Principles of Effective Paint Inspection Lighting

Charles Lloyd and Peter Boyce

From a 1996 technical report written for the Lighting Research Center, Troy, New York.


This document brings together the results of several analyses and evaluations conducted over the past year for the Ford Paint Inspection Lighting research program. The purpose of the report is to provide a "snapshot" of the state of our knowledge. The work on this program is by no means complete and several recommendations for continued research are provided. In this report we draw interim conclusions, illustrate concepts, and develop a "best current estimate" of the essential attributes of paint inspection facilities optimized for the inspection of small topographical defects.

Early in this research program the decision was made to concentrate on those factors which affect inspection performance. Thus, relatively little attention has been paid to the issues of visual comfort and fatigue during the first year of this program. Several factors which are thought to be determinants of discomfort and fatigue are briefly discussed here. However, the verification that these factors are important remains to be completed.

In this research effort we have concentrated almost exclusively on inspection for small (e.g., less than a few mm in diameter) topographical defects including embedded dirt and fibers, runs, and sags. This decision is supported by warranty return data which indicate that repairs of topographical defects account for the largest portion of warranty repair costs.

Attention has also been restricted to the inspection of high-gloss surfaces. We expect that the effective inspection of diffuse (matte) surfaces will require the use of a lighting system with different characteristics. It is not yet clear whether the results of these evaluations will apply to semi-gloss paints such as those used in primers.

Finally, we must caution the reader that the lighting system described here is likely inappropriate for the inspection of many types of defects which occur below the first­ surface of the paint. These other types of defects include color and reflectance non­uniformities in the pigment and metal flake. Similarly, the lighting system described here may be inappropriate for the inspection of the interior of the vehicle.

If a paint inspection facility is designed based on the recommendations provided here the designer can expect significantly increased visibility of topographical defects relative to the visibility found in typical inspection facilities. This increased visibility will lead to an increase in the number of defects repaired and a decrease in the number and severity of defects that get through the inspection facility.

With significantly improved visibility of topographical defects it is expected that the full length of the inspection deck will not be needed for finding topographical defects. If this turns out to be the case, then some portion of the inspection deck can be designed specifically for the detection of other types of defects such as polish swirl and pigment non­uniformities.