Computer Calculations versus Field Measurements for Tunnel Lighting Design
The purpose of this article is to discuss the differences between the design (calculated) and field-measured luminance of a recent tunnel lighting project. Field-measured illuminance values agreed reasonably well with the design (calculated) values. However, the field-measured luminance values, which are based on the reflected light from surfaces, did not concur with the design (calculated) values.
A roadway’s reflective characteristics are defined by its physical surface properties. Sufficient data have been collected on the reflective characteristics of different pavement types to allow them to be described by reflectance tables or “R” tables. Investigation and calculations for the tunnel lighting design were performed using the average luminance coefficient (Q0) from the roadway lighting standard RP-8-00 [ANSI/IES 2000], R-tables, and the Memphis (a mobile gonio-reflectometer) field measurements. An empirical factor was developed to account for the differences in the design and measured luminance results.
Although established standard parameters are helpful to start a tunnel lighting design calculation, it is important to note that not all design parameters associated with new construction are appropriate. Some tunnel lighting systems are rehabilitated without new roadway paving and/or without newly painted walls and ceiling. Using an established standard R-table may be economical but may produce inaccurate results. Therefore, various factors must be taken into account based on existing conditions of the tunnel proper.
Based on the findings from this project, it is critical to use the actual roadway and surface reflectance measurements within the tunnels to obtain meaningful computer calculations for tunnel rehabilitation projects; otherwise, the results can be substantially inaccurate and result in a design that does not satisfy the design criteria and/or industry standards.