The dust lamp: a simple tool for observing the presence of airborne particles mdhs82

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Scope 

1 

This guide is written for occupational hygienists, ventilation engineers, health 

and safety practitioners and others interested in how exposures to airborne 

particles occur. It briefly explains the principles of the dust lamp, its use in 

observing the presence of airborne particles, and identifies its advantages and 

limitations

2 

The presence of many different types of particles, both solids (eg dusts, fumes 

and fibres) and liquids (organic or inorganic mists), can be revealed by the dust 

lamp. However, it does not give a quantitative measure of either particle 

concentration or size.

Summary

3 

In the absence of effective control measures, airborne particles are released 

into the workplace atmosphere by many industrial processes. Such particle clouds 

can be invisible under normal lighting conditions, but may be made visible by the 

use of a high-intensity beam of light. This technique is commonly referred to as the 

dust lamp. Use of the lamp enables the existence of particle release at a process to 

be simply demonstrated, or the performance of an extractor system to be 

qualitatively assessed.

4 

The dust lamp is a simple qualitative tool for making fine particle clouds visible 

or enhancing the visibility of partially visible clouds. With a certain amount of 

experimentation, observations can be recorded on still or video film. The dust lamp 

is a powerful tool in experienced hands and can be used in a variety of ways to 

gain understanding of how work processes cause exposure or controls fail to 

prevent emissions. The very fact that the technique makes the invisible visible 

explains the impact it can have on employers and employees.

Prerequisites

5 

The dust lamp is a useful tool in the investigation of processes, controls and 

exposure but it must be seen in context. It is an occupational hygiene tool that can 

be applied to exposure and control problems. As with any other occupational 

hygiene investigation, the user should have a good understanding of the process 

and work method, and be able to relate dust lamp observations to other 

occupational hygiene data and findings.

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The dust lamp

A simple tool for observing the presence of airborne particles

Methods for the 

Determination of 

Hazardous Substances

 

Health and Safety 

Laboratory

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Safety

6 

Users of this guide should be familiar with the equipment used and carry out a 

suitable risk assessment for the particular application. It is the user’s responsibility 

to establish appropriate health and safety practices and to ensure compliance with 

regulatory requirements.

Principles of use of the dust lamp

7 

The main use of the dust lamp is to make fine airborne particles visible  

(ie particles below ~10μm aerodynamic diameter, usually termed respirable

1

). Some 

of the important general properties of fine particles can be summarised as follows:

 

■

they move with the air in which they are suspended;

 

■

they settle out of the air slowly and can remain airborne for long periods of 

time.

8 

They can be revealed by the beam of the dust lamp as particles swirling and 

flowing with the air in which they are suspended. The dust lamp in this context 

usually shows that particle clouds occupy a far greater volume of air than is 

suspected.

9 

Although the main use of the dust lamp is to make fine airborne particles 

visible, it can also enhance the visibility of dust clouds containing coarser particles, 

such as those generated from woodworking operations. Through experience and 

careful use of the dust lamp it is possible to observe the extent, pattern and 

direction of a particle cloud’s movement.

10  For fine particles, the intensity of the scattered light is greatest at a small angle 

to the incident light beam, as shown in Figure 1 (the actual values of intensity differ 

for different dusts and incident light beams and the units are therefore not 

specified). As the angle is increased, the intensity of the scattered light falls rapidly, 

consistent with diffraction theory, but at angles greater than about 120° the 

intensity increases somewhat as the diffracted light is augmented by reflection. For 

particles smaller than 0.1 μm, scattering by mechanisms other than diffraction 

occurs and the variation in intensity with angle is reduced; the intensity of the light 

scattered by such mechanisms is relatively low, however, and is not important in 

dust lamp use.

11  The curve in Figure 1 was obtained from photometer readings. To the human 

eye, however, the visibility of the particle cloud depends not only on the intensity of 

the scattered light (I

s

) but also on that of the background (I

b

), ie on the contrast, 

defined as (I

s

 - I

b

)/I

b

. Thus, if viewing at a small angle results in appreciable 

background illumination, or exposes the observer to glare from the direct light 

beam, it may be preferable to view the cloud at a larger angle. In practice, an 

observer must make a judgement as to the viewing angle that provides maximum 

contrast between the scattered light and the background.

