Lighting calculations, how accurate are they?
Kalgoorlie, 30th June 2010
Waiting for my flight back to Perth… It has been a good trip, happy and satisfied clients and some new work in the pipeline, what more can I wish! I wrote last night about our site measurements to confirm that the lighting installation was compliant with our original design specifications. References for our compliance check were the lighting calculations that we had executed with the actual manufacturers photometric IES data files. So you would expect in this case that what you calculate is what you would measure on site.
Fact is that calculations are as good as the operators input. If you don’t understand the workings of light and the impact that variable input components have on the outcome of the calculations, calculations can be way of the reality on site. Some manufacturers paint rosy pictures of lighting level results to promote their fittings by omitting depreciation, light output or utilization factors (maybe purposely or just because they don’t know). There are many other variable parameters such as reflection coefficients, grid calculation spacing and height, aiming angles, light fitting position coordinates, etc, etc. Quite frankly if you know how to operate a calculation program you can pretty much manipulate the outcome the way you want it! So as a professional lighting designer it is up to you to make sure the lighting calculations are as accurate as possible as clients have a habit of taking your calculations as a biblical reference for assessing your work!
So step one is to make sure your input parameters are as realistic as possible and this very much comes down to the lighting designer’s experience, but still the results are theoretical. Assuming the calculations are realistic you then need to check the practical implementation and again there are many variables. Lighting positions maybe off, aiming angles maybe slightly different (some lights have only a very rough scale for reference), measurement points may be different, etc. But these can easily be checked. What is more difficult to the untrained person are the “hidden” variables. Voltage drops, wrong lamp or incorrect lamp mounting (out of focus), wrong reflector and so on. All these little things add up to the real lighting level results.
It is therefore easy to see that the final accuracy depends on input as well as output variables. Garbage in is garbage out!