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In Parts One and Two of this blog, I described the basic situation of a photosensor mounted high in a space not working and the Inverse Square Law being invoked as the obvious reason why.  Now let’s follow this to a logical conclusion.

The oft cited explanation is that the photosensor is seeing a light source such as a desk in its field of view.  That desk is a light source by virtue of its reflecting daylight from its top surface.  The desk isn’t truly a point source, but we can overlook that for now.  If we are twice as far from that desk, our photosensor will receive approximately one-fourth the light from it.  And the less light the photosensor receives, the harder it is to calibrate accurately.  That makes sense, doesn’t it?

Not really.  Something that does not change no matter how far the photosensor is above the floor is the sensor’s field of view.  Imagine I have a photosensor mounted high on a ceiling pointed downward.  If it has a field of view of 30 degrees and is 16 feet high, it “sees” a floor area of 57.74 square feet.   If I were to move that photosensor down to 8 feet, it would “see” only 14.43 s.f of floor.  That photosensor at 8 feet above the floor would probably only see one student desk.   But when the same photosensor moves to 16 feet above the floor there would be four desks in its field of view.  The math is pretty obvious.  When the photosensor is twice as far away, each desk contributes one-fourth the reflected light.  But the four desks in the field of view perfectly balance that loss, and the photosensor receives the same amount of light!  It should work just as well at 16 ft. as it does at 8 ft.!

If you want to learn more about what is really going wrong with photosensor performance, check out Part Four.


Roofs:  roofs can be steep, gently sloped, or flat (which still have a slight slope to facilitate drainage) and take many forms, gable, gambrel, hipped, stepped gable, shed, pent or Mansard.  The roof type is an important key to identifying the style of a building.

Parapet:  a wall-like barrier at the edge of a roof that extends above the roof.

  • Where extending above a roof, it may simply be the portion of an exterior wall that continues above the line of the roof surface, or may be a continuation of a vertical feature beneath the roof such as a fire wall.
  • Parapets were originally used to defend buildings from military attack, but in contemporary architecture they are primarily used to terminate flat roofs into walls and/or prevent the spread of fires.

Soffit: the underside of a part or member of a building.

  • On a roof a soffit is the underside of the horizontal overhanging edge of the roof.  In most cases on buildings with sloping roofs the wall terminates into the soffit.  This overhang can be different widths depending on the design and preference. This will provide protection from the weather as well as add to the aesthetics of the building.
  • As opposed to the fascia that is the vertical face at the edge of a sloping roof.
  • the underside of an architectural feature, as a beam, arch, ceiling, vault, or cornice.
  • the underside of a part of a building or a structural component, such as an arch, beam, stair, etc.

Eaves: the overhanging edge of a roof that projects beyond the wall at the bottom of sloping roof.

  • Typically consists of a soffit and fascia.
  • Gutters are typically placed at the eaves to collect and channel water away from the building walls and foundations.
  • As opposed to a rake which is the overhang on the side of a sloping roof as seen on a typical gable end for example.

Shed:    A sloping roof that consists of only one sloping plane.

Gable:  The generally triangular section of wall at the end of a pitched roof, occupying the space between the two slopes of the roof

  • A triangular, usually ornamental architectural section, as one above an arched door or window.

Hipped Roof: A hip roof, or hipped roof, is a type of roof where all sides slope downwards to the walls, usually with a fairly gentle slope. Thus it is a building with no gables or other vertical sides to the roof.

  • A square hip roof is shaped like a pyramid. Hip roofs on the houses could have two triangular sides and two trapezoidal one. A hip roof on a rectangular plan has four faces. They are almost always at the same pitch or slope, which makes them symmetrical about the centerline.
  • Hip roofs have a consistent level fascia, meaning that a gutter can be fitted all around. Hip roofs often have dormer slanted sides.

Gambrel: A gambrel is a usually symmetrical two-sided roof with two slopes on each side. The upper slope is positioned at a shallow angle, while the lower slope is steep. This design provides the advantages of a sloped roof while maximizing headroom on the building’s upper level.  This efficient use of space is one reason gambrel roof are found on many barns.  The name comes from the Medieval Latin word gamba, meaning horse’s hock or leg.

