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Illuminating the Landscape

Color temperature and color rendering are two of the most critical aspects in understanding how light and landscape design is so very intertwined. If you read a technical manual on lighting and look up the term "color temperature", you will get a description similar to: "Color temperature is a comparison of the color produced by a light source to a standard (black body radiator) source heated to a particular temperature." This is a tough concept to comprehend.What does it really mean? The bottom line is this: all lamps emit a color; that color affects the colors you choose in your design. Understanding this interrelationship could dramatically alter how you select your finishes for any project.

Color Temperature is measured in the Kelvin (K) temperature scale. Understanding color temperature means unlearning your concept of temperature in terms of heat. For example: If an apple were placed in an oven at 5,000°, what would happen to it? It would burn up. Yet, 5,000° in terms of a Kelvin rating is a frigid, freezing temperature. Confused? Don’t worry. Once you’ve finished this article, you’ll have a clearer understanding.

The silvery tones of the bleached wood gate and the gray slate pavers are enhanced, along with the greenery, using 20 watt MR11's fitted with daylight blue filters. A lower wattage was selected so that the lamps could stay undimmed in order to keep the light as white as possible.

Let’s start with the sun. If you were handed a box of crayons and asked to draw the sun, what color would you choose? Most people would choose a deep yellow. In reality, sunlight is a yellow-white light. It looks yellow compared to the blue sky around it. Daylight is blue-white, because it is a combination of sunlight and bluish skylight. Yes, in the early and waning hours of the day, there is a golden glow that occurs as the sun passes at an oblique angle to the horizon and is filtered by the atmosphere. But this is not what we are talking about in terms of color temperature.

You will hear the term: daylight fluorescent, it means that this lamp comes close to the color temperature of daylight (sunlight + skylight = daylight), which is true. However, the proportions of colors that make up this blue-white light may be significantly different from the proportions in true daylight, so the color rendering may surprise you. If you expect a warm, sunlight-yellow color of light from these lamps, you will be very disappointed. The color will be much closer to that of moonlight, which is a wonderful color for plants. More fluorescent sources are being used in landscape lighting design than ever before.

Have you ever gotten dressed before the sun was out or in a closet with no natural light? If so, have you ever looked down later in the day and realized that what you thought was navy blue was really black, or that two reds you chose didn’t really go together? You mentally blame yourself for not being careful enough. The truth is that under incandescent light, the colors shift so dramatically that you couldn’t tell the difference between navy blue and black, white and yellow, or a blue-red from an orange-red. Trading your incandescent light source for a good color rendering daylight-fluorescent source will allow you to choose your colors more carefully. The same applies to plants. They tend to look rather sickly under incandescent sources. When you do use incandescent sources consider adding a daylight blue lens which can help filter out the amber quality of incandescent light.

The image below shows an entry garden located in Palm Springs, California that becomes too yellow when illuminated with standard halogen sources. The image above shows the same garden with the addition of daylight blue filters which enhance the crisp green color of the cacti and succulents.

Why do we all rush to the window when selecting a carpet sample, paint color or even a sweater? Because we want to see the real color. This is the correct thing to do — daylight does the best job of color rendering. The quality of light is intense and includes the complete color spectrum, so the material which sunlight hits will be able to reflect its own special hue of the spectrum. All other light sources get rated on their ability to show color hue as compared to the daylight itself. This is called CRI rating (Color Rendering Index). Daylight is the best, so it gets a 100 (a 100% score).

The closer the other sources come to daylight, the higher their score is in the CRI. An 85-90 rating is considered pretty high.

How does this affect your design practices? Looking at your planting samples in daylight is fine - for daylight situations - but they can be completely different looking in nighttime settings.

A gargoyle peeks out from the surrounding ferns. Two low voltage fixtures by FX Luminaire are mounted on an overhanging branch above to help create the dappled pattern of light and shadow. Facing page: This gentle water feature is highlighted with recessed adjustable fixtures mounted on the underside. The boulder itself has been lit from below with a low voltage linear light source to create a subtle look.

Much of the time, landscape designers use incandescent sources (including halogen), that can vary as much as 2200°K from daylight. Under incandescent light, your color of your plant material can shift tremendously. White can go to yellow, red can turn to orange, blue shifts towards green, and grays can turn to tan. Then all your hard work, all your hours of color selection, are for naught.

