Using the Solarmeter 6.2 UVB Meter

Instructions for Use     Solar Recordings      Recordings in the vivarium

Setting up a Test Bench   Spread Charts   Comparing Meters      Problems?

Feature: Make Yourself a UVB Spread Chart

The Solarmeter 6.2 UVB meter (fig.1) is easy to use and we have found the readings to be consistent and reliable. We welcome your recordings to add to our database.

A little more detail...

The Solarmeter Model 6.2 UVB
The meter we used in all our tests, the Solarmeter 6.2 from Solartech, Inc. (www.solarmeter.com) is claimed to be the most accurate hand held UVB meter on the market. This very instrument has been used in current research into vitamin D3 synthesis at the Vitamin D, Skin and Bone Laboratory at Boston University Medical Center.19

The device is small, fitting easily in one hand, and is powered by an ordinary 9v battery. The sensor is at the top of the meter, and to take a reading, the operator simply aims the meter directly at the UVB source and presses a button on the front of the unit. The amount of UVB reaching the sensor at that moment is displayed on the LCD panel in microwatts per square centimetre (uW/cm²). If, for example, the reading was 20uW/cm², this would mean that a square centimetre of reptile skin would be getting 20 microwatts of UVB at that point.

The sensor is extremely sensitive, with minute changes in the angle between sensor and source causing variations in the readings. Logically, the highest readings will be seen when the sensor is most accurately aligned in the beam of light; when taking hand-held recordings, it is adviseable to "scan" for the best alignment with very slow, very small movements. With practice, lining up the sensor can be done fairly quickly and very accurately. When taking recordings, we always use the highest readings obtained consistently (i.e. on at least 2 to 3 "scans") on each occasion.

Although it measures the complete UVB range (280 to 320nm) the Solarmeter's peak sensitivity is between 290-300nm, right in the middle of the range of wavelengths responsible for D3 synthesis, which makes it ideal for checking UVB reptile lamps. 

Our Solarmeters were imported from the USA. However, there are now several Solarmeter distributors in the UK, and Zoo Med have just brought out a very similar meter, also made by Solartech, Inc., in the UK, which can be obtained from British Zoo Med stockists. (see our links page for details of all distributors and suppliers)

Measuring UVB with the new meters
What we have been able to do, with our hand-held radiometers, is to look at the entire UVB output of the sun, and of any light source, between 280 and 320nm of the electromagnetic spectrum and investigate in detail the initial output of a new lamp; the decay in the output of a lamp with time; and the characteristics of the UVB "beam" for each type of lamp - how far the UVB light extends from the lamp and in what directions. 

Between us we have taken many thousands of readings both outdoors in natural sunlight and shade, and indoors from a wide range of UV products. 

There's more detail about our project, the accuracy of our readings and comparisons with other work in this field in the Introduction to the 2005 Lighting Survey  

The sections below describe the methods of recording we have found the most useful when using the meter outdoors (for solar recording), indoors in the vivarium, and on a simple "test bench".

The page concludes with a short section on comparing different UVB meters, and a discussion of one or two problems which might be encountered when using the Solarmeter 6.2, along with their solutions.

Solar Recordings

The readings collected for our studies on UV light in nature are mainly direct readings, for which the meter is aimed directly at the sun. These give a picture of the maximum UVB which a reptile could obtain if it was fully exposed to sunshine at that location.

When taking solar recordings, never look directly at the sun. It is easiest to align the meter whilst facing the sun. Hold the meter out at a comfortable distance at just below shoulder level and "scan" carefully by tipping the meter towards the sun, searching for the highest reading.

Readings need to be taken out in the open, where trees or buildings do not block a clear view of the sky. The sensor's field of view is a full 180° with a cosine response curve - this means that although it is most sensitive to whatever is directly in line with it (in this case, the sun), it will include diffused UVB from the sky around the sun in the reading. If this "sky" includes the dark silhouette of a building or tree, a lower reading will be obtained. Conversely, if there are large reflective surfaces such as water or snow beneath the sky, reflected and scattered UV from these may result in a higher meter reading.

Close to sunrise and sunset, readings from the sky above the sun may be slightly higher than those from the sun itself. This is because light from closer to the horizon must pass through a thicker layer of atmosphere. Water vapour in the air scatters light of shorter wavelengths more strongly than longer wavelengths; close to the horizon this thicker layer scatters enough short-wavelength light to remove most of the ultraviolet and blue. (This is why, of course, the rising or setting sun appears only red or orange.)

