In flying operations, two visual ranges are of importance. One is the MOR, or meteorological optical range (*R _{M}*); the other is Allard's runway visual range, or

*R*[Middleton, 1952]. The values of

_{A}*R*and

_{M}*R*are respectively, the maximum distances at which non-luminous entities (

_{A}*e.g*., runway markings), and luminous objects (

*e.g*., runway edge lights) are barely identifiable. The MOR is operationally significant only under daylight conditions,

*i.e*., when the atmospheric brightness

*B*exceeds

*50 cd m*.

^{-2} Visual range (both *R _{M}* and

*R*) is of central importance to flight operations. Hence it is important to remove all ambiguities in determining and reporting its value. To avoid confusion between

_{A}*R*and

_{M}*R*, just one quantity

_{A}*viz*., Runway Visual Range or RVR (

*R*) is now reported [World Meteorological Organization (WMO)]. As defined by the WMO,

*R*takes on the value of the greater of

*R*and

_{M}*R*in daylight, and the value of

_{A}*R*

_{A}_{ }at night. Thus only the single value

*R*, as defined above, is reported to the pilot as the RVR, thereby eliminating ambiguity.

In daylight it thus becomes important to delineate the boundary between the regimes *R _{M }> R_{A }*

*and*

_{ }*R*<

_{M}*R*to make the correct choice of

_{A}*R*or

_{M}*R*to represent RVR. Qualitatively, it is well known that

_{A}*R*under conditions of high atmospheric or background brightness (

_{M}> R_{A}*B*), and/or low runway edge light intensity (

*I*). Under such conditions, runway markings can be perceived at greater distances than can runway lights. In this communication, we develop an explicit criterion to determine the boundary of the regime

*R*so that the choice of

_{M}> R_{A }*R*or

_{M}*R*to be reported to the pilot can be made automatically by the instrument, without the introduction of human error.

_{A} As recently reported [Pichamuthu, 2005], the MOR varies in different directions according to the atmospheric brightness in the direction of view. The criterion for *R _{M}*

*to be greater than*

*R*

_{A}_{ }is given in Eq. (1). The effects of directional variations of brightness on

*R*. are incorporated in the equation. The derivation of Eq. (1) will be presented in the paper. The visual range at the boundary of the regime

_{M}*R*is represented by

_{M}> R_{A }*R*, and given by the criterion:

_{eq}

* R _{eq} = {(IC*

*b/E*(1)

_{t})^{1/2 }

*R _{eq}* is the visual range at which

*R*and thus delineates the boundary separating the two regimes. The anisotropy of atmospheric brightness is represented by the factor

_{eq}= R_{M}= R_{A}*b.*If, as in the classic Koschmieder theory, uniform atmospheric brightness is assumed,

*b = 1*. The contrast between the background and the object being viewed is

*C*,

*I*is the runway edge or center line light intensity and

*E*the illuminance threshold of the eye. Equation (1) thus provides a quantitative basis for demarcating the regime

_{t}*R*. It also provides a simple test of the software to verify that

_{M}> R_{A}*R*or

_{M}*R*is correctly selected as the RVR according to the prevailing conditions of background brightness and runway edge light intensity. Because

_{A}*b ³ 1*, the anisotropy increases

*R*, while it reduces

_{eq}*R*. Thus the changeover from

_{M}*R*to

_{A}*R*as the RVR will occur at higher values of background brightness (which means higher values of

_{M}*E*) than if the atmosphere were to be uniform.

_{t}