Letters on safety signs should be 100mm tall to be safely read within 25 meters - according to the old Guidance to the Fire Precautions. But the guidelines were set out in the days when signs read just EXIT and FIRE EXIT.

What about the new graphical symbol-based signs as required by the Health and Safety (Safety Signs and Signals) regulations?

Guidance on the correct use of fire safety signs has been omitted by the otherwise excellent BS 5499: Part 4 2000 that so expertly meets all the other requirements of the regulations.

Perhaps I can offer some help.

Legibility Data From Eyesight Testing

Luckily there is plenty of research data available from eyesight tests on millions of people. Indeed this is the basis for the old and widely accepted 100mm for 25 meter observation distance rule. It has been found that people with normal (or corrected to normal) vision can reliably resolve a detail that subtends an angle of 1 minute. Using simple geometry, we can work out that the relationship of the observation distance (D) to the detail height (d) for normal sighted, is given by the relationship ran 1 minute = d/D. This gives D = 3437.75 d.

So what about graphical symbols?
Are they more difficult to 'read' than letters of the alphabet?

The height of the alphabet letter E (h) has five details to resolve, i.e.. three lines and two spaces so, for normal sighted people, the letter E will be reliably resolved when D = 687.55h.

Letters such as B and D are a little more difficult to resolve than the Letter E and a margin of 33% was placed on this extra difficulty.

So, for reliable resolution of all the letters in the alphabet, again for normal sighted individuals, the relationship becomes D = 687.55 x 0.75 x h i.e... D = 515.66h.

For good measure, at the time of the first edition of BS 5499, a further safety factor of 2 was introduced to give the famous D = 250 h or for the purists, D = 257.83h.

Incidentally this data refers to letters which have a font without serif such as Helvetica Medium which should be used for supplementary text signs accompanying the new graphical symbol safety signs.

This means that the height of text on a sign is designed for a population with a vision that is half that of normal sighted individuals.

Again, from the data collected from eye testing, using the famous 'Snellen' letter chart, this level of visual acuity is present in 95% of people aged between 18 and 79 with vision corrected. If these people were not wearing their glasses, the percentage of the population reliably catered for would drop to 76%.

Legibility of graphical symbols

So what about graphical symbols? Are they more difficult to 'read' than letters of alphabet?

Much of the research on eyesight has been carried out in Japan with audiences asked to resolve characters which are apparently much more complex than the English alphabet.

But data suggests that the legibility distances for people reading characters are not significantly different from the legibility distances for those reading letters of the alphabet.

So the human eye, working with the brain to match pattern detail to memory, can recognise and understand symbols. The important point, however is to use graphical symbols with excellent comprehensibility credentials.

For some years, graphical symbols forming part of International Standards for public information, have been tested before standardisation. The majority of the graphical symbols used for the fire safety signs of BS 5499 Part 4 2002 have been tested and form the basis for ongoing International Standards work in this area.

Using work done by a number of researchers, including KFH Munrel (1965) and D Fletcher (1972) using numerals, 'a reasonable' size based on a number of 10 was established making some allowance for viewers with poorer vision.

Introducing a further safety factor of 2, the visual angle to be subtended for a well understood graphical symbol should therefore be 20 minutes.

This would then give a viewing distance, for reliable understanding of 17 metres for a symbol of 100mm in height or the relationship D = 172s (where s is the symbol height).

Given that the graphical symbol occupies at least 75 - 80% the overall height of the safety sign, then as a function of the height of the sign, the observation distance of the safety signs given by the relationship D = 130h where h is the overall height of the sign.

Data for recognising the existence of a safety sign in the area

In an occupied area, it is important to recognise the existence of a safety sign in peripheral vision, before it can be read. Is the sign conspicuous?

Luckily this area has been well researched too, since the armies of the world need to understand the detection threshold of targets and resolution/recognition threshold.

The colour and shapes of the sign, as well as contrast with the surroundings, have an important part to play in making the sign conspicuous.

The colour and shapes of the sign, as well as contrast with the surroundings, have an important part to play in making the sign conspicuous.

In particular, the sign's border must be of contrasting colour to the sign colour and its surroundings.

Given these features, it has been established that the resolution threshold size beyond which reliable resolution of the target will not be assured 26 minutes of visual angle for 15 degrees of eccentricity from the direct line of sight.

