Science of Light

  1. What is LED. LED stands for light emitting diode. LEDs are a solid state device and do not require heating of a filament to create light. Rather, electricity is passed through a chemical compound that is excited and generates light. LEDs are not bulbs or lamps in the true sense of the word and application. LEDs require a lot of work to make them ready to be used by the consumer. They are placed on a circuit board or other material that allows electricity to pass through at a specific voltage and current and with components required to operate them at specific voltages.
  2. What is a footcandle? The unit is defined as the amount of illumination the inside surface of an imaginary 1 foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Basically, the amount of light that a single candle would provide to a 1 foot radius sphere.
  3. What is lumen and lux? The unit of luminous flux in the International System is a lumen, which is equal to the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions. It is used to measure light bulbs as standalone light sources. Lighting fixtures are measured by lux output which is lumens per square meter. The higher the lux reading the more light the fixture is producing over a given area.
  4. What is a kelvin temperature? A measure of the color of a light source relative to a black body at a particular temperature expressed in degrees Kelvin (K). Incandescent lights have a low color temperature (approx. 2800K) and have a red-yellowish tone. Warm white LED Lamps have a color temperature between 2700-3500K. Lamps rated between 5000K and 6000K are viewed as white, while lamps above 6000K tend to have a blue cast.
  5. What is an LED driver? LED drivers are current control devices that replace the need for resistors. LED drivers respond to the changing input voltage while maintaining a constant amount of current (output power) to the LED as its electrical properties change with temperature.
  6. What is a PCB or Printed Circuit Board? PCB’s are made from various materials including fiberglass, copper and aluminum. The PCB has an electrical circuit imprinted in silver or copper etching. That circuit says how the LED will operate. The PCB is also the platform by which LEDs are employed in various applications. It can be rigid or flexible.
  7. What does “Full Spectrum” Mean? A light source or lamp that produces a light spectrum that covers the entire range of visible light (400-700nM) without gaps in its spectral output. White LEDs are inherently a full spectrum light source. Most HID lamps have many spectra that are weak or not present.
  8. What is Average Rated Life? An average rating, in hours, indicating when a percentage of a large group of lamps have failed, when operated at nominal lamp voltage and current. The life of an LED is defined as the operating time in hours for the lamp to reach L70 which designates 70% lumen maintenance (or 30% reduction in initial light output). Every lamp type has a unique mortality curve that depicts its average rated life.
  9. What is Correlated Color Temperature (CCT)? Color temperature is defined as the absolute temperature (expressed in Kelvin units) of a theoretical black body whose chromaticity most nearly resembles that of the light source. The CCT rating is an indication of how “warm” or “cool” the light source appears. The higher the CCT value, the cooler the lamp color will appear. The lower the number, the warmer the lamp color will appear. However, CCT is only one aspect of color. The true “color” of a light source is derived from a complicated relationship of CCT, CRI and spectral distribution.
  10. What is Color Rendering Index (CRI)? In general, CRI is a numeric indication of a lamp’s ability to render individual colors accurately relative to a standard. The CRI value is derived from a comparison of the lamp’s spectral distribution to the standard (e.g. a black body or the daytime sky) at the same color temperature. However, CRI is only one aspect of color. The true “color” of a light source is derived from a complicated relationship of CCT, CRI and spectral distribution. When we look at a light source, the eye “perceives” a single color. In reality, we are seeing literally thousands of colors and hues made up of a combination of different wavelengths of light. These different combinations and the relative intensity of various wavelengths of light are used to determine the CRI of a light source.
  11. What are Photopic Lumens? Photopic lighting is light measurement based upon the response of the human eye under daytime conditions. Published lumen ratings reflect photopic lumens.
  12. What are Scotopic Lumens? Scotopic lighting is the light measurement based upon the response of the human eye under nighttime conditions. Scotopic lumens are higher for high CCT light sources.
  13. What do Rods and Cones Have to do with Lighting? There are two types of light detectors in the eye, the cones and the rods. Cones dominate during daylight conditions and rods dominate at night and at low light levels. Photopic lumens are based on cone response and scotopic lumens are based on rod responses.
  14. What is an S/P Ratio? An S/P ratio is the ratio of light measured by the scotopic and photopic methods. Some common S/P ratios are listed below. Lamps with high S/P ratios (high scotopic content) also improve one’s ability to see both outdoors and indoors.
    – The S/P ratio of 5000K daylight light sources is 2.1.
    – The S/P ratio of 4000K light sources is 1.65.
    – The S/P ratio of 3000K light sources is 1.4.
    – The S/P ratio of yellow HPS light sources is 0.64.
  15. What does the S/P Ratio Tell Us? Lamps with high S/P ratios (high scotopic content) also improve one’s ability to see. Human studies have shown that the S/P values, though previously applicable to only nighttime conditions, can be used to describe the “effective lumens” of indoor lighting as well.
    The following examples describe the effect of S/P in relation to outdoor and indoor lighting.Outdoor Lighting: The result is that a 4000K light source is at least twice as effective for viewing at night as HPS sources. Similarly, a 5000K light source is more than 3 times as effective as HPS Sources.Indoor Lighting: If two rooms are lit separately with 4000K and 5000K lamps to equal photopic luminance, the 5000K room will appear 13% brighter [(S/P)0.5 = 1.13].

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