Light Quality

Once acclimatized to the reduced light intensity of the interior, the plantscape may still prove unsatisfactory if the light quality is incorrect. Light quality is the color of light emitted by a particular source. The sun emits all colors of light, some of which the human eye can perceive and others that are imperceptible to humans but beneficial to plants. The green-yellow light most comfortable for humans is of little use in pho­tosynthesis by plants. They depend on light from the blue and red bands of the visible light spectrum. Visible light is only a narrow region of the radiant light spectrum (Figure 8-1). The unit of measurement for light wavelengths is the nanometer.

As long as both humans and plants can derive their light energy from the sun, the needs of each are satisfied. Indoors, however, where light energy is usually created by artificial means, the quality of the light can vary considerably. Light preferred by an interior plant specialist con­cerned with the health of plantings may cause the skin tones of human beings to appear ashen and deathly. In similar fashion, an interior deco­rator may specify a quality of lighting that gives the human complexion a healthy glow while making nearby plantings appear brown and dead. Clearly, someone seeking a career in interior plantscaping needs to know the types of lamps currently available and the quality of light they provide. Categories and examples of lamps that have some use in inte­rior plant illumination are shown in Table 8-1. Table 8-2 compares the lamps in all areas important to interior plant survival. You should study these tables thoroughly before proceeding further.

Selection of the proper lamp for the illumination of an interior planting will depend on the answers to several questions:

Visible light spectrum
(Wavelength in millimicrons)

figure 8-1. Electromagnetic spectrum and spectral distribution of visible light (Delmar/Cengage Learning)

Lamps for Interior Plant Illumination

Tungsten Filament Incandescent Lamps

• standard (the familiar household lightbulb)

• reflector (spot or flood lights)

• parabolic aluminized reflector (a weather-resistant type of floodlight with a more precise beam)

• incandescent plant lamps (not proven to be any better than the standard incandescent)

Fluorescent Lamps

• cool white

• warm white

• plant lamps

• wide spectrum plant lamps High-Intensity Discharge Lamps

• mercury

• metal halide

• high-pressure sodium

• How extensive is the planting?

• Are the plants to be encouraged to grow or merely to be maintained at their current size?

• Will the plants receive any sunlight? If so, how much and for how long?

• How far will the artificial light source be from the plants?

• What types of lamps are being used for general lighting of the area and what is the intensity of surface illumination provided?

For example, consider the plantscape of a typical office. The plants may be permanently located or in movable planters. Their functions may be to serve as room dividers, establish a mood, or relieve a cluttered desk top. The ceiling may be 8 or 10 feet high. Side windows or a skylight may admit some natural light. In such a setting, cool white fluorescent lighting would be ideal for both general lighting and the growth of the plants. People, plants, and furnishings look natural beneath cool white light due to its excellent color rendition, and the plants receive the right quality of light for photosynthesis. If additional task lighting is needed, small desk lights should be used. Special effects such as shadows or textural highlights can be created with incandescent lights installed beneath the plants and directed upward. (These are called uplights.) Some benefit will accrue to the plants from the addition of lighting at the base. However, if supplemental lighting is needed for photosynthe­sis, it is most efficient when applied from overhead because chloroplasts are concentrated in the upper leaf surface.