12  At small viewing angles, the wavelength of the scattered light will be that of 

the incident light beam, ie a white light will show the dust as white, regardless of 

the true colour of the dust. At larger viewing angles, however, reflected light may 

show the true colour of the dust.

13  The greater the intensity of the incident light beam, the greater will be the 

intensity of the scattered light and the more visible will be the particle cloud; thus 

the principal requirement for a dust lamp is that it should give a defined beam of 

high intensity. A parallel or near parallel beam is desirable, so that the intensity does 

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not fall off rapidly with distance, thus giving a greater range. Such a beam is 

produced by a parabolic reflector having a small, compact filament at its focus and 

a plane lens, solely for protection. However, if the narrow beam of a spot lamp 

does not illuminate a sufficient area it may be necessary to use a divergent beam 

lamp. The latter usually requires mains power to obtain the necessary illuminance, 

and may require a dark background to give sufficient contrast.

14  It is difficult to specify a minimum intensity requirement for a dust lamp since 

this depends on the nature and concentration of the particles, the distance of the 

lamp from the cloud, the background illumination and the skill of the operator.

Equipment 

Battery powered lamps 

15  Battery powered lamps are convenient and easy to handle, making them 

particularly convenient for routine hygiene surveys or when visiting a number of 

different installations. Most battery powered lamps produce a narrow beam of high 

intensity light based on the use of a halogen bulb and a parabolic reflector. The 

limited area illuminated is not usually a problem as the lamp can be readily panned 

around the area under investigation.

Mains operated lamps 

16  These are mostly diverged beam devices providing even illumination over a 

wide area. The use of this type of lamp enables the whole volume of the particle 

cloud to be illuminated at the same time. 

Still photography and video recording of particle 

clouds 

17  With the use of a still camera (with the flash switched off) or a video camera, 

permanent records of the observations can be made. The technique of taking good 

images can be developed by experience and experimentation. However, this 

should not deter those with a limited knowledge of photography from using the 

dust lamp.

Practical applications 

18  The dust lamp can be used in a variety of ways:

 

■

as a tool to investigate work operations and processes to gain an 

understanding of the potential for exposure before any air sampling is done;

 

■

where air sampling has demonstrated significant over exposure, as an aid to 

understanding how and why exposure is occurring; and

 

■

as a tool in investigating the effectiveness of controls during their development 

in confirming effectiveness after installation and as part of routine monitoring 

of controls.

19  The method of operation is demonstrated in Figures 2 and 3. A bright beam 

of light is shone through the area where it is thought a particle cloud may be 

present. The observer’s eyes are shielded from the main beam by means of a flat 

panel, eg a piece of card, or by using the worker’s body or a convenient piece of 

machinery as a shield. The particle cloud should be observed looking up the beam 

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towards the source of illumination, preferably at a small angle off the centre line of 

the beam, and, if possible, against a darker background, for instance a black ‘pop-

up’ background. Dense clouds can be made visible with the dust lamp under 

normal lighting conditions but to see a small leak, for instance, or to trace the 

extended movement of a cloud as far as possible, the ambient lighting may need to 

be suppressed.

20  The four examples of the use of the dust lamp which follow illustrate the 

diversity of application in terms of particle composition, concentration, particle size 

and industrial process. 

21  The photographs in Figure 4 show a finishing job in a woodworking shop, 

using a vertical belt sander. The operator is holding the piece of hard wood against 

a fast-moving sanding belt, and dust particles up to 2 mm in diameter are 

generated. A LEV system at the end of the table collects most of the dust. (The 

LEV was shut off during photography to show the technique to good effect.)  

Figure 4a shows the positions of the camera, the machine and the dust lamp. The 

camera was shielded from the main beam by parts of the machinery. Using a 

mains-operated diverging beam, a wide area was illuminated, and a dark 

background was utilised to enhance the contrast (see Figure 4b and 4c). The 

technique shows the large volume into which the dust disperses and the swirling 

motion of the cloud. Some large individual particles can be identified.

22  The second example shows a typical bagging operation in a mineral 

processing plant. The operator fills a bag with the finely ground mineral by holding 

it over the filling spout. The feed stops automatically when a pre-set weight has 

been bagged and the operator then withdraws the bag and tucks in the flap to seal 

the bag. 

