Learn photometric principles for solar lighting design

What is Photometry?

Photometry is the measurement of light visible to the human eye.

Photometrics are of particular importance when installing lighting fixtures as they measure how bright a light will appear to onlookers. They allow us to ascertain the visibility that lighting will provide to a given site.

Photometric performance may be especially important in solar fixtures because available power may be dependent on the hours of sunlight.  Lighting designers should consider average and minimum hours of sunlight, the hours when light is needed, and what level of visibility is required. These will help choose necessary lighting profiles. Proper placement of solar fixtures will ensure proper photometric performance and that an adequate level of safety and security is provided.

Several photometric principles should be taken into consideration during site planning.

Lighting Distribution

Many lighting fixtures offer several standard patterns: symmetrical and asymmetrical. Symmetrical lighting delivers light in a condensed, even circular pattern. They are commonly used in landscaping applications or to brighten medians and contained areas. Lights with an asymmetrical distribution pattern deliver light in an elongated form that is often used for pathway lighting. The following diagram demonstrates how light from both patterns will fall on a surface surrounding a lighting bollard:

Chart showing symmetrical and asymmetrical light distribution options
Select lighting distribution at time of order.

Luminous Flux

Luminous flux is a measurement of the overall light output that a solar bollard provides. Luminous flux is measured in lumens and can provide an indication of how effective the illumination of lighting  will be. Along with lumen output, it is also important to consider where the lumen output will be distributed. As previously mentioned, symmetrical and asymmetrical distributions are effective for different applications. 


Illuminance is a measure of the illumination provided by a solar bollard light at various points throughout its distribution pattern. Light emitted from a solar bollard will appear more intense at certain points of distribution than others and different distribution patterns will deliver light at different levels of intensity. This means that light emitted from these differing patterns will often appear more intense at certain points of distribution than others. As discussed above, lighting bollards provide light in symmetrical or asymmetrical light distribution patterns and each offers a different illuminance. Footcandles (lm/ft²) is the unit of measurement used to calculate the luminance cast on a surface by a one candela source one foot away. The following diagram demonstrates the differing points of light intensity in two possible lighting distributions. Symmetrical distributions always circle the light source, but asymmetrical profiles can change based on the fixture.


Lighting intensity measured in foot candles


For most solar bollard lighting applications, the average and minimum illuminance values and the maximum to minimum uniformity ratio for the area to be illuminated are of the most interest. Developing a lighting layout that takes both of these values into consideration will ensure that an adequate level of light will be provided and that there will be no breaks caused by lighting level drops. To obtain the minimum illuminance value and maximum to minimum uniformity ratio of your site, you may wish to consult a lighting specialist, engineer or architect. 

Solar Insolation

The term solar insolation refers to the amount of sunlight that a solar bollard receives in one day. It is commonly measured in the amount of hours the bollard’s solar panel will be directly exposed to sunlight. The amount of sunlight light the bollard will receive will determine the level of power the bollard will have to operate. The less sunlight a lighting bollard receives, the less energy it will have to operate. Innovations in solar panels and the use of LEDs as a light source mean that solar lights today are able to run on far fewer insolation hours, making solar more and more accessible to northern or cloudy locales.


Depending on the area in which solar fixtures are installed, there may be days when they do not receive full sun insolation. Advances in panel sensitivity means that solar fixtures are often able to store enough power to function even with ambient or reflected light. This process allows solar lighting to run autonomously in many more locations than was possible even a decade ago. 

Please note that it is necessary to judge the solar insolation that a site receives prior to installation. Each site is different and, although Reliance Foundry’s solar bollards expend energy efficiently, certain base levels of solar insolation are required to ensure that they operate effectively throughout the year. Northern environments should pay particular attention to the average number of no-sun days experienced. Shading may also affect a solar bollard’s ability to receive sunlight and thus its autonomy. If you have question on the typical levels of solar insolation you region receives and if your site can facilitate the use of solar bollards, please contact the Sales Department.


The term “dark sky” refers to efforts to reduce light pollution. Light pollution derives from light that is delivered ineffectively or in an uncontrolled manner. Cut-off lighting fixtures control light delivery so that less than 2.5% of the light output is allowed to escape the fixture above 90 degrees. Cut-off lights may contribute to a site's LEED status. For more information on how lighting design can help a project qualify for LEED credits, please visit the US Green Building Council’s guidelines on light pollution reduction.