Hi there!

I'm wondering if there is any advantage to using one of the clear top cover PC's on a Nema style fixture? My thinking is that the clear cover will allow maximum light exposure to the actual PC sensor thus reducing it's sensitivity from dusk to dawn.

I have a Nema fixture thats in my backyard and is somewhat shaded by some tree branches, so its a little more dark compaired to if it was out in the open.

This causes the fixture to come on early (before dusk) and shut off late (about an hour or so after sunrise) so if I use this clear PC, maybe it will not behave like the Fisher Pierce PC thats currenly installed?

A fellow Lighting Gallery Member (gmercury2000) was very kind and sent me some BT-25 100w Westie Lifeguards, a few PC's and a ultra rare and very cool Westinghouse 100w Nema  head!

Will be installing this to replace my Landmark custom built 100w MV fixture that has been up for a  year now doing the dusk to dawn thing.

What you want from the photocell actually is, to switch on and off at certain levels of light (which may not necessary be the same level for switching on and switching off)

We can look at it as the absolute light level for switching on and absolute light level for switching off, but instead, lets look at it as the approximate light level for both switching actions, and the difference between them

The absolute light level goes up and down in any photocell, When the receptor (LDR/phototransistor/whatever) is exposed to direct sunlight it will get more light, and when it is covered it will get less. But the sensitivity of the receptor can be different as well, so the 2 photocells with different design may be made to switch at the same light levels

The difference between switch-on and switch-off is more interesting story :

In a basic design, there would be no difference - same level for switching on and off. The problem with such design is, that near the border lighting level (say somewhat cloudy day at dusk), it may switch more then once, and that would have quite significant effect on the life of a HID lamp

To prevent that, in an improved design hysteresis is built in - The cell swith on at light level X, but to switch back off the light would have to be somewhat higher then X. The difference in switching levels is chosen so that minor distractions (light cloud coming and going, ...) won't cause the redundant switching

(in some cells a time delay is added as well - This is more to prevent switching from short time disturbances, like a bird sitting on the light for a second or car headlights hitting the cell from distance along the road. This feature comes free in the cells with thermal switching, but can be made in electronic cells too)

But is that the best way for energy efficiency ?

In dusk, we are after day, so want to continue to see fairly bright light. As the dusk comes down, we want the light to switch early on. In dawn, we are after dark night, and can see pretty well with the first light of the sun starting to come up, so can switch the light off early, at much lower light level then the switch on, and save the energy of the extra time at dawn where we dont really need the light

In addition, the HID lamp is not immediately at full brightness when first switched on, so e want to switch it on even earlier, but not switch it off later

So, we actually want a reverse hysteresis - switch on at some light level, then back off at even lower light level. And we still want to get it right, so without redundant extra switching on and off around borderline light levels....

The difference in light levels for switching may be achieved in few ways :

 - In "normal" hysteresis all it takes is some tipping-point mechanical system, that can be tweaked for the wanted difference in levels

 - With a photocell that have a side window, we can try to turn thw window to some side - for example to east, to try to make some difference

 - With electronic photocell, we can make pretty much any switching schedule we want electronically, then there is no need to care as much for the position of the receptor



But those are not the only considerations :

Deterioration of the cell :
Sunlight contains a lot of UV (in different bands), that damages most plastics (so the cover or window of the cell casing) as well as the materials used in the receptor of the cell (cadmium sulfide, epoxy materials and so on)

Exposing or not exposing the parts to direct sunlight will affect the way the materials deteriorate - When turned to North (where the sun does not go in the Northern hemisphere) or South (in Australia & New Zealand), the materials of the cell will last longer. So it makes sense to instruct the installation of the cell with the window pointed to North and make it more sensitive accordingly, to react at light levels seen to the North direction (so basically the brightness level of the blue sky)

Sensitivity spectrum of the cell :
The receptors can be made in ways to react to only part of the spectrum. For example. a photocell used in a fluorescent uplighter may be made sensitive to IR only by n IR filter, then it is oblivious to the light of the lamp used in the same lantern - So you dont have problem of the lantern making itself cycle, and dont need to position the cell remotely. On the other hand, such cell won't react very well to the blue sky either, as the IR which it senses goes mostly with direct sunlight and not with the scattered light from the sky....