If taking a series of readings throughout a single day, ideally all the readings should be taken from the same spot, so that the readings are comparable one with another. It is also important to record the exact time of each recording. As the sun rises in the sky, the reduction in the thickness of atmosphere through which the light is passing causes an extremely rapid increase in UVB. We have recorded changes of a microwatt a minute. For example, between 7.30am and 8am on June 10th 2005, in Wales, UK, under a clear sky, the reading climbed from 40 to 70uW/cm². By 8.30am it was 105uW/cm² and by 9am it was 135uW/cm².

When taking solar recordings, it's useful to note the weather conditions; in particular, if the sun is shining, is it in a clear, deep blue sky or is there high cloud or haze? The increase in water vapour in the sky due to these will reduce the UVB levels reaching the ground.

If there is total cloud cover, direct solar recordings may not be possible. So-called "global" recordings usually give the highest readings with 100% overcast skies. To obtain these, the meter is held vertically, to record from the zenith (directly above the observer).

When recording for a database, you may also wish to record the location (latitude and altitude) at which the recording was made. If your country is using Daylight Saving Time this too needs adding to your report; and remember, if so, the sun will reach its highest point in the sky, and UVB its maximum level, around 1.00pm Local Time rather than at noon.

Fig.4 shows recordings made during the course of one clear day in August in Wales, UK. Faint high cloud and haze in front of the sun has caused slight "dips" in the graph at various times but the pattern is distinctive. More details on solar recordings, and the ongoing Solar Recording Project, are in our feature UV light in nature and in the files section of the UVB_Meter_Owners internet mailing group. We welcome your solar recordings from anywhere on the planet, to add to our database.

Recordings in the Vivarium

Safety Precaution: Always wear eye protection when studying UV lamps.

However careful you are, it is very difficult to take recordings without looking directly at the lamp, especially when measuring the distance from the lamp to the meter. Look for glasses which state clearly that they offer 100% protection from UV light.

Whereas some fluorescent tubes, for example, emit (at 12" distance) less UV light than is reflected by sunlit trees and grass, and are thus unlikely to cause harm, some mercury vapour lamps, at 12" distance, emit as much UVB as the sun at the centre of their beam.

Intense visible light may also damage your eyes. Even when wearing glasses, avoid staring directly into any very bright lamp, whether or not it emits UV, at close range.

Taking readings

The simplest recordings are taken using just a meter and a steel tape measure, ruler or measuring stick. The end of the tape measure is positioned at the surface of the tube or face of the lamp, and the meter is positioned so that the sensor is at the required distance from the light.

Because the tape measure or ruler will inevitably block some light from the sensor, it must be moved away before the reading is taken. This manoeuvre takes a little practice, as the meter must remain at the correct distance while the tape measure is removed.

With fluorescent tubes, the highest readings will be found when the meter is positioned perpendicular to the axis of the tube, half-way along its length.

With lamps and bulbs, the beam is usually focused by internal reflectors and although, in general, the highest readings are found directly in front of the lamp face, this cannot be relied upon. Often, the beam is at a slight angle from the perpendicular owing to minute discrepancies in the alignment of the arc tube, glass envelope, etc. The path of the UVB beam usually closely follows that of the visible light and this can be used as a first step in aligning the meter within the beam; thereafter, careful scanning (in the same way as previously described for a solar reading) will locate the "hot-spot" at the chosen distance.

Fig.5 shows a steel tape measure being used to take a reading from a fluorescent tube fitted with a reflector. First the meter is positioned at the required distance (in this example, 6 inches). Next, the meter is switched on and moved around a little, at that distance, whilst scanning for the maximum reading (i.e., aligning the meter). Then, the tape measure is moved out of the way, a final alignment made and the reading taken. In this example, the reading increased from 102uW/cm² to 105uW/cm² with the removal of the tape measure.

Sets of readings may be taken from each lamp tested, at increasing distances from the lamp, to give an indication of how far the UVB penetrates into the vivarium. If readings are repeated at intervals (for example, once a month), the decay of the lamp throughout its lifespan may also be recorded.

If there is more than one UV source in a vivarium, don't forget to switch off the one you are not recording from, or it will affect your results.

Watch out for metal reflections in the vivarium. UVB is hardly reflected at all from glass or perspex, which absorbs the rays. Glass mirrors likewise reflect almost no UVB. However, bare metal surfaces such as polished or brushed aluminium reflect UVB strongly. You may obtain false high readings from a lamp if the sensor picks up UV from the lamp's reflection in a metal surface.

Setting Up a Test Bench

If you want to take your study further, you may wish to set up a simple test bench for the rapid, accurate measurement of the UVB output of your lamps. New lamps can be burned-in whilst on the bench, making it easy to repeat daily measurements precisely. Lamps on long-term testing can be temporarily removed from the vivarium and placed on the bench for individual tests, as well.

