Traditionally, lighting engineers and those in the design professions have been concerned with lighting in terms of vision or aesthetics. Until recently, the biological significance of light has been overlooked. The fabrication of incandescent and fluorescent lamps is based on the assumption that people will be exposed to sunlight as a normal part of each day and not be confined to a habitation of electrical illumination. These lamps emit a narrow spectrum of light that does not include ultraviolet.
Fluorescent light is light without heat, whereby ultraviolet radiation is converted into radiation of a longer wavelength. (Since the human eye is not sensitive to ultraviolet radiation, these wavelengths are lengthened by phosphors to which the eye is sensitive.) Different phosphors create different tints of fluorescent light. Thus, fluorescent lamps, in simplistic terms, are nothing more than glass tubes, the inner surface of which has been coated with phosphor powders, which, when excited by ultraviolet energy created within the arc stream, give off visible light. Most fluorescent lamp tints cost the same to manufacture, although, due to marketing demand, cool white always costs less than more appealing colors.
If people are to be confined for long periods away from sunlight, a balanced light that emits a fairly full spectrum of wavelengths is desirable. The illumination of our environment acts both to induce and to time glandular and metabolic functions affecting, among other things, milk produced, the quality and quantity of eggs laid, and stimulation or inhibition of sexual activity. Light dilates blood vessels, thereby increasing circulation. Sudden exposure to bright light stimulates the adrenal gland. Our biological time clocks—our circadian rhythms—are manipulated by light. Studies have shown that subjects who are forced to live in darkness for prolonged periods suffer sensory deprivation. The loss of environmental cues that tell the body what to do throws body systems out of kilter.
As populations increase and pollution keeps pace, those in urban centers spend increasingly more time in indoor environments. People confined to nursing homes or institutions who are not able to get outdoors similarly depend on their indoor environment to supply well-balanced light that includes some ultraviolet. Those who design such environments must be aware of not only the biological effects of light, but also the psychological effects, as well as the visual quality of the light.
Perhaps the optimal solution for lighting offices, homes, restaurants, hospitals, and hotels would be a system of changing light levels and tints. Since natural light changes throughout the day (warm and rosy at dawn and dusk, bright with a bluish cast at midday), should we not try to imitate these day-night cycles in our indoor environments?
The sensations that we call color and light are our psychological interpretations of certain portions of the electromagnetic spectrum. How well we see colors depends on how closely the ingredients of artificial (electric) light sources match the ingredients of sunlight. Electric sources of light have varying degrees of each color—some have more warm wavelengths and some more cool. An incandescent bulb, for example, is high in orange and red and low in blue and violet; thus, it imparts a warm glow, but it is far from the color of daylight.
Typically, fluorescents produce about 72 lumens (the amount of light generated at the light source) per watt, compared with 6 to 24 lumens per watt produced by an incandescent lamp and fluorescents have an average life of 15,000 hours. Although fluorescent lamps have been the mainstay of commercial lighting, the proliferation of fixtures using LEDs has increased exponentially in just the past 18 months. Despite this, because of the increased cost of LED lamps and fixtures, fluorescents are likely to be the lamp of choice in numerous settings for the foreseeable future. Designers and architects, however, have been strongly embracing LEDs. And, with so much interest in green design and LEED®, LEDs enable more creative lighting design within the allowable watts per square feet. LEDs have been in use for many years in electronic devices, outdoor signs, traffic signals, the exteriors of buildings and monuments, but the last frontier to conquer has been interior architectural lighting. While they are an appealing lighting modality, there are problems with consistency and reliability according to a recent report by the U.S. Department of Energy (see LED Lamps below).
Fluorescent lamps are continually being improved to offer higher efficiency and numerous proprietary colors are available. However, careful selection must be made after consulting manufacturers’ lamp specification catalogs, because bulbs of the same wattage do not necessarily have equal lumen counts. There are often other quantitative and qualitative differences about a specific lamp from one manufacturer to another.
Fluorescent lamps are selected on the basis of lumen output, color temperature, and color rendition. The color temperature is expressed in Kelvins. The higher the color temperature, the bluer the appearance and the closer to daylight; the lower the color temperature, the redder the appearance. The color rendering index (CRI) describes the ability of a lamp to render objects as they would be seen in outdoor sunlight, which has a CRI of 100. Thus, a lamp with a CRI of 80 renders the object 80 percent as accurately as outdoor sunlight. The closer the CRI to 100, the better the color rendition of the lamp. Below is a list of the most commonly used fluorescent lamps plus a few unique ones. The optimal color temperature for most areas of medical and dental facilities, with the exception of surgeries and dental operations, is 3,500 Kelvins with a CRI of 85 or higher.
- Cool White lamps are approximately 4,100 Kelvins with a CRI of 68. They intensify white, gray, blue, and green and do not blend well with incandescent. These are not recommended.
- Deluxe Cool White lamps are 4,100 Kelvins with a CRI of 89. Their color rendition is a big improvement over Cool White lamps, but the lumens per watt are reduced considerably, so more of them are needed to achieve the same level of illumination as with cool white. They produce a white light with a slightly pink tint.