  • The cross-section of a gambrel roof is similar to that of a mansard roof, but a gambrel has vertical gable ends instead of being hipped at the four corners of the building. A gambrel roof overhangs the façade, whereas a mansard normally does not.

Mansard Roof:  named after the French architect Francois Mansart (1598-1666); a double sloped roof with the lower slope being longer and steeper, with a concave curve.  Can be sloped on all four sides or just two sides (front and back).

  • A mansard or mansard roof (also called a French roof) is a four-sided gambrel-style hip roof characterized by two slopes on each of its sides with the lower slope, punctured by dormer windows, at a steeper angle than the upper. The roof creates an additional floor of habitable space, such as a garret. The upper slope of the roof may not be visible from street level when viewed from close proximity to the building.

Pediment:  a triangular space created by a front facing gable roof, often seen in Classical Revival style buildings.

  • A pediment is a classical architectural element consisting of the triangular section found above the horizontal structure (entablature), typically supported by columns. The gable end of the pediment is surrounded by the cornice molding.  The tympanum, or triangular area within the pediment, was often decorated with sculptures and reliefs demonstrating scenes of Greek and Roman mythology or allegorical figures.
  • (In classical architecture) a low gable, typically triangular with a horizontal cornice and raking cornices, surmounting a colonnade, an end wall, or a major division of a façade.
  • Any imitation of this, often fancifully treated, used to crown an opening, a monument, etc., or to form part of a decorative scheme.

Facade:  A facade is generally one exterior side of a building, usually, but not always, the front. The word comes from the French language, literally meaning “frontage” or “face.”

  • In architecture, the facade of a building is often the most important from a design standpoint, as it sets the tone for the rest of the building.  Many facades are historic, and local zoning regulations or other laws greatly restrict or even forbid their alteration.
  • The front of a building, especially an imposing or decorative one.

In Part One of this blog, I described a common explanation for photosensor  failure – being mounted too high in a space.  When I ask why the sensor won’t work properly because it’s too high, the inevitable answer is “the inverse square law, of course!”  A look of smug satisfaction usually accompanies the recall of this tidbit from high school physics.  Case closed!  Everybody knows there is no defense against the infallible inverse square law of photosensor failure.  It’s as good as the Latvian Gambit in chess.  Or for you Trekkies, the Corbomite Maneuver.

Having taught Daylighting at the University of Colorado for many years, I’ve had the opportunity to ponder this control challenge.  And I don’t buy the ISL explanation for a nanosecond (as we’re talking about photons, a minute would be entirely inappropriate).  Here’s why.

It’s true that our most common light sources, whether the sun, an incandescent bulb, or a fluorescent lamp, emit light that weakens as the inverse square of the distance from the light source.  Twice as far from the light source, one fourth the light.  Three times as far, one ninth the light.  Pretty basic stuff.

Oops, that’s all for Part Two.  Continue on to Part Three for the conclusion.

We have recently experienced some problems with daylight photosensors inside buildings not properly controlling the electric lights within their zones.  Most often, the sensors fail to detect daylight accurately and they therefore leave lights on or fail to dim them adequately.  This is a significant problem because electric lights account for a large share of energy use in a school building.  When we have both daylight and electric light, we are wasting resources.

As we began investigating the issue, we heard a variety of explanations.  Some made sense, others less so.  One very common excuse was that “the sensors are only rated for a certain distance above the floor.  Higher than that, they don’t work.”

This response drives me nuts!!!  I can just imagine the picture inside the mind of the sales rep, electrician, or even electrical engineer.  Itty bitty photons leave the surface of a desk, heading upward.  They valiantly attempt to reach the photosensor 16 feet above the floor, like Thomas the Tank Engine climbing the hill on the island of Sodor.  But the higher they get, the more they fight gravity, saying to themselves “I think I can, I think I can.”  Somehow knowing the photosensor is only rated for 12 feet, they fall back just short of the sensor.  And the lights stay on.

If you want to know what’s wrong with this scenario, look for Part Two early next week.