A simple test bench for fluorescent tubes and compact lamps is easily constructed on a table top or work surface. Fig. 6 shows recordings being made from a fluorescent tube. Dark coloured card has been placed on, and behind, the work surface to reduce reflections.
A measuring line has been drawn on the card and the tube mounted vertically, so that the zero for the measuring line is at the surface of the lamp. Compact lamps may be place in simple lamp-holders positioned at the end of a measuring line, in the same way.

To make life easy and eliminate the need for a tape measure, the UVB meter has been temporarily attached to a set square such that a pointer fixed to the base of the square, resting on the measuring line, indicates the position of the sensor directly above it. The distance from the tube to the meter sensor can thus be measured accurately without a tape measure. The meter is held steadily against the set-square while it is moved along the measuring line, enabling rapid recording of a set of measurements with no errors due to hand-shake.

Mercury vapour lamps, because of their directional beam, are more conveniently tested whilst hanging pendant-fashion. If the lamp-holder is secured to a ceiling or a horizontal beam close to a wall, the measuring line may be drawn on the wall, or on card affixed to the wall (Fig. 7.)

Spread Charts

A further development to the recording of the output of a lamp in this simple linear style, is the production of a "spread chart". This is a two-dimensional map of the shape of the beam, constructed by plotting the UVB gradient under the lamp. Spread charts for each type of lamp are featured in the 2005 Lighting Survey reports.

To construct a spread chart is not difficult, but it is time-consuming. If you would like to make one yourself, full details may be found in our article:
Make Yourself a UVB Spread Chart

Are all meters equal?

All our UVB data has been compiled from readings taken with the Solarmeter 6.2 UVB meter. These Solarmeters are individually calibrated by the manufacturer, by reading transfer from a standard meter calibrated to NIST standards. They are claimed to be accurate to at least ±10%. We have tested a number of meters together, and have found that individual meters are consistent with each other in the readings they give from the same light source (as demonstrated in fig. 9 with models of the Solarmeter 6.2 and the ZooMed UVB meter, made with the same components by the same company). We therefore believe that it is valid to collate, combine and compare data from all Solarmeter 6.2 UVB meters and we are interested in collecting data from owners of these meters worldwide.

For technical reasons, however, it is not possible to combine or compare raw data from two different brands of radiometer, or even models from the same manufacturer with different specifications. Experiments conducted by Gehrmann et al (see references 18 and 19 on our reference page) have shown that the most accurate UV radiometers from different manufacturers may give very different readings from the same light source. Reasons for this may include differences in the response to different wavelengths by different components in the sensors, or in their calibration. Gehrmann and his team used measurements of the in-vitro production of vitamin D3 under UV light to devise conversion factors for the readings from four different meters. (Such work is outside the scope of this project.)

Problems?

Low Battery

The Solarmeter runs off a standard 9-volt DC battery which may, according to the manufacturer, last for about two years of "normal" use. Battery operation voltage is 9V down to 6.5V. Below 6.5V the LCD numbers will begin to dim, indicating the need for battery replacement. The readings remain accurate until the numbers begin to dim.

High Humidity and Heat

The meters are sensitive to excessive humidity and heat. If the meter is stored or used in a very hot or humid situation, there can be a temporary false elevation of the reading by several microwatts. This is easily detected, as the meter will not read "zero" with the cover over the sensor, but give a reading of a few microwatts instead. Allowing the meter to cool or dry out will resolve the problem. In very humid environments, the manufacturer suggests storing the meter in a plastic bag with a sachet of silica gel.

When testing a high-wattage lamp, at close range the meter may be affected by the heat. Our tests using two 100 watt mercury vapour lamps showed that even as close as 4" from the lamp surface, provided the readings are taken within 1-2 minutes there is little or no effect upon the reading. Longer exposure to heat does cause a slow artificial rise in the readings as the front of the meter gets hot. A change in temperature from around 21-22°C, to around 44-50°C produced a change in readings from 1 - 6uW/cm2 in a set of 4 experiments- a change of between 2-6%.

It is worth bearing in mind that readings taken close to lamps should therefore be taken fairly rapidly, within maybe a half a minute or so, as holding a meter close to a hot bulb for any length of time will cause a slow false rise in the readings as the sensor is heated.

The meter sensor should not be touched or held against the surface of a lamp. Contact with any solid object can produce minute electrical fields which create brief false readings. More seriously, contact with the extremely hot glass of some incandescent or mercury vapour lamps could easily damage the sensor.