Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye, as opposed to radiometry, which is the science of measuring the entire electromagnetic spectrum.
The photometric performance of a luminaire is comparable to the spray of a sprinkler watering a lawn. The efficiency of a sprinkler is measured by the quantity and spread of water it delivers all around it, just as the photometric efficiency of a luminaire is measured by the quantity and distribution of light it delivers.
Just as with the sprinkler, what we do not want is a puddle of water in one spot and dry dying grass all around. Too much light (or water) is not beneficial, it needs to be spread out as evenly as possible over as large an area as possible.
Studies show that photometric performance ranks high on selection criteria for luminaires but they also show that this measurement remains somewhat of a mystery to many professionals in the field.
Here are a few advantages of better photometric performance to consider:
Using quality refractors and reflectors creates larger pole spacing; this means that a project owner will need to purchase less luminaires to illuminate any given area. This in turn causes ripple effects in the form of maintenance (less is needed because there are less luminaires), less light source replacement, fewer public works man-hours, a reduction in gas for the maintenance fleet, and so on.
Proper distribution creates better, and safer, visibility at night. The human eye is very sensitive to light. In fact, it is so specialized that it can detect a single photon in total darkness. There are 3 types of vision.
Scotopic vision is the monochromatic vision of the eye in dim light (that of a full moon or less). Since cone cells are nonfunctional in low light, scotopic vision is produced exclusively through rod cells so therefore there is no color perception (which is why red cars appear black at night).
Mesopic vision works in intermediate lighting conditions (like a lit highway at night) and is effectively a combination of scotopic and photopic vision.
Photopic vision is what we use in elevated light situations (as in daytime) when the vision of cone cells dominates.
The improper distribution of light at night renders seeing more difficult, bordering on dangerous. In the past, all lamps were calibrated to photopic vision to take advantage of the cone receptors in our eyes, while mesopic vision (the vision that uses both cones and rods) is the one we really use in illuminated nighttime conditions.
An interesting fact to note is that the lighting industry is gearing up to calculate a nighttime correction factor for mesopic vision that should improve illuminated nocturnal visibility considerably.
Ideal photometric performance is not just about intensity; it's is also about creating the best uniform lighting conditions that permit the human eye to see in darkness.
Although often misunderstood, this measurement assures citizen safety as well as lighting and energy efficiency, and is not something that ever should be sacrificed for cost. The implications of erroneous photometric calculations or sub-performing products not only create negative side effects for projects, they are downright dangerous.