Are you confused with these terms? Well you are not alone. Today we’re going to discuss these terms and integrate physics concepts with Dark Sky Lighting Principles for Responsible Outdoor Lighting.
Kelvin temperature is used to describe the "color temperature" of artificial light. It is not the actual temperature of the light. A lower Kelvin (K) value corresponds to warmer, more yellowish light, while a higher Kelvin (K) value corresponds to cooler, bluer light. This is called Correlated Color Temperature (CCT) for modern light sources like LEDs. Wein’s Law is a physics principle that further explains this concept and will be presented by Dr. Bob Grossman in an upcoming Western Slope Skies broadcast. For now, the important concept to grasp is that color temperature is not an actual temperature. The Kelvin (K) value only indicates the color of the light. Light in the blue spectrum, associated with electronic devices, is a contributing factor for Circadian Rhythm disturbances, while light in the warmer red-yellow spectrum is considered better for health.
Watt (W) is the standard unit of power, defined as the rate at which energy is transferred or work is done. One watt is equal to one joule of energy per second (1 W = 1 J/s). This means a device consuming 100 watts uses 100 joules of energy every second; it's a fundamental concept for understanding how quickly energy is converted or used in any system, from a light bulb to a motor.
Lumens measure a light bulb's brightness, while watts measure the energy it consumes. With older incandescent bulbs, higher wattage also meant higher brightness. However, with modern, energy-efficient technology like LEDs, this is no longer the case. You should now choose bulbs based on the lumen rating, not the wattage, to get the brightness you want. Remember, Watts measure energy consumption and Lumens measure brightness. Also in relationship to the concept of Kelvin temperature, you can have two bulbs that are equally bright but have very different color temperatures.
Rule of thumb to compare incandescent and LED lights:
• A 100W incandescent bulb equals an LED bulb of about 1,600 lumens.
• A 75W incandescent bulb equals an LED bulb of about 1,100 lumens.
• A 60W incandescent bulb equals an LED bulb of about 800 lumens.
• A 40W incandescent bulb equals an LED bulb of about 450 lumens.
International Dark Sky recommends Five Lighting Principles for Responsible Outdoor Lighting:
1) Useful. Use light only if needed. All light should have a clear purpose. Consider how the use of light will impact the area, including wildlife and their habitats. For example, is it really necessary to light up your deck like a cruise ship or the Las Vegas Strip?
2) Targeted. Direct light so that it falls only where it is needed. Use shielding and careful aiming to target the direction of the light beam so that it points downward and does not spill beyond where it is needed. Light trespass is defined as the unwanted spill of artificial light that extends beyond the boundaries of the property where it is intended to illuminate, causing annoyance, loss of privacy, or other nuisances.
3) Low Level. Light should be no brighter than necessary. Use the lowest light level required. This is where the concept of lumens applies. Be mindful of surface conditions, as some surfaces may reflect more light into the night sky than intended.
4) Controlled. Use light only when it is needed. Use controls such as timers or motion detectors to ensure that light is available when it is needed, dimmed when possible, and turned off when not needed.
5) Warm Colored. Use warmer color lights where possible. Limit the amount of shorter wavelength (blue-violet) light to the least amount needed. The recommended Kelvin (K) (temperature) level is less than 3000 Kelvin (K).
The Western Slope Dark Sky Coalition (WSDSC) appreciates your assistance in protecting our magnificent night sky. (We are available to share information with you at https://www.westcoloradodarksky.org and https://darksky.org)