Color of Light
One factor to consider when choosing the right light is the color of that light. All lamps, whether it is a light bulb or a complete fixture, produces light of a single color or range of color often referred to a color changing or color tuning. For white light, the color ranges from warm white light around 2700K (K stands for Kelvin) to cool white around 5000K. Warm white light between 2700K-3000K is more yellow in tone and commonly found in residential applications. Cool white light around 4200-5000K is bluer in tone and similar to daylight.
In addition to the degree of warmth or coolness of the white light, you will also want to consider how well the light renders color. The accuracy of color is measured by a Color Rendering Index (CRI) on a scale from 1-100. The higher the number, the more effective the light is at rendering color. Good color rendering is 80 CRI and above. Excellent color rendering is 90 CRI and above.
Efficacy – The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt.
Ambient Temperature – The temperature of the surrounding air that comes into contact with the lamp and ballast. Ambient temperature affects the light output and active power of fluorescent lamp/ballast systems. Each fluorescent lamp-ballast system has an optimum ambient temperature at which it produces maximum light output. Higher or lower temperatures reduce light output. For purposes of lamp/ballast tests, ambient temperature is measured at a point no more than 1 meter (3.3 feet) from the lamp and at the same height as the lamp.
Application – The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications.
Average Rated Life – The number of hours at which half of a large group of product samples fail under standard test conditions. Rated life is a median value; any lamp or group of lamps may vary from the published rated life.
Beam Angle – The angle at which luminous intensity is 50 percent of the maximum intensity.
Beam Spread – The width of a light beam, expressed in degrees. The beam of light from a reflector-type lamp (PAR, R, ER, or MR) can be thought of as a cone. The beam spread is the angular width of the cone. Common beam spreads are known as spot, narrow, narrow flood, and flood.
Bulb Shape – An abbreviation of the shape and size of a lamp’s outer envelope. The letter or letters indicate the shape and the numbers indicate the bulb’s maximum diameter in eighths of an inch.
Correlated Color Temperature (CCT) – A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT.
Color Rendering Index (CRI) – A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural.
Fixture – A complete lighting unit consisting of lamp or lamps and the parts designed to distribute the light, position and protect the lamp(s), and connect the lamp(s) to the power supply.
Kelvin – Color temperature is measured in degrees Kelvin, which indicate the hue of a specific type of light source. Higher temperatures indicate whiter, “cooler” colors, while lower temperatures indicate yellower, “warmer” colors.
Lamp Life – The median lifespan of a very large number of lamps (also known as the average rated life). 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. For discharge light sources, such as fluorescent and HID lamps, lamp life depends on the number of starts and the duration of the operating cycle each time the lamp is started.
Lumen (lm) – A unit 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.
PAR Lamp – A lamp with a glass bulb and an interior reflecting surface, led chip, and a lens to control beam spread. The lens is hermetically sealed to the reflector.
R Lamp – A common reflector lamp in which the sides of the outer blown-glass bulb are coated with a reflecting material to direct the light. The light-transmitting region may be clear, frosted, or patterned.