Foot Lamberts – Explained
Written by John Higo
Over the last year, we’ve alluded to dedicating an entire newsletter to explaining the difference between foot candles and foot lamberts. So, the time has finally come to demystify the foot lambert.
To begin with, we need to understand how our eye works, and perceives light.
The diagram above illustrates that what we see is reflected light.
To apply to this, the terms that we are familiar with in our lighting world, we modify this diagram slightly.
Looks pretty simple right?
Okay…all done, foot lamberts demystified…but wait…how do we apply this to what we do every day?
Most of us are familiar with footcandles, and if you read our August newsletter, you would know how to read a standard photometric ‘V’ chart. But as we just discovered, this doesn’t tell us how the light will be perceived…it only tells us how much light is landing on our subject.
If what we see is reflected light, we need to know how much light will be reflected by our subject, in order to know how the light will be perceived.
The following chart shows some reflectance values of common building materials for your reference.
How does this relate?
Imagine you are lighting a garden, and you have a cement statue that you would like to use as a focal point. Also in this view, you have a stuccoed cabana. You want your focal point to be the brightest in the scene. If you were to use the same lamp, and distance from your subject on the statue, as you did on the wall of the cabana, the cabana would appear 48% brighter.
Let’s assume you were thinking of using an Illumicare LED 6W-WW-45-FL, 45 degree flood to illuminate both the wall and the statue, and from a distance of 4 feet. We know from our photometric ‘V’ chart that at this distance, with this lamp, we would be getting 15.6fc falling on our subject, but we know from the reflectance chart, that the walls would reflect more light – 40% compared to 27% for the cement. Thus, what we perceive is 6.25 foot lamberts from the walls, vs 4.2 foot lamberts from the statue.
Same lamp, same distance from the subject, but one appears brighter than the other, because more light is being reflected.
Changing the lamp on the statue from the 45 degree to the LED 6W-WW-30-NFL, 30 degree lamp, would change the amount of perceived light to 18.36 foot lamberts, but that may be too much light. Adding a second fixture to the statue, would double the amount of light, making it 30% brighter than the wall. The real difficulty comes in when lighting plant material.
The reflectance chart only gives one number for vegetation (two if you include grass), but are all plants created equally?
A variegated dogwood will reflect more light than a rhododendron, which would reflect more light than a purple smokebush. A silver maple will reflect more light than a Norway maple. It would be a monumental task to undertake measuring and categorizing all common plant material, but all hope is not lost…there are a few simple guidelines that will help you along the way.
It should come as no surprise that light colors reflect more light, while dark colors absorb more light.
Shiny or waxy leaves may reflect more light than fuzzy or hairy leaves. Deciduous trees tend to reflect more light than conifers. Thin leaves may allow light to pass through, while thicker leaves will block light. After that, it’s really trial and error, and experience.
When designing a lighting job, look at the different plant materials, look at all the materials available, and plan accordingly.
Take a few sample leaves home with you, and see how they react to light, you may come up with some creative new ideas on what and how to light. Bear these principles in mind when planning traverse lighting, so that adequate illumination levels are achieved.
And apply these principles to bring cohesion to your scene, by not exceeding the recommended 4:1 uniformity ratio.
For more detailed information about foot lamberts, please feel free to contact our sales team at firstname.lastname@example.org