Overview:
This is a quick way of estimate the quantity of fixtures needed to illuminate a specific
area. It works because there are many physical constraints on a layout. These include (1)
where poles can be located, (2) row and columns needed for a uniform appearance, (3) the
need to have uniform number of fixtures on a pole for uniformity of appearance, and (4)
there is no such thing as 5.3 fixtures.
The concept:
1. It all works because all you need to know are (1) the number of Lumens needed to light
the area and (2) the number of lumens that a single luminaire can deliver to the surface
2. Simply dividing the total lumens needed to light the area by the net lumens coming from
one luminaire will tell you approximately how many fixtures you need.
3. Once you have the raw quantity, that quantity must be quickly massaged to give a
reasonable solution.
For example, if you reach an answer of 5.3 fixtures, then you would
increase the quantity to a whole fixture (6 fixtures).
Without considering the details of the layout, you can tell your Client
that it will take about 6 fixtures to light the job.
Easy?
To continue. . .
Lumens needed to light the area:
Knowing the average illumination level needed and the area being lighted
determines the number of Lumens needed.
Lumens = (FC(avg) * AREA)
For example, to light a 250 ft by 500 ft area to an average level of 2
foot-candles (fc) will require
Lumens = 2 * (250 * 500) = 2 * 125,000 sq ft = 250,000 Lumens
But, what if your specification calls out 1/2 fc Minimum? Well, if this is to be a uniform
lighting layout, then we know that the relationship of average fc to minimum fc is about 4
to 1. Thus 1/2 fc minimum is also stated as 2 fc average. To go a bit further: if the
requirement is 5 fc Minimum, then the average fc level is 20. Many national retailers are
demanding these and even greater illuminance levels.
Here's how:
All you need to do is:
1. determine the number of lumens needed to illuminate the area in
question.
2. determine the amount of light (lumens) that one fixture can produce.
3. Divide (1) by (2) to find the quantity of fixtures required.
That's easy!!
However, to make it work for you, you need a bit more information and you must make
several assumptions.
Net Lumens from 1 Luminaire
Now you know the number of lumens needed to illuminate the whole area (250 ft by 500 ft)
to 2 fc average (125,000 Lumens)
Now you need to figure the net number of lumens coming from 1 luminaire.
(This is really simple ... it just sounds a little complex).
The formula to figure the net lumens from a fixture is: Net Lumens = (Rated Lumens * LLF *
CBU)
The following assumptions I use for the estimate. They have nothing to do with the
performance (or goodness) of one luminaire over another.
1. What lamp to use?
High pressure sodium ( that yellow light) or Metal Halide (that beautiful high quality
color light)?
What size lamp (watts)? 1000 watt for large areas, lower wattage for smaller areas and
where mounting heights are restricted. Some towns limit the mounting height to 14 or 16
feet.
Choose the lamp you wish to try and look up the rated initial lumens in a lamp catalog. To
save time, I made a lumen table, below.
2. We must adjust the number of luminaire needed to deliver maintained illumination. Being
that lamps blacken with age, the lumen output decreases. Thus for Light Loss Factor
(LLF)
use 0.80
3. We must estimate how many lumens from the lamp actually leave the luminaire and reach
the ground. For this, I assume a Coefficient of Beam Utilization (CBU) of 0.50.
To make things easy, here is a table of net lumens emitted from a fixture to the surface,
based on initial lamp lumens and applying the LLF (0.80) and CBU (0.50).
For example:
For a 1000 watt metal halide, which is rated at 107,800 initial lumens. Net Lumens =
(107,800 * 0.80 * 0.50) = 43,120 lumens (round to 43,000 lumens)..
Here is the table for a few popular lamps:
| HPS |
Initial Lumens |
Net Lumens |
| 100 |
9,500 |
3,800 |
| 150 |
15,800 |
6,300 |
| 250 |
29,000 |
11,600 |
| 400 |
50,000 |
20,000 |
| 1000 |
130,000 |
52,000 |
| Metal Halide |
Initial Lumens |
Net Lumens |
| 100 |
8,500 |
3,400 |
| 150 |
13,000 |
5,200 |
| 175 |
15,000 |
6,000 |
| 250 |
23,000 |
9,200 |
| 400 |
39,000 |
15,600 |
| 1000 |
107,800 |
43,000 |
Let's try one example
1. The parking lot measures 600 feet by 250 feet
2. This is a security-conscious tenant who requires 5 foot-candles minimum (remember -
that's 20 fc average)
3. The tenant wants the excellent color rendition of of metal halide (he can never find
his brown Chevy Malibu under HPS).
4. There is no limit on pole height. Let's use 30 ft poles)
Intuitively, you choose to select the 1000 Metal Halide lamp (43,000 net lumens)
The number of lumens needed to light the area is (fc times area) = 20 X 600 X 250 =
3,000,000 lumens
The number of lumens from 1 luminaire = 43,000 lumens
Thus Quantity of luminaires needed to light the area is (3,000,000 lumens / 43,000 lumens)
= 69.8 (70) luminaires
You can stop here and report the estimated quantity. You are finished if all you want is a
quick estimate.
But, you can go further to get a quick reading on the number of poles needed, while still
meeting the spacing-to-mounting height requirements dictated by the luminaire
photometrics.
To get a quick read on whether you can meet the spacing requirements for an even
illuminated layout, you simply calculate the area covered by a single pole. Whether a pole
has 1, 2, 3 or 4 luminaires hanging from it, this area and the subsequent spacing can be
quickly estimated.
Divide the total area by the number of pole locations to obtain the area covered by
each pole. Take the Square Root of the area to estimate the length of a side. Divide this
length by the mounting height to determine whether the spacing criteria is met.
Try this: 70 luminaires are needed to illuminate this area (150,000 sq ft).
The number of poles needed is:
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