As with most accurate test equipment, initial calibration and routine recalibration must be done properly; this is something we can carry out with speed and efficiency. As you would expect Megatron instruments comply fully with the accuracy requirements of BS667:2005 Specification for Portable Photoelectric Photometers. A Calibration Certificate shows that the instrument has been compared with Normals that have themselves been checked against standards tested by the National Physical Laboratory; the instrument calibration therefore complies with the common requirements that it is to a "traceable standard". We recommend that instruments should be returned to us on a regular basis for a calibration check and routine servicing. We can calibrate nearly all types and makes of illuminance meter and repair most of them.

Even with a lightmeter in good repair, there are two other problems which can give rise to serious inaccuracies.

1. Reflection from the surface of the cell which increases as the angle of incidence increases, so that the reading will be too low, eg. when measuring the horizontal illuminance from a window at the far end of a room; this can give errors of 20% or more

We use a "cosine correcting mount" to correct this error; there are two basic types: the mount with the clear 'Perpex' dome, and the photocell with a flat opal 'Perspex' diffuser. As can be seen from the graph the flat cell gives rather better results, but both methods are much better than using an uncorrected cell.

2. The spectral response of the photocell differs from that of the human eye, causing light of equal illuminance from two different sources such as tungsten and say mercury to produce very different readings; this can cause an error of 25% or more.

Both the digital(DL) and analogue(DA) lightmeters use the Megatron Type M photocell, which is colour correct ie. it has a response very similar to the human eye - see the Graph and Table of Spectral Response. This avoids the need for correction filters or factors with their own inaccuracies and inconveniences. The EA2 and EA22 are for botanical work and use Type B photocells which are better suited for this application.