Within these two rather extreme ends of the spectrum are many the most commonly used lamps. There are colored lamps, such as red or yellow and some colors of H.I.D. (High Intensity Discharge) sources, such as low-pressure sodium, that are off this scale. The H.I.D. sources are sometimes used in large-scale residential design. They are most commonly selected to light large public green spaces.

Metal halide and improved-color mercury vapor are the two H.I.D. sources most commonly used in landscape projects. Slowly but surely fluorescents are making appearances in commercial settings as well. They are less costly to operate, the replacement lamps are less costly than H.I.D. sources, and they don't have a delayed start-up time.

Rule of Thumb — The higher the Kelvin rating, the whiter (cooler) the light; the lower the rating, the more yellow (warmer) the light.

What is available now, which wasn’t the case in the recent past, are a huge variety of lamps with color temperatures that fall within the two extremes of warm and cool.

One of the first was halogen (also known as quartz or tungsten halogen). It is promoted as a white light source. This is true when compared to regular household bulbs. Standard incandescent is 2800°K, halogen is 200°K cooler (3000°K). Also note that halogen is an incandescent source and becomes more amber as you dim it, like all incandescent lamps. It is only whiter when operating at full output. Yet compared to daylight it is 2,000°K more yellow. That’s a huge difference. So white light is a relative term. Daylight is the definitive white light.

So how do you choose colors for a landscape project that will be used in both daylight and evening situations? The answer is actually very straightforward. Simply look at your samples under an incandescent source, as well as, a daylight source. Choose those hues that are acceptable in both situations. That way, you end up with a design that looks right both at night and in the daytime hours. The addition of the daylight blue filters go a long way towards a better color of light for plants.

The climbing greenery of the columns and the low level plantings look a bit sickly when illuminated with straight incandescent light. The addition of color correction filters would be a quick solution to the problem. The warm glow of the lantern lighting will add a pleasing contrast. Facing page: This lush garden takes on a fantasy appearance at night. Directional low voltage fixtures are positioned to throw light into the stone urn and the reflecting ball without revealing the location of the light source.

Color-corrected and daylight do not mean the same thing. Color-corrected means that the color-rendering of the lamp is good to excellent, intended to compliment the color and/or skin-tone of the people and surfaces within the space. Daylight means the source is close to the color temperature of daylight, but it does not render skin tone pleasantly.

Many lighting showrooms now have light boxes, which are display cubicles that show a variety of color temperatures and color-rendering abilities from a number of different light sources. This is a great way of seeing how the numerous lamps compare to each other.

Color Temperature and Plants

Color temperature is a way of describing the degree of whiteness of a light source. Those sources that produce a bluish-white light have a high color temperature and those that produce a yellowish-white light have a low color temperature.

Plants love white light. They look lush and healthy under white light sources. Unfortunately, most lights that are used to light plants are incandescent, which, as you now know, has an amber hue. This yellowish light turns the green color muddy and the plants look unhealthy.

What can you do on a job that already exists? The answer is to change the color temperature of the light source to a cooler version. Here are some techniques:

Solution One-

A Buddha figure sits among the foliage of this Northern California garden. Low voltage ground mounted fixtures by B.K. Lighting are hidden in the greenery to provide accent light for the statue and surrounding plantings. Facing page: The grey tones of the iron wood carved figure benefit form the blue-white light of color corrected halogen sources.

Have you seen the grow-light bulbs that they sell in hardware stores? These are incandescent sources that have been coated to filter out the green-yellow wavelengths emitted by incandescent. Replace the existing lamp with a reflector type or A-lamp version, depending on the luminaire it is going into.

Solution Two-

Use that color-correcting filter to alter the color temperature of the lamp. They are known as daylight-blue filters or ice-blue filters, among other names, and come in sizes to fit everything from MR11's to MR16’s to H.I.D. luminaires.

Solution Three-

Install new luminaires that accommodate fluorescent or H.I.D. sources that come in cool color temperatures.

Remember that the sun is white light and the moon is the same, because it is just a reflection of the sun’s illumination. So, if you want to create a moonlighting effect with your landscape lighting, here again you need to use a cool color temperature lamp.

This principle also applies to interior design work. As long as you let your clients see your selections under both lighting conditions, then there will be no surprises or disappointments when the project is installed.

About the Author: Randall Whitehead, IALD has written five books on the subject of lighting design, including “Lighten Up! A Practical Guide to Residential Lighting” and “The Art of Outdoor Lighting” available at

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October 17, 2019, 9:08 am PDT

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