In 2001, I attended a training course conducted by Dr. Warren Julian, from University of Sydney, which created a new understanding of lighting. Me and my friend became ardent fans of the professor and peppered him with questions after questions and did not even let him eat peacefully during the breaks. But, he was so kind and nice that he tolerated us.

I had completed 3 years in Philips in 2001 and was part of a team which did complete street lighting illumination of the twin cities of Hyderabad and Secunderabad and architectural and landscape illumination of Ramoji rao film city, Hyderabad. They were such prestigious and high value projects that this training program was supposed to be a reward and only two of us were sent for this course from Philips. My friend was from the design department and I was from the sales department.

I thought I had learnt everything there is, in lighting, to learn, in those 3 years. While I had learnt the subject by rote from the "the lighting bible" (Philips lighting guide!) I lacked the understanding of real life implications of light.

I will try to explain the first and most important principle that I learnt (understood) that day. Have a look at the picture below and check if you can see all the light in the picture. This is a picture from NASA website, of our earth and moon.

Can you see the light in the above picture? Yes, of course. But, what light do we see? The light or the reflected light? Does all the black color in the picture represent darkness?

Now, see this picture…

Do you see the light or the reflected light? How do we see the light ray here? It is the surfaces of dust particles which reflect the light.

From big planets to minuscule dust particles, without SURFACES, we do not "see" light.

Since there were only the surfaces of earth and moon in the previous picture, we saw darkness all around the surfaces, in spite of tremendous amount of light from the sun.

Sun emits a complete spectrum of electromagnetic waves and we see only the visible part of that spectrum. Infra-red and ultra violet waves are on either side of that visible spectrum. Other parts of the spectrum of electromagnetic waves have given us the modern lifestyle that we live today!  

Back to our discussion, I knew that there is a difference between illuminance and luminance. I could have told you the definitions of these terms, to the letter at that time! I knew that lux is for illuminance and candela per square meter for luminance.

Lumens or photometric flux or visible flux density, is amount of light emitted by a light source

When we measure the amount of light falling(incident) on an unit area, we call it lux or foot candles.

Lux or foot candles is lumens/ per square meter or lumens per square foot, respectively.

Multiply foot candles by 10.76 and you will get the equivalent value in lux.

So far, we have been discussing only about the light falling (incident) on an unit area, illuminance.

Luminance is the brightness of an illuminated surface or luminous surface as perceived by the human eye. It is measured in units of luminous intensity per unit area. If you consider lumens per steradian per square meter, it is candela per square meter! (enough showing off, no need to mention, photopic flux and scotopic flux etc…).

But knowing these definitions were of no practical use for real life applications. I had to simplify it to - amount of light falling on an unit area is illuminance and the portion of reflected light that is perceive by our eye is luminance.

Any incident light will get absorbed, reflected, transmitted, refracted etc and then a portion of it will come towards you!. And depending on your age, health etc, the amount of light finally entering your eyes, reach the rods and cones, which convert them in to electrical signals. The optic nerve carries these signals to the brain, where the image is produced/perceived.

This is one of the high order brain functions and takes a considerable amount of brain capacity for this image processing. Hence, closing your eyes for a few minutes is a big relaxation technique, if you are tired. Then, imagine, what a good sleep can do to relax us!

(How we see light and health impacts of light are topics for future blogs!)

So we can understand that the amount of light we have in a volume of space and what we perceive are entirely different and are not necessarily directly proportional. Depending on the surface finishes, textures, colors, materials, direction of light (Check out steradians, if you want to do Phd in lighting!), viewing angles etc. only a fraction of the incident light is perceived by us.

Even today, after 13 years of realizing this differentiation, it continues to teach me new things every day.

If I go to an office, a restaurant, shop, gardens or any other functional space, I constantly make an effort to differentiate between the illuminance and luminance.

Have a look at the video (end of page) I came across, some time back. The video features a light source going all around the model's face.

So every frame has a different angle of incident light. View it frame by frame (drag on the time bar, instead of pressing play) to see how the same face and same expression, transforms in cycles between friendly to threatening to scary as the light goes around.

It is said that more than 70% (not exact number, but, surely, a huge portion) of our communication is visual. Imagine an office setting or a social setting like a restaurant where you are having a conversation with somebody important. If the lighting creates harsh shadows on their face or worse still, like in some restaurants, where the light is coming from below.

Without even realizing it, because of improper lighting, subconsciously, our communications could be distorted by what we see and hence what we hear.

While illuminance is used often, for simplicity, luminance is an important aspect of lighting design.

What we do not see, is much greater than what we see!

What do you think? Share your comments below.

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I aim to spread awareness on the subject of Healthy Lighting designs to facilitate co-creation of healthy lighting at home, work and other social spaces. My posts can be accessed at Archinect  and Linkedin Group. I also have a self published book "Healthy Lighting of your Homes" at Amazon.