23  Clouds of dust are produced during feeding, as the bag is withdrawn from the 

spout and when tucking in the flap. The particle size (~90% less than 25 μm) of the 

material made the airborne dust almost invisible under ambient lighting. Figure 5a is 

a photograph of the operation taken with flash photography which shows no sign 

of the dust cloud. Figure 5b is a similar shot but this time a narrow beam lamp has 

been shone from the far side of the operator. A dust cloud within the operator’s 

breathing zone is now clearly visible. When shot from the same side as the lamp 

(Figure 5c) there is little evidence of the dust cloud apart from reflected light 

showing up some particulate. For best results, however, the set-up in Figure 5b 

was repeated but without flash. As shown in Figure 5d, the swirling dust cloud is 

clearly seen enveloping the operator. 

24  The third example (Figure 6) shows the products (tennis balls) of a moulding 

press where rubber fume is released for a short time when hot. In this process the 

rubber contained only a small amount of oil so the volume of fume released was 

low. Using black and white photography and a dark background, the fume 

appeared as a hazy white cloud hanging above the products. A narrow beam dust 

lamp was used in this instance. Under ambient light it would be impossible to see 

such a low-level emission. 

25  Solder fume exposure can be visualised using the dust lamp, as shown in 

Figure 7. In this example, the operator was enveloped in a plume of fume as he 

was soldering components on an electronic circuit board. The fume was only just 

visible in ambient light or using flash photography (see Figure 7a). 

26  When the process was repeated in reduced light without flash but with a 

narrow beam dust lamp looking into the main beam at a small angle and using the 

operator’s body to obscure the main beam to get the maximum contrast, the 

plume of fume appeared like a flame (Figure 7b). For best results, Figure 7c was 

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taken in identical conditions but with a separate flash unit pointing slightly 

downward behind the operator’s shoulder, acting as an additional wide area dust 

lamp. The flash was used to freeze the motion of the fume. The photograph 

revealed the whole volume and detail movement of the fume. 

Figure 1 Intensity of scattered light as a function of angle

Figure 2 The use of the dust lamp to observe the presence of airborne particles

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Figure 3 Diagram of an example of the dust lamp in use

Figure 4a The arrangement of the lamp, sanding machine and camera for dust lamp 

photography, set up on a vertical belt wood sanding machine

Figure 4b Photograph showing dust cloud from the sanding operation

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Figure 4c Wood dust cloud from sanding

Figure 5a Bagging operation in a mineral processing plant: under ambient light and with 

flash, dust cloud is not visible

Figure 5b View from small angle towards lamp and with flash: small volume of dust cloud  

is visible

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Figure 5c View from the same side of the lamp and with flash: dust cloud is not visible

Figure 5d View from small angle towards the beam of light and without flash: a dust cloud is 

clearly seen enveloping the operator

Figure 6 Rubber fume is revealed using the dust lamp

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Figure 7a Solder fume viewed under ambient light and with flash

Figure 7b View using dust lamp, under reduced light, without flash

Figure 7c View from small angle of the dust lamp, under reduced light

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Published by the Health and Safety Executive                MDHS82/2                01/15 

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References 

BS EN 481: 1993 Workplace atmospheres: Size fraction definitions for 

measurement of airborne particles British Standards Institution

You should use the current edition of any standards listed.

Further information

For information about health and safety, or to report inconsistencies or inaccuracies 

in this guidance, visit www.hse.gov.uk/. You can view HSE guidance online and 

order priced publications from the website. HSE priced publications are also 

available from bookshops.

British Standards can be obtained in PDF or hard copy formats from BSI:  

http://shop.bsigroup.com or by contacting BSI Customer Services for hard copies 

only Tel: [phone] email: [e-mail].

This guidance is issued by the Health and Safety Executive. Following the guidance 

is not compulsory, unless specifically stated, and you are free to take other action. 

But if you do follow the guidance you will normally be doing enough to comply with 

the law. Health and safety inspectors seek to secure compliance with the law and 

may refer to this guidance. 

This MDHS is available at: www.hse.gov.uk/pubns/mdhs

For further information about this method or other MDHS methods, please visit 

HSL’s website: www.hsl.gov.uk or email: [e-mail]

© Crown copyright If you wish to reuse this information visit  

www.hse.gov.uk/copyright.htm for details. First published 01/15