Robert E. Levin

Position Index in VCP Calculations

Position Index (P), the spatial sensitivity factor used in the determination of the index of sensation for a source, is not unambiguously defined for all directions in the visual field. The working definition is a set of curves indicating contours of constant position index. Numerical values have been tabulated for the standard room tables of visual comfort probability (VCP) and serve as a precise definition for a limited number of directions in the visual field. It is desirable to define the position index by a functional relation over the entire field of view. Here, four analytic representations are considered in terms of spatial coverage, errors with respect to known representations, simplicity, and the like.

Cavities, Coefficients, and Direct Ratios

A critical examination of the Zonal-Cavity coefficient concept identifies misconceptions that are widely held with regard to the direct ratio. The significance of independent parameters other than luminaire spacing-to-mounting height ratio are investigated for the direct ratio weighting factors (zonal multipliers). These include room length-to-width ratio and luminaire-to-wall spacing. It is shown that the presently neglected luminaire-to-wall ratio is of equal or greater significance than the luminaire mounting height parameter now used.

Field Study of Luminaire Dirt Depreciation

A field study of luminaire dirt depreciation was conducted over a three year period and included over 200 sites in the U.S. Recessed 2 x 2 and 2 x 4 fluorescent luminaires, both louver and lens-panel types, were incorporated in the study. The sites included offices, schools and retail establishments. The IESNA recommended luminaire dirt depreciation factors now in use were formulated in the middle of the last century. Analysis of the results from this present study shows these existing recommendations significantly overestimate the light loss due to dirt deposited on luminaire surfaces. The improvement found in luminaire dirt depreciation factors is believed to be principally due to changes in the indoor environment since the 1950s when the original luminaire dirt studies were made. The significance of these results is that the increase in the initial design light level over the required maintained light level is less in those applications where the results of this study are applicable.

An Evaluation of VCP Calculations

There have been many indications that VCP values calculated at various laboratories may differ significantly. The objectives of this paper are (1) to illustrate the magnitude of such differences under the present state-of-the art, (2) to describe the more important causes of such differences, and (3) to indicate some of the steps which can be taken to reduce such differences in the future.

On the Non-Rectangular Cavity Approximation

This paper shows the origin of the approximation of effective cavity reflectance for rectangular cavities is based on a two surfact flux transfer model independent of the surface shape. Results of the approximation are compared to the three surface Zonal-Cavity flux transfer model demonstrating, for a range of cavity forms, small error for cavities of nonuniform surface reflectance and negligible error for cavities of uniform surface reflectance. It is also shown that the approximation is an exact solution for specific cavity forms. Further, the results of the approximation are compared to solutions without the Zonal-Cavity restriction of uniform exitance and illuminance over each surface.

On Fluorescent Photometry

Calibration for relative fluorescent luminaire goniophotometry is generally derived from the flux to intensity ratio for fluorescent lamps. Conventional photometric practice is to use a generic factor of 9.25, but the factor for a specific lamp is sometimes utilized.