Circadian Index Ac

Circadian Activation Index


Ac is a comparison of a light source and a standard daylight (D65) effect on activating effect on human nervous system.

Various types of light at a given illuminance have different effect on melatonin blood level and their impact on human cyrcadian rhythm is different.

Ac of D65 daylight is 100.


Ac definition (1)


Ac definition (2)


  • X(λ) is relative spectral distribution of radiation power,
  • V(λ) is spectral luminous efficiency according to IEC 50(845),
  • C(λ) is melatonin supression action spectrum sms(λ) according to DIN V 5031-100.

Examples of Ac values for various sources of light.

Ac examples


  • GALL, Dietrich; LAPUENTE, Virginia. Beleuchtungsrelevante Aspekte bei der Auswahl eines förderlichen Lampenspektrums : Teil 1 & 2.
  • DIN V 5031-100. Optical radioation physics and illuminating engineering: Prart 100: Non-visual effect of ocular light on human beings - Quantities, symbols and action spectra. Berlin: Deutsches Institut für Normung, 2009, 16 p. DIN V 5031-100:2009-06.
  • IEC 50(845) Internation Electrotechnical Vocabulary: Chapter 845: Lighting.
  • FUKSA, Antonín. Light and the Biological Clock. Světlo (light). 2010, nubmer 6, p. 56-58. English translation available online.