Workplace Luminance Ratios: Do They Affect Subjective Fatigue and Performance?
Authors: Cushman William H. , Dunn James E. , Peters Kathleen A. 
Published: January 2012

Although recent advances have made it possible to automate many photofinishing processes, print-making machines that require operator judgments are still widely used, particularly where volume is relatively low. Operators of these machines examine back-lighted negatives to determine whether they require density and/or color corrections. The luminance of the task (in this case a photographic negative) is often as much as 200 times as great as the surround (i.e., luminance ratio = 200:1) Ergonomie guidelines (such as those issued by the Illuminating Engineering Society of North America) recommend that the task-to-surround luminance ratio not exceed 3:1 or 5:1 to ensure visual comfort.

One way to reduce the task-to-surround luminance ratio at printer workplaces is to increase the luminance of the area around the aperture of the printer gate (device that holds the negative). The luminous surround must be visible from the operator's seated position but not from above the gate where light sensitive cells that determine the initial exposure are located. In addition, any device used to improve visual comfort should not adversely affect operator productivity. A device which meets these requirements has been designed. It consists of a very thin electroluminescent panel and louver overlay (louvered EL panel, hereafter). Controls for varying the luminance of the EL panel have been added so that the task-to-surround luminance ratio may be changed.

In this study the EL panel was evaluated in a simulated production environment. A Graeco-latin square experimental design was used with task-to-surround luminance ratio, order of conditions, subjects, and negative sets as independent variables and operator performance (printing rate and errors) and subjective ratings (ocular discomfort and general fatigue) as dependent variables. Trained operators printed for four 2-hour periods using the EL panel. During three of the sessions the task-to-surround luminance ratio was fixed: 3.4:1 (near ideal), 15:1, or 110:1 (EL panel turned off). During the remaining session the operator was free to adjust the task-to-surround luminance ratio to any preferred value and make changes at any time.

As the luminance ratio was decreased (i.e., luminance of the area around the negative was increased), both the printing rate (number of prints per hour) and error rate declined slightly. In other words, the operators made fewer prints, but they also made fewer errors. Ocular discomfort and general fatigue were lowest when the subject was allowed to adjust the task-to-surround luminance ratio, but the differences between this condition and those involving fixed task-to-surround luminance ratios were very small. Printing rates increased slightly from session one through session four. Less ocular discomfort was reported during the last two sessions as compared with the first two sessions.

The results show that task-to-surround luminance ratios far in excess of those usually recommended have only a small effect on operator performance and reported subjective discomfort. However, designing a workplace so that operators may adjust the task-to-surround luminance ratio to accommodate individual differences in lighting preferences may have beneficial effects.