Ricco's law

Threshold data from Table 8 of Blackwell (1946)[1] plotted as Figure 4 of Crumey (2014).[2] Curves are for background luminances ranging from 3.426 × 10−5 cd m−2 (top) to 3.426 × 103 cd m−2 (bottom) at intervals of one log unit. The straight dotted sections correspond to Ricco's law.

Riccò's law, discovered by astronomer Annibale Riccò, is one of several laws that describe a human's ability to visually detect targets on a uniform background.[3][4] It says that for visual targets below a certain size, threshold visibility depends on the area of the target, and hence on the total light received. The "certain size" (called the "critical visual angle"), is small in daylight conditions, larger in low light levels. The law is of special significance in visual astronomy, since it concerns the ability to distinguish between faint point sources (e.g. stars) and small, faint extended objects ("DSOs").

  1. ^ Blackwell, H. Richard (1946). "Constant thresholds of the human eye". Journal of the Optical Society of America. 36 (11): 624–643. Bibcode:1946JOSA...36..624B. doi:10.1364/JOSA.36.000624. PMID 20274431.
  2. ^ Crumey, Andrew (2014). "Human contrast threshold and astronomical visibility". Monthly Notices of the Royal Astronomical Society. 442 (3): 2600–2619. arXiv:1405.4209. doi:10.1093/mnras/stu992.
  3. ^ Riccò, A. (1877). "Relazione fra il minimo angolo visuale e l'intensità luminosa". Memorie della Societa Degli Spettroscopisti Italiani. 6. Bibcode:1877MmSSI...6B..29R.
  4. ^ Schwartz, Steven H. (2004). Visual Perception: A Clinical Orientation (3 ed.). McGraw-Hill Professional. pp. 46–47. ISBN 0-07-141187-9.