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_A [Middleton, 1952]. The values of R_M and R_A are respectively, the maximum distances at which non-luminous entities (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_A) 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 R_M and R_A, just one quantity 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_M and R_A in daylight, and the value of 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_A  to make the correct choice of R_M or R_A to represent RVR. Qualitatively, it is well known that R_M > R_A under conditions of high atmospheric or “background” brightness (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_M > R_A so that the choice of R_M or R_A to be reported to the pilot can be made automatically by the instrument, without the introduction of human error.

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_M. 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 R_M > R_A is represented by R_eq, and given by the criterion:

                                        R_eq = {(ICb/E_t) ^1/2                                                                              (1)

R_eq is the visual range at which R_eq = R_M = R_A and thus delineates the boundary separating the two regimes. The anisotropy of atmospheric brightness is represented by the factor 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_t the illuminance threshold of the eye. Equation (1) thus provides a quantitative basis for demarcating the regime R_M > R_A. It also provides a simple test of the software to verify that R_M or R_A is correctly selected as the RVR according to the prevailing conditions of background brightness and runway edge light intensity. Because b ³ 1, the anisotropy increases R_eq, while it reduces R_M. Thus the changeover from R_A to R_M as the RVR will occur at higher values of background brightness (which means higher values of E_t) than if the atmosphere were to be uniform.