Hey thanks for the reply. Wow I must say your knowledege of light fixtures is very extensive. I think with my Nema head MV fixture being installed around trees it's going to be difficult to obtain the on and off times or light levels that most PC's would see.

My thinking with the clear top PC's, there design would allow maximum exposure of light to strike the sensor thus decreasing it 's operating time because the sunlight can enter at all angles?

Thanks for the write up

The cells with clear top allow more light to the receptor, but this is allready taken into account in their design - so the receptor is made less sensitive compared to the ones with the side window instead. They might switch at the same light levels

The switching levels of the cell have ratings : light level (typically around 30..60 Lux or so), and the difference between the light levels for the on and off switching, stated as proportions (1:2, 1:0.5....). But what light level will hit the cell at what time of the day, that is the matter of the location you install it, trees around and other stuff



You can get around in many ways.....

Use a more sensitive cell. It does not have to be a clear top cell

Turn the cell with side window in a direction where there is more light (but not where it can get direct sunlight, this may shorten the cell lifetime)

Remove the cell from the lantern and place it in another location where the lighting level is right (and from there connect the cable to the lantern)

Use a time switch, manual switch, ripple control device (if your electrical utility company provides the control signal for it), computer controlled switch and so on

I've never really liked the clear Photocontrols.  The LC-120 from Area Lighting Research Inc. started off with a clear cap in the early-mid 80s, then in the early 90s, they switched to a gray cap, and then in 1994-1995, they went back to the clear cover.  The BF-120 was the Utility version of the LC-120, and it kept the gray cap.  The problem with the ALR Photocontrols was they did not last long before the gray cap turned a dark lemon-lime color, and the window became so fogged up that it was almost impossible for the darned thing to work.

The ones that come with the newer lowes utilitech lights are complete crap. They will dayburn during overcast times and would only kick off when it was completely sunny. I decided to buy a intermatic pc and it did service for three years until a power surge cooked it.

The ones here in my part of the UK are sort of greenish color they work fine even in our cloudy climate. I must ad!it though it's been very warm and sunny lately

Different plastics, doped with different additives (that include additives to slow down the deterioration), decay in different speed....

Also, the extent of clouding of the plastic that we see (in visible light) is not allways same as what the chip inside sees (in whatever wavelength it may be sensitive to, some work in the IR range for example)

Does anybody remember the Pre-1992 Fisher Pierce 6660 Series Photocontrols.  The plastic cover was made out an entirely different material, and it aged fairly quickly, in that it wasn't long before the gray cap turned greenish/yellowish.  1991 was the last year I believe they used that material.  The 1992 6660s used a new plastic that did not yellow at all, and I believe it is the same material plastic they use on the cap to this day.  I have an 88, 89, and 91 Fisher Pierce that have never been used, and I have a 1991 Fisher Pierce that was used from 1991-1998 and in the 7 years that it was used, the cap looked 20+ years old.

I think I recall what your talking about. The gray plastic cover would turn a tan brownish sorta crispy sunburnt look and it was from just that, the suns UV rays. I have seen many like that and I even have one in my collection. The date is 1991!!

Around 1999, I got an email from a guy named Ben Crutcher, who worked for Fisher Pierce at their Weymouth, MA location.  He told me that when they moved from their Braintree, MA location to the Weymouth, MA location, they started planning an entirely different assembly process.  Starting in 1993, it went from 90% Handmade, 10% Automation to 10% Handmade, and 90% Automation.  Which obviously saved them money, but at the same time, I have yet to see a Fisher Pierce PE Control made after 1993 that lasted as long as some of the pre-1993 models (from 6660 to 7760).

I think I recall what your talking about. The gray plastic cover would turn a tan brownish sorta crispy sunburnt look and it was from just that, the suns UV rays. I have seen many like that and I even have one in my collection. The date is 1991!!

I have a 1991 that is old, and brown, and I also have a 1991 that is brand new.  It's awesome to compare the two side by side.