Comfort and Cut-off Angle

May 9, 2008

Have you ever been in an interior environment in which you feel you need a baseball cap to get the light out of your eyes?   There are two clear reasons that might be the cause of this.  One, the fixtures might be aimed in a way that did not take into regard your position in the space and they are directed right into your face.   If this is the case, it may need to be re-aimed.    Or two, the fixtures may not have an appropriate cut-off angle.  

Cut-Off is a term that refers to the visibility of the bare light bulb (lamp) in the fixture.  The cut-off angle describes the position at which you can no longer see the bulb when directly viewing the light fixture.   This is an important metric in evaluating the quality of a fixture because it is a performance criteria by which comfort can be predicted and controlled.  

Quality lighting fixtures are designed in a way that optimizes the cut-off angle for the application it will be used. 

The cut-off angle is a published detail in most lighting fixture specifications.   It’s measured up from Nadir, directly beneath the fixture, to the point at which you can no longer directly see the bulb.    Cut-off means you avoid glare when walking or sitting in a space.  A “sharp cut-off angle” means that the visibility of the lamp disappears quickly as you move away from the fixture.    A  cut-off angle or 45 degrees or less is generally considered to be “sharp”.  

There are various recommended cut-off angles that lighting designers strive to achieve.  This is based on systematic research about comfort level and other user based criteria such as needs when viewing a computer or driving a car.   Desired cut-off angle is also a function of the ceiling or mounting height of the luminaire.

 Sketch of Cut-off Angle of Lighting Fixture

Another relevant factor to consider is the reflected image of the lamp in the trim or cone portion of the fixture.  Sometimes fixtures uses highly polished reflectors and depending on the effectiveness of the design, the lamp and thus the glare, may be visible in the reflector.  What good does it do to not see the lamp directly if a near mirror image of the lamp is showing up in the fixture cone? 

This lamp image is often experienced by the user of a space as a flash that occurs when walking underneath the fixtures.  If you’re walking in a hallway, you may notice that you are experiencing a flash from the fixtures long before you are close enough to directly see the bulb.   These flashes my occur in the fixture cone from the top down or from the bottom up.  Images that occur from the top down are generally more comfortable and appear less chaotic than images that appear from the bottom up.  Egg shaped images are less noticeable than rabbit ear type images.   A good optical designer at a lighting fixture manufacturer takes this all into account.  

So what’s the big deal if you have never before noticed that this existed?  While this may not be as uncomfortable as sitting in a space with glare,  it does attract your eye,  conscious or not conscious.  This interrupts you’re experience of a space.  You begin to be conscious of the light source and what it’s lighting.   For most applications, the goal is to feel the effect of the lighting on the textures and surfaces in the space, not the lighting itself.

We also absorb an unbelievable amount of information from lighting whether we realize it or not.   Rabbit ear flashes in a reflector cone may actually be giving you a cue of economy or cheapness – since the last place you likely saw it was in a cost cutting environment.

Written by editor · Filed Under Consumer, Facts and Terms for the Lighting Consumer, Lighting Design Notebook 

Comments

One Response to “Comfort and Cut-off Angle”

  1. Darren on November 17th, 2011 4:12 am

    Hi, this is the first article I have found to make a distinction between “rabbit ear” and “egg” forms of reflector glare.
    I would like to understand more about this. Do you have diagrams or videos?

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