Daylight Factors

Ian Ashdown, P. Eng., FIES

Chief Scientist, Lighting Analysts Inc.

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Nine out of ten daylight simulation programs agree … and therein lies a story worth retelling.

Daylight in History

The story begins in the sixth century with the publication of Corpus Juris Civilis (“Body of Civil Law”) by order of the eastern Roman emperor Justinian I [Scott 1932]. Written in four volumes, it included the Digest, being extracts from the writings of earlier Roman jurists. Book VIII, Title 2, “Concerning Servitudes of Urban Estates,” includes this legal distinction between daylight and views:

Light is the power of seeing the sky, and a difference exists between light and view; for a view of lower places may be had, but light cannot be obtained from a place which is lower.

In a time when artificial lighting consisted of oil lamps, access to daylight was a critical issue. The Digest therefore had some 40 legal rulings on the rights of property owners concerning daylight. In some cases, a property owner whose newly-constructed building blocked a neighbor’s access to daylight could be legally compelled to tear the building down.

Somewhat surprisingly, these rulings survived over the centuries to become what is referred to as the “ancient lights” law in European legal traditions. Modern use of this concept dates back to the British Prescription Act of 1832, which reads in part:

When the access and use of light to and for any dwelling house, workshop, or other building shall have been actually enjoyed therewith for the full period of twenty years without interruption, the right thereto shall be deemed absolute and indefeasible, any local usage or custom to the contrary notwithstanding, unless it shall appear that the same was enjoyed by some consent or agreement expressly made or given for that purpose by deed or writing.

This enactment led of course to a new profession: Rights to Light surveyors. These chartered professionals served as expert witnesses in legal disputes, and offered advice to architects. The surveyor Robert Kerr wrote a book on the topic in 1865, in which he took 55 pages to explain the practice of surveying access to daylight [Kerr 1865].

Sky Factors

Complexity begets uncertainty however, and so it was that a chartered surveyor and lighting engineer named Percy J. Waldram proposed a much simpler way of determining adequate access to daylight [Waldram 1909]. It is today known in the United Kingdom as a “sky factor,” and is defined as:

Sky factor: the ratio of the illuminance Eindoor of a horizontal plane at a given point inside a building due to the light received directly from an overcast sky of uniform luminance sky, to the illuminance Eoutdoor of the point due to the unobstructed sky.

DF = ( Eindoor / Eoutdoor ) * 100 %


Waldram and his son later suggested that a sky factor of 0.2 percent was sufficient “for ordinary purposes, comparable to clerical work” [Waldram and Waldram 1923]. Their seemingly offhand comment that this was the level at which “average reasonable persons would consistently grumble” became what is now referred to by British light surveyors in all seriousness as the “grumble point” [Chynoweth 2004].

The sky factor metric was accepted by the Commission Internationale de L’Eclairage in 1932 [CIE 1932], and is still widely used in the United Kingdom by chartered surveyors. (It is however no longer recognized as a unit of light measurement by the British Standards Institution.)

Waldram’s choice of 0.2 percent was simply a rule-of-thumb guess [Waldram and Waldram 1923], with no supporting research [Chynoweth 2005]. The Royal Institution of Chartered Surveyors recommends at least 0.5 percent (which on average is about 25 lux) [RICS 2010], but the British legal system still works on the assumption of 0.2 percent, or 10 lux [Chynoweth 2009]. This leads to the curious situation of a complex legal system that deliberately encourages and enforces poor daylighting practices.

Daylight Factors

The sky factor metric is important in the United Kingdom because “rights to light” is a still-valid legal concept as an “easement right” that has descended from the rulings of ancient Roman jurists. This is not however the situation in American civil law, where access to daylight is considered a right only in exceptional circumstances [Unger 2005].

Regardless, the closely related “daylight factor” metric is widely used [CIE 1970]. This has the same basic definition as the sky factor above, with the exception that the CIE Standard Overcast Sky is used instead of a uniform luminance sky [IES 2013]. It also takes into account ground reflections, window transmittance, and interreflections from room surfaces.

