Color Considerations in Alarm Design

I’ve known Emerson’s Cindy Scott a long time in our days building the DeltaV brand, and even earlier. I’ve not known Cindy to be versed in the use of the Queen’s English by spelling words such as color, colour. I can only assume the article I read, Don’t let colours hide the alarms!, had the hand of an editor to convert Cindy’s words.

The technology change for operator display screens has been dramatic from the introduction of distributed control systems in the 1970s. The early versions had eight colors choices to choose from including “blink”. So red and blinking red were two of your eight choices.

Today’s human machine interface (HMI) software supports what the current PC graphics cards support, which is typically 16.7 million colors. And of course, these 16.7 million colors can blink, or bounce, or rotate, etc. Also, display resolution has increased from a typical 640×480 pixel grid, to 1280×1024 pixels, up to 1680×1050 widescreen.

Many company and plant standards were developed while the display technologies were less mature. The ANSI/ISA-S5.5 Graphic Symbols for Process Displays standard came out in 1985, which was also during this early display technology stage. These standards have not kept pace with the technology changes and are in revision by the ISA101 committee. Cindy wrote:

When systems were upgraded, the plant display standards were not updated to incorporate the capabilities of the new display technologies, so the basic display layout and colour schemes were maintained. Although this makes sense to minimise cost and operator disruption, it forgoes many of the potential benefits of current graphics capabilities.

Having all these technology advances and extra colors does not necessarily improve the performance of operators. The article asked some thought-provoking questions when it comes to color and the design of plant operator screens:

  • How many colors are too many?
  • Which are the ‘right’ colors to use?
  • When is it appropriate to use dynamic color?
  • How much data is too much data?
  • What alarms are needed and where should they be shown?

The article does not answer all these tough questions, but does illuminate some of the existing research and does offer ideas for process manufacturers to consider in their operator display design standards.

Cindy cited the research of North Carolina State University’s Christopher Healey in ‘preattentive” visual processing:

Healey points out that for a particular item to be picked out rapidly from a group of socalled distracters, there must be a single difference between the desired item and the distracters. Thus a red item can be spotted readily among a group of blue items of the same size and shape, while a round item can quickly be picked out of a group of square items of the same size and colour. On the other hand, a red circle in a group of red squares and blue circles will not leap out from the screen and must be found by searching.

Cindy also shared the work of the University of Southern California’s iLab, which researched the effect of color, shape, and object orientation. Changes in an object’s color can be lost if the color matches other background colors. If the change has a unique color and shape, it is much more easily detectable.

Applying the results of this research is not always easy in operator display alarming. Operators are used to the status quo and may resist changes. Automation suppliers may also want to keep the existing standards that may favor their automation systems. Change occurs through a process of analysis, review, and buy-in among the operations, maintenance, and plant engineering staffs.

Cindy’s key takeaway is to be judicious in the use of colors. She concludes:

It is important for control system display designers to resist the temptation to add colour to everything, and to use it only where it conveys useful information. Bright colours should be reserved for things that must be seen such as alarms, and must not be masked by using the same colours for non-critical items. Reducing operator errors and accidents hinges on an understanding of visual perception mechanisms.

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