Light & Color: A Q+A with Lighting Professor Asha Hegde

A look at the science behind seeing

Q. In order to understand how we see, we need to understand color and light. What is the relationship between the two? 

A. Light is electromagnetic energy. It is the “visible” portion of this energy that enables the mechanism of sight. Simply put, the perception of seeing occurs when the “visible” light (which is composed of different wavelengths—colors such as red, orange, yellow, green, blue, violet) hits an object and is then reflected back and received by the eyes and brain to complete the perception of seeing.

Light And Color

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It is hard to separate light from color and color from light since one can’t be explained without the other. As mentioned above, light is made up of different wavelengths (i.e., colors) in the visible portion of the spectrum. Light that includes all the visible wavelengths will appear as white or a variation of white. It is important to understand that the color makeup of the light source affects the perceived color of objects. Objects absorb certain wavelengths and reflect others back to the viewer. We perceive these wavelengths as color.

For example, if you have a red sofa in your living room that is lit by an incandescent light (which is a warm white light and also includes red wavelength), the sofa will appear red because it reflected light in the red portion of the spectrum and absorbed light of other wavelengths. However, if I used a light source that lacked the red wavelength, the sofa will reflect very little light and will appear black.

Q. When we talk about measuring color in light, we often hear the phrases “color temperature” and “color rendering index.” Can you explain the difference between the two?

A. Color temperature (CT) and color rendering index (CRI) are two color attributes of light. When in a store or office space, have you ever noticed the light sources in the ceiling with varying shades of whiteness? One light may appear a bit bluish white while the other might appear yellowish white. What you noticed is the color appearance of light.

Light And Color

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This color appearance—visual cool (bluish white) or warm (yellowish/orange/reddish) light—is referred to as color temperature. The color temperature for lamps is listed in Kelvin degrees. For example, an incandescent lamp is assigned a CT of 2,700K, while daylight can range from 5,500K and higher. The lower the CT number, the warmer the light will appear. The higher the number, the cooler the light will appear.

CRI informs us how true a particular light source renders the color of objects. Have you ever purchased a sweater at the store because you liked the color, and when you brought it home the color looked totally different? What you experienced is the effect of light source color on the sweater color under two different light sources.

Light and Color

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CRI is ranked using a scale of 0-100. A lamp with a CRI rating of 80 or above is good to use because it will render skin tone and object colors well. An explanation of how CRI and CT systems were arrived at can be found at Both these color attributes are important because they enable the user, designer, or architect to specify the right white light that is appropriate for a specific application.

Q. What does it mean for light to be warm vs. cool?

A. This refers to the human perception and psychological associations of light and color. Humans associate the colors and light sources that appear reddish/orange/yellowish as warm colors, while light sources that appear bluish white are perceived as cool. Incandescent, candlelight, and sunset are good examples of white light sources that have a warm appearance. Cool white fluorescent light and daytime sunlight are good examples of white light that appear cool. With current lighting technology, homeowners can achieve either warm or cool light with fluorescent and LED sources.

Q. Where would we want warm light in our homes? What about cool light? 

A. Having a home appear warm or cool is a personal preference, but most homeowners generally prefer a warm look for their home since psychologically we associate the warm look (orange glow) with comfort, intimate settings, and relaxation. Warm-appearing light is generally used in living, dining, and bedroom settings while cooler-appearing light sources are found in utilitarian areas such as the kitchen, home office, garage, etc. It is important to understand that if you have an open plan where the living, dining, and kitchen areas flow together it is best to keep the appearance (warm or cool) of the light color consistent in all these areas so that they appear unified visually.

Q. What technology do you use most often in your work (LEDs, CFLs, incandescent, halogen, etc.)? What are the color differences in each of the commonly used sources?

A. Residential clients still like the look and feel of the incandescent/halogen (warm, CRI 100 and CT 2,700-3,000K) light source, which they are so accustomed to in their homes. However, many clients are becoming aware of environmental and energy needs and are open to the use of compact fluorescent (CFL) and LED. Color quality and appearance are the most common reasons why a homeowner might be unsure about the use of the CFL or LED. With current technologies, both the CFL and LED come in an array of white light that can range in appearance from warm to cool (CT range 2,700-6,500K) with superb ability to render colors (CRI range for CFL 80-90 and LED 65-90 ) along with added benefits of energy savings and long lamp life.

Q. Where do you see lighting technology going in the next 5-10 years in terms of color?

A. Currently most lamp and fixture manufacturers are investing their time, resources, and money in LED solid-state technology. The driving force behind this is the low energy consumption by LED sources, as well as the ability to provide a wide range of white light and colored light. Researchers in the areas of vision and lighting have discovered that light not only affects visibility and seeing (visual aspects) but it also influences non-visual aspects such as circadian rhythm and other health aspects of an individual. Research is informing us that the blue wavelength of the light plays a huge role in setting our day-to-day circadian rhythms. Based on this knowledge, it seems natural that in the future we will have the ability to replicate the day/night cycle as it appears in nature with LED artificial lighting in our home (residential and eldercare facilities) and work environment. LED technology is the perfect light source for providing a wide range of color in a fluid fashion.

About Asha Hegde

Asha Hegde is an Associate Professor of Interior Design and Lighting Design at Texas State University. Besides research and teaching she conducts continuing education seminars to architects and interior designers on the topic of light and color across the nation. She has a B.S. in Interior Design from Florida State University and a M.S. and Ph.D. in Human Environmental Sciences with an emphasis in lighting from Oklahoma State University. She has over 25 referred publications and invited presentations in the field of light and color. She also serves on the IES National Committee of Lighting for Aging and Partially Sighted.

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