Light Painting Tips and Techniques: Angle of Reflection Equals Angle of Incidence

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In my light painting workshops and presentations, I talk a lot about lighting theory, and how I believe that it is easier to understand how light works if you can break lighting theory down into just a few basic principles. In this post, I’ll use an image that I just made this past weekend at the workshop I was teaching. I always do a demonstration on the first day of the workshop, and this time, I decided to shoot a vintage industrial blower that I found a few months ago. I love the patina that these old machines and tools have gathered over many years of use.

One of the most important principles in lighting theory is that when light strikes an object at a certain angle, it reflects at the same (yet opposite) angle. And so we say “The angle of incidence is equal to the angle of reflection”. Think of a billiard table. When a billiard ball strikes the bumper on the side of the table, it bounces off at exactly the same (but opposite) angle. Of course, this happens only if there is no spin on the ball. I have yet to figure out how to put a spin on light!

Please see the short tutorial video below the image!

If you’re interested in learning about light painting (and so much more!), please consider attending one of my upcoming workshops! You can find the information HERE.



Photographer Harold Ross' light painted image "Vintage Industrial Blower"

Photograph by Harold Ross

~ by Harold Ross on March 8, 2017.

4 Responses to “Light Painting Tips and Techniques: Angle of Reflection Equals Angle of Incidence”

  1. Great tips Harold. Thanks again!

  2. NIIIIIIIIIIIIIIIIIIIIICE job on the video …


  3. Thanks Harold. This tip was very helpful. I’m reading a book on light theory and it is very focused on the Family of Angles based on this law. I’m curious on how the Family of Angles relates to light sculpting?

    • Hi Michael, Nice to hear from you! Basically, “Family of Angles” takes into account the focal length of the lens. When we use a shorter lens (usually not recommended in the studio) we “gather” more reflections (direct reflection), or put another way, the camera “sees” a broader area in direct reflections, meaning that we would be more likely to pick up a specular reflection in a flat reflective surface. Conversely, with a longer lens, the lens “sees” a narrower angle, which can afford the photographer more ease in avoiding specular reflections in a flat reflective surface. In light painting, we sometimes want those reflections (such as when we light a black or very dark object, as those objects are, in a sense, defined by their reflections. Since light painting is more dynamic (we are, after all, constantly moving to light to make it softer), “Family of Angles” is not as important for us to think about, as we do not use a light source of given size and placement. It is more important to think about “Family of Angles” when using a soft box or scrim, and knowing how to change focal length to help. In light painting, it is a bit more direct (and simpler) to think about “The angle of incidence equals the angle of reflection”. We simply place the light in the correct position to give us the specular highlight (or, if we don’t want one, we place the light where it won’t give us the reflection). Further, Since most objects have a radius, or rounded edge (these radii vary in size, of course), the specular reflection will reveal the curvature of that surface. Also, in light painting, we light from a close distance (for several reasons) and this helps to make those specular highlights (which define the shape in a very dark object) softer and more beautiful. Hope this helps! – H

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