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Photmetric Science

Photometric Science is the science of light measurement. It involves identifying distribution properties and their calculative characteristics. This is based on verified measured values.

Measured values. Not projected or estimated values. It is science, not speculation.

Here's how it works:

A luminaire is mounted to a device called a 'goniometer'. Basically it's a base that the luminaire is fixed to, with a rotating arm that has a light meter mounted across from a mirror - and this arm swings in designated angles around the luminaire, in different planes, and records all this data. The data is then plotted out as a performance curve with intensity values at different angles. Other characteristics such as luminous intensity values etc are charted too. This text file is then entered into an IES format recognized by lighting calculation programs for projecting performance.

Many lighting manufacturers distribute product "photometry" that is derived from ray-trace reflector design programs - not actual finished product testing. At The Light Edge, we rely on third-party, independent test lab evaluation of our physical products. The difference? One is representative of intent, one (ours) is the actual testing of an actual product.

Many lighting manufacturers have "in-house" photometric test labs. These often contain highly calibrated, certified instruments, run by smart people who understand this stuff. They are also part of a large corporation that includes input from the marketing department. Again, The Light Edge relies on third-party, independent testing labs for an actual, unbiased photometric report. Can you trust in-house testing? Maybe.

Fixture Efficiency - worthless values.

We have seen manufacturers claim high 'fixture efficiencies' - even some in excess of 100%! One doesn't need to be a scientist to understand that there is no such possibility under the laws of physics. Losses are inherent in every energy exchange. How can a luminaire boast 93% fixture efficiency, when it has a 95% reflective specular reflector and an average 12% inter-reflected light loss (light directed back into the lamp, or lost to dead spots & corners in the housing), and a lens (with 10% average transmission loss)? Remember, the first luminaire ever developed - Edison's keyless socket, with an incandescent lamp (at about 10 lumens per Watt), distributed 100% of the light produced by the source, so it was 100% efficient! Kind of takes away some of the credibility of the importance of "fixture efficiency", doesn't it? We should be more concerned with the way light produced by a given luminaire is distributed - it's uniformity and intensity on the horizontal and vertical planes.

 

The Light Edge, Inc. recommends careful review and interpretation of manufacturer's photometric data. Add a requirement for Independent Test Reports to your master spec - this will eliminate in-house generated IES files that favor the manufacturer's marketing department. Question the reality of what you are offered as photometric performance - is it a true metered report, or a ray trace program report? The Specifier should be aware and in charge.