This distinction is important. Both sky types are defined in CIE Standard S 011 [CIE 2003], where the CIE Standard Overcast Sky (Standard Sky Type 1) is described as, “steep luminance gradation towards zenith, azimuthal uniformity.” The sky factor metric however assumes CIE Standard Sky Type 5, “sky of uniform luminance.” (The “traditional” CIE Standard General Sky is now referred to as CIE Standard Sky Type 16; its luminance distribution near the horizon varies slightly from Sky Type 1.)

The advantage of a uniform luminance sky is that determining the sky factor for a given room is simply a matter of geometry. That is, the value of the sky factor is completely independent of the sky luminance distribution. It can in theory be calculated with the aid of Waldram diagrams using photographs or hand drawings [e.g., RICS 2010].

Waldram Diagram

The same is not true of course for the calculation of daylight factors. The luminance distribution of the CIE Standard Overcast Sky varies continuously from the horizon to zenith, and so computer calculations with a 3D CAD model of the room are essential. (Accurate calculation of interreflections from room surfaces also requires computer calculations.)


As a lighting design tool, the daylight factor metric has numerous limitations. As noted by IES RP-5-13, Recommended Practices for Daylighting Buildings [IES 2013], it addresses only a single sky condition that is prevalent in its country of origin. It does not consider such daylighting design issues as direct and reflected sunlight, latitude, building orientation, time and date, or climatic conditions.

These are serious limitations in that satisfying a daylight factor requirement may result in excess daylight under clear sky conditions. IES RP-5-13 therefore recommends modern climate-based annual daylight performance metrics such as spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE) in combination with Typical Meteorological Year (TMY) weather data files specific to a given geographical location [IES 2012].

As an aside, it is a myth that the sky factor metric was developed specifically for northern European climates. Prior to the widespread availability of electric lighting, access to diffuse daylight was preferred to that of direct sunlight. (The studios of artists and photographers for instance had north-facing windows and skylight wherever possible.) As the Victorian-era Keller wrote [Keller 1865]:

In fact, it is this diffused daylight which is constituted, by the express intent of Nature, the standard medium of human vision; for where there is one purpose of sight specially served by the direct and unobscured light of the sun, there must be a thousand for which the eye prefers the more genial agency of the diffused light of the atmosphere.

(Is it any wonder that Keller needed 55 pages to explain the practice of rights to light surveying?)

Even by the time of Waldram a half-century later, instruments for measuring daylight were not commonly available. Assuming a uniform luminance sky that allowed purely geometric calculations was therefore a matter of practical necessity.

Limitations aside, reports of the death of the daylight factor metric have been greatly exaggerated – it is still a useful if primitive tool, especially for students learning the basics of daylighting design. Climate-based annual daylight performance metrics such as sDA and ASE may be recommended for detailed analysis of architectural designs, but the daylight factor metric provides the necessary sanity checks.

Nine Out Of Ten

As noted previously, calculating daylight factors requires a 3D CAD model and daylight simulation software, if only to accurately model the spatial luminance distribution of the CIE Standard Overcast Sky and room surface interreflections. The obvious question is, how accurate are daylight simulation programs?

This question was investigated some sixteen years ago, and the results were not encouraging. Point-by-point errors for clear sky conditions were as much as 18 times, and for overcast sky conditions as much as 10 times the measured values. Much worse, the average error for clear sky conditions was about ten times for two of the four programs investigated.

Today, the situation is markedly different. A recent study [Iverson et al. 2013] investigated the ability of nine daylight simulation programs to calculate the daylight factor metric in five typical rooms. Going well beyond the basic requirements of CIE 171, “Test Cases to Assess the Accuracy of Lighting Computer Programs” [CIE 2006], this study is exemplary of how the accuracy of lighting design and simulation software should be assessed.

The study included nine daylight simulation programs:

  • Radiance
  • Daysim
  • Desktop Radiance
  • IESve
  • DIALux
  • Relux
  • Ecotect
  • LightCalc

Given that these programs use a wide variety of radiosity and ray tracing techniques, you might hope to see at least reasonable agreement among their daylight factor predictions.

What the study revealed however was stunning: all but one of the programs agreed to within a few percent of each other.

