Bollard Lighting – Vertical and Horizontal Illuminance

Illuminance, also known as incident light or brightness, refers to the amount of light that falls on a surface. Vertical illuminance is defined as the amount of light on a vertical plane or surface, usually measured in footcandles. When most people ask “how many footcandles do I need?” for a particular lighting project, they are usually asking about horizontal illuminance, which is the amount of light on the ground from the light source, or the horizontal plane. However, vertical illuminance matters in lighting more than you might think, especially in regards to bollard lighting and other architectural lighting styles. In this article, we will also explore how something as small as changing the lensing of a bollard light can completely affect light distribution and illuminance patterns. 

Bollard Lighting and Horizontal Illuminance

Horizontal illuminance is what most people think of when they think of lighting. Many facilities require that bollard lighting must illuminate at least one footcandle on walkways and sidewalks. The image to the lower right shows a photometric analysis of an OPPE 37 with an opal polycarbonate lens from a bird’s eye view. The footcandles shown on the diagram are the footcandles on the ground radiating out from the center, where the bollard light itself is. 

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If a customer needed to maintain one footcandle of horizontal illuminance on a walkway, they could space out these particular bollard lights about fifteen feet apart. Horizontal illuminance  is crucial when discussing spacing of fixtures such as area lights and bollard lights. For more information on bollard light spacing, click here.

Bollard Lighting and Vertical Illuminance

When choosing a bollard light for a commercial or residential project, customers often ask themselves multiple questions. The most common question that Access Fixtures lighting specialists receive is “How many bollard lights do I need to meet my footcandle requirement?” Customers are usually talking about horizontal illuminance, or the amount of footcandles on the ground. However, vertical illuminance is just as important in selecting effective and safe bollard lighting for any given project. 

Uneven vertical illuminance on a surface can result in unnecessary and unwanted contrast. This can create shadows and glare on objects such as cars or even people walking by. In instances where a bollard light mostly projects light to the ground, it can be hard to see people’s faces in high foot traffic areas, creating potential safety concerns. The right lensing can play a large part in fixing this problem.

Vertical illuminance is also extremely important in sports lighting, where in many cases, a small object (a ball) is flying through the air at various heights at very high speeds. In cases where vertical illuminance is uneven or too low, games such as baseball and tennis can quickly become a safety hazard to players and spectators alike.

Bollard Lighting and Lensing

Optics and lensing matter. Using the example of our OPPE bollard lights, which are available in 12 different styles and wattages, we compared the light distribution of the standard clear polycarbonate lens with the light distribution from the opal polycarbonate lens option. The results were quite different and may surprise you.

The image to the right shows the vertical illuminance of an OPPE 37 with a clear round polycarbonate lens. The lighting was tested using a 3′ x 6′ flat object 10′ away from the light source. The highest number of footcandles is 2.2 directly on the ground, and it gradually decreases as it descends. The image directly below it, however, shows the vertical illuminance of an OPPE 37 with an opalescent polycarbonate lens. The only difference here is the lensing, but you can immediately note that the OPPE 37 with a polycarbonate lens has much more even vertical illuminance. Note also that both of these photometric studies were run with each bollard light ten feet away from a 3′ x 6′ flat vertical object (made to represent a human in real-world situations). 

 Overall, if getting the most light as far out as it can go is the customer’s goal, then the clear lens makes the most sense. If facial recognition, aesthetics, and evenness of lighting is preferred, then the OPPE 37 with opal polycarbonate lens is the best choice.

Generally speaking, this principle can be applied to other Access Fixtures bollard lights —  opal polycarbonate lenses generally produce more even vertical illuminance while clear lenses and louvered bollard lights provide greater reach to the ground.

Click below for the full photometric comparisons of the OPPE 37 clear lens and OPPE 37 opalescent lens.