- Warm White lamps are approximately 3,000 Kelvins with a CRI of 56. They slightly distort all colors and have a pink glow, but mix well with incandescent. These are not recommended.
- Deluxe Warm White lamps are 3,000 Kelvins with a CRI of 74. They greatly intensify warm colors, are not as pink as standard warm white, and blend well with incandescent. These are not recommended.
- Daylight lamps are 5,000 to 6,500 Kelvins and usually have a CRI of 75. This lamp produces a cold blue-white light, not enhancing to warm colors and incandescent light, but useful in a room where a large quantity of natural light is present.
- Full-Spectrum lamps range from 5,000 to 6,500 Kelvins with a CRI of 90 to 98. This is a high-quality lamp ideal for color-critical applications such as dental operatories. It produces a bright white light that simulates the full color and ultraviolet spectrum of sunlight. These are manufactured by several sources: Lumichrome® by Lumiram is 6,500 Kelvins with a CRI of 98 and 5,000 Kelvins with a CRI of 96; Verilux® by Verilux, Inc., is 6280 Kelvins with a CRI of 94.5; Sylvania Octron 950 is 5000 Kelvins with a CRI of 90. These lamps are ideal for dental operatories where exact color matching is critical. Specify high-frequency electronic ballasts to eliminate flicker.
- The Ultralume 3000 lamp is 3,000 Kelvins with a CRI of 85. Made by Phillips, this lamp enhances warm colors and has better color rendition than the warm white deluxe.
- The Pentron™ T5 lamp is available in various color temperatures with a CRI of 82. Made by Sylvania, the ⅝-inch diameter lamp gives 104 lumens per watt with 95 percent maintenance (constancy) over the life of the lamp. The high output (HO) version gives twice the lumen output of a T8 of similar size.
- The SP35 lamp is 3,500 Kelvins with a CRI of 73. Made by General Electric (GE), this lamp renders skin tones very well, making it ideal for medical offices. It has the good color rendering properties of cool white deluxe and warm white deluxe, but has considerably more light output. Cool white deluxe has 56 lumens per watt, while the SP35 offers 83 lumens per watt. The SP35 complements both cool and warm color palettes, producing a crisp light midpoint between cool white and warm white although a CRI of 73 is low.
- The SPX35 is 3,500 Kelvins with a CRI of 82. Manufactured by GE, it is an enhanced version of the SP35 and is more expensive, but makes colors appear more vivid. It greatly enhances interiors.
- Compact fluorescent lamps are available in standard color temperatures from 2,700 Kelvins (with a CRI of 81) to 4,100 Kelvins. If one does not specify a color temperature, usually 2,700 Kelvins will be supplied as it most closely resembles warm white or incandescent. Manufactured by many lamp companies, this type of fluorescent is a small twin or quad tube in a U shape, available in 7-, 9-, 13-, and 26-watt lamps. The color rendition is somewhat similar to incandescent, but tends to be a bit more pink, rather than yellow. The 7-watt lamp is equivalent to a 40-watt incandescent; the 9-watt lamp, to a 60-watt incandescent; and the 13-watt, to a 75-watt incandescent. These lamps are very popular because they combine the high efficiency and long life of fluorescent lamps with fairly good color rendition. Their size allows them to be used in downlights, wall sconces, and other types of fixtures that previously required incandescent bulbs. It is important to match the color temperature of compact fluorescents to the other sources of light being used.
Light-emitting diode (LED) lamps are rapidly becoming the lamp of choice for their energy efficiency, making them a great choice for LEED projects in particular. Selection of fixture types is no longer limited with new entries to the market almost monthly. There are fixtures for general ambient light, accent lighting, and downlights along with more specialty pendants and decorative fixtures. Because the light is so very bright white it doesn’t take the place of halogens, for example, in all situations and it is susceptible to color shifts when dimmed. Today LED lamps have a typical life of 20,000 to 50,000 hours at full output, but see below for some critical discussion about this.
LED lamps can be dimmed with the appropriate driver and control equipment. The current light output of LED lamps is 25 to 75 lumens per watt, although that number is typically reduced by at least 15 percent depending on the light system. At the end of an LED lamp life, the light source will not turn off/burn out as in fluorescent or incandescent sources. LED light degrades over time. At a 70 percent lumen output compared to original output, an LED is considered at the end of its life and should be replaced. This can be very important when light levels are directed by code or essential to critical tasks. Therefore, LED drivers that indicate when light output has degraded below a certain level are available.
Due to the manufacturing process of LEDs, the electrical and photometric characteristics may vary between individual diodes. In order to provide luminaire manufacturers and consumers with consistency in the appearance of light, LEDs are tested and sorted, or “binned,” in accordance with certain characteristics such as color and brightness. LED luminaire manufacturers then order LEDs by bins to create fixtures that have a consistent and specific quality of light. When specifying LED luminaires or lamps, designers should be sure manufacturers tightly control bins.