Constructing an Iso-irradiance Chart:

1. A Mercury Vapour or Metal Halide Lamp

A UVI Iso-irradiance Chart is a particularly useful tool for visualising the extent of the UV coverage of a lamp. This chart is useful as it enables predictions to be made of the UV levels which various basking platforms, etc, will receive in a vivarium and therefore it can be used to help choose the right lamp for a given species in a particular vivarium or enclosure.

To map the UV gradient, 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 map the UV gradient

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 UV from a hanging mercury vapour or metal halide 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 UVI 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. 1, 2 and 3.)

Plotting the Iso-irradiance 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 UVI "contours" - the furthest points at which given outputs (eg. UVI 0.5, 1, 2, 4, 6 and so on) are found.

To detect the UVI 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 and then "joining up the dots" (see Figs. 1 and 2.)

Fig. 1 shows a chart being plotted for a mercury vapour lamp; the readings are being taken for the UVI 8.0 contour. Fig. 4 shows a completed chart for a metal halide lamp; the meter is being held on the UVI 2.0 contour. If you find it difficult to hold the meter against the wall while pressing the meter button and simultaneously marking the points with the pencil, you can keep the button on the meter permanently depressed by placing a coin (or other small object) over it and wrapping a rubber band around the meter to hold the coin down.

All the iso-irradiance charts in the reports featured on this website were constructed in this way. From this full size chart, a scale diagram may be prepared, if required.

Completing the chart for inclusion in a report

If the chart is small enough, and drawn onto card or paper, it may be scanned, in sections, into an image-editing program. If it is too large, or drawn onto a wall, then the easiest way to transfer the data to the computer is to 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. I first draw out the grid, then trace the contours - for this, the Photoshop "paths" tools make life easier.

Fig. 5 (below) shows a typical example of the original scale drawing of an iso-irradiance chart at completion. Fig. 6 shows the finished chart.

The final result can also be converted into a useful visualisation of the UV gradient of the lamp in question by over-laying it onto a diagram of a vivarium.

2. The Fluorescent Tube and Compact Fluorescent Lamp

Iso-irradiance charts for most fluorescent tubes and compact lamps are smaller, since the UV 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.

Compact fluorescent lamps may be mounted horizontally, in vivarium hoods or lampholders, or hung pendant-fashion in lampholders or in a dome reflector. Each type of fixture will alter the shape of the beam, owing to the presence or absence of reflected UV from aluminium reflectors and/or shading by the frame of the lampholder or dome. If the lamp is to be hung pendant-fashion, it may be easier to map the UVI gradient against a wall, as described above for the mercury vapour and metal halide lamps.

Fig. 7 (above) shows a chart being constructed for a fluorescent tube. The UV gradient is being plotted parallel to the axis of the lamp. Fig. 8 shows a typical completed chart for a new ZooMed Reptisun 5.0 tube. A scale drawing of a reptile may be added to make the relative size of the beam easier to understand. In this example, a small tortoise with a plastron length of 15cm is placed with its eye level 30cm beneath the tube.