older bulbs of the same wattage.
- Room size and shape.
- Availability of natural light (daylight).