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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.


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.

Casement: in a window refers to a vertical window hinged on its vertical side, meant to open either out or in.

  • a window containing frames hinged at the side or at the top or bottom
  • a poetic word for window
  • hinged window: a window that opens on hinges located at one side, as distinct from one that slides up and down

Casing:  Casing is a type of trim moulding used to trim out windows and doors. This trim is called door casing or window casing depending on the application. Casing will come in different sizes and profiles. Two poplar casing are colonial and tear drop.

  • frame for door or window: a frame containing a door, window, or stairway

Clearstory (or Clerestory):  the upper level of a room that extends beyond the single-story height; often found in churches and penetrated by windows.

  • May also refer to the upper row of windows that is close to the ceiling
  • In modern usage, clerestory refers to any high windows above eye level; the purpose is to bring outside light, fresh air, or both into the space.
  • A clerestory is a high wall with a band of narrow windows along the very top. The clerestory wall usually rises above adjoining roofs.
  • Originally, the word clerestory referred to the upper level of a church or cathedral.
  • Pronunciation: Clerestory is pronounced clear story.

Fenestration:  The arrangement of windows across the facade of a building.

  • Fenestration, refers to the design and/or disposition of openings in a building or wall envelope.
  • Fenestration products typically include: windows, doors, louvers, vents, wall panels, skylights, storefronts, curtain walls, and slope glazed systems.
  • the arrangement, proportioning, and design of windows and doors in a building
  • an opening in a surface (as a wall or membrane)

Louver: A louver (American English) or louvre (British English), from the French l’ouvert; (“the open one”) is a window, blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain, direct sunshine, and noise. The angle of the slats may be adjustable, usually in blinds and windows, or fixed.

Glazing:  Glazing, which derives from the Middle English for ‘glass’, is a part of a wall or window, made of glass. Glazing also describes the work done by a professional “glazier“.

  • Common types of glazing that are used in architectural applications include clear and tinted float glass, tempered glass, and laminated glass as well as a variety of coated glasses, all of which can be glazed singly or as double, or even triple, glazing units. Ordinary clear glass has a slight green tinge but special clear glasses are offered by several manufacturers.
  • Glazing can be mounted into a window sash or door stile, usually made of wood, aluminium or PVC. The glass is fixed into a rabbet (rebate) in the frame in a number of ways including triangular glazing points, putty, etc.. Toughened and laminated glass can be glazed by bolting panes directly to a metal framework by bolts passing through drilled holes.
  • Glazing is commonly used in low temperature solar thermal collectors because it helps retain the collected heat.

So now the big question, is that “window” a window, storefront, or curtainwall?  Generally speaking all three are means to insert glazing, i.e. glass, in the exterior envelope of the building, but there are important technical and cost differences between the three.

Window:  A window is a complete manufactured unit that includes glass, frame, and componentry all in one.  Windows are delivered to the job site ready to install in a framed opening in the building.

  • Windows are most common in residential construction.
  • Big box home improvement stores carry windows, but not storefront or curtainwall.
  • Generally speaking windows are the least expensive of the three systems.

Storefront:  Storefront is different from windows in that it typically does not come to the job site assembled as a complete glazed unit.  The frames are either be pre-assembled and delivered to the site, or can be assembled in a framed opening on site.  The glazing is later installed in the frame on site.  This allows storefront to be more customizable and much larger than a window.

  • In most cases storefront will consist of multiple windows and/or doors in one framed opening in the building envelope.  While multiple windows and doors can be attached to one and other the effect is not as seamless or stable as can be achieved with storefront.
  • Storefront is generally more expensive and durable than windows.
  • Although large windows are available, typically storefront or curtainwall are used when larger individual glazed areas or a large combination of windows and doors is desired.  Ultimately the size of any individual area of glazing is limited by available glass sizes.
  • Storefront is typically used in openings up to two stories in height.  Larger openings require the use of curtainwall to resist wind loads.
  • The name here is telling.  Storefront is typically used at the front of retail stores to allow the store to feel open and welcoming to customers by maximizing visibility.  That being said storefront is also used in many other commercial building applications and even occasionally in custom residential work.

Curtainwall:  Curtainwall is the most complex, durable, and expensive glazing system of the three. That being said it shares one important similarity with windows.  It is typically delivered to a site in fully assemble panels that are ready to hang from the structure.  Where it differs is that the assemblies are large enough to require a crane for installation.

  • Curtainwall also differs from windows and storefront in that instead of being installed in framed openings in the exterior envelope it typically is the building envelope.
  • Curtain Wall Systems are typically designed with extruded aluminum members, although the first curtain walls were made of steel. The aluminum frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as daylighting
  • Curtain walls differ from store-front systems in that they are designed to span multiple floors, and take into consideration design requirements such as: thermal expansion and contraction; building sway and movement.
  • Although it is accurate to say that curtainwall is non-structural in the sense that it is not part of the structural frame of the building, it is the strongest of the three glazing systems in its resistance to wind loads.
  • Curtain wall is typically suspended from a buildings structural frame.
  • It is typically used on high rise buildings and other large commercial buildings.
  • On smaller projects that do not require the use of curtainwall to resist wind loads it is typically avoided due to its high cost.