This corresponds to a sign height given by the relationship D = 132h where h is the overall height of the sign.

We can therefore conclude that a safety sign that has a well understood graphical symbol, that is designed according to the standard practices of conspicuous colour and shape, will be reliably conspicuous within 15 degrees of the direct line of sight and, when noticed and positioned in the direct line of sight, will be legible and understood if D = 130h.

Safety signs with supplementary text alongside the graphical symbol, often with additional arrow are increased in length and, therefore, visibility. This type of safety sign is significantly more conspicuous and can be resolved well outside the limits set out above.

Illumination of safety signs

A discussion of reliable resolution would not be complete without some mention of illumination.

Here once again, there has been a great deal of research in establishing the relationship that exists between visual acuity, optical brightness and brightness contrast of the information to be read and understood.

If an area is in need of a safety sign then adequate lighting should be installed.

In brief, the above relationship of D = 130h does not apply for lighting conditions that are better or poorer than good lighting. As we will see later, signs in poorly lit areas may need to be up to 25% larger and, in extremely well illuminated signs, a smaller size could be used and still be conspicuous and legible. However to understand and use this information effectively, it is important to know a little more about lighting and the response of the human eye.

Many people are conditioned to think mainly in terms of lux (the measure of illuminance) as the key measure of lighting and visibility. In fact, illuminance, in lux, is the amount of light that is incident or falling onto a surface. The eye actually actually perceives what is reflected or emitted from that surface - its optical brightness or luminance. The units for luminance are candelas/m2.

Good general lighting of 100 lux, on a clear white surface would provide some 10 million times this threshold level at 30cd/m2. Excellent lighting as required for detailed work, such as a shaving mirror, would be 130 cd/m2 and for surgery some 300 - 400 cd/m2. The human eye starts to feel glare and some discomfort, above 1,000 cd/m2.-

With these numbers in mind and using the research of the Japanese, we can apply corrections to the normal relationship of D = 130 h as shown in the table below.

It is of course, obvious that if an area is in need of a safety sign, then adequate lighting should be installed. The table should in no way be used to overcome inadequate normal lighting of a risk or a safe area such as an evacuation route.

Luminance contrast of safety signs

By design and well dealt with by the British Standard 5378 Part 1, the safety sign has colours that provide excellent luminance contrast. The luminance contrast is obtained by comparing the luminance of the graphical symbol detail with the luminance of the background.

For a safety sign that is illuminated by reflecting light, then this comparison is given by the ratio of the luminance factors of the colours and surface of the sign. A luminance contrast of 80% is considered excellent and is part of the design of safety signs set out in BS 5378 or International Standard 3864.

With safety signs illuminated by transmitted light from rear illumination, often luminance contrast is lost if luminance is high or too little consideration has been given to the light transmission properties of the safety sign image. A reduction of luminance contrast to 60% would reduce the reliable observation distance by some 20%. Many rear illuminated safety signs are appearing as part of emergency lighting provisions of the nonmaintained variety. It is clear that these sings are neither conspicuous nor translucent, their luminance contrast well in excess of the excellent 80%, giving an improvement in legibility of a further 10 - 15%.

Safety signs need to be cleaned as part of the standard safety maintenance schedule to avoid deterioration of the light transmission properties.

Unobstructed view and safety by numbers

Theoretical information on viewing distances is in valuable, but the actual viewing conditions of the premises must be taken into account. The principal locations fore fire exit signs may be at the end of long corridors with direct line of sight. But these corridors may be filled with people.

It is important to note other viewing obstacles such as pillars, furniture and partitioning. The solution is to add extra directional signs, to be viewed at shorter distances rather than relying on one larger sign.

Conclusions

Professional use of safety signs is a combination of design and site knowledge.

The design criteria have, for the most part, been adequately handled by the current safety sign standards and the real job required is to find manufacturers committed to these principles and rules. Here, we have given the necessary information for correct sizing according to position, illumination and viewing distance as well as information to cater for a population with some poorer visual acuity than the normal. The site knowledge and risk assessment of specific locations still remains an essential element in the final design of effective signing systems, in particular along escape routes. The relationship of (h) sign size to observation distance of D = 130 h has excellent foundations provided all the other criteria are adequately considered.

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