Constructing a Spread Chart:

1. The Mercury Vapour Lamp

A UVB Spread Chart is a particularly useful tool for visualising the extent of the UVB coverage of a lamp. This chart is useful as it enables predictions to be made of the UVB levels which various basking platforms, etc, will receive in a vivarium. It enables estimation of the level of UVB a reptile would expose itself to, if it sat in any given spot relative to the lamp (Fig. 1.is an example of how a chart can be used in this way.)

To make a Spread Chart, it is necessary to record the output of the lamp in a two-dimensional plane directly beneath and to the sides of the lamp face (Fig.2). Direct readings are taken from several hundred points in this plane, and plotted on a chart so that a two-dimensional visualisation of the three-dimensional “cone” of radiation emitted by the lamp can be visualised.

Preparing to Record the Spread Chart

It is easiest to work beneath a bright lamp, since the light does not shine directly into your eyes. You will need to hang the lamp a short distance from a wall (8 - 10 inches away should prove satisfactory).

The beam of UVB light from a hanging mercury vapour lamp may extend a considerable distance beneath the lamp. If direct recordings have already been taken from below the lamp, you will already have a clear idea of how far below the lamp the useful beam extends. Angling the meter towards the lamp from the side will give you a rough idea of how wide the beam will be. Fasten a suitably large sheet of non-reflective paper or card to the wall - you will be plotting the full-size chart onto this. If the lamp is very powerful (such as a lamp for zoos) the chart may be so large that you might consider drawing straight onto the wall. Use a water-soluble pencil in this case!

The most difficult part of this enterprise is ensuring that the UVB meter is kept in the same plane throughout the recording session. The easiest way to do this is to tape or blue-tack the meter to a set square or spirit level rule, which is then held against the wall. A pointer of some sort may be affixed to the other end of the set square or rule, such that its tip corresponds to the relative position of the meter sensor. (see Figs. 2 and 3.)

Plotting the Contours

You are now going to take readings from the meter, angled so that it is aiming directly at the lamp surface (direct readings) whilst keeping the meter exactly the same distance from the wall – so all your readings are in the same vertical plane as the wall..it is then a fair cross section of the lamp’s output. You are going to plot the UVB "contours" - the furthest points at which given outputs (eg. 50, 100, 200, 300uW/cm² and so on) are found.

To plot the UVB contours, move the meter back and forth, tipping it gently to scan for the furthest distance from the lamp at which it is possible to record the output you are working on. Use the rule to transfer your data to the card on the wall, by marking each reading at the corresponding point on the card (see Fig. 2.)

Fig. 4 shows a spread chart being plotted for a low-output mercury vapour lamp; the readings are being taken for the 20uW/cm² contour.

You can then literally “join up the dots” to obtain the spread chart and visualise the shape of the UVB beam. From this full size chart, a scale diagram may be prepared, if required. All the Spread Charts featured on this website were constructed in this way.

Completing the Spread Chart

If the chart is small enough, it may be scanned, in sections, into an image-editing program. If it is too large, then the easiest way to transfer the data to the computer is to draw a grid onto the full-size chart and produce an accurate, scaled-down copy of the whole chart on a sheet of graph paper, which can then be scanned into an image-editing program.

Most editing programs which utilise layer technology would be suitable; I use Photoshop Elements. Over the scanned image, I first construct the grid, and then trace the contours - for this, a graphics tablet and pen makes life easier.

Fig. 5 (below) shows a typical example of a full size spread chart at completion. This used three A1-sized sheets of card and measured nearly 4ft across. Fig. 6 shows the scale diagram of the spread chart, obtained from the full-size version as described above .

The final result can, of course, be coloured and converted into a useful visualisation of the UVB beam of the lamp in question by over-laying it onto a photograph of the lamp in use in a vivarium. This chart, for example, is the one featured in Fig.1 (at the top of the page). Great care must be taken to record key measurements within the vivarium, such as the distance from the lamp surface to the basking shelf, so that the chart may be scaled up accurately over the photograph.

2. The Fluorescent Tube and Compact Lamp

Spread charts for most fluorescent tubes and compact lamps are smaller, since the UVB output is usually relatively low. In most cases, the chart may be plotted on a single A1-size sheet of paper or card. This is placed on a table or test bench to which the lamp is mounted, either horizontally or vertically depending upon the desired orientation of the chart. The principle is exactly the same as for the mercury vapour lamp.

Fig. 7 shows a spread chart being constructed for a fluorescent tube. The tube is held in position at a suitable working height over a test bench, to enable the UVB gradient to be plotted parallel to the axis of the lamp. Fig. 8 shows the spread chart for this lamp, a new ZooMed Reptisun 5.0 tube.

If the tube is mounted vertically on the test bench, the UVB gradient at right angles to the axis of the lamp may be plotted. This orientation was used in the construction of some of the spread charts and vivarium overlays featured in the section on fluorescent tubes.

Fig. 9 shows a spread chart being constructed for a compact fluorescent lamp, using an early version of the DIY "set square" meter holder, fixed to a plastic block and ruler aligned with the sensor. In this example the UVB gradient is being plotted in the horizontal plane, i.e., perpendicular to the axis of the lamp. The sensor is positioned at a level half-way up the lamp face.

Spread charts plotted for horizontally-mounted and vertically-mounted compact lamps are featured in the section on compact lamps.

Contribute your recordings!

We welcome your input. If you have made recordings using your Solarmeter 6.2 or 6.4 meter - either simple direct recordings or a spread chart like any of these - please consider contributing a copy. We would very much like to hear from you. Email us.