Work Station Design For VDU Operators: Part II

Flicker

A flicker image on a screen will diminish the legibility and cause fatigue and asthenopia, depending upon the length of time that the screen is viewed.
The flicker on the VDU screen is dependent upon design characteristics of the display and personal factors.
The design characteristics to be taken into account include the phosphor type, refresh rate, character size and color, area of screen illuminated and viewing angle. The personal factors include the critical fusion frequency (CFF) and tolerance to a flickering display. CFF is the lowest frequency at which a flickering stimulus is perceived as continuous. It varies between individuals and is affected by such factors as pupil size, age and general health.

The use of dark characters on a light background (positive contrast or polarity) ensures good legibility, as reflections are less obvious and edges of the characters appear sharper. However, positive contrast tends to enhance the flicker effect. It has therefore been suggested that negative contrast displays should be used. These have another advantage in that the legibility is superior for people with low vision.

Color contrast

Color contrast is not essential in determining the legibility of a display; it is the overall contrast that is more important. The color of the phosphors used for VDUs varies. Green phosphors are usually used for monochromatic displays as they are most readily available. From surveys it appears that white, yellow or green on a neutral background are the preferred combinations, with green being favored most of all. The reason for the preference for green phosphor is the relationship between accommodation and chromatic aberration. It has been shown that the wavelength in focus on the retina varies with the state of accommodation.

When the eye is un-accommodated and fixating a distant target, a wavelength of about 650nm is in focus. However, when the eye accommodates for targets at closer distances, the wavelength in focus gradually shifts towards the short wavelengths. At a distance of 0.5m the wavelength in focus is about 520m, i.e. green phosphors and would be most suitable. Blue phosphors would induce a small amount of myopia if the operator accommodates by the amount normally appropriate for 0.5m.

Alpha-numeric displays

The characters on the displays should be:

1. Visible; characters should be readily detectable from the background. 
2. Legible; it should be easy to identify each character. 
3. Readable; spacing of characters, etc. should permit easy reading.

Careful consideration has to be given to the design of the various features of the characters and their arrangement.
It is possible to generate scrolling on a VDU screen so that the text moves rapidly up the screen. It has been suggested that increased rates of scrolling lead to poor performance and increased stress.

Ergonomics

To ensure that the design of the work station is operational, the VDU operator should:

1. Have easy access and regress,
2. Be able to reach and operate all controls with ease,
3. Be able to see and reach all the displays,
4. Be able to work comfortably.

The very common complaints of back- and neck-ache may be due to incorrect work station design, perhaps due to chair position or the height/angle of the VDU being fixed without any method of adjustment/alteration.


Proceed to: Part III
Back to: Part I