A Comparison of Artificial Lighting Sources for Interior Plantscapes

TABLE 8-2. F7 Lamp Type How Light Is Produced Quality of Light Produced Percent of Visible Light Radiation Color Rendition Initial Cost Operating Cost Incandescent (all types) Current flows through a tungsten filament heating it and making it glow. High in red light; low in blue light 7-11 Good Low High Cool White Fluorescent Phosphor coating inside the glass tube is acted on by radiation from a mercury arc. High in blue and yellow-green light; low in red light 22 Good (blends with natural daylight Moderate Moderate Warm White Fluorescent Phosphor coating inside the glass tube is acted on by radiation from a mercury arc. Low in blue and green light; more yellow and red light 22 Poor (blends with incandescent light) Moderate Moderate Fluorescent Plant Growth Lamps Same as other fluorescents. Special phosphors transmit most light energy in blue and red light regions of the spectrum. High in red and blue light; low in yellow-green light 22 Average (enhances red and blue colors; darkens green colors) Moderate Moderate Wide Spectrum Plant Growth Same as other fluorescents. Special phosphors transmit most light energy in blue and red light regions of the spectrum. Less blue and red than standard plant growth lamps; more far – red and yellow – green light 22 Average (favors red and blue colors; darkens green colors) Moderate Moderate Mercury Deluxe White Model, for Interior Plants An electric arc is passed through mercury vapor. High in yellow – green light; less red and blue light, but still usable for plant growth 13 Poor (favors blue and green colors) High Moderate Metal Halide Similar to mercury lamps but with metal and gas additives to produce a different spectrum High in yellow – green light; less red and blue light, but still usable for growth 20-23 Good (similar to CW fluorescent) High Low High- Pressure Sodium Sodium is vaporized into an arc. High in yellow – orange-red light 25-27 Poor (similar to WW fluorescent) High Low Low- Pressure Sodium Sodium is vaporized into an arc. High in yellow – orange-red light 31-35 Poor High Low

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Placement Life of the Lamp Height Above Plants Plant Responses Major Advantages Major Disadvantages 750 to 2000 Hours At least 3 feet to avoid foliage burn Plants become long and spindly with pale foliage. Flowering is promoted and senescence is accelerated. • Good for special lighting effects • Compact source of light • Simple installation • Energy inefficient; too much lost as heat • Light does not distribute evenly over a surface. • Glass blackens with time and light output reduced. • Frequent replacement is needed. Up to 20,000 Hours 10 feet or less Plants stay short and compact. Side shoots develop. Flowering extends over a longer period. • Energy efficient • Heat is radiated over the length of the lamp, allowing closer proximity to plant foliage. • Light distributed more evenly over a flat surface • Light does not focus well. • They are difficult to start when line voltage drops or humidity is high. • Installation is expensive. • Special fixtures are needed. Up to 20,000 Hours 10 feet or less Same as CW fluorescent • Same as CW fluorescent • Same as CW fluorescent Up to 20,000 Hours 10 feet or less Rich green foliage color. Large leaf size. Side shoots develop. Plants stay short. Flowering is delayed. • Same as CW fluorescent • Light emission is from the region of the spectrum most important to photosynthesis. • Same as CW fluorescent • Greater expense with little increase in benefit to the plants Up to 20,000 Hours 10 feet or less Stems elongate. Side shoots are suppressed. Flowering is promoted. Plants age rapidly. • Same as CW fluorescent • Same as CW fluorescent • Growth may not be desired. • Poor color rendition on nonplant materials Up to 24,000 Hours 10-15 feet or more Plants respond in a manner similar to CW fluorescent. • Long life; useful for inaccessible fixtures • Medium energy efficiency • Not interchangeable with other lamps • Warm-up time required Up to 20,000 Hours 10-15 feet or more Plants respond in a manner similar to CW fluorescent. • High energy efficiency, surpassing the mercury lamp • Good for both plant and general lighting • Warm-up time required • Color and light quality change with operating hours.

CN 1 ОЭ Ш v ___ 1 1	(Continued)
Placement
Life of the Height Above
Lamp Plants		Plant Responses	Major Advantages	Major Disadvantages
Up to 10-15 feet or	Typical red-light	• High energy efficiency.	• Yellow color makes
24,000 more		plant responses;	When combined with	them unsatisfactory for
Hours		similar to	blue light sources (such	general indoor lighting by
		fluorescent plant	as metal halide), they	themselves.
		growth lamps	provide good lighting for
		when compared	plants and people.
		on equal energy.	• Long life

A shopping mall presents different problems. The corridors may have ceilings too high to permit the use of fluorescent lamps, which are not good for illumination when the ceiling is much beyond 10 feet. There may be skylights as well as decorative architectural lighting. Overall illumination by mercury or metal halide lamps would be best. As in an office, supplemental or decorative lighting might also be desirable for special effects or the health of the plants. When uplights are used, they should be installed directly into the planters and waterproofed. When supplemental lights are added for overhead illumination, they should be positioned to light the plants fully without shining in the eyes of viewers.