Frequently Asked Questions About Lighting
Posted Saturday, December 21, 2002
Why is a lamp's CRI or CCT important?
The Color Rendering Index (CRI) is the reason that the shirt and pants you thought matched so well at home seem to clash in the restroom at work. The closer the CRI of a lamp is to 100, the more "true" it renders colors in the environment. For individuals, such as graphic artists who perform tasks that require color precision or discrimination, lamps with a high CRI are recommended. Full-spectrum lamps are available that offer a CRI of over 90.
Warm and inviting are some of the words used to describe environments that contain lamps with a Correlated Color Temperature (CCT) of 3000K. Coffee shops, restaurants, and hotel lobbies are a few applications in which cozy lighting environments are desirable. Hospitals, cafeterias, classrooms, and conference rooms are areas where an image of neatness is important. Lamps that appear cool - measuring a CCT of 4100K - are most appropriate for these types of applications. By comparison, daylight measures 5000K, and neutral lamps have a CCT of 3500K.
What are the differentiating characteristics of incandescent, fluorescent, and high-intensity discharge (HID) lamps?
- Appear "warm" in color and have excellent color rendering.
- Are the least efficient of general lamp types due to the amount of energy consumed heating the filament in order for the lamp to turn incandescent.
- Have a short lamp life of between 500 and 3,000 hours.
- Are easy to install because no ballast is required.
- Are available in a complete range of color combinations.
- Produce low heat.
- Require a more extensive installation because ballasts are a necessary part of the fixture.
- Have an extremely long life of up to 24,000 hours.
- Due to smaller sizes and screw base features, fluorescents can replace incandescent lamps.
High-intensity discharge (HID) lamps:
- Are ideal for large stores, warehouses, auditoriums, outdoor parking areas, and applications where efficiency is a priority.
- Have a warm-up period, which results in slower start-up.
- Deliver a large amount of light over a wide area.
- Have a long life of between 5,000 and 24,000 hours.
- Require ballasts.
How do T12, T10, T8, and T5 fluorescent lamps differ?
These four lamps vary in diameter (ranging from 1.5 inches to 0.625 or five-eighths of an inch in diameter). Efficacy is another area that distinguishes one from another. T8 lamps offer a 5-percent increase in efficacy over 34-watt T12 lamps, and have become the most popular choice for new installations.
Choosing Light Sources for General Lighting, published in 1998 by the Illuminating Engineering Society of North America (IESNA), New York City, states that although they are available in lengths similar to the T12, T8s require a different ballast. For retrofit, the T10 is compatible with T12 ballasts. The T10 also uses a higher-efficiency phosphor and has a greater efficacy than the T12.
T5s are straight tube lamps with a high efficacy. Due to their unique sizes (available only in "metric" lengths) and special ballast requirements, the T5 is a favorable choice for retrofits for VHO fixtures.
GLOSSARY OF LIGHTING TERMS
Color rendering index (CRI) - a method for describing the effect of a light source on the color appearance of objects being illuminated, with a CRI of 100 representing the reference condition (and the maximum CRI possible). In general, a lower CRI indicates that some colors may appear unnatural when illuminated by a lamp.
Color temperature - see correlated color temperature.
Constant-wattage autotransformer (CWA) - the most common type of ballast used for HID lamps, it maintains a constant power (wattage) supply to the lamp when system input voltage fluctuates.
Correlated color temperature (CCT) - a description of the color appearance of a light source in terms of warmth or coolness, as measured on the Kelvin scale (K). As the temperature rises, the color appearance shifts from yellow to blue. Thus, lamps with a low CCT (3000K or less) have a yellow-white color appearance and are described as "warm"; lamps with a high CCT (4000K and higher) have a blue-white color appearance and are described as "cool".
Ballast factor (BF) - the ratio of light output of a lamp operated by a given ballast to the light output of the same lamp when operated by a reference ballast. Lamps operated by a ballast with BF of 0.90 will provide 90 percent of their rated light output (lumens). BFs between 0.85 and 1.0 are the most common.
Disability glare - light that is relatively bright compared to the background, making vision measurably worse. The inability to see clearly as a result of the brightness of headlights from an oncoming car at night is an example.
Discomfort glare - the type of glare that is uncomfortable and distracting, yet less obvious than disability glare. A bright light source in an individual's peripheral vision is an example.
Efficacy - the ratio of light output (lumens) to input power (watts) expressed in lumens per watt (LPW).
High pressure sodium (HPS) - a high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element.
Illuminance - The amount of light that reaches a surface. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience the IESNA uses 10 lux as the equivalent.
Instant-start - a method of starting fluorescent lamps. The voltage, which is applied across the electrodes to strike the electric arc, is up to twice as high as it is with other starting methods. The higher voltage is necessary because the electrodes are not heated prior to starting.
Lamp life - the median life span of a very large number of lamps. Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life.
Lamp lumen depreciation (LLD) - the reduction in lamp light output that progressively occurs during lamp life.
Lumen (lm) - a unit of measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation.
Luminance - the photometric quantity most closely associated with the perception of brightness, measured in units of luminous intensity (candelas) per unit area (feet squared or meters squared).
Luminaire efficiency - the ratio of the light emitted by luminaire to the light emitted by the lamp or lamps within it. Components of a luminaire, such as reflectors and diffusers, absorb some of the light from the lamps(s). A highly efficient luminaire emits most of the light that the lamp(s) emits.
Mercury vapor (MV) - a high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements.
Metal halide (MH) - a high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements.
Open-circuit voltage - the voltage applied across the output terminals of a ballast when no load is connected. This is the voltage applied across a lamp circuit to start a lamp. After starting, the voltage rapidly decreases and stabilizes at the operating voltage.
Preheat - a method of starting fluorescent lamps in which the electrodes are heated before a switch opens to allow a starting voltage to be applied across the lamp. With preheat starting, the lamp flashes on and off for a few seconds before staying lit.
Power factor - a measure of how effectively a ballast converts current and voltage into usable power to operate the lamps. A power factor of 0.9 or greater indicates a high power-factor ballast.
Prismatic lens - an optical component of a luminaire that is used to distribute the emitted light. It is usually a sheet of plastic with a pattern of pyramid-shaped refracting prisms on one side. Most ceiling mounted luminaries in commercial buildings use prismatic lenses.
Rapid-start - a method of starting fluorescent lamps in which the ballast supplies voltage to heat the lamp electrodes for one to two seconds prior to starting and, in most cases, during lamp operation. A rapid-start system starts smoothly, without flashing.
Reflected glare - often called veiling reflections, glare that results from light shining off polished or glossy surfaces.
Restrike time - the time required for a lamp to restrike, or start, after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted.
Visual comfort probability (VCP) - a system for estimating discomfort glare that predicts the percentage of people who are likely to find the lighting comfortable, against the percentage that find it uncomfortable.