Lighting Analysts’ AGi32 was not included in the study, but the lead author of the report kindly provided the CAD files for the test rooms so that Lighting Analysts could perform its own tests. The results were the same: agreement to within a few percent.

Test Room Example

Nine out of ten lighting programs agree … this is important news for lighting designers and architects involved in daylighting design.

The test models used in the study took into consideration room dimensions, surface reflectances, glass transmittance, and exterior obstructions. What this study in effect says is that whatever daylight simulation program (with one important exception – and it was not LightCalc) is chosen, daylight factor calculations will be within the ±10 percent accuracy range expected for such programs [Reinhart and Andersen 2006].

The full “Daylight Calculations in Practice” study is available here, and the Lighting Analysts follow-up study is available here.


Chynoweth, P. 2004. “Progressing the Rights to Light Debate – Part 1: A Review of Current Practice,” Structural Survey 22(3):131-137.

Chynoweth, P. 2005. “Progressing the Rights to Light Debate – Part 2: The Grumble Point Revisited,” Structural Survey 23(4):251-264.

Chynoweth, P. 2009. “Progressing the Rights to Light Debate – Part 3: Judicial Attitudes to Current Practice,” Structural Survey 27(1):7-19.

CIE. 1932. Recueil des Travaux et Compte Rendu des Seances, Huiteme Session (Collection of Works and Minutes of Sittings, Eighth Session). Cambridge, UK: Cambridge University Press.

CIE. 1970. Daylight. CIE 016-1970. Vienna, Austria: CIE Central Bureau.

CIE. 2003. Spatial Distribution of Daylight – CIE Standard General Sky. CIE S011/E:2003. Vienna, Austria: CIE Central Bureau.

CIE. 2006. Test Cases to Assess the Accuracy of Lighting Computer Programs. CIE 171:2006. Vienna, Austria: CIE Central Bureau.

IES. 2012. IES Spatial Daylight Autonomy (sDA) and Annual Sunlight Exposure (ASE). IES LM-83-12. New York, NY: Illuminating Engineering Society of North America.

IES. 2013. Recommended Practice for Daylighting Buildings, IES RP-5-13. New York, NY: Illuminating Engineering Society of North America.

Iversen, A., N. Roy, M. Hvass, M. Jørgensen, J. Christoffersen, W. Osterhaus, and K. Johnsen. 2013. Daylight Calculations in Practice. Technical Report SBi 2013:26. Copenhagen, Denmark: Danish Building Research Institute, Aalborg University.

Kerr, R. M. 1865. On Ancient Lights and the Evidence of Surveyors Thereon. (Available as

Reinhart, C. F., and M. Andersen. 2006 “Development and Validation of a Radiance Model for a Translucent Panel,” Energy and Buildings 38(7):890-904.

RICS. 2010. Rights of Light – Practical Guidance for Chartered Surveyors in England and Wales, First Edition. Conventry, UK: Royal Institution of Chartered Surveyors. (Available as Rights of light guidance note 2010.pdf.)

Scott, S. P. The Civil Law, Including the Twelve Tables, the Institutes of Gaius, the Rules of Ulpian, the Opinions of Paulus, the Enactments of Justinian, and the Constitutions of Leo. English Translation. Cincinnati, OH: The Central Trust Company. (Available from

Ubbeholde, M. S., and C. Humann. 1998. “A Comparative Evaluation of Daylighting Software: Superlite, Lumen Micro, Lightscape and Radiance,” Proceedings of the International Daylighting Conference (Daylighting ’98), pp. 97-104.

Unger, S. C. 2005. “Ancient Lights in Wrigleyville: An Argument for the Unobstructed View of a National Pastime,” Indiana Law Review Vol. 38, pp. 533-564.

Waldram, P. J. 1909. “The Measurement of Illumination, Daylight and Artificial, with Special Reference to Ancient Light Disputes,” Journal of the Society of Architects 3:131-140.

Waldram, P. J., and J. M. Waldram. 1923. “Window Design and the Measurement and Predetermination of Daylight Illumination,” The Illuminating Engineer, Vol. XVI, pp. 90-122.


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