What is the difference between a D65 illuminant and lux?

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FAQ: “I am a little confused on the distinction between D65 and lux. Can you explain?”

“D65″ refers to a CIE definition of the spectral quality of a specific type of white daylight source representing noon daylight on a sunny day rated at 6500 kelvin.

It is part of the CIE "D" series of illuminants representing daylight at different times of the day.

  • At sunrise and sunset, what is called "horizon light", daylight is much warmer, typically around 5000 kelvin.
  • At mid-morning and mid-afternoon, the spectral quality of daylight is cooler, rated at a higher 5500 K color temperature.
  • Noon daylight is rated 6500 K and overcast at 7500 K, moving towards to "cooler daylight" and a higher color temperature rating.

A measurement in lux is the intensity of the light expressed in absolute lux units.

D65 is a standardized measurement of the spectral quality of light while lux is a measurement of the intensity of the light. They are both terms used to rate different aspects of visible light.

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